[Show abstract][Hide abstract] ABSTRACT: We report the detection of As V resonance lines observed in the Far
Ultraviolet Spectroscopic Explorer (FUSE) spectra of three hot DA white dwarfs:
G191-B2B, WD0621-376, and WD2211-495. The stars have effective temperatures
ranging from 60,000 K to 64,000 K and are among the most metal-rich white
dwarfs known. We measured the arsenic abundances not only in these stars, but
also in three DO stars in which As has been detected before: HD149499B, HZ21,
and RE0503-289. The arsenic abundances observed in the DA stars are very
similar. This suggests that radiative levitation may be the mechanism that
supports arsenic. The arsenic abundance in HZ21 is significantly lower than
that observed in HD149499B, even though the stars have similar atmospheric
parameters. An additional mechanism may be at play in the atmospheres of these
two DO stars.
[Show abstract][Hide abstract] ABSTRACT: The origin of a system of metal lines observed in addition to
interstellar lines in the hot DA star CD-38 10980 has been subject to
debates. The interpretation of the high dispersion ultraviolet spectra,
originally obtained by IUE, led to evidence in favor of the
circumstellar origin of the observed silicon and carbon lines although
the alternate view arguing for a photospheric origin of the same lines
was still defended by some groups. This circumstellar interpretation was
supported by the evidence of a significant velocity shift of these lines
relative to what was then assumed to be the radial velocity of the white
dwarf. In order to explore and possibly settle this question, we have
performed an analysis of the HST STIS and GHRS spectra of this white
dwarf. These spectra should, in principle, have a more reliable and
accurate calibration of the wavelength solution and lead to improved
velocity measurements of the detected metal lines. Our analysis suggests
that there was an unaccounted zero-point offset in the IUE spectra which
led to spurious velocity shifts. Our new results show that when we
adjust the velocity of the interstellar lines detected in the IUE
spectrum to match those of the interstellar lines measured in the GHRS
and STIS spectra, we get an excellent agreement of the velocity of the
metal lines with the extensive radial velocity measurements made by
Maxted et al. (2000) for this white dwarf. This, in our opinion, is
definitely confirming the photospheric origin of the metal lines in
CD-38 10980. The measured abundances of silicon and carbon are in
excellent agreement with the predictions of the radiative levitation
[Show abstract][Hide abstract] ABSTRACT: WD1337+705 (EGGR 102, G 238-44)) is a DAZ white dwarf. Previous optical
and UV observations have revealed high abundances of heavy elements in
its atmosphere in spite of being in an effective temperature range where
selective radiative acceleration of metals is not expected to be
significant. Unlike many DAZ with high metal abundances, it does not
exhibit a clear infrared excess that would normally indicate the
presence of a dust disk around the star. In order to further our
understanding of this star and related objects, we have acquired high
resolution ultraviolet spectra of this star with the goal of
characterizing the metallic content of its atmosphere through a detailed
model atmosphere analysis of these spectra. Specifically, we focus on
the analysis of the spectra recently obtained with the Cosmic Origin
Spectrograph (COS) and STIS from which we report the detection of Al,
Mg, and Fe. By combining these new measurements with time-dependent
simulation of diffusion in presence of accretion, we put stringent
constraints on the accretion rates for the different species detected
and on the composition of the accreted matter.
[Show abstract][Hide abstract] ABSTRACT: We consider a sample of five stars with effective temperatures lower than 25,000 K and showing a range of metallicities. One star exhibits a host of metal lines from multiple species while the others show lines from one or two species. The most likely origin of metals detected in stars of this type is the accretion of matter from circumstellar material, although none of the objects considered in this study bears the signature of a dust disk frequently detected through IR excess in DAZ. The aim of this paper is to explain the peculiar abundance patterns observed in these stars in terms of the accretion-diffusion model and infer the source of the circumstellar matter. To achieve this, we perform time-dependent diffusion calculations accounting for radiative levitation, which is not entirely negligible for some of the light species (C, Si, and Al).
[Show abstract][Hide abstract] ABSTRACT: Elements heavier than hydrogen or helium that are present in the atmospheres of white dwarfs with effective temperatures lower than 25,000 K, are believed to be the result of accretion. By measuring the abundances of these elements and by assuming a steady‐state accretion, we can derive the composition of the accreted matter and infer its source. The presence of radiative levitation, however, may affect the determination of the accretion rate. We present time‐dependent diffusion calculations that take into account radiative levitation and accretion. The calculations are performed on C, N, O, Ne, Na,
Mg, Al, Si, S, Ar, and Ca in hydrogen‐rich white dwarf models with effective temperatures lower than 25,000 K and a gravity of
g = 8.0.
We show that in the presence of accretion, the abundance of an element supported by the radiative levitation is given by the equilibrium between the radiative and gravitational accelerations, unless the abundance predicted by the steady‐state accretion is much greater than the abundance supported by the radiative acceleration.
[Show abstract][Hide abstract] ABSTRACT: We present an analysis of optical spectra for 29 DAO white dwarfs. First, we present our new up-to-date model atmosphere grids computed without the assumption of local thermodynamic equilibrium in which we have included carbon, nitrogen, and oxygen at solar abundances. We demonstrate that the addition of these metals in the model atmospheres is essential in overcoming the Balmer-line problem, which manifests itself as an inability to fit all the Balmer lines simultaneously with consistent atmospheric parameters. We then present the spectroscopic analysis of our sample of DAO white dwarfs for which we determine the effective temperature, surface gravity, and helium abundance. We also present 18 hot DA white dwarfs that also suffer from the Balmer-line problem. We analyze these stars with models analogous to those for the DAO white dwarfs save for the presence of helium. Systematic differences between our newly determined atmospheric parameters with respect to previous determinations are explored. Far-ultraviolet spectra from the FUSE archive are then examined to demonstrate that there exists a correlation between higher metallic abundances and instances of the Balmer-line problem. The implications of these findings for all hot, hydrogen-rich white dwarfs are discussed. Specifically, the possible evolutionary scenario for DAO white dwarfs is revised and post-extreme horizontal branch evolution is no longer needed to explain the evolution for the majority of the DAO stars. Finally, we discuss how the presence of metals might drive a weak stellar wind which in turn could explain the presence of helium in DAO white dwarfs.
Preview · Article · Aug 2010 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: We present a spectral synthesis analysis of the hot subdwarf OB star LB 3241. The analysis is based on spectra obtained by the Far Ultraviolet Spectroscopic Explorer (FUSE). With an effective temperature of 41,000 K and a gravity of log g = 5.7, the position of LB 3241 in a Teff-log g diagram suggests that it has evolved from the extreme horizontal branch. Such stars evolve into white dwarfs without ascending the asymptotic giant branch after the helium core exhaustion. Arsenic (Z = 33), selenium (34), and tellurium (52) are observed in the atmosphere of LB 3241, and are a first for a hot subdwarf star. LB 3241 shows peculiar chemical abundances that exhibit trends observed in cooler sdB stars. The content of its atmosphere in light elements is about a factor ten lower than that of the Sun, except for nitrogen which has a solar abundance. The Fe abundance is consistent with a solar abundance, but abundances of elements beyond the iron peak (As, Se, Te, Pb) show enrichments over the solar values by factors ranging from 10 to 300. These observations suggest that competing mechanisms must counterbalance the effects of the downward diffusion. The FUSE observations also suggest that LB 3241 is a radial velocity variable.
[Show abstract][Hide abstract] ABSTRACT: Heavy elements detected in the atmospheres of hot white dwarfs are commonly explained in terms of a diffusion model involving an equilibrium between radiative forces and gravity. In spite of the declining relative importance of radiative accelerations at lower effective temperatures, an increasing number of cooler DA white dwarfs are also found to host significant abundances of metals in their atmosphere. In these cases, accretion of circumstellar matter appears to be the source of detected metals. To better assess the potential influence of radiative accelerations on the metallicity of cooler DA white dwarfs, we performed new radiative accelerations calculations in DA envelopes with effective temperatures between 12,500K and 25,000K for C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, and Fe. The results are compared with a metal abundance analysis of a sample of 19 stars observed with FUSE. The main elements predicted to be supported by radiative acceleration over this temperature range are C, Si, and Al, in line with the FUSE observations where only C and Si have been detected in most objects. For the other elements, we confirm that radiative acceleration can be safely neglected when computing steady-state abundances in the accretion-diffusion framework.
[Show abstract][Hide abstract] ABSTRACT: Over the course of the Far Ultraviolet Spectroscopic Explorer (FUSE) mission, many DA white dwarfs have been observed either as background sources for local interstellar medium studies or for detailed investigations of white dwarf atmospheres. The FUSE sample provides a rather complete snapshot of the population of DA's with stars as cool as ZZ Ceti stars or as hot as stars that have barely initiated their descent on the white dwarf cooling tracks. Based on this large sample of stars, we present a FUV spectroscopic analysis of DA white dwarfs. With the help of detailed model atmospheres, we determine the effective temperatures and surface gravities of DA white dwarfs by fitting their Lyman line profiles. We also measure or set abundances upper limits of ionic species that are predicted by the radiative levitation theory. We compare our atmospheric parameters to those obtained by fitting the Balmer lines, and critically review our abundances and upper limits in terms of the diffusion theory.
[Show abstract][Hide abstract] ABSTRACT: We present results from our analysis of a sample of DA white dwarfs having effective temperature below 25,000 K observed with the FUSE satellite with the goals of better understanding the origin of metals detected in the atmosphere of these stars. When possible, we combine the FUSE spectra with the IUE spectra and determine atmospheric parameters by fitting the Lyman line profiles. In general we find a good agreement with published values based on fits of the Balmer series. We observe that the continuum in the blue wing of the Lyman alpha line profile is generally lower in comparison with model spectra and that the discrepancy appears to become less important at higher effective temperature. The agreement between models and observations is excellent at wavelengths shorter than 1100 Å, which gives us confidence in the determination of atmospheric parameters. Finally, using adopted atmospheric parameters; we have performed a detailed analysis of the composition of these stars. In several instances, we have observed the presence of silicon and in one case that of carbon. For each star in the sample we have either measured or set an upper limit on the presence of key species such as CII, CIII, SiIII, and SiIV. We then compare the measured abundances with equilibrium abundance predicted by radiative levitation theory for each star. In this limited sample, we find that when detected, the abundance of silicon is in good agreement with theory. However there are several cases where the upper limits are smaller than the predictions and one case where it is considerably larger.
No preview · Article · Jun 2009 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: The FUSE flux calibration is defined by synthetic spectra of six DA white dwarfs with effective temperatures Teff ranging from 32,800 K to 61,200 K. The LWRS observations of these stars were reprocessed using the final calibration, and the results compared against synthetic spectra. Discrepancies were generally 5% or less, apart from the well-known ``worm'' feature in LiF1B. Discrepancies for GD 246 were significant for the Teff used previously (53,000 K), but are comparable to those for the other stars for a model computed at Teff = 58,700 K. The effects of quasi-molecular satellite features are significant for the coolest star in this sample, and appear to modify the Lyman line profiles at all Teff. Blanketing by weak metal lines in G 191-B2B make use of this star problematic for calibrations accurate to a few percent or better in the FUV.
[Show abstract][Hide abstract] ABSTRACT: One of the most puzzling phenomena observed at the surface of sdB stars is their abundance anomalies. For instance, high‐resolution optical spectroscopy shows that the C abundance is lower than the solar abundance by about a factor of ten on average. A few stars, however, show a solar abundance or an abundance higher than solar. Although optical spectroscopy allows the measurement of relatively high C abundance, it fails to provide stringent upper limits when no C lines are detected. The Far Ultraviolet Spectroscopic Explorer (FUSE)
comes to the rescue when no C lines are detected in the optical, because it is 1,000 times more sensitive than the VLT for the purpose of measuring very small amounts of C in the atmospheres of sdB stars. We measure the C abundance in the
spectra of 30 stars. We confirm the C abundance trend reported in previous studies, but report the disappearance of C in five stars. The upper limits that we measure for these stars are about four orders of magnitude lower than the solar abundance. The observation of very low C abundance suggests that the normal equilibrium between gravitation and radiative levitation has been disrupted, perhaps by a weak stellar wind.
[Show abstract][Hide abstract] ABSTRACT: With its unprecedented sensitivity and spectral resolution, FUSE has been a source of important data that have helped us further our knowledge and understanding of white dwarf stars. In order to illustrate the crucial contribution of FUSE to the study of white dwarfs, we present an abundance analysis of Si based on FUSE observations of 83 hydrogen-rich (DA) white dwarfs. Studying the Si abundances in the atmospheres of white dwarfs is important for understanding the mechanisms that counteract the downward diffusion of elements heavier than hydrogen. The FUSE observations show that Si is present in about 25% of white dwarfs. The Si abundances show a large scatter that is difficult to interpret in terms of a strict equilibrium between the gravitation and the radiative levitation.
[Show abstract][Hide abstract] ABSTRACT: We present new calculations for the Stark broadening of the hydrogen line profiles in the dense atmospheres of white dwarf stars. Our improved model is based on the unified theory of Stark broadening from Vidal, Cooper & Smith, but it also includes non-ideal gas effects from the Hummer & Mihalas occupation probability formalism directly inside the line profile calculations. This approach improves upon previous calculations that relied on the use of an ad-hoc free parameter to describe the dissolution of the line wing opacity in the presence of high electric microfields in the plasma. We present here the first grid of model spectra for hot Teff >~ 12,000 K DA white dwarfs that has no free parameters. The atmospheric parameters obtained from optical and UV spectroscopic observations using these improved models are shown to differ substantially from those published in previous studies. Comment: 8 pages, 8 figures, to appear in Journal of Physics Conference Proceedings for the 16th European White Dwarf Workshop
Preview · Article · Apr 2009 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: The hot white dwarf in the close binary EUVE J1016-053 (=RE 1016-053) has been classified as a DAO white dwarf; its mixed H/He composition has been attributed to steady accretion from the close red dwarf companion. We have obtained extreme-ultraviolet (EUV) photometric and spectroscopic observations of EUVE J1016-053 with the Extreme Ultraviolet Explorer (EUVE). We report the discovery of a ≈ 30% EUV flux variation over a period of 57.3 minutes, which we attribute to surface abundance inhomogeneities modulated over the stellar rotation period, Prot. The EUVE spectrum shows the effect of heavy-element opacities on the white dwarf EUV energy distribution. Spectral synthesis including trace opacities of helium and a group of heavy elements (C, N, O, Si, S, Fe) in the otherwise hydrogen-rich atmosphere constrains abundances to Y/Y☉ = Z/Z☉ = 2 × 10-3, in support of a simple accretion model. The low surface-averaged helium abundance measured in the white dwarf atmosphere limits the accretion rate to between 10-19 and 10-18 M☉ yr-1, i.e., much lower than the Bondi-Hoyle accretion rate, which is of the same order as the red dwarf mass-loss rate (≥10-14 M☉ yr-1), therefore invalidating a simple wind-accretion model. We speculate that weak mass loss from the white dwarf or interaction with a magnetosphere may inhibit accretion onto the white dwarf. Accretion of heavy elements may also be restricted to smaller areas, possibly correlating to magnetic poles.
Full-text · Article · Jan 2009 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: We present new far-ultraviolet observations of the hot, hydrogen-rich, white dwarf (DA) HZ 43 obtained with the Berkeley spectrograph during the ORFEUS-SPAS II mission. The spectrum, which covers the range between 892 and 1200 Å at a spectral resolution of 3300, is dominated by the Lyman series of hydrogen and shows no evidence of photospheric absorption lines from other ionic species often detected in hot white dwarf stars. The photospheric Lyman line profiles are modeled using synthetic spectra based on improved Stark-broadening calculations; narrow-line cores of interstellar origin help constrain the velocity dispersion in the line of sight. We find an effective temperature of 50,400 ± 100 K and a log g of 7.91±0.06. We also take advantage of ORFEUS's high spectral resolution in order to put stringent upper limits on the abundance of C, N, Si, S, P, and Cl. We find upper limits on the number abundance relative to hydrogen of 10−8.0 for C and N, 10−8.5 for Si, P, and Cl, and 10−7.0 for S. These upper limits are at least an order of magnitude below the abundances predicted by radiative levitation calculations. Our analysis refines considerably our knowledge of HZ 43's atmospheric parameters, and we discuss the possible physical mechanisms that might explain the purity of HZ 43's atmosphere. We have also measured the H I velocity dispersion (b-value) in the line of sight of HZ 43 and find a value of 9±2 km s−1.
Full-text · Article · Jan 2009 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: The Berkeley spectrograph aboard the ORFEUS telescope made its second flight on the 14 day ORFEUS-SPAS II mission of the Space Shuttle Columbia in 1996 November/December. Approximately half of the available observing time was dedicated to the Berkeley spectrograph, which was used by both principal and guest investigators. The spectrograph's full bandpass is 390-1218 Å; here we discuss its in-flight performance at far-ultraviolet wavelengths, at which most of the observations were performed. The instrument's effective area peaks at 8.9±0.5 cm2 near 1020 Å, and the mean spectral resolution is 95 km s-1 FWHM for point sources. Over most of the spectral range, the typical nighttime background event rate in each spectral resolution element was ~0.003 s-1. Simultaneous background observations of an adjacent blank field were provided through a secondary, off-axis aperture. The Berkeley spectrograph's unique combination of sensitivity and resolution provided valuable observations of approximately 105 distinct astronomical targets, which ranged in distance from the Earth's own Moon to some of the brightest active galactic nuclei.
Full-text · Article · Jan 2009 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: We analyze high-dispersion ultraviolet spectroscopic observations of the peculiar helium-rich white dwarf MCT 0501-2858. Ultraviolet spectra obtained during the 1996 November ORFEUS-SPAS II mission show the complete He II (n=2) line series as well as narrow heavy-element lines; the spectrum constrains the effective temperature (68,600 ± 1800 K) and surface gravity (log g=7.20±0.07), imposing a low mass (0.4 M) and short cooling age (105 yr) for the star. A study of the complete spectral energy distribution, based on EUVE, ORFEUS, and IUE spectra, shows that carbon is the second most abundant element [log (C/He) = -1.9] and that lower abundances are measured for other elements [log (N/He) = -4.0, log (O/He) = -3.3, log (Si/He) = -4.6, and log (P/He) = -6.2]. The new atmospheric parameters (temperature, gravity, and chemical composition) suggest a possible evolutionary filiation with the class of PG 1159 stars.
Full-text · Article · Jan 2009 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: We present spectroscopic and timing analyses of the hot DA white dwarf GD 394 showing abundance inhomogeneities across its surface. Lick Hamilton echelle, International Ultraviolet Explorer (IUE), HST GHRS, and Hopkins Ultraviolet Telescope (HUT) spectra show dominant Si III and Si IV features, while Extreme Ultraviolet Explorer (EUVE) spectra show evidence of a host of trace element opacities. We report the detection of Si III λ4552 triplet with a measured radial velocity of 27 km s-1 in agreement with far ultraviolet (FUV) line velocities, but indicating a silicon abundance (Si/H = 2 × 10-5) a factor of 10 larger than measured in the FUV range [Si/H = (2-7) × 10-6]. Effective temperature measurements based on Lyman continuum (EUVE) and Lyman line series (HUT) are systematically cooler (ΔT ~ -4000 K) than measurements based on medium-dispersion Balmer line spectroscopy, an effect attributed to yet unidentified opacities. A timing analysis of EUVE deep-survey and scanner time series, as well as spectrometer data, shows GD 394 to be variable in the extreme ultraviolet (EUV) with an amplitude of 25% and a period of 1.150 ± 0.003 days. The EUV variability suggests abundance inhomogeneities in the atmosphere, and we explore different models to explain its origin.
Preview · Article · Dec 2008 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: We present far-UV and UV spectroscopic observations of Proxima Centauri obtained as part of our continued investigation into the optically thin approximation assumed for the transition regions of late-type stars. Significant opacity is found in the C III lines during both the quiescent and flaring states of Proxima Cen, with up to 70% of the expected flux being lost in the latter. Our findings cast some doubt on the suitability of the C III λ977 line for estimating the electron density in stellar atmospheres. However, the opacity has no significant effect on the observed line widths. We calculate optical depths for homogeneous and inhomogeneous geometries and estimate an electron density of 6 × 1010 cm-3 for the transition region using the O IV line ratios at 1400 Å. The combination of electron density and optical depth indicates path lengths as low as ≈10 km, which are in excellent agreement with estimates of the small-scale structure seen in the solar transition region.
Full-text · Article · Dec 2008 · The Astrophysical Journal