Search Publications
A Comprehensive Spectroscopic Analysis of DB White Dwarfs
Fontaine, G.; Bergeron, P.; Wesemael, F. +9 more
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 M sun for our sample, with a dispersion of only 0.09 M sun. White dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample, and the…
Rocky Extrasolar Planetary Compositions Derived from Externally Polluted White Dwarfs
Koester, D.; Zuckerman, B.; Klein, B. +1 more
We report Keck High Resolution Echelle Spectrometer data and model atmosphere analysis of two helium-dominated white dwarfs, PG1225-079 and HS2253+8023, whose heavy pollutions most likely derive from the accretion of terrestrial-type planet(esimal)s. For each system, the minimum accreted mass is ~1022 g, that of a large asteroid. In PG1…
Emission Lines from the Gas Disk around TW Hydra and the Origin of the Inner Hole
Pascucci, I.; Najita, J.; Hollenbach, D. +1 more
We compare line emission calculated from theoretical disk models with optical to submillimeter wavelength observational data of the gas disk surrounding TW Hya and infer the spatial distribution of mass in the gas disk. The model disk that best matches observations has a gas mass ranging from ~10-4 to 10-5 M sun fo…
The Effect of Helium-enhanced Stellar Populations on the Ultraviolet-upturn Phenomenon of Early-type Galaxies
Chung, Chul; Yoon, Suk-Jin; Lee, Young-Wook
Recent observations and modeling of globular clusters (GCs) with multiple populations strongly indicate the presence of super-helium-rich subpopulations in old stellar systems. Motivated by this, we have constructed new population synthesis models with and without helium-enhanced subpopulations to investigate their impact on the UV-upturn phenomen…
Ultraviolet and X-ray Variability of the Seyfert 1.5 Galaxy Markarian 817
Brandt, W. N.; Schneider, Donald P.; Shull, J. Michael +9 more
We present an investigation of the ultraviolet and X-ray spectra of the Seyfert 1.5 galaxy Markarian 817. The ultraviolet analysis includes two recent observations taken with the Cosmic Origins Spectrograph (COS) in 2009 August and December, as well as archival spectra from the International Ultraviolet Explorer and the Hubble Space Telescope. Twe…
A Method for the Study of Accretion Disk Emission in Cataclysmic Variables. I. The Model
Hillier, D. John; Hubeny, Ivan; Puebla, Raúl E. +1 more
We have developed a spectrum synthesis method for modeling the ultraviolet (UV) emission from the accretion disk from cataclysmic variables (CVs). The disk is separated into concentric rings, with an internal structure from the Wade & Hubeny disk-atmosphere models. For each ring, a wind atmosphere is calculated in the comoving frame with a ver…
A Light-curve Model of the Symbiotic Nova PU Vul (1979): A Very Quiet Explosion with Long-lasting Flat Peak
Cassatella, Angelo; Kato, Mariko; Hachisu, Izumi +1 more
We present a light-curve model of the symbiotic nova PU Vul (Nova Vulpeculae 1979) that shows a long-lasting flat peak with no spectral indication of wind mass loss before decline. Our quasi-evolution models consisting of a series of static solutions explain both the optical flat peak and ultraviolet (UV) light curve simultaneously. The white dwar…
A Hard and Variable X-ray Emission from the Massive Emission-line Star HD 157832
Motch, C.; Lopes de Oliveira, R.
We report the discovery with XMM-Newton of a hard-thermal (T ~ 130 MK) and variable X-ray emission from the Be star HD 157832, a new member of the puzzling class of γ-Cas-like Be/X-ray systems. Recent optical spectroscopy reveals the presence of a large/dense circumstellar disk seen at intermediate/high inclination. With a B1.5V spectral type, HD …
The Hot Components of AM CVn Helium Cataclysmics
Sion, Edward M.; Godon, Patrick; Linnell, Albert P. +1 more
We present the results of a multi-component synthetic spectral analysis of the archival far-ultraviolet spectra of the hot components of several AM CVn double degenerate interacting binaries with known distances from trigonometric parallaxes. Our analysis was carried out using the code BINSYN, which takes into account the donor companion star, the…