Publications (16)10.92 Total impact
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Article: Constraints on porosity and mass loss in O-star winds from modeling of X-ray emission line profile shapes
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ABSTRACT: We fit X-ray emission line profiles in high resolution XMM-Newton and Chandra grating spectra of the early O supergiant Zeta Pup with models that include the effects of porosity in the stellar wind. We explore the effects of porosity due to both spherical and flattened clumps. We find that porosity models with flattened clumps oriented parallel to the photosphere provide poor fits to observed line shapes. However, porosity models with isotropic clumps can provide acceptable fits to observed line shapes, but only if the porosity effect is moderate. We quantify the degeneracy between porosity effects from isotropic clumps and the mass-loss rate inferred from the X-ray line shapes, and we show that only modest increases in the mass-loss rate (<~ 40%) are allowed if moderate porosity effects (h_infinity <~ R_*) are assumed to be important. Large porosity lengths, and thus strong porosity effects, are ruled out regardless of assumptions about clump shape. Thus, X-ray mass-loss rate estimates are relatively insensitive to both optically thin and optically thick clumping. This supports the use of X-ray spectroscopy as a mass-loss rate calibration for bright, nearby O stars.05/2013; -
Article: A generalised porosity formalism for isotropic and anisotropic effective opacity and its effects on X-ray line attenuation in clumped O star winds
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ABSTRACT: We present a generalised formalism for treating the porosity-associated reduction in continuum opacity that occurs when individual clumps in a stochastic medium become optically thick. We consider geometries resulting in either isotropic or anisotropic effective opacity, and, in addition to an idealised model in which all clumps have the same local overdensity and scale, we also treat an ensemble of clumps with optical depths set by Markovian statistics. This formalism is then applied to the specific case of bound-free absorption of X-rays in hot star winds, a process not directly affected by clumping in the optically thin limit. We find that the Markov model gives surprisingly similar results to those found previously for the single clump model, suggesting that porous opacity is not very sensitive to details of the assumed clump distribution function. Further, an anisotropic effective opacity favours escape of X-rays emitted in the tangential direction (the `venetian blind' effect), resulting in a 'bump' of higher flux close to line centre as compared to profiles computed from isotropic porosity models. We demonstrate how this characteristic line shape may be used to diagnose the clump geometry, and we confirm previous results that for optically thick clumping to significantly influence X-ray line profiles, very large porosity lengths, defined as the mean free path between clumps, are required. Moreover, we present the first X-ray line profiles computed directly from line-driven instability simulations using a 3-D patch method, and find that porosity effects from such models also are very small. This further supports the view that porosity has, at most, a marginal effect on X-ray line diagnostics in O stars, and therefore that these diagnostics do indeed provide a good `clumping insensitive' method for deriving O star mass-loss rates.11/2011; -
Article: Chandra X-ray spectroscopy of the very early O supergiant HD 93129A: constraints on wind shocks and the mass-loss rate
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ABSTRACT: We present analysis of both the resolved X-ray emission line profiles and the broadband X-ray spectrum of the O2 If* star HD 93129A, measured with the Chandra HETGS. This star is among the earliest and most massive stars in the Galaxy, and provides a test of the embedded wind shock scenario in a very dense and powerful wind. A major new result is that continuum absorption by the dense wind is the primary cause of the hardness of the observed X-ray spectrum, while intrinsically hard emission from colliding wind shocks contributes less than 10% of the X-ray flux. We find results consistent with the predictions of numerical simulations of the line-driving instability, including line broadening indicating an onset radius of X-ray emission of several tenths Rstar. Helium-like forbidden-to-intercombination line ratios are consistent with this onset radius, and inconsistent with being formed in a wind-collision interface with the star's closest visual companion at a distance of ~100 AU. The broadband X-ray spectrum is fit with a dominant emission temperature of just kT = 0.6 keV along with significant wind absorption. The broadband wind absorption and the line profiles provide two independent measurements of the wind mass-loss rate: Mdot = 5.2_{-1.5}^{+1.8} \times 10^{-6} Msun/yr and Mdot = 6.8_{-2.2}^{+2.8} \times 10^{-6} Msun/yr, respectively. This is the first consistent modeling of the X-ray line profile shapes and broadband X-ray spectral energy distribution in a massive star, and represents a reduction of a factor of 3 to 4 compared to the standard H-alpha mass-loss rate that assumes a smooth wind.04/2011; -
Article: X-ray spectral diagnostics of activity in massive stars
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ABSTRACT: X-rays give direct evidence of instabilities, time-variable structure, and shock heating in the winds of O stars. The observed broad X-ray emission lines provide information about the kinematics of shock-heated wind plasma, enabling us to test wind-shock models. And their shapes provide information about wind absorption, and thus about the wind mass-loss rates. Mass-loss rates determined from X-ray line profiles are not sensitive to density-squared clumping effects, and indicate mass-loss rate reductions of factors of 3 to 6 over traditional diagnostics that suffer from density-squared effects. Broad-band X-ray spectral energy distributions also provide mass-loss rate information via soft X-ray absorption signatures. In some cases, the degree of wind absorption is so high that the hardening of the X-ray SED can be quite significant. We discuss these results as applied to the early O stars zeta Pup (O4 If), 9 Sgr (O4 V((f))), and HD 93129A (O2 If*). Comment: To appear in the proceedings of IAU 272: Active OB Stars09/2010; -
Article: Modeling Broadband X-ray Absorption of Massive Star Winds
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ABSTRACT: We present a method for computing the net transmission of X-rays emitted by shock-heated plasma distributed throughout a partially optically thick stellar wind from a massive star. We find the transmission by an exact integration of the formal solution, assuming that the emitting plasma and absorbing plasma are mixed at a constant mass ratio above some minimum radius, below which there is assumed to be no emission. This model is more realistic than either the slab absorption associated with a corona at the base of the wind or the exospheric approximation that assumes that all observed X-rays are emitted without attenuation from above the radius of optical depth unity. Our model is implemented in XSPEC as a pre-calculated table that can be coupled to a user-defined table of the wavelength-dependent wind opacity. We provide a default wind opacity model that is more representative of real wind opacities than the commonly used neutral interstellar medium (ISM) tabulation. Preliminary modeling of Chandra grating data indicates that the X-ray hardness trend of OB stars with spectral subtype can largely be understood as a wind absorption effect.The Astrophysical Journal 08/2010; 719(2):1767. · 6.02 Impact Factor -
Article: A mass‐loss rate determination for ζ Puppis from the quantitative analysis of X‐ray emission‐line profiles
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ABSTRACT: We fit every emission line in the high-resolution Chandra grating spectrum of ζ Pup with an empirical line profile model that accounts for the effects of Doppler broadening and attenuation by the bulk wind. For each of 16 lines or line complexes that can be reliably measured, we determine a best-fitting fiducial optical depth, , and place confidence limits on this parameter. These 16 lines include seven that have not previously been reported on in the literature. The extended wavelength range of these lines allows us to infer, for the first time, a clear increase in τ* with line wavelength, as expected from the wavelength increase of bound–free absorption opacity. The small overall values of τ*, reflected in the rather modest asymmetry in the line profiles, can moreover all be fitted simultaneously by simply assuming a moderate mass-loss rate of 3.5 ± 0.3 × 10−6 M⊙ yr−1, without any need to invoke porosity effects in the wind. The quoted uncertainty is statistical, but the largest source of uncertainty in the derived mass-loss rate is due to the uncertainty in the elemental abundances of ζ Pup, which affects the continuum opacity of the wind, and which we estimate to be a factor of 2. Even so, the mass-loss rate we find is significantly below the most recent smooth-wind Hα mass-loss rate determinations for ζ Pup, but is in line with newer determinations that account for small-scale wind clumping. If ζ Pup is representative of other massive stars, these results will have important implications for stellar and Galactic evolution.Monthly Notices of the Royal Astronomical Society 07/2010; 405(4):2391 - 2405. · 4.90 Impact Factor -
Article: A mass-loss rate determination for zeta Puppis from the quantitative analysis of X-ray emission line profiles
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ABSTRACT: We fit every emission line in the high-resolution Chandra grating spectrum of zeta Pup with an empirical line profile model that accounts for the effects of Doppler broadening and attenuation by the bulk wind. For each of sixteen lines or line complexes that can be reliably measured, we determine a best-fitting fiducial optical depth, tau_* = kappa*Mdot/4{pi}R_{\ast}v_{\infty}, and place confidence limits on this parameter. These sixteen lines include seven that have not previously been reported on in the literature. The extended wavelength range of these lines allows us to infer, for the first time, a clear increase in tau_* with line wavelength, as expected from the wavelength increase of bound-free absorption opacity. The small overall values of tau_*, reflected in the rather modest asymmetry in the line profiles, can moreover all be fit simultaneously by simply assuming a moderate mass-loss rate of 3.5 \pm 0.3 \times 10^{-6} Msun/yr, without any need to invoke porosity effects in the wind. The quoted uncertainty is statistical, but the largest source of uncertainty in the derived mass-loss rate is due to the uncertainty in the elemental abundances of zeta Pup, which affects the continuum opacity of the wind, and which we estimate to be a factor of two. Even so, the mass-loss rate we find is significantly below the most recent smooth-wind H-alpha mass-loss rate determinations for zeta Pup, but is in line with newer determinations that account for small-scale wind clumping. If zeta Pup is representative of other massive stars, these results will have important implications for stellar and galactic evolution. Comment: Accepted for publication in the Monthly Notices of the Royal Astronomical Society. 17 pages, including 14 figures (7 color)03/2010; -
Article: Quantitative analysis of resolved X-ray emission line profiles of O stars
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ABSTRACT: By quantitatively fitting simple emission line profile models that include both atomic opacity and porosity to the Chandra X-ray spectrum of $\zeta$ Pup, we are able to explore the trade-offs between reduced mass-loss rates and wind porosity. We find that reducing the mass-loss rate of $\zeta$ Pup by roughly a factor of four, to 1.5 \times 10^{-6} M_sun/yr, enables simple non-porous wind models to provide good fits to the data. If, on the other hand, we take the literature mass-loss rate of 6 \times 10^{-6} M_sun/yr, then to produce X-ray line profiles that fit the data, extreme porosity lengths -- of $h_{\infty} \approx 3$ Rstar -- are required. Moreover, these porous models do not provide better fits to the data than the non-porous, low optical depth models. Additionally, such huge porosity lengths do not seem realistic in light of 2-D numerical simulations of the wind instability.01/2008; -
Article: Evidence for the importance of resonance scattering in X-ray emission line profiles of the O star $\zeta$ Puppis
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ABSTRACT: We fit the Doppler profiles of the He-like triplet complexes of \ion{O}{7} and \ion{N}{6} in the X-ray spectrum of the O star $\zeta$ Puppis, using XMM-Newton RGS data collected over $\sim 400$ ks of exposure. We find that they cannot be well fit if the resonance and intercombination lines are constrained to have the same profile shape. However, a significantly better fit is achieved with a model incorporating the effects of resonance scattering, which causes the resonance line to become more symmetric than the intercombination line for a given characteristic continuum optical depth $\tau_*$. We discuss the plausibility of this hypothesis, as well as its significance for our understanding of Doppler profiles of X-ray emission lines in O stars.11/2006; -
Article: Measurements and analysis of helium-like triplet ratios in the X-ray spectra of O-type stars
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ABSTRACT: We discuss new methods of measuring and interpreting the forbidden-to-intercombination line ratios of helium-like triplets in the X-ray spectra of O-type stars, including accounting for the spatial distribution of the X-ray emitting plasma and using the detailed photospheric UV spectrum. Measurements are made for four O stars using archival Chandra HETGS data. We assume an X-ray emitting plasma spatially distributed in the wind above some minimum radius R_0. We find minimum radii of formation typically in the range of 1.25 < R_0 / R_* < 1.67, which is consistent with results obtained independently from line profile fits. We find no evidence for anomalously low f/i ratios and we do not require the existence of X-ray emitting plasmas at radii that are too small to generate sufficiently strong shocks.06/2006; -
Article: Wind signatures in the X-ray emission line profiles of the late O supergiant $\zeta$ Orionis
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ABSTRACT: X-ray line profile analysis has proved to be the most direct diagnostic of the kinematics and spatial distribution of the very hot plasma around O stars. In this paper we apply several analysis techniques to the emission lines in the Chandra HETGS spectrum of the late-O supergiant zeta Ori (O9.7 Ib), including the fitting of a simple line-profile model. We show that there is distinct evidence for blue shifts and profile asymmetry, as well as broadening in the X-ray emission lines of zeta Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for zeta Ori are very similar to those for the earlier O star, zeta Pup, which we have previously shown to be well-fit by the same wind-shock line-profile model. The more subtle effects on the line-profile morphologies in zeta Ori, as compared to zeta Pup, are consistent with the somewhat lower density wind in this later O supergiant. In both stars, the wind optical depths required to explain the mildly asymmetric X-ray line profiles imply reductions in the effective opacity of nearly an order of magnitude, which may be explained by some combination of mass-loss rate reduction and large-scale clumping, with its associated porosity-based effects on radiation transfer. In the context of the recent reanalysis of the helium-like line intensity ratios in both zeta Ori and zeta Pup, and also in light of recent work questioning the published mass-loss rates in OB stars, these new results indicate that the X-ray emission from zeta Ori can be understood within the framework of the standard wind-shock scenario for hot stars.03/2006; -
Article: X-ray spectroscopy of Eta Carinae with XMM-Newton
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ABSTRACT: We present XMM-Newton observations of the luminous star Eta Carinae, including a high resolution soft X-ray spectrum of the surrounding nebula obtained with the Reflection Grating Spectrometer. The EPIC image of the field around Eta Car shows many early-type stars and diffuse emission from hot, shocked gas. The EPIC spectrum of the star is similar to that observed in previous X-ray observations, and requires two temperature components. The RGS spectrum of the surrounding nebula shows K-shell emission lines from hydrogen- and helium-like nitrogen and neon and L-shell lines from iron, but little or no emission from oxygen. The observed emission lines are not consistent with a single temperature, but the range of temperatures observed is not large, spanning ~0.15-0.6 keV. We obtain upper limits for oxygen line emission and derive a lower limit of N/O > 9. This is consistent with previous abundance determinations for the ejecta of Eta Car, and with theoretical models for the evolution of massive, rotating stars. Comment: 20 pages, 8 figures, accepted by ApJ09/2002; -
Article: Wind signatures in the X-ray emission line proflles of the late O supergiant ‡ Orionis
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ABSTRACT: X-ray line proflle analysis has proved to be the most direct diagnostic of the kine- matics and spatial distribution of the very hot plasma around O stars. The Doppler- broadened line proflles provide information about the velocity distribution of the hot plasma, while the wavelength-dependent attenuation across a line proflle provides in- formation about the absorption to the hot plasma, thus providing a strong constraint on its physical location. In this paper we apply several analysis techniques to the emission lines in the Chandra HETGS spectrum of the late-O supergiant ‡ Ori (O9.7 Ib), including the fltting of a simple line-proflle model. We show that there is distinct evidence for blue shifts and proflle asymmetry, as well as broadening in the X-ray emission lines of ‡ Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for ‡ Ori are very similar to those for the earlier O star, ‡ Pup, which we have previously shown to be well-flt by the same wind-shock line-proflle model. The more subtle efiects on the line-proflle morphologies in ‡ Ori, as compared to ‡ Pup, are consistent with the somewhat lower density wind in this later O super- giant. In the context of the recent reanalysis of the helium-like line intensity ratios in this star, and also in light of recent work questioning the published mass-loss rates in OB stars, these new results indicate that the X-ray emission from ‡ Ori can be understood within the framework of the standard wind-shock scenario for hot stars. -
Article: Measuring mass-loss rates and constraining shock physics for ten OB stars using Chandra X-ray emission line profiles
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ABSTRACT: X-ray spectral lines provide a powerful diagnostic of massive star winds. An X-ray emission line's characteristic shape is affected by the kinematics of the hot plasma where the X-rays are produced and by the properties of the attenuating bulk material of the wind, so X-rays can be used as a probe of both wind components. Qualitatively, X-ray lines in massive stars have been observed to lack the signatures of absorption that are predicted by other diagnostics. In this paper, following the methods of Cohen et al. (2009), we analyze the spectra of ten stars from the Chandra archive in order to 1) quantify the amount of absorption by determining mass-loss rates for all stars and 2) test predictions of the wind-shock X-ray production scenario by determining onset radii and terminal velocities for the X-ray producing material. We obtain mass-loss rate reductions of between a factor of 2 and an order of magnitude, and find that our results are in general agreement with the predictions of the wind-shock model. -
Article: Quantifying mass-loss rate reduction and wind porosity using Chandra HETGS emission line proflles of ‡ Puppis
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ABSTRACT: We explore the joint efiects of mass-loss rate reduction and wind porosity on symmetrizing X-ray emission line proflles, using the Chandra grating spectrum (Co-authors: we may add XMM data to the analysis) of ‡ Puppis as a test case. We focus on three relatively high signal-to-noise, representative lines { Ne X at 12.13 "A, Fe XVII at 15.01 "A, and O VIII at 18.97 "A (Co-authors: we can add more lines to the analysis) { and show that when the data are of high enough quality, small values of wind optical depth, ¿⁄, and porosity length, h, are preferred over porous models with larger optical depths. We quantitatively explore the trade- ofis between ¿⁄ and h1 { the terminal velocity porosity length { in those cases where models with high porosity and high wind optical depth provide acceptable flts. We flnd that h1 … 5 R⁄ before the ¿⁄ values implied by the literature mass- loss rate can be accommodated. We also flnd that porosity has little efiect on the line-proflle flts for h1 . 1 R⁄. Co-authors: A sentence or two about introducing the non-isotropic porosity model in the context of the porosity length formalism, and the results of flts with this model. By both examining the porosity length (Co-authors: we haven't done this yet, and it may prove to be too problematic and/or not worth doing) and by synthesizing line proflles from the output of two- dimensional numerical simulations of the wind instability, we show that physically realistic clumpy and porous wind structure does not have large enough porosity lengths to signiflcantly afiect the emergent X-ray line proflle shapes. We also explore the overall level and wavelength dependence of the atomic opacity via detailed modeling and show that relatively ∞at opacities between 0.5 and 1 keV are easily obtained in a smooth wind. -
Article: Quantifying mass-loss rate and wind porosity using the X-ray emission line proflles of ‡ Puppis
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ABSTRACT: We flt X-ray line proflle models, including the efiects of large-scale wind porosity, to the high-resolution Chandra spectrum of the O4 supergiant ‡ Pup. We probe the importance of porosity in two speciflc ways: by comparing the flt quality of porous and non-porous models for individual line proflles, and also by studying the trend in proflle shape for many proflles as a function of wavelength. Individual lines can be flt by both non-porous models and porous models that assume spherical clumps, although for the higher signal-to-noise lines, the non-porous models are always preferred. As the mass-loss rate and the porosity length are nearly degenerate parameters, we explore the trade-ofis between the parameters and flnd that the porosity lengths required to accommodate the traditional mass-loss rate of ‡ Pup, 8 £ 10¡6 Mfl yr¡1, are very high, with h1 > R⁄. Porous models that assume oblate, or ∞attened, clumps, produce proflles with a difierent overall shape, and one that does not provide good flts to the data. We also flnd that there is in fact a signiflcant trend in optical depth as a function of wavelength over the range 6 to 22 "A. This trend is consistent with the expected atomic opacity, but is inconsistent with a highly porous medium, in which the optical depth is governed by the geometrical cross-section of the clumps. From the flts to these lines under the assumption that porosity does not afiect the opacity, we derive a mass-loss rate of 3:0£10¡6 Mfl yr¡1, which represents a factor of » 3 reduction of the traditional mass-loss rate derived assuming no wind clumping, and is consistent with more recent determinations that include small-scale clumping. Note to coauthors: I wonder if we should add a sentence about the CNO abundances and how the sub-solar net abundances make the mass-loss rate determination about a factor of two higher than we'd flnd if we assumed a solar C + N + O.
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2010
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National Polytechnic Institute
Gustavo A. Madero, The Federal District, Mexico
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