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The Physical Properties and Effective Temperature Scale of O-Type Stars as a Function of Metallicity. I. A Sample of 20 Stars in the Magellanic Clouds
Abstract
We have obtained HST and ground-based observations of a sample of 20 O-type stars in the LMC and SMC, including six of the hottest massive stars known (subtypes O2-3) in the R136 cluster. In general, these data include (a) the HST UV spectra in order to measure the terminal velocities of the stellar winds, (b) high signal-to-noise, blue-optical data where the primary temperature- and gravity-sensitive photospheric lines are found, and (c) nebular-free H-alpha profiles, which provide the mass-loss rates. The line-blanketed non-LTE atmosphere code FASTWIND was then used to determine the physical parameters of this sample of stars. We find good agreement between the synthetic line profiles for the hydrogen, He I, and He II lines in the majority of the stars we analyzed; the three exceptions show evidence of being incipiently resolved spectroscopic binaries or otherwise spectral composites. One such system is apparently an O3 V+O3 V eclipsing binary, and a follow-up radial velocity study is planned to obtain Keplerian masses. Although we did not use them to constrain the fits, good agreement is also found for the He I $\lambda 3187$ and He II $\lambda 3203$ lines in the near-UV, which we plan to exploit in future studies. Our effective temperatures are compared to those recently obtained by Repolust, Puls & Herrero for a sample of Galactic stars using the same techniques. We find that the Magellanic Cloud sample is 3,000-4,000$^\circ$K hotter than their Galactic counterparts for the early through mid-O's. These higher temperatures are the consequence of a decreased importance of wind emission, wind blanketing, and metal-line blanketing at lower metallicities. Comment: Accepted for publication in the Astrophysical Journal. A postscript version with the figures embedded can be found at ftp://ftp.lowell.edu/pub/massey/haw.ps
... Similar investigations have been performed for OB-stars in the Magellanic Clouds (MCs), in particular to study metallicity effects on the effective temperatures and mass-loss rates. Massey et al (2004Massey et al ( , 2005) investigated a large sample of MC O-stars by means of FASTWIND, and provided a spectral-type-T eff calibration for the SMC. For the LMC, the situation remained unclear, since their sample was concentrated towards the hottest objects, O2-O4. ...
... Mokiem et al (2006Mokiem et al ( , 2007a) 20 studied the O-/early B-star targets of the FLAMES survey in the SMC and LMC, respectively. They confirmed the basic results from Massey et al (2004Massey et al ( , 2005, but refined the spectral-type-T eff scale, particularly with respect to the LMC objects. They showed that, at least for O-dwarfs (which are not "contaminated" by additional wind-effects), the effective temperatures for a given spectral sub-type decrease with increasing metallicity, i.e., T eff (SMC) > T eff (LMC) > T eff (MW). ...
... (iv) For a given luminosity, the mass-loss rates of SMC-stars are lower than for their Galactic counterparts, consistent with theory (Massey et al 2004(Massey et al , 2005. A more precise quantification of the metallicity dependence of the winds from O-/early B-stars was possible within the FLAMES survey. ...
Mass loss is a key process in the evolution of massive stars, and must be understood quantitatively to be successfully included in broader astrophysical applications. In this review, we discuss various aspects of radiation driven mass loss, both from the theoretical and the observational side. We focus on winds from OB-stars, with some excursions to the Luminous Blue Variables, Wolf- Rayet stars, A-supergiants and Central Stars of Planetary Nebulae. After reca- pitulating the 1-D, stationary standard model of line-driven wind, extensions accounting for rotation and magnetic fields are discussed. The relevance of the so-called bi-stability jump is outlined. We summarize diagnostical methods to infer wind properties from observations, and compare the results with theore- tical predictions, featuring the massloss-metallicity dependence. Subsequently, we concentrate on two urgent problems which challenge our present understanding of radiation driven winds: weak winds and wind- clumping. We discuss problems of measuring mass-loss rates from weak winds and the potential of NIR- spectroscopy. Wind-clumping has severe implications for the interpretation of observational diagnostics, as derived mass-loss rates can be overestimated by factors of 2 to 10 if clumping is ignored, and we describe ongoing attempts to allow for more uniform results. We point out that independent arguments from stellar evolution favor a moderate reduction of present- day mass-loss rates. We also consider larger scale wind structure, interpreted in terms of co-rotating interacting regions, and complete this review with a discussion of recent progress on the X-ray line emission from massive stars, highlighting as to how far the analysis of such X-ray line emission can give further clues regarding an adequate description of wind clumping. (Abridged abstract)
... Its SED, also shown in Figure 10(c), has a significant excess at long wavelengths most likely due to PAH emission from the surrounding H ii region, but no evidence for hot or warm circumstellar dust. Based on its O9.5 Ia spectral type, we would expect a temperature near 30,000 K (Martins et al. 2005;Massey et al. 2004). With the adopted A v of 0.7, however, it is clear that a 30,000 K blackbody is not a good fit to the SED, and a significantly lower temperature ≈20,000 K is implied. ...
... The Of/WN star, UIT 008 in M33 has an unusually slow wind speed (Table 5) for this type of star, and thus may be an example of star that has shed a lot of mass. Its very high temperature corresponding to its O7-O8 spectral type of ≈35,000 K (Martins et al. 2005;Massey et al. 2004) and bolometric luminosity of −10.7 place it near the top of the S Dor instability strip near stars like AG Car and AF And. On this basis, it may be a candidate LBV and a star worth watching although it is not a known variable. ...
An increasing number of non-terminal eruptions are being found in the numerous surveys for optical transients. Very little is known about these giant eruptions, their progenitors and their evolutionary state. A greatly improved census of the likely progenitor class, including the most luminous evolved stars, the luminous blue variables (LBVs), and the warm and cool hypergiants is now needed for a complete picture of the final pre-supernova stages of very massive stars. We have begun a survey of the evolved and unstable luminous star populations in several nearby resolved galaxies. In this second paper on M31 and M33, we review the spectral characteristics, spectral energy distributions, circumstellar ejecta, and evidence for mass loss for 82 luminous and variable stars. We show that many of these stars have warm circumstellar dust including several of the Fe II emission line stars, but conclude that the confirmed LBVs in M31 and M33 do not. The confirmed LBVs have relatively low wind speeds even in their hot, quiescent or visual minimum state compared to the B-type supergiants and Of/WN stars which they spectroscopically resemble. The nature of the Fe II emission line stars and their relation to the LBV state remains uncertain, but some have properties in common with the warm hypergiants and the sgB[e] stars. Several individual stars are discussed in detail. We identify three possible candidate LBVs and three additional post-red supergiant candidates. We suggest that M33-013406.63 (UIT301,B416) is not an LBV/S Dor variable, but is a very luminous late O-type supergiant and one of the most luminous stars or pair of stars in M33.
... We also present C/O estimates from population synthesis fits to the UV and optical nebular lines as described in Section 5.1 and adopted in the remainder of the fits presented in this paper. Massey et al. 2004;Crowther et al. 2016). Likewise, rather than the nebular gas emission typically observed in He ii λ1640 at metallicities below ∼ 0.2Z , the target systems all show broad emission with FWHM ∼ 1500-2000 km/s. ...
As deep spectroscopic campaigns extend to higher redshifts and lower stellar masses, the interpretation of galaxy spectra depends increasingly upon models for very young stellar populations. Here we present new HST/COS ultraviolet spectroscopy of seven nearby (<120 Mpc) star-forming regions hosting very young stellar populations (∼4–20 Myr) with optical Wolf–Rayet stellar wind signatures, ideal laboratories in which to benchmark these stellar models. We detect nebular C iii] in all seven, but at equivalent widths uniformly <10 Å. This suggests that even for very young stellar populations, the highest equivalent width C iii] emission at ≥15 Å is reserved for inefficiently cooled gas at metallicities at or below that of the SMC. The spectra also reveal strong C iv P-Cygni profiles and broad He ii emission formed in the winds of massive stars, including some of the most prominent He ii stellar wind lines ever detected in integrated spectra. We find that the latest stellar population synthesis prescriptions with improved treatment of massive stars nearly reproduce the entire range of stellar He ii wind strengths observed here. However, we find that these models cannot simultaneously match the strongest wind features alongside the optical nebular line constraints. This discrepancy can be naturally explained by an overabundance of very massive stars produced by a high incidence of binary mass transfer and mergers occurring on short ≲10 Myr time-scales, suggesting these processes may be crucial for understanding systems dominated by young stars both nearby and in the early Universe.
... We also present C/O estimates from population synthesis fits to the UV and optical nebular lines as described in Section 5.1 and adopted in the remainder of the fits presented in this paper. Massey et al. 2004;Crowther et al. 2016). Likewise, rather than the nebular gas emission typically observed in He ii λ1640 at metallicities below ∼ 0.2Z , the target systems all show broad emission with FWHM ∼ 1500-2000 km/s. ...
As deep spectroscopic campaigns extend to higher redshifts and lower stellar masses, the interpretation of galaxy spectra depends increasingly upon models for very young stellar populations. Here we present new HST/COS ultraviolet spectroscopy of seven nearby ($<120$ Mpc) star-forming regions hosting very young stellar populations ($\sim$ 4-20 Myr) with optical Wolf-Rayet stellar wind signatures, ideal laboratories in which to test these stellar models. We detect nebular C III] in all seven, but at equivalent widths uniformly $< 10$ {\AA}. This suggests that even for very young stellar populations, the highest equivalent width C III] emission at $\geq 15$ {\AA} is reserved for inefficiently-cooled gas at metallicities at or below that of the SMC. The spectra also reveal strong C IV P-Cygni profiles and broad He II emission formed in the winds of massive stars, including some of the most prominent He II stellar wind lines ever detected in integrated spectra. We find that the latest stellar population synthesis prescriptions with improved treatment of massive stars nearly reproduce the entire range of stellar He II wind strengths observed here. However, we find that these models cannot simultaneously match the strongest wind features alongside the optical nebular line constraints. This discrepancy can be naturally explained by an overabundance of very massive stars produced by a high incidence of binary mass transfer and mergers occurring on short $\lesssim 10$ Myr timescales, suggesting these processes may be crucial for understanding the highest-sSFR galaxies in the early Universe. Reproducing both the stellar and nebular light of young systems such as these will be a crucial benchmark for the next generation of stellar population synthesis models.
Observations of individual massive stars, super-luminous supernovae, gamma-ray bursts, and gravitational wave events involving spectacular black hole mergers indicate that the low-metallicity Universe is fundamentally different from our own Galaxy. Many transient phenomena will remain enigmatic until we achieve a firm understanding of the physics and evolution of massive stars at low metallicity ( Z ). The Hubble Space Telescope has devoted 500 orbits to observing ∼250 massive stars at low Z in the ultraviolet (UV) with the COS and STIS spectrographs under the ULLYSES programme. The complementary X-Shooting ULLYSES (XShootU) project provides an enhanced legacy value with high-quality optical and near-infrared spectra obtained with the wide-wavelength coverage X-shooter spectrograph at ESO’s Very Large Telescope. We present an overview of the XShootU project, showing that combining ULLYSES UV and XShootU optical spectra is critical for the uniform determination of stellar parameters such as effective temperature, surface gravity, luminosity, and abundances, as well as wind properties such as mass-loss rates as a function of Z . As uncertainties in stellar and wind parameters percolate into many adjacent areas of astrophysics, the data and modelling of the XShootU project is expected to be a game changer for our physical understanding of massive stars at low Z . To be able to confidently interpret James Webb Space Telescope spectra of the first stellar generations, the individual spectra of low- Z stars need to be understood, which is exactly where XShootU can deliver.
Context. The origin of the observed population of Wolf-Rayet (WR) stars in low-metallicity galaxies, such as the Small Magellanic Cloud (SMC), is not yet understood. Standard, single-star evolutionary models predict that WR stars should stem from very massive O-type star progenitors, but these are very rare. On the other hand, binary evolutionary models predict that WR stars could originate from primary stars in close binaries.
Aims. We conduct an analysis of the massive O star, AzV 14, to spectroscopically determine its fundamental and stellar wind parameters, which are then used to investigate evolutionary paths from the O-type to the WR stage with stellar evolutionary models.
Methods. Multi-epoch UV and optical spectra of AzV 14 are analyzed using the non-local thermodynamic equilibrium (LTE) stellar atmosphere code PoWR. An optical TESS light curve was extracted and analyzed using the PHOEBE code. The obtained parameters are put into an evolutionary context, using the MESA code.
Results. AzV 14 is a close binary system with a period of P = 3.7058 ± 0.0013 d. The binary consists of two similar main sequence stars with masses of M 1, 2 ≈ 32 M ⊙ . Both stars have weak stellar winds with mass-loss rates of log Ṁ /( M ⊙ yr ⁻¹ ) = −7.7 ± 0.2. Binary evolutionary models can explain the empirically derived stellar and orbital parameters, including the position of the AzV 14 components on the Hertzsprung-Russell diagram, revealing its current age of 3.3 Myr. The model predicts that the primary will evolve into a WR star with T eff ≈ 100 kK, while the secondary, which will accrete significant amounts of mass during the first mass transfer phase, will become a cooler WR star with T eff ≈ 50 kK. Furthermore, WR stars that descend from binary components that have accreted significant amount of mass are predicted to have increased oxygen abundances compared to other WR stars. This model prediction is supported by a spectroscopic analysis of a WR star in the SMC.
Conclusions. Inspired by the binary evolutionary models, we hypothesize that the populations of WR stars in low-metallicity galaxies may have bimodal temperature distributions. Hotter WR stars might originate from primary stars, while cooler WR stars are the evolutionary descendants of the secondary stars if they accreted a significant amount of mass. These results may have wide-ranging implications for our understanding of massive star feedback and binary evolution channels at low metallicity.
We present the spectroscopic analysis of 333 OB-type stars extracted from VLT-MUSE observations of the central 30 × 30 pc of NGC 2070 in the Tarantula Nebula on the Large Magellanic Cloud, the majority of which are analysed for the first time. The distribution of stars in the spectroscopic Hertzsprung-Russell diagram (sHRD) shows 281 stars in the main sequence. We find two groups in the main sequence, with estimated ages of 2.1 ± 0.8 and 6.2 ± 2 Myr. A subgroup of 52 stars is apparently beyond the main sequence phase, which we consider to be due to emission-type objects and/or significant nebular contamination affecting the analysis. As in previous studies, stellar masses derived from the sHRD are systematically larger than those obtained from the conventional HRD, with the differences being largest for the most massive stars. Additionally, we do not find any trend between the estimated projected rotational velocity and evolution in the sHRD. The projected rotational velocity distribution presents a tail of fast rotators that resembles findings in the wider population of 30 Doradus. We use published spectral types to calibrate the He I λ 4921/He II λ 5411 equivalent-width ratio as a classification diagnostic for early-type main sequence stars when the classical blue-visible region is not observed. Our model-atmosphere analyses demonstrate that the resulting calibration is well correlated with effective temperature.
Analysis of late O-type stars observed in the Large Magellanic Cloud (LMC) by the VLT-FLAMES Tarantula Survey revealed a discrepancy between the physical properties estimated from model-atmosphere analysis and those expected from their morphological classifications. Here we revisit the analysis of 32 of these puzzling objects using new hydrogen-helium-silicon FASTWIND models and a different fitting approach to re-evaluate their physical properties. Our new analysis confirms that these stars indeed have properties that are typical of late O-type dwarfs. We also present the first estimates of silicon abundances for O-type stars in the 30 Dor clusters NGC 2060 and NGC 2070, with a weighted mean abundance for our sample of ɛSi = 7.05 ± 0.03. Our values are ~0.20 dex lower than those previously derived for B-type stars in the LMC clusters N 11 and NGC 2004 using TLUSTY models. Various possibilities (e.g. differences in the analysis methods, effects of microturbulence, and real differences between stars in different clusters) were considered to account for these results. We also used our grid of FASTWIND models to reassess the impact of using the Galactic classification criteria for late O-type stars in the LMC by scrutinising their sensitivity to different stellar properties. At the cool edge of the O star regime the He IIλ 4686/He Iλ 4713 ratio used to assign luminosity class for Galactic stars can mimic giants or bright giants in the LMC, even for objects with high gravities (log g > 4.0 dex). We argue that this line ratio is not a reliable luminosity diagnostic for late O-type stars in the LMC, and that the Si IVλ 4089/He Iλ 4026 ratio is more robust for these types.
Hot star winds are driven by the radiative force due to light absorption in lines of heavier elements. Therefore, the amount of mass lost by the star per unit of time, i.e., the mass-loss rate, is sensitive to metallicity. We provide mass-loss rate predictions for O stars with mass fraction of heavier elements 0.2 <Z/Z ⊙ ≤ 1. Our predictions are based on global model atmospheres. The models allow us to predict wind terminal velocity and the mass-loss rate just from basic global stellar parameters. We provide a formula that fits the mass-loss rate predicted by our models as a function of stellar luminosity and metallicity. On average, the mass-loss rate scales with metallicity as (Z/Z ⊙ ) 0.59 . The predicted mass-loss rates agree with mass-loss rates derived from ultraviolet wind line profiles. At low metallicity, the rotational mixing affects the wind mass-loss rates. We study the influence of magnetic line blanketing.
The study of massive stars in different metallicity environments is a central topic of current stellar research. The spectral analysis of massive stars requires adequate model atmospheres. The computation of such models is difficult and time-consuming. Therefore, spectral analyses are greatly facilitated if they can refer to existing grids of models. Here we provide grids of model atmospheres for OB-type stars at metallicities corresponding to the Small and Large Magellanic Clouds, as well as to solar metallicity. In total, the grids comprise 785 individual models. The models were calculated using the state-of-the-art Potsdam Wolf-Rayet (PoWR) model atmosphere code. The parameter domain of the grids was set up using stellar evolution tracks. For all these models, we provide normalized and flux-calibrated spectra, spectral energy distributions, feedback parameters such as ionizing photons, Zanstra temperatures, and photometric magnitudes. The atmospheric structures (the density and temperature stratification) are available as well. All these data are publicly accessible through the PoWR website.
We provide mass-loss rate predictions for O stars from Large and Small Magellanic Clouds. We calculate global (unified, hydrodynamic) model atmospheres of main sequence, giant, and supergiant stars for chemical composition corresponding to Magellanic Clouds. The models solve radiative transfer equation in comoving frame, kinetic equilibrium equations (also known as NLTE equations), and hydrodynamical equations from (quasi-)hydrostatic atmosphere to expanding stellar wind. The models allow us to predict wind density, velocity, and temperature (consequently also the terminal wind velocity and the mass-loss rate) just from basic global stellar parameters. As a result of their lower metallicity, the line radiative driving is weaker leading to lower wind mass-loss rates with respect to the Galactic stars. We provide a formula that fits the mass-loss rate predicted by our models as a function of stellar luminosity and metallicity. On average, the mass-loss rate scales with metallicity as $ \dot M\sim Z^{0.59}$. The predicted mass-loss rates are lower than mass-loss rates derived from H$\alpha$ diagnostics and can be reconciled with observational results assuming clumping factor $C_\text{c}=9$. On the other hand, the predicted mass-loss rates either agree or are slightly higher than the mass-loss rates derived from ultraviolet wind line profiles. The calculated \ion{P}{v} ionization fractions also agree with values derived from observations for LMC stars with $T_\text{eff}\leq40\,000\,$K. Taken together, our theoretical predictions provide reasonable models with consistent mass-loss rate determination, which can be used for quantitative study of stars from Magellanic Clouds.
We use a combination of BVJHK and Spitzer [3.6], [5.8] and [8.0] photometry to determine IR excesses for a sample of 58 LMC and 46 SMC O stars. This sample is ideal for determining IR excesses because the very small line of sight reddening minimizes uncertainties due to extinction corrections. We use the core-halo model developed by Lamers & Waters (1984a) to translate the excesses into mass loss rates and demonstrate that the results of this simple model agree with the more sophisticated CMFGEN models to within a factor of 2. Taken at face value, the derived mass loss rates are larger than those predicted by Vink et al. (2001), and the magnitude of the disagreement increases with decreasing luminosity. However, the IR excesses need not imply large mass loss rates. Instead, we argue that they probably indicate that the outer atmospheres of O stars contain complex structures and that their winds are launched with much smaller velocity gradients than normally assumed. If this is the case, it could affect the theoretical and observational interpretations of the "weak wind" problem, where classical mass loss indicators suggest that the mass loss rates of lower luminosity O stars are far less than expected.
Stellar evolution models are most uncertain for evolved massive stars. Asteroseismology based on high-precision uninterrupted space photometry has become a new way to test the outcome of stellar evolution theory and was recently applied to a multitude of stars, but not yet to massive evolved supergiants.Our aim is to detect, analyse and interpret the photospheric and wind variability of the O9.5Iab star HD 188209 from Kepler space photometry and long-term high-resolution spectroscopy. We used Kepler scattered-light photometry obtained by the nominal mission during 1460d to deduce the photometric variability of this O-type supergiant. In addition, we assembled and analysed high-resolution high signal-to-noise spectroscopy taken with four spectrographs during some 1800d to interpret the temporal spectroscopic variability of the star. The variability of this blue supergiant derived from the scattered-light space photometry is in full in agreement with the one found in the ground-based spectroscopy. We find significant low-frequency variability that is consistently detected in all spectral lines of HD 188209. The photospheric variability propagates into the wind, where it has similar frequencies but slightly higher amplitudes. The morphology of the frequency spectra derived from the long-term photometry and spectroscopy points towards a spectrum of travelling waves with frequency values in the range expected for an evolved O-type star. Convectively-driven internal gravity waves excited in the stellar interior offer the most plausible explanation of the detected variability.
The VLT-FLAMES Tarantula Survey has observed hundreds of O-type stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). We study the properties of 105 apparently single O-type dwarfs. To determine stellar and wind parameters, we used the IACOB-GBAT package, an automatic procedure based on a large grid of atmospheric models calculated with the FASTWIND code. In addition to classical techniques, we applied the Bayesian BONNSAI tool to estimate evolutionary masses. We provide a new calibration of effective temperature vs. spectral type for O-type dwarfs in the LMC, based on our homogeneous analysis of the largest sample of such objects to date and including all spectral subtypes. Good agreement with previous results is found, although the sampling at the earliest subtypes could be improved. Rotation rates and helium abundances are studied in an evolutionary context. We find that most of the rapid rotators (vsini higher than 300 km/s ) in our sample have masses below 25 MSun and intermediate rotation-corrected gravities (log gc between 3.9 and 4.1). Such rapid rotators are scarce at higher gravities (i.e. younger ages) and absent at lower gravities (larger ages). This is not expected from theoretical evolutionary models, and does not appear to be due to a selection bias in our sample. We compare the estimated evolutionary and spectroscopic masses, finding a trend that the former is higher for masses below 20 MSun. This can be explained as a consequence of limiting our sample to the O-type stars, and we see no compelling evidence for a systematic mass discrepancy. For most of the stars in the sample we were unable to estimate the wind-strength parameter (hence mass-loss rates) reliably, particularly for objects with luminosity lower than logL/LSun about 5.1. Ultraviolet spectroscopy is needed to undertake a detailed investigation of the wind properties of these dwarfs.
The Tarantula region in the Large Magellanic Cloud contains the richest population of spatially resolved massive O-type stars known so far. This unmatched sample offers an opportunity to test models describing their main-sequence evolution and mass-loss properties. Using ground-based optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to determine stellar, photospheric and wind properties of 72 presumably single O-type giants, bright giants and supergiants and to confront them with predictions of stellar evolution and of line-driven mass-loss theories. We apply an automated method for quantitative spectroscopic analysis of O stars combining the non-LTE stellar atmosphere model {{\sc fastwind}} with the genetic fitting algorithm {{\sc pikaia}} to determine the following stellar properties: effective temperature, surface gravity, mass-loss rate, helium abundance, and projected rotational velocity. We present empirical effective temperature versus spectral subtype calibrations at LMC-metallicity for giants and supergiants. In the spectroscopic and classical Hertzsprung-Russell diagrams, our sample O stars are found to occupy the region predicted to be the core hydrogen-burning phase by Brott et al. (2011) and K\"{o}hler et al. (2015). Except for five stars, the helium abundance of our sample stars is in agreement with the initial LMC composition. The aforementioned five stars present moderate projected rotational velocities (i.e., $v_{\mathrm{e}}\,\sin\,i\,<\,200\,\mathrm{km\,s^{-1}}$) and hence do not agree with current predictions of rotational mixing in main-sequence stars. Adopting theoretical results for the wind velocity law, we find modified wind momenta for LMC stars that are $\sim$0.3 dex higher than earlier results. [Due to the limitation of characters, the abstract appearing here is slightly shorter than that in the PDF file.]
We provide an observational view of evolutionary models in the Hertzsprung--Russell diagram, on the main sequence. For that we computed evolutionary models with the code STAREVOL for 15 < M/Msun < 100. We subsequently calculated atmosphere models at specific points along the evolutionary tracks, using the code CMFGEN. Synthetic spectra obtained in this way were classified as if they were observational data. We tested our spectral classification by comparison to observed spectra of various stars. We also compared our results with empirical data of a large number of OB stars. We obtain spectroscopic sequences along evolutionary tracks. In our computations, the earliest O stars (O2-3.5) appear only above ~50 Msun. For later spectral types, a similar mass limit exists, but is lower. A luminosity class V does not correspond to the entire main sequence. This only holds for the 15 Msun track. As mass increases, a larger portion of the main sequence is spent in luminosity class III. Above 50 Msun, supergiants appear before the end of core-hydrogen burning. Dwarf stars do not occur on the zero-age main sequence above 80 Msun. Consequently, the distribution of luminosity class V in the HR diagram cannot be used to constrain the size of the convective core. The distribution of dwarfs and giants in the HR diagram agrees well with the location of stars analyzed by means of quantitative spectroscopy. For supergiants, there is a slight discrepancy in the sense that luminosity class I is observed slightly earlier than our predictions. This is mainly due to wind densities that affect the luminosity class diagnostic lines. We predict an upper mass limit for dwarf stars (~60 Msun) that is found consistent with the rarity of O2V stars in the Galaxy. Stars with WNh spectral type are not predicted by our models. Stronger winds are required to produce the characteristic emission lines of these objects.
The search of extragalactic regions with conspicuous presence of Wolf-Rayet (WR) stars outside the Local Group is challenging task due to the difficulties in detecting their faint spectral features. In this exploratory work, we develop a methodology to perform an automated search of WR signatures through a pixel-by-pixel analysis of integral field spectroscopy (IFS) data belonging to the Calar Alto Legacy Integral Field Area survey, CALIFA. This technique allowed us to build the first catalogue of Wolf-Rayet rich regions with spatially-resolved information, allowing to study the properties of these complexes in a 2D context. The detection technique is based on the identification of the blue WR bump (around He II 4686 {\AA}, mainly associated to nitrogen-rich WR stars, WN) and the red WR bump (around C IV 5808 {\AA} and associated to carbon-rich WR stars, WC) using a pixel-by-pixel analysis. We identified 44 WR-rich regions with blue bumps distributed in 25 galaxies of a total of 558. The red WR bump was identified only in 5 of those regions. We found that the majority of the galaxies hosting WR populations in our sample are involved in some kind of interaction process. Half of the host galaxies share some properties with gamma-ray burst (GRB) hosts where WR stars, as potential candidates to being the progenitors of GRBs, are found. We also compared the WR properties derived from the CALIFA data with stellar population synthesis models, and confirm that simple star models are generally not able to reproduce the observations. We conclude that other effects, such as the binary star channel (which could extend the WR phase up to 10 Myr), fast rotation or other physical processes that causes the loss of observed Lyman continuum photons, are very likely affecting the derived WR properties, and hence should be considered when modelling the evolution of massive stars.
We introduce a HST/STIS stellar census of R136a, the central ionizing star cluster of 30 Doradus. We present low resolution
far-ultraviolet STIS/MAMA spectroscopy of R136 using 17 contiguous 52×0.2 arcsec slits which together provide complete coverage
of the central 0.85 parsec (3.4 arcsec). We provide spectral types of 90% of the 57 sources brighter than mF555W = 16.0 mag within a radius of 0.5 parsec of R136a1, plus 8 additional nearby sources including R136b (O4 If/WN8). We measure
wind velocities for 52 early-type stars from C ivλλ1548–51, including 16 O2–3 stars. For the first time we spectroscopically classify all Weigelt & Baier members of R136a,
which comprise three WN5 stars (a1-a3), two O supergiants (a5-a6) and three early O dwarfs (a4, a7, a8). A complete Hertzsprung-Russell
diagram for the most massive O stars in R136 is provided, from which we obtain a cluster age of 1.5$^{+0.3}_{-0.7}$ Myr. In addition, we discuss the integrated ultraviolet spectrum of R136, and highlight the central role played by the most
luminous stars in producing the prominent He ii λ1640 emission line. This emission is totally dominated by very massive stars with initial masses above ∼100M⊙. The presence of strong He iiλ1640 emission in the integrated light of very young star clusters (e.g A1 in NGC 3125) favours an initial mass function extending
well beyond a conventional upper limit of 100 M⊙. We include montages of ultraviolet spectroscopy for LMC O stars in the Appendix. Future studies in this series will focus
on optical STIS/CCD medium resolution observations.
We present a spectroscopic analysis of HST/COS observations of three massive
stars in the low metallicity dwarf galaxies IC 1613 and WLM. These stars, were
previously observed with VLT/X-shooter by Tramper et al. (2011, 2014) who
claimed that their mass-loss rates are higher than expected from theoretical
predictions for the underlying metallicity. A comparison of the FUV spectra
with those of stars of similar spectral types/luminosity classes in the Galaxy,
and the Magellanic Clouds provides a direct, model-independent check of the
mass-loss - metallicity relation. Then, a quantitative spectroscopic analysis
is carried out using the NLTE stellar atmosphere code CMFGEN. We derive the
photospheric and wind characteristics, benefiting from a much better
sensitivity of the FUV lines to wind properties than Ha. Iron and CNO
abundances are measured, providing an independent check of the stellar
metallicity. The spectroscopic analysis indicates that Z/Zsun = 1/5, similar to
a SMC-type environment, and higher than usually quoted for IC 1613 and WLM. The
mass-loss rates are smaller than the empirical ones by Tramper et al. (2014),
and those predicted by the widely used theoretical recipe by Vink et al.
(2001). On the other hand, we show that the empirical, FUV-based, mass-loss
rates are in good agreement with those derived from mass fluxes computed by
Lucy (2012). We do not concur with Tramper et al. (2011, 2014) that there is a
breakdown in the mass-loss - metallicity relation.
New integral field spectroscopy has been obtained for IZw18, the nearby
lowest-metallicity galaxy considered our best local analog of systems forming
at high-z. Here we report the spatially resolved spectral map of the nebular
HeII4686 emission in IZw18, from which we derived for the first time its total
HeII-ionizing flux. Nebular HeII emission implies the existence of a hard
radiation field. HeII-emitters are observed to be more frequent among high-z
galaxies than for local objects. So investigating the HeII-ionizing source(s)
in IZw18 may reveal the ionization processes at high-z. HeII emission in
star-forming galaxies, has been suggested to be mainly associated with
Wolf-Rayet stars (WRs), but WRs cannot satisfactorily explain the
HeII-ionization at all times, in particular at lowest metallicities. Shocks
from supernova remnants, or X-ray binaries, have been proposed as additional
potential sources of HeII-ionizing photons. Our data indicate that conventional
HeII-ionizing sources (WRs, shocks, X-ray binaries) are not sufficient to
explain the observed nebular HeII4686 emission in IZw18. We find that the
HeII-ionizing radiation expected from models for either low-metallicity
super-massive O stars or rotating metal-free stars could account for the
HeII-ionization budget measured, while only the latter models could explain the
highest values of HeII4686/Hbeta observed. The presence of such peculiar stars
in IZw18 is suggestive and further investigation in this regard is needed. This
letter highlights that some of the clues of the early Universe can be found
here in our cosmic backyard.
We examine the relation between polycyclic aromatic hydrocarbon (PAH)
emission at 8 microns and far-infrared emission from hot dust grains at 24
microns and from large dust grains at 160 and 250 microns in the nearby spiral
galaxies NGC 2403 and M83 using data from the Spitzer Space Telescope and
Herschel Space Observatory. We find that the PAH emission in NGC 2403 is better
correlated with emission at 250 microns from dust heated by the diffuse
interstellar radiation field (ISRF) and that the 8/250 micron surface
brightness ratio is well-correlated with the stellar surface brightness as
measured at 3.6 microns. This implies that the PAHs in NGC 2403 are intermixed
with cold large dust grains in the diffuse interstellar medium (ISM) and that
the PAHs are excited by the diffuse ISRF. In M83, the PAH emission appears more
strongly correlated with 160 micron emission originating from large dust grains
heated by star forming regions. However, the PAH emission in M83 is low where
the 24 micron emission peaks within star forming regions, and enhancements in
the 8/160 micron surface brightness ratios appear offset relative to the dust
and the star forming regions within the spiral arms. This suggests that the
PAHs observed in the 8 micron band are not excited locally within star forming
regions but either by light escaping non-axisymmetrically from star forming
regions or locally by young, non-photoionising stars that have migrated
downstream from the spiral density waves. The results from just these two
galaxies show that PAHs may be excited by different stellar populations in
different spiral galaxies.
Massive stars likely played an important role in the reionization of the
Universe, and the formation of the first black holes. Massive stars in
low-metallicity environments in the local Universe are reminiscent of their
high redshift counterparts. In a previous paper, we reported on indications
that the stellar winds of low-metallicity O stars may be stronger than
predicted, which would challenge the current paradigm of massive star
evolution. In this paper, we aim to extend our initial sample of six O stars in
low-metallicity environments by four. We aim to derive their stellar and wind
parameters, and compare these to radiation-driven wind theory and stellar
evolution models. We have obtained intermediate-resolution VLT/X-Shooter
spectra of our sample of stars. We derive the stellar parameters by fitting
synthetic fastwind line profiles to the VLT/X-Shooter spectra using a genetic
fitting algoritm. We compare our parameters to evolutionary tracks and obtain
evolutionary masses and ages. We also investigate the effective temperature
versus spectral type calibration for SMC and lower metallicities. Finally, we
reassess the wind momentum versus luminosity diagram. The derived parameters of
our target stars indicate stellar masses that reach values of up to 50
$M_{\odot}$. The wind strengths of our stars are, on average, stronger than
predicted from radiation-driven wind theory and reminiscent of stars with an
LMC metallicity. We discuss indications that the iron content of the host
galaxies is higher than originally thought and is instead SMC-like. We find
that the discrepancy with theory is lessened, but remains significant for this
higher metallicity. This may imply that our current understanding of the wind
properties of massive stars, both in the local universe as well as at cosmic
distances, remains incomplete.
We are now in an era of large spectroscopic surveys of OB-type stars.
Quantitative spectroscopic analysis of these modern datasets is enabling us to
review the physical properties of blue massive stars with robust samples, not
only revisiting mean properties and general trends, but also incorporating
information about the effects of second-order parameters.
We investigate the spectral type -- effective temperature (SpT - Teff)
calibration for O-type dwarfs, and its claimed dependence on metallicity, using
statistically-meaningful samples of stars extracted from the IACOB and VFTS
surveys.
We perform a homogeneous differential spectroscopic analysis of 33 Galactic
and 53 LMC O~dwarfs (spanning spectral types of O4 -- O9.7) using the
IACOB-GBAT package, a $\chi^2$-fitting algorithm based on a large pre-computed
grid of FASTWIND models, and standard techniques for the hydrogen/helium
analysis of O-type stars. We compare the estimated effective temperatures and
gravities as a function of (internally consistent) spectral classifications.
While the general trend is that the temperature of a star increases with
earlier spectral types and decreasing metallicity, we show that the large range
of gravities found for O-type dwarfs -- spaning up to 0.45-0.50 dex in some
spectral bins -- plays a critical role on the dependence of the effective
temperature calibrations as a function of spectral type and metallicity.
This result warns us about the use of SpT - Teff calibrations for O-dwarfs
which ignore the effects of gravity, and highlights the risks of employing
calibrations based on small samples. The effects of this scatter in gravities
(evolutionary status) for O-type dwarfs should be included in future recipes
which employ SpT - Teff calibrations.
We present the first quantitative ultraviolet spectroscopic analysis of resolved OB stars in IC 1613. Because of its alleged very low metallicity (1/10 Z
☉, from H II regions), studies in this Local Group dwarf galaxy could become a significant step forward from the Small Magellanic Cloud (SMC) toward the extremely metal-poor massive stars of the early universe. We present HST-COS data covering the ~1150-1800 Å wavelength range with resolution R ~ 2500. We find that the targets do exhibit wind features, and these are similar in strength to SMC stars. Wind terminal velocities were derived from the observed P Cygni profiles with the Sobolev plus Exact Integration method. The v
∞-Z relationship has been revisited. The terminal velocity of IC 1613 O stars is clearly lower than Milky Way counterparts, but there is no clear difference between IC 1613 and SMC or LMC analog stars. We find no clear segregation with host galaxy in the terminal velocities of B-supergiants, nor in the v
∞/v
esc ratio of the whole OB star sample in any of the studied galaxies. Finally, we present the first evidence that the Fe-abundance of IC 1613 OB stars is similar to the SMC, which is in agreement with previous results on red supergiants. With the confirmed ~1/10 solar oxygen abundances of B-supergiants, our results indicate that IC 1613's α/Fe ratio is sub-solar.
We study stellar wind properties of selected late O stars in the Small
Magellanic Cloud (SMC). We calculate NLTE line-driven wind models for
these stars and compare predicted wind parameters with observed values.
We found satisfactory agreement between theoretical and observed
terminal velocities. On the other hand, predicted and observed mass-loss
rates are in a good agreement only for higher mass-loss rates. For
mass-loss rates lower than approximately {10^{-7} M_⊙ year^{-1}} we
found large discrepancy between theoretical and observed values. We
propose a new explanation of this effect based on dynamical decoupling
of some atoms. Finally, we study the dependence of wind terminal
velocities and mass-loss rates on metallicity.
Motivated by the idea that a subset of high-velocity clouds (HVCs) trace dark matter substructure in the Local Group, we search
for signs of star formation in the Smith Cloud, a nearby ∼2 × 106 M⊙ HVC currently falling into the Milky Way. Using GALEX NUV and WISE/2MASS NIR photometry, we apply a series of colour and apparent magnitude cuts to isolate candidate O and B stars that are
plausibly associated with the Smith Cloud. We find an excess of stars along the line of sight to the cloud, but not at a statistically
significant level relative to a control region. The number of stars found in projection on the cloud after removing an estimate
of the contamination by the Milky Way implies an average star formation rate surface density of 10−4.8±0.3 M⊙ yr−1 kpc−2, assuming the cloud has been forming stars at a constant rate since its first passage through the Milky Way ∼70 Myr ago.
This value is consistent with the star formation rate expected based on the average gas density of the cloud. We also discuss
how the newly discovered star-forming galaxy Leo P has very similar properties to the Smith Cloud, but its young stellar population
would not have been detected at a statistically significant level using our method. Thus, we cannot yet rule out the idea
that the Smith Cloud is really a dwarf galaxy.
The model atmosphere programs FASTWIND and CMFGEN are both elegantly designed to perform non-LTE analyses of the spectra of hot massive stars, and include sphericity and mass-loss. The two codes differ primarily in their approach toward line blanketing, with CMFGEN treating all of the lines in the co-moving frame and FASTWIND taking an approximate approach which speeds up execution times considerably. Although both have been extensively used to model the spectra of O-type stars, no studies have used the codes to independently model the same spectra of the same stars and compare the derived physical properties. We perform this task on 10 O-type stars in the Magellanic Clouds. For the late-type O supergiants, both CMFGEN and FASTWIND have trouble fitting some of the He I lines, and we discuss causes and cures. We find that there is no difference in the average effective temperatures found by the two codes for the stars in our sample, although the dispersion is large, due primarily to the various difficulties each code has with He I. The surface gravities determined using FASTWIND are systematically lower by 0.12 dex compared to CMFGEN, a result we attribute to the better treatment of electron scattering by CMFGEN. This has implications for the interpretation of the origin of the so-called mass discrepancy, as the masses derived by FASTWIND are on average lower than inferred from stellar evolutionary models, while those found by CMFGEN are in better agreement.
We compile the first comprehensive census of hot luminous stars in the 30
Doradus (30 Dor) star forming region of the LMC. The census extends to a radius
of 10arcmin (150pc) from the central cluster, R136. Stars were selected
photometrically and combined with the latest spectral types. 1145 candidate hot
luminous stars were identified of which >700 were considered genuine early type
stars that contribute to feedback. We assess the spectroscopic completeness to
be 85% in outer regions (>5pc) but fall to 35% in the vicinity of R136, giving
a total of 500 hot luminous stars with spectroscopy. Stellar calibrations and
models were used to obtain their physical parameters before integrated values
were compared to global observations and the population synthesis code,
Starburst99. The 31 W-R and Of/WN stars made large contributions to the total
ionising and wind luminosities of ~40% and ~50%, respectively. Stars with
Minit>100Msun also showed high contributions to the global feedback, ~25% in
both cases. Such massive stars are not accounted for by the current Starburst99
code, which underestimated the ionising and wind luminosities of R136 by
factors of ~2 and ~9, respectively. The census inferred a SFR of
0.073+/-0.04Msun/yr for 30 Dor, typically higher than results from popular SFR
calibrations. However, it remained consistent with a far-UV luminosity tracer
and a combined Halpha and mid-infrared tracer, but only after correcting for
Halpha extinction. The global ionising output exceeded measurements from the
associated gas and dust, suggesting ~6(+55/-6)% of ionising photons escape the
region. When studying very luminous star forming regions, it is therefore
essential to include the most massive stars to ensure a reliable energy budget.
If 30 Dor is typical of other large star forming regions, estimates of the SFR
will be underpredicted if this escape fraction is not accounted for.(abridged)
We review the empirical mass-loss properties of early-type stars in the upper part of the Hertzsprung-Russell diagram. Specifically, we focus on the relation between mass loss and chemical composition by comparing properties of massive stars in our galaxy with those in the Magellanic Clouds. The first robust empirical calibration of this relation is discussed, yielding Mdot ∝ Z0.83 ± 0.16, where Z is the metallicity. This compares well with theoretical expectations. It is pointed out that the largest uncertainties in the power-law index of this relation are likely connected to uncertainties in abundances, in particular of objects in the Small Magellanic Cloud. The role of clumping in stellar winds is discussed. Using Halpha as the prime diagnostic, a comparison of the empirical and predicted Mdot(Z) suggests that the outflows are only modestly clumped, with clumping factors in the Halpha line forming region of about 3--5. If the properties of clumping are independent of chemical environment, the Mdot(Z) power-law index is not affected by the actual value of the clumping factor.
We present in this contribution a review of recent results of parameters of massive OB stars in the Milky Way, the Magellanic Clouds and nearby spirals. The review is timely because of the strong lowering of the temperature scale of Galactic OB stars produced by the new model atmospheres, including sphericity, blanketing and mass-loss, that are also discussed, and because of the boost suffered by extragalactic stellar physics in the last years. The effects on the determination of mass-loss and the Modified Wind Momentum-Luminosity Relationship are discussed, and a comparison with evolutionary models is provided. We finally present a comparison of abundances from OB stars and H II regions in Orion and M 33.
We have determined column densities of H I and/or H_2 for sight lines in the
Magellanic Clouds from archival HST and FUSE spectra of H I Lyman-alpha and H_2
Lyman-band absorption. Together with some similar data from the literature, we
now have absorption-based N(H I) and/or N(H_2) for 285 LMC and SMC sight lines
(114 with a detection or limit for both species) -- enabling more extensive,
direct, and accurate determinations of molecular fractions, gas-to-dust ratios,
and elemental depletions in these two nearby, low-metallicity galaxies. For
sight lines where the N(H I) estimated from 21 cm emission is significantly
higher than the value derived from Lyman-alpha absorption (presumably due to
emission from gas beyond the target stars), integration of the 21 cm profile
only over the velocity range seen in Na I or H_2 absorption generally yields
much better agreement. Conversely, N(21 cm) can be lower than N(Ly-alpha) by
factors of 2--3 in some LMC sight lines -- suggestive of small-scale structure
within the 21 cm beam(s) and/or some saturation in the emission. The mean
gas-to-dust ratios obtained from N(H_tot)/E(B-V) are larger than in our Galaxy,
by factors of 2.8--2.9 in the LMC and 4.1--5.2 in the SMC -- i.e., factors
similar to the differences in metallicity. The N(H_2)/E(B-V) ratios are more
similar in the three galaxies, but with considerable scatter within each
galaxy. These data may be used to test models of the atomic-to-molecular
transition at low metallicities and predictions of N(H_2) based on comparisons
of 21 cm emission and the IR emission from dust.
We present the results of a spectroscopic analysis of the Trapezium cluster stars inside the Orion nebula. The rotational velocities were obtained using the Fourier analysis method, and we found agreement with values derived by the usual method based on linewidth measurements. The rotational velocity derived for theta1 Ori C by this method is consistent with the variability of some of its spectral features that have a period of 15.42 days. By means of the fit of H, He I, and He II observed profiles with Fastwind synthetic profiles, stellar parameters and wind characteristics were derived. This methodology let us estimate the errors associated with these parameters, and we found that macroturbulence effects have to be included for a good fit to the He I-II lines in the spectrum of theta1 Ori C. By means of a very accurate study, oxygen abundances were derived for the three B0.5V stars theta1 Ori A, D, and theta2 Ori B. Final abundances are consistent with the nebular gas-phase results presented in Esteban et al. (2004) and are lower than those given by Cunha & Lambert (1994). Our results suggest a lower dust depletion factor of oxygen than previous estimations for the Orion nebula.
We have obtained new spectrophotometric data for 28 H II regions in the spiral galaxy NGC 300, a member of the nearby Sculptor Group. The detection of several auroral lines, including [O III] λ4363, [S III] λ6312, and [N II] λ5755, has allowed us to measure electron temperatures and direct chemical abundances for the whole sample. We determine for the first time in this galaxy a radial gas-phase oxygen abundance gradient based solely on auroral lines, and obtain the following least-square solution: 12 + log(O/H) = 8.57(±0.02) – 0.41(±0.03)R/R
25, where the galactocentric distance is expressed in terms of the isophotal radius R
25. The characteristic oxygen abundance, measured at 0.4 × R
25, is 12 + log(O/H) = 8.41. The gradient corresponds to –0.077 ± 0.006 dex kpc–1, and agrees very well with the galactocentric trend in metallicity obtained for 29 B and A supergiants in the same galaxy, –0.081 ± 0.011 dex kpc–1. The intercept of the regression for the nebular data virtually coincides with the intercept obtained from the stellar data, which is 8.59(±0.05). This allows little room for depletion of nebular oxygen onto dust grains, although in this kind of comparison we are somewhat limited by systematic uncertainties, such as those related to the atomic parameters used to derive the chemical compositions. We discuss the implications of our result with regard to strong-line abundance indicators commonly used to estimate the chemical compositions of star-forming galaxies, such as R
23. By applying a few popular calibrations of these indices based on grids of photoionization models on the NGC 300 H II region fluxes, we find metallicities that are higher by 0.3 dex (a factor of 2) or more relative to our nebular (Te based) and stellar ones. We detect Wolf-Rayet stellar emission features in ~1/3 of our H II region spectra, and find that in one of the nebulae hosting these hot stars the ionizing field has a particularly hard spectrum, as gauged by the "softness" parameter η = (O+/O++)/(S+/S++). We suggest that this is related to the presence of an early WN star. By considering a larger sample of extragalactic H II regions we confirm, using direct abundance measurements, previous findings of a metallicity dependence of η, in the sense that softer stellar continua are found at high metallicity.
We present data for single and binary massive stars, both from new analyses (Trapezium stars, O stars in NGC 6611, and the massive binary system V478 Cyg) and collected from the literature. We show that data for single stars depart from the theoretical ZAMS, while those for binary systems are consistent with a ZAMS formed through a linearly mass-dependent accretion rate. In spite of this difference, spectroscopic and dynamical analyses give consistent results in several binary systems.
In this review the stellar and wind properties of OB stars in the MW and the MCs derived by means of modern-era quantitative spectral analyses are summarised and briefly discussed with a particular emphases on the results inferred from optical observations.
Although our knowledge of the descendants of the most massive O stars, WR stars, has greatly improved in the last few decades, there are still a few physical parameters for which we have only very little information. In this paper, we present preliminary results of a project aimed at measuring two of these, namely rotation rates and magnetic fields, for a sample of WR stars.
In this review, the physical and wind properties of OB-stars are discussed, with particular emphasis on metallicity dependence and recent results from the flames survey of massive stars. We summarize the relation between spectral type and Teff, discuss the status quo of the "mass-discrepancy", refer to the problem of "macro-turbulence" and comment on the dis- tribution of rotational velocities. Observational constraints on the efficiency of rotational mixing are presented, and magnetic field measurements summarized. Wind properties are reviewed, and problems related to weak winds and wind-clumping highlighted. tures, gravities, wind-properties, chemical composition of the outer layers) has (and will be) obtained by quantitative spectroscopy, i.e., the analysis of stellar spectra by means of atmospheric models. These have to be calculated in nlte due to the intense radiation field and low densities in the line-forming regions. With respect to such models, the last IAU-symposium on massive stars in Lanzarote (2002) saw the begin of two important developments which are "standard" nowadays, namely the incorporation of metal-line
We discuss theoretical predictions and observational findings obtained for radiatively driven winds of massive stars, with emphasis on their dependence on metallicity. If these winds are not strongly clumped or the clumping properties are independent of metallicity z, theory and observations agree very well, and mass-loss rates and terminal velocities scale as ú M / z0.62±015 and v∞ / z0.13, respectively. This dependence could be validated only for winds with solar and sub-solar abundances, due to missing super-solar metallicity test cases. The actual values for the mass-loss rates are uncertain, due to unknown clumping properties of the wind, and currently accepted numbers might be overestimated, by factors in between �2 and 10.
Wind models of very massive stars with metallicities in a range from
10-4 to 1.0 solar are presented using a new treatment of
radiation driven winds with depth dependent radiative force multipliers
and a comprehensive list of more than two million of spectral lines in
NLTE. The models yield mass-loss rates, wind velocities, wind momenta
and wind energies as a function of metallicity and can be used to
discuss the influence of stellar winds on the evolution of very massive
stars in the early universe and on the interstellar medium in the early
phases of galaxy formation. It is shown that the normal scaling laws,
which predict stellar mass-loss rates and wind momenta to decrease as a
power law with metal abundance break down at a certain threshold. The
new wind models are applied to calculate ionizing fluxes and observable
UV-spectra of very massive stars as a function of metallicity using the
WM-basic code developed by Pauldrach et al., 2001, and the effects of
metallicity are discussed.
Optical and ultraviolet spectroscopic analysis of a sample of Galactic B
supergiants is presented here. Fundamental parameters such as
temperature, luminosity, mass loss rate and CNO abundances are derived
for individual stars using the non-LTE, line-blanketed model atmosphere
code of Hillier & Miller (1998). The implications of this Galactic B
supergiant temperature scale and the derived mass loss rates and CNO
abundances are discussed and compared to other results.
Empirical analysis of the ionization conditions of early B supergiant
winds has also been carried out (based on SEI modeling) and compared to
model predictions from the stellar atmosphere code of Hillier &
Miller (1998). Intriguingly we find that the values of the empirical
q_{i}'s are much lower than expected, with none of the ions approaching
unity. We discuss our most recent findings and their implications for
clumping and structure in the wind. In particular we argue that these
results point for a downward revision in mass loss rates by at least an
order of magnitude. There is therefore a clear need to review mass loss
rate determinations for massive stars.
We review the observational evidence for a dependence of the mass loss
through stellar winds of hot massive stars on the metal content of their
atmospheres. The metal content of stars in the Small Magellanic Cloud
is discussed, and an overview is given of state-of-the-art Mdot
determinations of OB stars in SMC and the Milky-Way. Assuming a
powerlaw dependence of mass loss on metal content, Mdot ∝ Z^{m},
and adopting the theoretical result ǐnf ∝ Z^{0.13}
tep{1992ApJ...401..596L} for the relation between wind terminal flow
velocity and metal content, we find from an analysis of the wind
momentum luminosity relation (WLR) that m = 0.67 ± 0.22 for stars
more luminous than 10^{5.25} lsun. This compares very well with the
prediction m = 0.69 ± 0.10 by te{2001A&A...369..574V}. For
stars of lower luminosity the winds are so weak that their strengths can
no longer be derived from Hα fitting and one must rely on the
analysis of ultraviolet lines. In this regime the observed WLR appears
much steeper than expected from theory, leading to an overprediction of
the wind strength by up to a factor 100. We discuss possible
explanations. UV analyses also suggests that about half of the stars
brighter than ˜ 10^{5.15} have clumped winds, implying that their
wind strengths may be overestimated by factors 3 to 10. This would imply
that also strong winds are overpredicted by current wind theory.
We report the discovery of extended X-ray emission within the young star cluster NGC 602a in the Wing of the Small Magellanic Cloud (SMC) based on observations obtained with the Chandra
X-Ray Observatory. X-ray emission is detected from the cluster core area with the highest stellar density and from a dusty ridge surrounding the H II region. We use a census of massive stars in the cluster to demonstrate that a cluster wind or wind-blown bubble is unlikely to provide a significant contribution to the X-ray emission detected from the central area of the cluster. We therefore suggest that X-ray emission at the cluster core originates from an ensemble of low- and solar-mass pre-main-sequence (PMS) stars, each of which would be too weak in X-rays to be detected individually. We attribute the X-ray emission from the dusty ridge to the embedded tight cluster of the newborn stars known in this area from infrared studies. Assuming that the levels of X-ray activity in young stars in the low-metallicity environment of NGC 602a are comparable to their Galactic counterparts, then the detected spatial distribution, spectral properties, and level of X-ray emission are largely consistent with those expected from low- and solar-mass PMS stars and young stellar objects (YSOs). This is the first discovery of X-ray emission attributable to PMS stars and YSOs in the SMC, which suggests that the accretion and dynamo processes in young, low-mass objects in the SMC resemble those in the Galaxy.
From a survey of the 3400 Å region in the earliest O-type spectra, we have found that two of the four O2 giants observed in the Large Magellanic Cloud have O IV lines there that are stronger than the N IV lines, while the other two have the opposite. A Small Magellanic Cloud counterpart also has N IV stronger than O IV. Inspection of the blue spectra of these stars shows that the former pair have weaker N lines in all ionization states (III, IV, and V) present as well as lines of C IV λ4658, while the latter three have stronger N lines and greater He/H. Space ultraviolet observations of two of the N-strong stars show N V wind profiles substantially stronger than those of C IV, while in the N-weak stars the C IV features are equal to or stronger than the N V. The N-strong stars are now reclassified as ON2 III(f*), newly defining that category. These characteristics strongly suggest a larger fraction of processed material in the atmospheres of the ON2 stars, which we confirm by modeling the optical spectra. In the context of current models, it is in turn implied that the ON2 stars are in a more advanced evolutionary state than the others, and/or that they had higher initial rotational velocities. The recent formulation of the effects of rotation on massive stellar evolution introduces an additional fundamental parameter, which the CNO abundances are in principle able to constrain. We present some illustrative comparisons with current Geneva evolutionary models for rotating massive stars. It is possible that these very hot, nitrogen-rich objects are products of homogeneous evolution. Our results will provide motivation for further physical modeling of the atmospheres and evolutionary histories of the most massive hot stars.
We present a catalog of 1750 massive stars in the Large Magellanic Cloud (LMC), with accurate spectral types compiled from the literature, and a photometric catalog for a subset of 1268 of these stars, with the goal of exploring their infrared properties. The photometric catalog consists of stars with infrared counterparts in the Spitzer SAGE survey database, for which we present uniform photometry from 0.3 to 24 μm in the UBVIJHKs +IRAC+MIPS24 bands. The resulting infrared color-magnitude diagrams illustrate that the supergiant B[e], red supergiant, and luminous blue variable (LBV) stars are among the brightest infrared point sources in the LMC, due to their intrinsic brightness, and at longer wavelengths, due to dust. We detect infrared excesses due to free-free emission among ~900 OB stars, which correlate with luminosity class. We confirm the presence of dust around 10 supergiant B[e] stars, finding the shape of their spectral energy distributions (SEDs) to be very similar, in contrast to the variety of SED shapes among the spectrally variable LBVs. The similar luminosities of B[e] supergiants (log L/L ☉ ≥ 4) and the rare, dusty progenitors of the new class of optical transients (e.g., SN 2008S and NGC 300 OT), plus the fact that dust is present in both types of objects, suggests a common origin for them. We find the infrared colors for Wolf-Rayet stars to be independent of spectral type and their SEDs to be flatter than what models predict. The results of this study provide the first comprehensive roadmap for interpreting luminous, massive, resolved stellar populations in nearby galaxies at infrared wavelengths.
A truncated power-law distribution is fitted to the 29 largest stellar masses known in R136. Two different statistical techniques
are used, with comparable results. An upper limit to the mass distribution of the order of 140–160 M⊙ is derived, while the power-law exponent is in the approximate range 0.9–1.7. A power-law distribution with no upper limit
on the mass can be rejected with considerable confidence. It is recommended that the calculations be repeated when more reliable
mass estimates are available.
I investigate the conditions upon which atmospheric absorption may participate to the observation of a diffuse interstellar band (DIB), and the implications it would have. A necessary condition is that the spectrum of reddened stars comprises a few percent of starlight forward scattered by the interstellar cloud on the line of sight. Reciprocally, this scattered starlight could, in part, explain the complexity of the DIB spectrum and several observed DIB properties. It will also affect the interstellar extinction curve and the value of the RV parameter.
We calculate non-local thermodynamic equilibrium (NLTE) line-driven wind models of selected O stars in the spectral range of O4 to O9 in the Small Magellanic Cloud (SMC). We compare predicted basic wind properties, i.e. the terminal velocity and the mass-loss rate with values derived from observation. We found relatively good agreement between theoretical and observed terminal velocities. On the other hand, predicted mass-loss rates and mass-loss rates derived from observation are in a good agreement only for higher mass-loss rates. Theoretical mass-loss rates lower than approximately 10−7 M⊙ yr−1 are significantly higher than those derived from observation. These results confirm the previously reported problem of weak winds, since our calculated mass-loss rates are in fair agreement with predictions of Vink et al. We study multicomponent models for these winds. For this purpose we develop a more detailed description of wind decoupling. We show that the instability connected with the decoupling of individual wind elements may occur for low-density winds. In the case of winds with very low observed mass-loss rates the multicomponent effects are important for the wind structure, however this is not able to explain consistently the difference between the predicted mass-loss rate and the mass-loss rate derived from observation for these stars. Similar to previous studies, we found the level of dependence of the wind parameters on the metallicity. We conclude that the wind mass-loss rate significantly increases with metallicity as , whereas the terminal velocity of wind on average depends on metallicity only slightly, namely v∞∼Z0.06 (for studied stars).
We emphasize in this paper the importance of the UV range for our knowledge of massive stars and the fundamental role played by past and present space-based UV capabilities (IUE, HST, FUSE and others). Based on a review of the work developed in the last years and the state of the art situation for quantitative spectroscopy of massive stars, we present crucial advances which could be addressed by hypothetical future space-based UV missions. Advantages and unique data that these missions could provide are explained in the context of our present knowledge and theories on massive stars in the Milky Way and nearby galaxies. It is argued that these studies are our key to a correct interpretation of observations of more distant objects.
Context. Results from the theory of radiatively driven winds are incorporated
in stellar evolutionary and population synthesis models, and used in our
interpretation of the observations of the deep Universe. Yet, the theory has
been confirmed only until Small Magellanic Cloud metallicities. Analyses of
O-stars at lower metallicities are needed to prove the theory. Aims. We have
observed GHV-62024, an O6.5 IIIf star in the low-metallicity galaxy IC1613.
According to a previous preliminary analysis this star could challenge the
radiatively driven wind theory at low metallicities. Methods. Our observations
were obtained with VIMOS at VLT, at R~2000 and were analysed using the latest
version of the model atmosphere code FASTWIND, which includes N III Results. We
obtain the stellar parameters and conclude that the star follows the average
wind momentum-luminosity relationship (WLR) expected for its metallicity, but
with a high value for the exponent of the wind velocity law, beta. We suggest
that this high value may be reached because GHV-62024 could be a fast rotator
seen at a low inclination angle. While the derived beta value does not change
by adopting a lower wind terminal velocity, a wrong $V_\infty$ has a clear
impact on the position of the star in the WLR diagram. The N and He abundances
are very high, consistent with strong CNO mixing that could have been caused by
the fast rotation, although we cannot discard a different origin. We find again
the well-known mass-discrepancy. Conclusions. We conclude that the star follows
the WLR expected for its metallicity. The results are consistent with GHV-62024
being a fast rotator seen close to pole-on, contaminated at the surface with
CNO products and with a wind structure altered by the fast rotation without
modifying the global WLR. We suggest that this could be a general property of
fast rotators.
We present terminal velocities of M 33 B-supergiants, obtained from STIS HST spectra as part of our programme to investigate the Wind Momentum - Luminosity Relationship (WLR) in the Local Group. Terminal velocities are derived from their N V, C Iv, and Si Iv resonance lines in UV spectra. Comparing with IUE spectra of Galactic B-supergiants we found evidence of low metallicity in three of our objects. The terminal velocities are consistent with the corresponding values of Galactic stars, except for B-133. For this star we find a very large vinfty and a red Si Iv component deeper than the blue one, that might be an indication of binarity. The average ratio between terminal and turbulent wind velocities is 0.25, well above the value found for Galactic stars. Partly based on INES data from the IUE satellite.
We have re-analyzed the Galactic O-star sample from \citet{puls96} by means of line-blanketed NLTE model atmospheres in order to investigate the influence of line-blocking/blanketing on the derived parameters. The analysis has been carried out by fitting the photospheric and wind lines from H and He. In most cases we obtained a good fit, but we have also found certain inconsistencies which are probably related to a still inadequate treatment of the wind structure. These inconsistencies comprise the line cores of Hgamma and Hbeta in supergiants (the synthetic profiles are too weak when the mass-loss rate is determined by matching Halpha) and the ``generalized dilution effect'' (cf. \citealt{vo89}) which is still present in He I 4471 of cooler supergiants and giants. Compared to pure H/He plane-parallel models we found a decrease in effective temperatures which is largest at earliest spectral types and for supergiants (with a maximum shift of roughly 8000 K). This finding is explained by the fact that line-blanketed models of hot stars have photospheric He ionization fractions similar to those from unblanketed models at higher Teff and higher log g. Consequently, any line-blanketed analysis based on the He ionization equilibrium results in lower Teff-values along with a reduction of either log g or helium abundance (if the reduction of log g is prohibited by the Balmer line wings). Stellar radii and mass-loss rates, on the other hand, remain more or less unaffected by line-blanketing. We have calculated ``new'' spectroscopic masses and compared them with previous results. Although the former mass discrepancy \citep{h92} becomes significantly reduced, a systematic trend for masses below 50 Msun seems to remain: The spectroscopically derived values are smaller than the ``evolutionary masses'' by roughly 10 Msun. Additionally, a significant fraction of our sample stars stays over-abundant in He, although the actual values were found to be lower than previously determined. Also the wind-momentum luminosity relation (WLR) changes because of lower luminosities and almost unmodified wind-momentum rates. Compared to previous results, the separation of the WLR as a function of luminosity class is still present but now the WLR for giants/dwarfs is consistent with theoretical predictions. We argue that the derived mass-loss rates of stars with Halpha in emission are affected by clumping in the lower wind region. If the predictions from different and independent theoretical simulations (\citealt {Vink00, Paul03, puls03a}) that the WLR should be independent of luminosity class were correct, a typical clumping factor /2 ≈ 5 should be derived by ``unifying'' the different WLRs. Based upon observations obtained at the INT and the European Southern Observatory, La Silla, Chile. The INT is operated on the island of La Palma by the ING in the Spanish Observatorio de El Roque de los Muchachos of the Instituto de Astrofísica de Canarias. Appendix A in only available in electronic form at http://www.edpsciences.org
As a first step of a vigorous program to investigate the Wind Momentum - Luminosity Relationship (WLR) of Galactic O-stars by analyzing stars belonging to the same clus- ter we present UV HST observations of six supergiants and one giant in the Cyg OB2 association. Terminal and turbulent wind velocities, velocity laws and metal ion column densities are de- rived and mean ionization fractions are estimated. Turbulent velocities are mostly in the range 10 - 14 of . The termi- nal velocities agree well with the average vs. spectral class relationship compiled by Kudritzki and Puls (2000). We com- pare the observed vs. escape velocity (depending on the diagnostics of the stellar mass) correlation with the predictions of the radiatively driven wind theory and find better agreement with the spectroscopic masses rather than with the evolutionary ones. The velocity field exponents are in the range 0.7 - 0.8, without any trend towards larger values. We show that for a single luminosity class there is a tight relationship between and (the mean density at the point in the stellar wind, where half the wind terminal veloc- ity is reached). In consequence, the ionization fractions show the same trend with both, and : we find that N V increases with , Si IV decreases and C IV does not clearly correlate. As a byproduct, we also derive interestellar H I column densities towards Cyg OB2, which turn out to be quite large. For one object (Cyg OB2 2) we find inconsistencies making the association membership questionable.
A quantitative analysis of the extremely hot and massive galactic
O3If^*^ supergiant HD 93129A is carried out using stellar wind and
pseudo photospheric lines observed in the FUV, UV and optical spectrum
together with hydrodynamical NLTE model atmospheres. The analysis in the
FUV is combined with spectrum synthesis of the molecular and
atomic/ionic interstellar spectrum to disentangle stellar and
interstellar blends. It is demonstrated that the combined
stellar/interstellar spectrum synthesis technique is crucial for the
determination of both interstellar column densities and stellar
properties. The fraction of hydrogen atoms in molecular form in the
Carina interstellar clouds is found to be 0.1, smaller than one would
expect for its E(B-V) value of 0.54. We attribute this to dissociation
by the strong FUV radiation field of HD 93129A. The excitation
temperature of ortho-hydrogen (J=1) is about 80K, whereas the excitation
to higher levels requires temperatures up to 230K in accordance with
NLTE effects for interstellar H_2_ as discussed in the literature. The
abundance of HD relative to H_2_ is of the order of 10^-5^. For CO we
obtain an upper limit of 2.6x10^-5^. Abundances for the interstellar
atomic and ionic species are also derived. The terminal velocity of the
stellar wind of HD 93129A is 3200+/-200km/s and the rate of mass-loss is
18x10^-6^Msun_/yr. The ionization equilibrium of the optical
emission and P-Cygni lines of NIII, NIV and NV is used to determine the
effective temperature as T_eff_=52000+/-1000K in reasonable agreement
with previous values obtained from the helium ionization equilibrium.
This high temperature is confirmed independently by an analysis of the
ArVI/ArVII ionization equilibrium in the FUV. The luminosity of HD
93129A is log L/Lsun_=6.4+/-0.1 corresponding to a zero age
main sequence mass of slightly in excess of 120Msun_. This
very high mass is consistent with the mass determined from the stellar
gravity and with the mass derived from Vinfinity_ using the
theory of radiation driven winds. HD 93129A is thus the most luminous
and most massive star known in our galaxy. The abundance determinations
yield clear evidence of contamination with CNO-cycled matter in the
atmosphere. The abundances of heavier elements are about solar. The
presence of high ionization stages such as OVI can be explained by X-ray
emission due to stellar wind shocks of low temperature (2.5x10^6^K)
corresponding to the jump velocity of 500km/s obtained from UV and FUV
P-Cygni profiles. Their luminosity is 1.6 dex smaller than the
luminosity of the high temperature shocks (1.1x10^7^K) observed directly
with the ROSAT PSPC. Using effective temperature, gravity, radius and
abundances as input parameters we calculate radiation driven wind models
for HD 93129A. We find that the theory is able to reproduce the extreme
stellar wind properties very precisely.
The first consistent abundance analysis of both H II regions and SNRs in
the Magellanic Clouds is presented. Detailed modeling of the SNRs
results in abundance measures in good agreement with those from H II
regions, confirming the overall accuracy of the generalized modeling
code MAPPINGS for both photoionized and shock-excited nebulae. A
differential analysis performed between SNRs and the Galactic
Herbig-Haro (H-H) objects for elements heavier than Ar reveals that
there is no difference for grain depletion in either Cloud SNR or
Galactic H-H objects. It shows that there is no measurable zero point
error between the Fe abundances determined from supergiants and those
determined from SNRs. This permits the two traditionally separated
abundance scales of the H II regions and supergiants to be combined for
the first time into one comprehensive system for the first time. The
importance of this overall abundance pattern for the determination of
the star formation history in the Clouds is addressed.
Some recent developments in the optical classification of OB spectra are
reviewed in terms of a comprehensive atlas of new blue-violet digital
data from the CTIO 1-meter photon-counting system. These developments
include the O3 spectral type; luminosity criteria for the O stars;
OBN/OBC anomalies; and refined, interpolated late-O/early-B types.
Examples of these phenomena are included among extensive spectral- and
luminosity-class sequences, comprising 75 standard objects arranged into
27 montages and covering the wavelength range 3950-4750 A for types
O3-B3 (-B8 at Ia). It is intended that this atlas serve a reference
function analogous to that of the printed MK atlases, for morphological
investigations of OB spectra based on digital data, which will supersede
photographic techniques in most future applications.
A new method for calculating line profiles formed in spherically
symmetric stellar winds in which the velocity increases monotonically
outward is described. The source function is calculated with the escape
probability method, but the transfer equation is solved exactly. The
method allows the calculation of singlets and doublets with underlying
photospheric absorption components. Profile comparisons show that the
presented method often attains the same accuracy as profiles calculated
with the comoving frame method but has the advantages of the escape
probability method. It can be run on a small computer and requires
little CPU time, so that it can be used in an interactive analysis of
observed line profiles.
We present ultraviolet photometry of stars in the Large Magellanic Cloud
star cluster R136. The data were obtained with the refurbished Wide
Field/Planetary Camera on the Hubble Space Telescope through an
ultraviolet filter centered at 1750 Å (F170W). These data,
combined with optical HST observations presented previously, are used to
explore the hot, luminous stellar population within the cluster. The
first question we wanted to address was whether the ultraviolet could
place better constraints on differential reddening and the degree of
coevality within the cluster than optical photometry alone. The stellar
sequence in the color magnitude diagram using the F170W- F555W color is
broader than it was in the optical and is broader than would be expected
from photometric uncertainties. Although there could be a modest age
spread among the massive stars, a large part of the intrinsic spread in
the color-magnitude diagram is likely to be due to reddening
differences. There is also some evidence that stars in the outer parts
of the cluster are affected by a larger range in reddening than those in
the smaller region of the cluster core. The second question we addressed
is whether ultraviolet photometry in combination with optical photometry
can distinguish the most massive stars more readily than is possible
with the optical colors. The F170W- F555W color is better at separating
B supergiants, of which there is only one in R136, from comparably
bright 0 and WoIf-Rayet stars. However, spectral classifications of the
stars within R136 will be necessary to properly identify stellar types
of the rest of the stars and to disentangle age and reddening effects.
Spectral classification of a sample of fainter OB stars in the Small
Magellanic Cloud (SMC) is used to derive a reliable distance estimate
for the galaxy. Spectra of 19 stars in the SMC and six galactic stars
were obtained with the CTIO 4-m Ritchey-Chretien image-tube
spectrograph, and classified by comparison with standard spectra, taking
into account the weakness of the metal lines in SMC spectra.
Calculations of the distance moduli of each star yield a distance
modulus of 19.1 magnitudes for the SMC, with a negligible foreground
reddening. Comparison of H-R diagrams indicates the SMC to be 0.5
magnitudes more distant than the LMC. Line intensity tracings for the
SMC stars reveal significant decreases in the line strengths of the C
III, N III and Si lines of the SMC relative to the LMC and galactic
stars, and allows the estimation of rotational velocities for the
individual stars.
We propose that the stellar initial mass function (IMF) is universal in the sense that its functional form arises as a consequence
of the statistics of random supersonic flows.
A model is developed for the origin of the stellar IMF that contains a dependence on the average physical parameters (temperature,
density and velocity dispersion) of the large-scale site of star formation. The model is based on recent numerical experiments
of highly supersonic random flows that have a strong observational counterpart.
It is shown that a Miller-Scalo-like IMF is naturally produced by the model for the typical physical conditions in molecular
clouds. A more ‘massive’ IMF in star- bursts is also predicted.
We derived photospheric parameters, mass-loss rates, and wind velocities of Galactic O6–O7 stars by ana-lyzing high-resolution spectra in the far-UV and UV ranges with line-blanketed, hydrodynamic, non-LTE spherical models. We combined spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE) in the range 905–1187 Å and International Ultraviolet Explorer (IUE) archival spectra (1150–3250 Å) and used the WM-BASIC code of Pauldrach et al. to compute model spectra. Lines in the FUSE range include high ionization stages (e.g., O vi) and lower abundance non-CNO elements (e.g., P v). Analyzed in addition to the N iv, N v, Si iv, and C iv lines in the IUE range, these features play a crucial role in uniquely constraining the stellar parameters, assessing the presence of shocks in the wind, and quantifying the effects of the resulting soft X-rays on the wind ionization. The effective temperatures derived from the consistent analysis of the far-UV and UV spectra are significantly (%6000 K or 15% on average, or between 4000 and 8000 K) lower than most values previously derived for some of our targets and lower than typical values assigned to their spectral types from different compilations. This result has great implications for our understanding of the evolution of massive stars and the characterization of young stellar populations, as well as for energy balance calculations of H ii regions.
We present photometric and spectroscopic observations of stars in the Large Magellanic Cloud OB associations LH 101 and LH 104, located in the HII region N158, which we have also imaged. From our observations we have constructed upper H-R diagrams for these OB associations, which we find to consist mainly of three populations, one of $2-6$ Myr for the stars inside the northern bubble (LH 104), and two populations in the southern HII region (LH 101), one of $\leq$ 2 Myr and the other one aged $3-6$ Myr. We have obtained for LH 101 a normal IMF, with a slope of $\Gamma = -1.29$ $\pm$ 0.20 whereas for LH 104 the IMF is flatter with a slope of $\Gamma = -1.05$ $\pm$ 0.12. These IMF slopes are consistent with that of other OB associations in the LMC. Our observations reveal in the region of LH 101 the presence of both unevolved and evolved very massive stars, whose ionizing flux is in excess of that derived from our H$\beta$ images of the HII region. The north-west nebulosity in the region of LH 101 thus appears to be matter bound.
Using the results of the most recent stellar atmosphere models applied to a sample of hot stars, we construct calibrations of effective temperature (T(sub eff)), and gravity (log(sub g)) with a spectral type and luminosity class for Galactic 0-type and early B-type stars. From the model results we also derive an empirical relation between the bolometric correction and T(sub eff) and log g. Using a sample of stars with known distances located in OB associations in the Galaxy and the Large Magellanic Cloud, we derive a new calibration of M(sub v) with spectral class. With these new calibrations and the stellar atmosphere models of Kurucz, we calculate the physical parameters and ionizing photon luminosities in the H(0) and He(0) continua for O and early B-type stars. We find substantial differences between our values of the Lyman- continuum luminosity and those reported in the literature. We also discuss the systematic discrepancy between O-type stellar masses derived from spectroscopic models and those derived from evolutionary tracks. Most likely, the cause of this 'mass discrepancy' lies primarily in the atmospheric models, which are plane parallel and hydrostatic and therefore do not account for an extended atmosphere and the velocity fields in a stellar wind. Finally, we present a new computation of the Lyman-continuum luminosity from 429 known O stars located within 2.5 kpc of the Sun. We find the total ionizing luminosity from this population ((Q(sub 0)(sup T(sub ot))) = 7.0 x 10(exp 51) photons/s) to be 47% larger than that determined using the Lyman continuum values tabulated by Panagia.
This book presents the latest understanding of the structure, evolution,
and dynamics of the Magellanic Clouds, based on observations in the
X-ray, FUV, infrared, and millimeter wavelengths, including results from
the Hubble Space Telescope. The general topics addressed include: the
distances of the Clouds, the Clouds as galaxies, the cluster population,
the youngest field population, the superassociations and supergiant
shells, the intermediate-age and oldest field populations, the
interstellar medium, X-ray emission and supernova remnants, the 30
Doradus complex, chemical abundances, and the structure and kinematics
of the Magellanic system.
The Magellanic Clouds continue to attract much interest. New data from observations at all wavelengths appear frequently and contribute valuable information regarding the Clouds as galaxies as well as their content of gas, dust and stellar generations. Detailed studies of individual objects and clusters add important clues to our knowledge of stellar evolution. Numerical simulations take us closer to understanding the interaction between the Small and the Large Clouds as well as between the Clouds and the Galaxy. Here, some recent results regarding the composition, structure and evolution of the Magellanic system are presented.
Preface; Part I: 1. Stellar taxonomy; 2. Spectral classification; 3.
Spectral classification systems; 4. Photometric classification; 5.
Photometric systems; 6. Comparison of classification methods; Part II:
7. Introduction; 8. O-type stars; 9. B-type stars; 10. A-type stars; 11.
F-type stars; 12. G-type stars; 13. K-type stars; 14. M-type stars; 15.
Degenerates; 16. Further developments.
An ultraviolet and optical spectral atlas of 15 O stars in the Small Magellanic Cloud (SMC) is presented and described. The echelle data have resolving powers of order 104; they were obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph in the UV, and at the Anglo-Australian Telescope or the European Southern Observatory 3.6 m in the optical. The ultimate objective is to develop metal-deficient templates for the interpretation of distant starbursts, but here we discuss interesting new properties of the SMC stars themselves, revealed by the high quality of these data. The SMC metal deficiency produces anomalously weak stellar-wind profiles along the entire O main sequence, as well as at intermediate luminosities; the first intermediate Si IV λ1400 wind profile in the SMC is shown. The second known Of star in the SMC displays wind peculiarities that are identical to those of its spectral classmate, again likely due to the low systemic metallicity. Several objects display marked CNO anomalies, including the first cases of C III λ4650 emission without N III λ4640 in O-type spectra. The N/C ratio appears to increase with mass, extent of evolution away from the zero-age main sequence, and/or rotational velocity in the young cluster NGC 346. In addition, the first examples of Onfp (Oef) and Of?p spectra in the SMC have been found (the latter being only the fourth member of its peculiar shell category known anywhere). The UV wind characteristics of these objects correlate with their optical peculiarities. All these spectroscopic phenomena provide diagnostics of the evolutionary status of metal-deficient massive stars.
This review deals with the winds from "normal" hot stars such as O-stars, B- and A-supergiants, and Central Stars of Planetary Nebulae with O-type spectra. The advanced diagnostic methods of stellar winds, including an assessment of the accuracy of the determinations of global stellar wind parameters (terminal velocities, mass-loss rates, wind momenta, and energies), are introduced and scaling relations as a function of stellar parameters are provided. Observational results are interpreted in the framework of the stationary, one-dimensional (1-D) theory of line-driven winds. Systematic effects caused by nonhomogeneous structures, time dependence, and deviations from spherical symmetry are discussed. The review finishes with a brief description of the role of stellar winds as extragalactic distance indicators and as tracers of the chemical composition of galaxies at high redshift.
Optimal techniques for reducing and calibrating stellar spectroscopy with a CCD can substantially improve the quality of the results. The methods described here are considered optimal because they consistently employ unbiased minimum-variance statistics and avoid resampling the raw data. Topics of particular interest include optimal spectrum extraction, robust rejection of cosmic ray hits on the spectrum, and elimination of the telluric absorption bands in near-infrared spectra.
Three new ultraviolet spectral libraries of massive, hot stars using high and medium resolution spectra of objects located in the solar neighbourhood and the Magellanic Clouds are presented. Massive stars display unique wind signatures which are relatively easy to study in the ultraviolet. These libraries are crucial tools when investigating the massive stellar population of distant star-forming galaxies.
The galaxies of the Local Group serve as important laboratories for understanding the physics of massive stars. Here I discuss what is involved in identifying various kinds of massive stars in nearby galaxies: the hydrogen-burning O-type stars and their evolved He-burning evolutionary descendants, the luminous blue variables, red supergiants, and Wolf-Rayet stars. Primarily I review what our knowledge of the massive star population in nearby galaxies has taught us about stellar evolution and star formation. I show that the current generation of stellar evolutionary models do well at matching some of the observed features and provide a look at the sort of new observational data that will provide a benchmark against which new models can be evaluated.
We combine new CCD UBV photometry and spectroscopy with those from the literature to investigate 19 Magellanic Cloud OB associations that contain Wolf-Rayet (W-R) and other types of evolved, massive stars. Our spectroscopy reveals a wealth of newly identified interesting objects, including early O-type supergiants, a high-mass, double-lined binary in the SMC, and, in the LMC, a newly confirmed luminous blue variable (LBV; R85), a newly discovered W-R star (Sk -69°194), and a newly found luminous B[e] star (LH 85-10). We use these data to provide precise reddening determinations and construct physical H-R diagrams for the associations. We find that about half of the associations may be highly coeval, with the massive stars having formed over a short period (Deltatau
Introduction Introducing the unevolved luminous stars Finding main-sequence luminous stars in the Local Group: methodology for a hard problem Finding the evolved descendants of massive stars: LBVs, WRs, and RSGs Secrets of star formation as revealed by luminous stars Secrets of stellar evolution revealed by luminous stars Summary: what to take away from all this Reference
The effect of radiation emitted or scattered by circumstellar material,
such as a stellar wind, into the stellar photosphere is investigated on
the basis of a gray model atmosphere generalized to include the effects
of an external radiation field and a surface boundary condition
describing the reflection of a specified fraction, depending on the
frequency, of the outgoing radiation. Substantial modifications both to
the temperature and flux distributions are found.
Preliminary to an extensive and detailed comparison of improved non-LTE
photospheric models with observations of hot stars made with high
photometric accuracy, non-LTE stellar atmospheres are constructed which
account for the radiation reflected back onto the photosphere by line
and electron scattering from the wind. The effects of this 'wind
blanketing' on the spectrum and internal structure of the atmosphere are
given for an example with an effective temperature T(eff) of 42,000 K,
and a wide range of wind density, gravity, and model assumptions.
Particular attention is given to the problem of determining T(eff).
Careful analysis of methods currently used to determine T(eff) from
continuous flux distributions, with and without interferometric angular
diameters, shows them to be unreliable in practice. Line profiles
continue to provide a legitimate means of determining T(eff) but only
when their dependence on gravity and mass loss is included. For the more
luminous OB stars spectral classification is truly three-dimensional,
with the mass loss rate, gravity, and effective temperature all playing
nearly equal roles in specifying the observed spectrum.
Based upon a sample of 140 stars observed over 20 years for which about
5,000 spectrograms are available, a classification scheme of Be stars is
presented. This is the first attempt to subdivide the Be star group into
physically significant subgroups, from which typical objects can be
selected for further study. The four groups proposed are based upon a
discussion of spectrum characteristics, multicolor photometry,
polarization, rotational velocities, UV spectral types and time
variability. Starting with the group membership of a Be star,
predictions can be made of the future behavior of it.
High S/N (~200), high spectral resolution (~25km/s) optical time series
spectroscopy of the extreme O8 supergiant HD151804 is presented. These
data were collected over ~5 nights in 1992 July in a coordinated
campaign between Australia (Mount Stromlo Observatory) and Chile (Cerro
Tololo Inter-American Observatory). Variability in the strong wind of
HD151804 is principally monitored via the HeI λ5876 P Cygni and
the Hα emission profiles, both of which are sensitive diagnostics
of density structures in the wind close to the star. Systematic changes
are present in the HeI profiles, which take the form of: (i) blueward
migrating optical depth enhancements, which travel from ~0.14 of the
terminal velocity (vinfinity_) to ~0.5vinfinity_
in about 24 hours; and (ii) blueward and redward motion of up to 50km/s
of the low-velocity emission portion of the HeI profile. Fluctuations
are also evident in the blue emission wing of Hα between -200km/s
and -750km/s, which are in sympathy with the systematic changes in HeI
absorption. These results indicate that the inner stellar wind of
HD151804, i.e., between 1.0 and 2.0 stellar radii, is unstable and
affected by coherent, evolving structures. Constraints on global
mass-flux variations and localised column density enhancements based on
steady-state stellar atmosphere modelling are presented.
I have used my newly calculated iron group line list together with my earlier atomic and molecular line data, 58,000,000 lines total, to compute new opacities for the temperature range 2000K to 200000K. Calculations have been completed at the San Diego Supercomputer Center for 56 temperatures, for 21 pressures, for microturbulent velocities 0, 1, 2, 4, and 8 km/s, for 3,500,000 wavelength points divided into 1221 intervals from 10 to 10000 nm, for scaled solar abundances [+1.0], [+0.5], [+0.3], [+0.2], [+0.1], [+0.0], [−0.1], [−0.2], [−0.3], [−0.5], [−1.0], [−1.5], [−2.0], [−2.5], [−3.0], [−3.5], [−4.0], [−4.5], and [−5.0]. I have rewritten my model atmosphere program to use the new line opacities, additional continuous opacities, and an approximate treatment of convective overshooting. The opacity calculation was checked by computing a new theoretical solar model that matches the observed irradiance. Thus far I have completed a grid of 7000 model atmospheres at 2 km/s for all the abundances, for the temperature range 3500K to 50000K, and for log g from 0.0 to 5.0. This grid will allow a consistent theoretical treatment of photometry from K stars to B stars. Fluxes are tabulated from .09 to 160 micrometers. Preliminary results are reported for many photometric systems. Work is underway on grids for other microturbulent velocities. Microturbulent velocity strongly affects the interpretation of Cepheid and RR Lyrae photometry. The models, fluxes, and colors are available on magnetic tape and will also be distributed on CD-ROMs.
Previously developed dynamical models of Of stars are used as the basis
of a set of statistical equilibrium calculations for hydrogen and
ionized helium, on the hypothesis of radiative equilibrium in the
stellar envelope. A number of spectral lines of these species are
observed in the expanding envelopes of Of stars, and thus they provide
diagnostic information about the stellar wind. It is found that the
H-alpha line has a strength that is quite sensitive to the rate of mass
loss by the star, and comparison with observation indicates loss rates
as high as 0.00001 solar mass/yr. The ratio H-alpha/He II 4686-A as
observed is in agreement with one of the two sets of theoretical values
that are computed; the choice is allowed by uncertainties in the
treatment of He II 304-A. Other lines of He II, for which the data are
more fragmentary, give only factor-of-2 agreement with theory. The
computed profiles of H-alpha and 4686-A are in qualitative agreement
with observation, but certain systematic differences exist; the effect
of absorption on the violet side of the emission profile is generally
more marked in the theoretical profile. The effect of assuming a higher
electron temperature in the stellar wind is considered. The primary
change would be an upward adjustment in the inferred rates of mass loss,
with some worsening of agreement between theory and observation of the
H-alpha/4686-A ratio in the later subclasses.
Chemical abundances in a large and representative sample of galactic H
II regions covering a wide range in galactocentric radius RG
were measured using radio and optical spectroscopy. Accurate electron
temperatures in 67 H II regions spanning the range RG =
3.5-13.7 kpc were determined using radio recombination lines and these
temperatures were applied to optical spectra of 33 of the same H II
regions in order to determine the abundances of O, N, S, Ne, Ar, and
He(+). Among other results, it is found that some H II regions have
electron temperatures below 5000 K and that the radio-determined
electron temperatures agree well with those obtained from the optical
line ratios, in the light of standard models of H II regions. A gradient
of H II region electron temperature with distance from the galactic
center is found which equals +433 + or - 40 K/kpc, while the oxygen
abundance gradient is -0.07 + or - 0.015 dex/kpc. The nitrogen abundance
gradient is similar to that of oxygen, -0.09 + or 0.015 dex/kpc, while
the sulfur abundance gradient (-0.01 + or - 0.02 dex/kpc) is
significantly flatter than that of oxygen. No significant gradient in
He(+)/H(+) is detected. In addition, evidence indicates that the
abundance gradients may be steeper over the inner regions of the
galactic disk.
We present new slit spectral types for 120 O and B stars in the SMC
which previously had only objective-prism classification. We discuss the
colors of O and B stars in the SMC. As in the LMC, we show that there is
no separation in colors between O and early B stars. The lower
metallicity of the SMC is particularly evident in the spectra of the B
stars. The different composition of the SMC may also be indirectly
responsible for apparently redder intrinsic colors, especially in (U -
B), compared with LMC stars. We also discuss the distance modulus of the
SMC, determined from spectroscopic parallax and ZAMS fitting. The second
method depends on a knowledge of the temperature/spectral-type relation
in the SMC, which has not yet been addressed in the literature. We adopt
DM = 19.0, but note how this might change with better understanding of
stellar parameters. The data in this paper are also used to re-examine
the Humphreys-Davidson limit, and an analytical relation is presented
for this upper luminosity limit.
Using 243 new coude spectrograms, a sample of 67 O type stars was
studied to determine the frequency of binaries. Binaries are detected in
36 percent of the sample, of which 85 percent have mass ratios less than
2.5. The binary frequency is lower than in earlier studies because a
number of stars, previously identified as variable, show only random
photospheric variations up to 30 km/s. It appears that mass ratios
greater than 3 may not be present in unevolved O type systems. The
relationship of the highly evolved X-ray binary systems to the unevolved
O type is discussed. From statistical arguments, a few of the 67 O stars
in the sample could contain neutron star companions and several
candidates are listed.
The radiation field is considered along with the equation of transfer,
the grey atmosphere, absorption cross-sections, the equations of
statistical equilibrium, and the solution of the transfer equation. A
description of model atmospheres is presented, taking into account the
classical model-atmospheres problem, Local Thermodynamic Equilibrium
(LTE) radiative-equilibrium models, convection and models for late-type
stars, the results of LTE model-atmosphere calculations for early-type
stars, non-LTE radiative-equilibrium models for early-type stars,
extended atmospheres, and semiempirical solar models. Attention is given
to the line absorption profile, classical treatments of line transfer,
non-LTE line transfer in the case of the two-level atom and the
multilevel atom, line formation with partial frequency redistribution,
radiative transfer in moving atmospheres, the equations of hydrodynamics
for an ideal compressible fluid, coronal winds, radiation hydrodynamics,
and radiatively driven winds.
The properties of the 29 galaxies presently known to be located within 1.5 Mpc of the Galaxy and dynamically associated with the Local Group are reviewed. The galactic distribution exhibits a marked concentration near M31, with all objects within about 100 kpc of M31 either ellipticals or dwarf spheroidals. Dwarf irregulars and dwarf spheroidals, the most numerous constituents of the Local Group, are primarily found in low-density environments and in the neighborhood of M31, respectively, suggesting that the dwarf spheroidals were originally irregulars transformed by the gaseous haloes of the parent galaxies. The luminosity function of the local Group galaxies yields a luminosity limit consistent with a Hubble constant of 100 km/sec per Mpc. Examination of the relative numbers of globular clusters in Local Group and other nearby ellipticals reveals that the specific frequency of globular clusters differs significantly from galaxy to galaxy. Dynamical friction has been shown capable of removing globular clusters and giant molecular clouds from the galactic disk, leading to the destruction of the disk within a Hubble time without counteracting processes. The oldest open star clusters in the Galaxy have been found to be concentrated near the galactic anticenter. Finally, it has been observed that the interstellar medium in the Small Magellanic Cloud is distributed much more smoothly than is that of the Large Magellanic Cloud, possibly due to the lower metallicity of the Small Cloud gas.
Spectral analysis of hot luminous stars requires adequate model atmospheres which take into account the effects of NLTE and radiation driven winds properly. Here we present significant improvements of our approach in constructing detailed atmospheric models and synthetic spectra for hot luminous stars. Moreover, as we regard our solution method in its present stage already as a standard procedure, we make our program package WM-basic available to the community (download is possible from the URL given below). The most important model improvements towards a realistic description of stationary wind models concern: (i) A sophisticated and consistent description of line blocking and blanketing. Our solution concept to this problem renders the line blocking influence on the ionizing fluxes emerging from the atmospheres of hot stars -mainly the spectral ranges of the EUV and the UV are affected -in identical quality as the synthetic high resolution spectra representing the observable region. In addition, the line blanketing effect is properly accounted for in the energy balance. (ii) The atomic data archive which has been improved and enhanced considerably, providing the basis for a detailed multilevel NLTE treatment of the metal ions (from C to Zn) and an adequate representation of line blocking and the radiative line acceleration. (iii) A revised inclusion of EUV and X-ray radiation produced by cooling zones which originate from the simulation of shock heated matter. This new tool not only provides an easy-to-use method for O-star diagnostics, whereby physical constraints on the properties of stellar winds, stellar parameters, and abundances can be obtained via a comparison of observed and synthetic spectra, but also allows the astrophysically important information about the ionizing fluxes of hot stars to be determined automatically. Results illustrating this are discussed by means of a basic model grid calculated for O-stars with solar metallicity. To further demonstrate the astrophysical potential of our new method, we first provide a detailed spectral diagnostic determination of the stellar parameters, the wind parameters, and the abundances by an exemplary application to one of our grid-stars, the O9.5Ia O-supergiant $\alpha$ Cam. Our abundance determinations of the light elements indicate that these deviate considerably from the solar values.
A review on the quantitative spectroscopy (QS) of hot stars is presented, with particular attention given to the study of photospheres, optically thin winds, unified model atmospheres, and stars with optically thick winds. It is concluded that the results presented here demonstrate the reliability of Qs as a unique source of accurate values of the global parameters (effective temperature, surface gravity, and elemental abundances) of hot stars.
We report on Planetary Camera observations of the central region of 30 Doradus in the Large Magellanic Cloud (LMC). These images of 30 Doradus are the first `deep' Hubble Space Telescope (HST) exposures that have appropriate photometric calibration. The B band (F439W) image, which shows R136a at the center of the PC6 charge coupled device (CCD) chip, reveals over 200 stars within 3 sec of the center of R13a, and over 800 stars in a 35 sec x 35 sec area. We used Malumuth et al.'s (1991) Point Spread Function (PSF)-fitting method to measure the magnitudes of all stars on the PC6 chip. These new B magnitudes, along with U and V magnitudes from archival PC images, yield a luminosity function, mass density profile, and initial mass function of the 30 Doradus ionizing cluster. The mass distribution is well fit by a King model with a core radius, R(sub c) = 0.96 sec (0.24 pc), a tidal radius, R(sub t) = 110 sec (28 pc), and a total mass, Mass = 16,800 solar mass. Both the luminosity function and initial mass function show evidence for mass segregation, in the sense that the central region has a higher fraction of massive stars than the outer regions. This is the first observational evidence for mass segregation in a very young cluster (age approximately 3 million years). The observations admit the hypothesis that the mass segregation occurred in the process of star formation and/or that the mass segregation is the result of dynamical evolution.
Hydrogen and helium line profiles with high signal-to-noise ratios were obtained for four stars of spectral type 09.5 (Alpha Cam, Xi Ori A, Delta Ori A,AE Aur) that form a sequence in luminosity: Ia, Ib, II, V. The basic stellar parameters of these stars are determined by fitting the observed line profiles of weak photospheric absorption lines with profiles from models which include the effect of radiation scattered back onto the photosphere from their stellar winds, an effect referred to as wind blanketing. For these stars, the inclusion of wind blanketing is significant only for the most luminous star, Alpha Cam, for which the effective temperature was shifted about -2000 K relative to an unblanketed model.