arXiv subjects

Thierry Lanz

Publications and source records attributed to Thierry Lanz.

36 records · Page 2Linked to original sources

arXiv2008

Argon Abundances in the Solar Neighborhood: Non-LTE Analysis of Orion Association B-type Stars

Argon abundances have been derived for a sample of B main-sequence stars in the Orion association. The abundance calculations are based on NLTE metal line-blanketed model atmospheres calculated with the NLTE code TLUSTY and an updated and complete argon model atom. We derive an average argon abundance for this young population of A(Ar) = 6.66 +- 0.06. While our result is in excellent agreement with a recent analysis of the Orion nebula, it is significantly higher than the currently recommended solar value which is based on abundance measurements in the solar corona. Moreover, the derived argon abundances in the Orion B stars agree very well with a measurement from a solar impulsive flare during which unmodified solar photospheric material was brought to flare conditions. We therefore argue that the argon abundances obtained independently for both the Orion B stars and the Orion nebula are representative of the disk abundance value in the solar neighborhood. The lower coronal abundance may reflect a depletion related to the FIP effect. We propose a new reference value for the abundance of argon in the solar neighborhood, A(Ar) = 6.63 +- 0.10, corresponding to Ar/O = 0.009.

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arXiv2007

Orbital Parameters and Chemical Composition of Four White Dwarfs in Post-Common Envelope Binaries

We present FUSE observations of the hot white dwarfs in the post-common envelope binaries Feige 24, EUVE J0720-317, BPM 6502, and EUVE J2013+400. The spectra show numerous photospheric absorption lines which trace the white dwarf orbital motion. We report the detection of C III, O VI, P V, and Si IV in the spectra of Feige 24, EUVE J0720-317 and EUVE J2013+400, and the detection of C III, N II, Si III, Si IV, and Fe III in the spectra of BPM 6502. Abundance measurements support the possibility that white dwarfs in post-common envelope binaries accrete material from the secondary star wind. The FUSE observations of BPM 6502 and EUVE J2013+400 cover a complete binary orbit. We used the FUSE spectra to measure the radial velocities traced by the white dwarf in the four binaries, where the zero-point velocity were fixed using the ISM velocities in the line of sight of the stellar systems. For BPM 6502 we determined a white dwarf velocity semi-amplitude of K_WD = 18.6+/-0.5km/s, and with the velocity semi-amplitude of the red dwarf companion (K_RD = 75.2+/-3.1 km/s), we estimate the mass ratio to be q = 0.25+/-0.01. Adopting a spectroscopic mass determination for the white dwarf, we infer a low secondary mass of M_RD = 0.14+/-0.01 M_solar. For EUVE J2013+400 we determine a white dwarf velocity semi-amplitude of K_WD = 36.7+/-0.7 km/s. The FUSE observations of EUVE J0720-317 cover approximately 30% of the binary period and combined with the HST GHRS measurements (Vennes et al. 1999, ApJ 523, 386), we update the binary properties. FUSE observations of Feige 24 cover approximately 60% of the orbit and we combine this data set with HST STIS (Vennes et al. 2000, ApJ, 544, 423) data to update the binary properties.

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arXiv2006

Modeling multidimensional effects in the propagation of radiative shocks

Radiative shocks (also called supercritical shocks) are high Mach number shock waves that photoionize the medium ahead of the shock front and give rise to a radiative precursor. They are generated in the laboratory using high-energy or high-power lasers and are frequently present in a wide range of astronomical objects. Their modelisation in one dimension has been the subject of numerous studies, but generalization to three dimensions is not straightforward. We calculate analyticaly the absorption of radiation in a grey uniform cylinder and show how it decreases with chi R, the product of the opacity chi and of the cylinder radius R. Simple formulas, whose validity range increases when chi R diminishes, are derived for the radiation field on the axis of symmetry. Numerical calculations in three dimensions of the radiative energy density, flux and pressure created by a stationary shock wave show how the radiation decreases whith R. Finally, the bidimensional structures of both the precursor and the radiation field are calculated with time-dependent radiation hydrodynamics numerical simulations and the influence of two-dimensional effects on the electron density, the temperature, the shock velocity and the shock geometry are exhibited. These simulations show how the radiative precursor shortens, cools and slows down when R is decreased.

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arXiv2006

A Grid of NLTE Line-Blanketed Model Atmospheres of Early B-type Stars

We have constructed a comprehensive grid of 1540 metal line-blanketed, NLTE, plane-parallel, hydrostatic model atmospheres for the basic parameters appropriate to early B-type stars. The BSTAR2006 grid considers 16 values of effective temperatures, 15,000 K <= Teff <= 30,000 K with 1,000 K steps, 13 surface gravities, 1.75 <= log g <= 4.75 with 0.25 dex steps, 6 chemical compositions, and a microturbulent velocity of 2 km/s. The lower limit of log g for a given effective temperature is set by an approximate location of the Eddington limit. The selected chemical compositions range from twice to one tenth of the solar metallicity and metal-free. Additional model atmospheres for B supergiants (log g <= 3.0) have been calculated with a higher microturbulent velocity (10 km/s) and a surface composition that is enriched in helium and nitrogen, and depleted in carbon. This new grid complements our earlier OSTAR2002 grid of O-type stars (Lanz & Hubeny, 2003, ApJS, 146, 417). The paper contains a description of the BSTAR2006 grid and some illustrative examples and comparisons. NLTE ionization fractions, bolometric corrections, radiative accelerations, and effective gravities are obtained over the parameter range covered by the grid. By extrapolating radiative accelerations, we have determined an improved estimate of the Eddington limit in absence of rotation between 55,000 and 15,000 K. The complete BSTAR2006 grid is available at the TLUSTY website (http://nova.astro.umd.edu).

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arXiv2006

Iron in Hot DA White Dwarfs

We present a study of the iron abundance pattern in hot hydrogen-rich (DA) white dwarfs. The study is based on new and archival far ultraviolet spectroscopy of a sample of white dwarfs in the temperature range 30,000 K < T_eff < 64,000 K. The spectra obtained with the Far Ultraviolet Spectroscopic Explorer along with spectra obtained with the Hubble Space Telescope Imaging Spectrograph and the International Ultraviolet Explorer sample FeIII to FeVI absorption lines enabling a detailed iron abundance analysis over a wider range of effective temperatures than previously afforded. The measurements reveal abundance variations in excess of two orders of magnitude between the highest and the lowest temperatures probed, but also show considerable variations (over one order of magnitude) between objects with similar temperatures and surface gravities. Such variations in cooler objects may be imputed to accretion from unseen companions or so-called circumstellar debris although the effect of residual mass-loss and selective radiation pressure in the hottest objects in the sample remain dominant.

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arXiv2006

Neon Abundances in B-Stars of the Orion Association: Solving the Solar Model Problem?

We report on non-LTE Ne abundances for a sample of B-type stellar members of the Orion Association. The abundances were derived by means of non-LTE fully metal-blanketed model atmospheres and extensive model atoms with updated atomic data. We find that these young stars have a very homogeneous abundance of A(Ne) = 8.27 +/- 0.05. This abundance is higher by ~0.4 dex than currently adopted solar value, A(Ne)=7.84, which is derived from lines produced in the corona and active regions. The general agreement between the abundances of C, N, and O derived for B stars with the solar abundances of these elements derived from 3-D hydrodynamical models atmospheres strongly suggests that the abundance patterns of the light elements in the Sun and B stars are broadly similar. If this hypothesis is true, then the Ne abundance derived here is the same within the uncertainties as the value required to reconcile solar models with helioseismological observations.

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arXiv2005

A Spectroscopic Analysis of Blue Stragglers, Horizontal Branch and Turn-Off Stars in Four Globular Clusters

We present a spectroscopic analysis of HST/STIS and FOS low- and intermediate-resolution spectroscopy of 55 stars (turn-off stars, horizontal branch stars and blue stragglers) in four globular clusters (47 Tucanae, M3, NGC6752, and NGC6397). Stars were analyzed with non-Local Thermodynamic Equilibrium model atmospheres, and values for their effective temperatures and gravities and some rotation rates were obtained. Using photometric fluxes, we also obtained radii, luminosities and spectroscopic masses.

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arXiv2004

Fundamental Properties of O-Type Stars

We present a comprehensive analysis of high-resolution, far-UV HST/STIS, FUSE, and optical spectra of 17 O stars in the SMC. Our analysis is based on NLTE metal line-blanketed model atmospheres calculated with our NLTE code TLUSTY. We systematically explore the sensitivity of various UV and optical lines to different stellar parameters. We have obtained consistent fits of the UV and the optical spectrum to derive the effective temperature, surface gravity, surface composition, and microturbulent velocity of each star. Stellar radii, masses, luminosities and ages then follow. Similarly to more limited recent studies, we derive cooler temperatures than the standard Teff calibration of O stars. We propose a new calibration between the spectral type and effective temperature based on our results from UV metal lines as well as optical hydrogen and helium lines. For stars of the same spectral subtype, we find a general good agreement between Teff determinations obtained with TLUSTY, CMFGEN, and FASTWIND models. We derive ionizing luminosities that are smaller by a factor of 3 compared to luminosities inferred from previous standard calibrations. The chemical composition analysis reveals that the surface of about 3/4 of the program stars is moderately to strongly enriched in nitrogen, while showing the original helium, carbon, and oxygen abundances. Our results support the new stellar evolution models that predict that the surface of fast rotating stars becomes N-rich during the main sequence phase because of rotationally-induced mixing. Most stars exhibit the ``mass discrepancy'' problem. This discrepancy too is a result of fast rotation which lowers the measured effective gravity. Our study thus emphasizes the importance of rotation in our understanding of the properties of massive stars. (abridged)

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arXiv2004

NLTE Model Atmosphere Analysis of the LMC Supersoft X-ray Source CAL 83

We present a non-LTE model atmosphere analysis of Chandra HRC-S/LETG and XMM-Newton RGS spectroscopy of the prototypical supersoft source CAL 83 in the Large Magellanic Cloud. Taken with a 16-month interval, the Chandra and XMM-Newton spectra are very similar. They reveal a very rich absorption line spectrum from the hot white dwarf photosphere, but no spectral signatures of a wind. We also report a third X-ray off-state during a later Chandra observation, demonstrating the recurrent nature of CAL 83. Moreover, we found evidence of short-timescale variability in the soft X-ray spectrum. We completed the analysis of the LETG and RGS spectra of CAL 83 with new NLTE line-blanketed model atmospheres that explicitly include 74 ions of the 11 most abundant species. We successfully matched the Chandra and XMM-Newton spectra assuming a model composition with LMC metallicity. We derived the basic stellar parameters of the hot white dwarf, but the current state of atomic data in the soft X-ray domain precludes a detailed chemical analysis. We have obtained the first direct spectroscopic evidence that the white dwarf is massive (Mwd > 1 Msun). The short timescale of the X-ray off-states is consistent with a high white dwarf mass. Our analysis thus provides direct support for supersoft sources as likely progenitors of SN Ia.

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arXiv2004

First Evidence of Circumstellar Disks around Blue Straggler Stars

We present an analysis of optical HST/STIS and HST/FOS spectroscopy of 6 blue stragglers found in the globular clusters M3, NGC6752 and NGC6397. These stars are a subsample of a set of ~50 blue stragglers and stars above the main sequence turn-off in four globular clusters which will be presented in an forthcoming paper. All but the 6 stars presented here can be well fitted with non-LTE model atmospheres. The 6 misfits, on the other hand, possess Balmer jumps which are too large for the effective temperatures implied by their Paschen continua. We find that our data for these stars are consistent with models only if we account for extra absorption of stellar Balmer photons by an ionized circumstellar disk. Column densities of HI and CaII are derived as are the the disks' thicknesses. This is the first time that a circumstellar disk is detected around blue stragglers. The presence of magnetically-locked disks attached to the stars has been suggested as a mechanism to lose the large angular momentum imparted by the collision event at the birth of these stars. The disks implied by our study might not be massive enough to constitute such an angular momentum sink, but they could be the leftovers of once larger disks.

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arXiv2003

Flash Mixing on the White Dwarf Cooling Curve: FUSE Observations of three He-rich sdB Stars

We present FUSE spectra of 3 He-rich sdB stars. Two of these stars, PG1544+488 and JL87, reveal extremely strong C III lines at 977 and 1176A, while the carbon lines are quite weak in the third star, LB1766. We have analyzed the FUSE data using TLUSTY NLTE line-blanketed model atmospheres, and find that PG1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL87 shows a similar surface enrichment of carbon and nitrogen, but some significant fraction of hydrogen still remains in its atmosphere. LB1766 has a surface composition devoid of hydrogen and strongly depleted of carbon, indicating that its surface material has undergone CN-cycle processing. We interpret these observations with new evolutionary calculations which suggest that He-rich sdB stars with C-rich compositions are the progeny of stars which underwent a delayed He-core flash on the white-dwarf cooling curve. During such a flash the interior convection zone will penetrate into the H envelope, thereby mixing the envelope with the He- and C-rich core. Such `flash-mixed' stars will arrive on the extreme horizontal branch (EHB) with He- and C-rich surface compositions and will be hotter than the hottest canonical (i.e., unmixed) EHB stars. Two types of flash mixing are possible: `deep' and `shallow', depending on whether the H envelope is mixed deeply into the site of the He flash or only with the outer layers of the core. Based on both their stellar parameters and surface compositions, we suggest that PG1544+488 and JL87 are examples of `deep' and `shallow' flash mixing, respectively. Flash mixing may therefore represent a new evolutionary channel for producing the hottest EHB stars. However, flash mixing cannot explain the abundance pattern in LB1766, which remains a challenge to current evolutionary models.

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arXiv2003

Evidence for Flash Mixing in He-rich sdB Stars

We present FUSE spectra of three He-rich sdB stars. Two of these stars, PG1544+488 and JL87, reveal extremely strong C III lines, suggesting that they have mixed triple-alpha carbon from the deep interior out to their surfaces. Using TLUSTY NLTE line-blanketed model atmospheres, we find that PG1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL87 shows a similar surface enrichment of C and N but still retains a significant amount of hydrogen. In contrast, the third star, LB1766, is devoid of hydrogen and strongly depleted of carbon, indicating that its surface material has undergone CN-cycle processing. We interpret these observations with new evolutionary calculations which suggest that He-rich sdB stars with C-rich compositions arise from a delayed helium-core flash on the white-dwarf cooling curve. During such a flash the interior convection zone will penetrate into the stellar envelope, thereby mixing the envelope with the He- and C-rich core. Such "flash-mixed" stars will arrive on the extreme horizontal branch (EHB) with He- and C-rich surface compositions and will be hotter than the hottest canonical EHB stars. Two types of flash mixing are possible: "deep" and "shallow", depending on whether the hydrogen envelope is mixed deeply into the site of the helium flash or only with the outer layers of the core. Based on both their stellar parameters and surface compositions, we suggest that PG1544+488 and JL87 are examples of "deep" and "shallow" flash mixing, respectively.

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arXiv2003

A Grid of NLTE Line-Blanketed Model Atmospheres of O-type Stars

We have constructed a comprehensive grid of 690 metal line-blanketed, NLTE, plane-parallel, hydrostatic model atmospheres for the basic parameters appropriate to O-type stars. The OSTAR2002 grid considers 12 values of effective temperatures, 27500K < Teff < 55000K with 2500K steps, 8 surface gravities, 3.0 < log g< 4.75 with 0.25 dex steps, and 10 chemical compositions, from metal-rich relative to the Sun to metal-free. The lower limit of log g for a given effective temperature is set by an approximate location of the Eddington limit. The selected chemical compositions have been chosen to cover a number of typical environments of massive stars: the galactic center, the Magellanic Clouds, Blue Compact Dwarf galaxies like I Zw 18, and galaxies at high redshifts. The paper contains a description of the OSTAR2002 grid and some illustrative examples and comparisons. The complete OSTAR2002 grid is available at our website at http://tlusty.gsfc.nasa.gov.

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arXiv2002

Isolating Clusters with Wolf-Rayet Stars in I Zw 18

We present UV images and spectra of the starburst galaxy I Zw 18, taken with the Space Telescope Imaging Spectrograph. The high spatial resolution of these data allows us to isolate clusters containing Wolf-Rayet stars of the subtype WC. Our far-UV spectra clearly show CIV 1548,1551 and HeII 1640 emission of WC stars in two clusters: one within the bright (NW) half of I Zw 18, and one on the outskirts of this region. The latter spectrum is unusual, because the CIV is seen only in emission, indicating a spectrum dominated by WC stars. These data also demonstrate that the HI column in I Zw 18 is strongly peaked in the fainter (SE) half of I Zw 18, with a column depth far larger than that reported in previous analyses.

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arXiv2002

A New Formation Mechanism for the Hottest Horizontal-Branch Stars

Stars with very large mass loss on the red-giant branch can undergo the helium flash while descending the white-dwarf cooling curve. Under these conditions the flash convection zone will mix the hydrogen envelope with the hot helium- burning core. Such ``flash-mixed'' stars will arrive on the extreme horizontal branch (EHB) with helium- and carbon-rich envelopes and will lie at higher temperatures than the hottest canonical (i.e., unmixed) EHB stars. Flash mixing provides a new evolutionary channel for populating the hot end of the EHB and may explain the origin of the high gravity, helium-rich sdO and sdB stars.

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arXiv2002

The Origin of Hot Subluminous Horizontal-Branch Stars in Omega Centauri and NGC 2808

Hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch (EHB) are found in the UV color-magnitude diagrams of omega Cen and NGC 2808. Such stars are unexplained by canonical HB theory. In order to explore the origin of these subluminous stars, we evolved a set of low-mass stars from the main sequence through the helium-core flash to the HB for a wide range in the mass loss along the red-giant branch (RGB). Stars with the largest mass loss evolve off the RGB to high effective temperatures before igniting helium in their cores. Our results indicate that the subluminous EHB stars, as well as the gap within the EHB of NGC 2808, can be explained if these stars undergo a late helium-core flash on the white-dwarf cooling curve. Under these conditions the flash convection will penetrate into the stellar envelope, thereby mixing most, if not all, of the envelope hydrogen into the hot helium- burning interior. This phenomenon is analogous to the "born-again" scenario for producing hydrogen-deficient stars during a very late helium-shell flash. "Flash mixing" greatly enhances the envelope helium and carbon abundances and, as a result, leads to an abrupt increase in the HB effective temperature. We argue that the EHB gap in NGC 2808 is caused by this theoretically predicted dichotomy in the HB morphology. Using new helium- and carbon-rich stellar atmospheres, we show that the flash-mixed stars have the same reduced UV flux as the subluminous EHB stars. Moreover, we demonstrate that models without flash mixing lie, at most, ~0.1 mag below the EHB and hence fail to explain the observations. Flash mixing may also provide a new evolutionary channel for producing the high gravity, He-rich sdO and sdB stars.

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arXiv2001

Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

We present a UV CMD spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data were obtained with the Space Telescope Imaging Spectrograph (STIS). Although previous optical CMDs of NGC 2808 show a high temperature gap within the hot HB population, no such gap is evident in our UV CMD. Instead, we find a population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster omega Cen. Our modeling indicates that the location of these subluminous stars in the UV CMD, as well as the high temperature gap along the HB in optical CMDs, can be explained if these stars underwent a late He-core flash while descending the white dwarf cooling curve. We show that the convection zone produced by such a late He flash will penetrate into the H envelope, thereby mixing H into the hot He-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "born again" scenario for producing H-deficient stars following a late He-shell flash. The flash mixing of the envelope greatly enhances the envelope He and C abundances, and leads, in turn, to a discontinuous increase in the HB effective temperatures at the transition between canonical and flash-mixed stars. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late He-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.

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arXiv1999

Hot stellar population synthesis from the UV spectrum: the globular cluster M79 (NGC 1904)

We have analyzed the far UV-spectrum of the globular cluster M79. We show that the nearly Gaussian mass distribution of zero-age horizontal-branch stars, as derived by Dixon et al.(1996), is able to reproduce the far-UV Hopkins Utraviolet Telescope (HUT) spectrum, if there is a luminous UV-bright star of about an effective temperature of 9,500 K within the HUT entrance slit, or, more likely, if the horizontal branch morphology becomes considerably redder in the core of M79, as observed in some other centrally condensed globular clusters. Agreement between the synthetic and observed far-UV spectra for M79 would also be improved if the surface abundances of the heavy elements in the hot horizontal-branch stars were enhanced by radiative diffusion. Contrary to Dixon et al. (1996) we do not need extremely low gravities to reproduce the width of Ly alpha.

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