Publications (17)59.21 Total impact
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Article: A Comprehensive Spectroscopic Analysis of DB White Dwarfs
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ABSTRACT: We present a detailed analysis of 108 helium-line (DB) white dwarfs based on model atmosphere fits to high signal-to-noise optical spectroscopy. We derive a mean mass of 0.67 Mo for our sample, with a dispersion of only 0.09 Mo. White dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample, and their mass distribution appears similar to that of DB stars. As in our previous investigation, we find no evidence for the existence of low-mass (M < 0.5 Mo) DB white dwarfs. We derive a luminosity function based on a subset of DB white dwarfs identified in the Palomar-Green survey. We show that 20% of all white dwarfs in the temperature range of interest are DB stars, although the fraction drops to half this value above Teff ~ 20,000 K. We also show that the persistence of DB stars with no hydrogen features at low temperatures is difficult to reconcile with a scenario involving accretion from the interstellar medium, often invoked to account for the observed hydrogen abundances in DBA stars. We present evidence for the existence of two different evolutionary channels that produce DB white dwarfs: the standard model where DA stars are transformed into DB stars through the convective dilution of a thin hydrogen layer, and a second channel where DB stars retain a helium-atmosphere throughout their evolution. We finally demonstrate that the instability strip of pulsating V777 Her white dwarfs contains no nonvariables, if the hydrogen content of these stars is properly accounted for.05/2011; -
Article: Multiwavelength Observations of the Hot DB Star PG 0112+104
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ABSTRACT: We present a comprehensive multiwavelength analysis of the hot DB white dwarf PG 0112+104. Our analysis relies on newly acquired FUSE observations, on medium-resolution FOS and GHRS data, on archival high-resolution GHRS observations, on optical spectrophotometry both in the blue and around Hα, as well as on time-resolved photometry. From the optical data, we derive a self-consistent effective temperature of 31,300 ± 500 K, a surface gravity of log g = 7.8 ± 0.1 (M = 0.52 M ☉), and a hydrogen abundance of log N(H)/N(He)< –4.0. The FUSE spectra reveal the presence of C II and C III lines that complement the previous detection of C II transitions with the GHRS. The improved carbon abundance in this hot object is log N(C)/N(He) = –6.15 ± 0.23. No photospheric features associated with other heavy elements are detected. We reconsider the role of PG 0112+104 in the definition of the blue edge of the V777 Her instability strip in light of our high-speed photometry and contrast our results with those of previous observations carried out at the McDonald Observatory.The Astrophysical Journal 07/2010; 718(2):647. · 6.02 Impact Factor -
Article: The Optical Spectrum of Extreme Helium Stars: A Hotbed of Forbidden Components of Neutral Helium
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ABSTRACT: We develop a simple criterion, based on the general properties of weakly excited forbidden components in neutral helium, to predict which forbidden components should be observable in the He I spectrum of extreme helium stars. On the basis of this criterion, we find that nearly a dozen such components should be visible. We then use the He I broadening data of Beauchamp et al. to calculate detailed synthetic spectra for effective temperatures and gravities representative of these objects. All the forbidden components predicted by our criterion are present in the synthetic spectra. Observation of these features in extreme helium stars will provide a valuable test of current theories of the Stark broadening of neutral helium lines.The Astrophysical Journal 01/2009; 496(1):395. · 6.02 Impact Factor -
Article: Spectroscopic Studies of DB White Dwarfs: The Instability Strip of the Pulsating DB (V777 Herculis) Stars
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ABSTRACT: We have secured optical spectra for the eight currently known variable DB, or V777 Her, stars. With the help of a new generation of synthetic spectra, spectroscopic effective temperatures are derived for these objects, as well as for 15 other DB or DBA stars above 20,000 K. We find that the location of the boundaries of the instability strip is sensitive to the atmospheric hydrogen abundance assumed for DB stars: the strip covers the range 22,400-27,800 K if atmospheres of pure helium are used and the range 21,800-24,700 K if undetectable traces of hydrogen are allowed for in the DB models. These determinations provide independent constraints for current seismological analyses of the V777 Her stars. More sensitive searches for weak hydrogen features in hot DB stars should help decide between the two temperature scales.The Astrophysical Journal 01/2009; 516(2):887. · 6.02 Impact Factor -
Article: Spectroscopic Studies of DB White Dwarfs: Improved Stark Profiles for Optical Transitions of Neutral Helium
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ABSTRACT: New Stark profiles for 21 He I lines and their forbidden components have been calculated for electron densities ranging from 1014 to 6 × 1017 cm-3, and for temperatures ranging from 10,000 K to 40,000 K. The profiles include quasi-static broadening by ions as well as electron broadening—the latter treated within the impact approximation near the core, and within the one-perturber approximation in the far wings. The calculated profiles also allow for line dissolution. These new profiles are the cornerstone of a recent reanalysis of the optical spectrum of a large sample of DB white dwarfs. Our calculations provide a very good match to the observed profiles of all the lines, provided that Stark broadening dominates other broadening mechanisms.The Astrophysical Journal Supplement Series 01/2009; 108(2):559. · 13.46 Impact Factor -
Article: LB 8827: A Variable, Magnetic DBA White Dwarf
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ABSTRACT: We present accurate CCD spectropolarimetry and spectra that show that the hot DB star LB 8827 (PG 0853+164) is a weakly magnetic, variable DBA white dwarf. We confirm Putney's detection of circular dichroism in the He I lines, but on three occasions we measure a weaker field strength and opposite sign of circular polarization, Be 100 kG. The Balmer line strengths appear to vary with time, indicative of changes in the apparent H/He abundance ratio or irregularities over the stellar surface. Fits to the observed energy distribution and to the optical He I line spectrum with model atmospheres yield effective temperatures in the 21,200-27,700 K range, with lower values favored. This suggests that LB 8827 falls within or near the instability strip of the V777 Herculis stars. High time resolution photometric observations presented here rule out white-light brightness variations with amplitudes larger than 1.2 mmag in the period window 20 s to 2 hr. The possibility that the magnetic field might tend to suppress pulsations is discussed.The Astrophysical Journal 12/2008; 554(2):1118. · 6.02 Impact Factor -
Chapter: New He I line profiles for synthetic spectra of DB white dwarfs
01/2006: pages 108-112; -
Conference Proceeding: Equilibrium Abundances of Heavy Elements Supported by Radiative Levitation in the Atmospheres of Hot DA White Dwarfs
IAU Colloq. 152: Astrophysics in the Extreme Ultraviolet; 01/1996 -
Article: Improved Calculations of the Equilibrium Abundances of Heavy Elements Supported by Radiative Levitation in the Atmospheres of Hot DA White Dwarfs
apj. 11/1995; 454:429. -
Article: Photometric Calibration of Hydrogen- and Helium-Rich White Dwarf Models
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ABSTRACT: Bolometric corrections and color indices on various photometric systems are provided for an extensive grid of hydrogen- and helium-rich white dwarf model atmospheres. Absolute visual magnitudes, masses, and ages are also obtained for each model from detailed evolutionary cooling sequences appropriate for these stars. The results of our calculations are briefly compared with published observational material. These calculations can easily be extended to any given photometric system. (SECTION: Stars)Publications of the Astronomical Society of the Pacific 10/1995; 107:1047. · 3.58 Impact Factor -
Article: Spectroscopic studies of DB white dwarfs: Confirmation and modeling of new forbidden components of He I
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ABSTRACT: Red optical spectrophotometry of three DB white dwarfs confirms the existence, first reported by Greenstein and Liebert, of new forbidden components of He I transitions near 6068 and 6632 A in the spectra of these stars. However, our search for the additional components tentatively reported earlier near 5379, 6337, and 6485 A remained unsuccessful. New model atmosphere calculations are presented which strengthen the identification of the confirmed features as, respectively, the forbidden 23P - 33P and 21P - 31P transitions in the He I ion, which arise from Stark perturbations of the upper atomic levels. Our calculations show, furthermore, that these features, which should be generally present in the red spectra of hot DB stars, are predicted with a strength comparable to that observed.The Astrophysical Journal 02/1995; 441:L85-L88. · 6.02 Impact Factor -
Article: A spectroscopic analysis of DAO and hot DA white dwarfs: The implications of the prescence of helium and the nature of DAO stars
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ABSTRACT: A comprehensive study of optical spectrophotometry of DAO white dwarfs is presented. A detailed analysis of the He II lambda 4686 line profiles demonstrates conclusively that the chemical composition in the atmospheres of DAO stars is not stratified, except in one object (PG 1305-017). On the other hand, line profiles calculated from models with homogeneous compositions are in excellent agreement with the observed profiles. This result indicates that the helium abundance distribution, in the line-forming region, is homogeneous and implies, in turn, that some physical mechanism is competing with gravitational settling to support helium in these layers. A similar analysis is then carried out for a sample of hot DA stars. In particular, the sensitivity of the atmospheric parameter determination to the presence of unobservable traces of helium in the photosphere is investigated in detail. The results show that, above approximately 40,000 K, the effective temperatures of DA stars can be overestimated by up to several thousand degrees if spectroscopically invisible traces of helium (N(He)/N(H) approximately less than 10-4) are present in their atmospheres. A comparison of the atmospheric parameters of the hot DA stars with those of the DAO stars reveal that the presence of helium in most DAO white dwarfs is directly related to their unusually low surface gravities (log g approximately less than 7.5), although several exceptions are found and considered individually. We present supporting evidence that the progenitors of most DAO stars are post-extended horizontal branch stars (M approximately 0.48 solar mass) which have evolved directly from the extended horizontal branch to the white dwarf state. Those DAO stars which are members of white dwarf + M dwarf composite systems represent an independent subclass with unique properties. PG 1305-017 is the only DAO star that shows evidence for stratification, while the variable spectrum of PG 1210+533 cannot be reproduced successfully with any of the models explored in this analysis. Finally, we offer observational evidence that the presence of heavy elements is the most plausible explanation for the origin of the so-called Balmer-line problem in DAO stars.The Astrophysical Journal 08/1994; 432:305-325. · 6.02 Impact Factor -
Article: Hot degenerates in the Montreal-Cambridge-Tololo survey. 2: Two new hybird white dwarfs, MCT 0128-3846 and MCT 0453-2933, and the nature of the DAB stars
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ABSTRACT: We report the discovery, in the course of a survey for high-latitude, ultraviolet-excess objects in the southern hemisphere, of two new hybird DAB stars. Both objects exhibit a weak He I 4471 A line superposed onto the usual Balmer line spectrum. Analysis of the Balmer series with both homogeneous and stratified models show these stars to have spectroscopic effective temperatures around 27,000 K. However, low-dispersion ultraviolet observations with the IUE reveal energy distributions considerably flatter than those of normal DA stars near that effective temperature. These energy distributions are inconsistent as well as those of stratified models calculated at the temperature and hydrogen-layer mass determined from the optical spectrum. We demonstrate instead that consistent fits to the optical and ultraviolet spectrum, as well as to the energy distribution, can be achieved if these objects are unresolved, composite systems consisting of a DA white dwarf together with a DB or DBA star. The DAB spectral class thus appears inhomogeneous, and may contain both composit systems and stratified stars.The Astrophysical Journal 06/1994; 429:369-379. · 6.02 Impact Factor -
Article: Discovery of two rare white dwarfs with hybrid spectra in the Montréal-Cambridge-Tololo Survey.
The Journal of the Royal Astronomical Society of Canada. Royal Astronomical Society of Canada 07/1993; 87:213. -
Article: The DB and DBA white dwarfs: epitomes of hydrogen-deficient stars
96:295. -
Article: A Search for H-α in a Sample of Hot DB stars
226:153. -
Article: Constraints on the Convective Efficiency in the Outer Layers of DB White Dwarfs from the Instability Strip of the V777 HER Stars
173:325.
Top co-authors
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Institutions
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2011
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Université du Québec à Montréal
- Department of Music
Montréal, Quebec, Canada
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1995–2009
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Université de Montréal
Montréal, Quebec, Canada
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1994
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The University of Arizona
Tucson, AZ, USA
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