9 research outputs found
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.Comment: 71 pages, 28 figures. Electronic figures only in the published
versio
A Comprehensive Spectroscopic Analysis of DB White Dwarfs
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.Comment: 74 pages including 30 figures, accepted for publication in the
Astrophysical Journa
A Comparative Study of the Mass Distribution of Extreme-Ultraviolet selected White Dwarfs
Original article can be found at: http://www.journals.uchicago.edu/ApJ/--Copyright American Astronomical SocietyWe present new determinations of e ective temperature, surface gravity, and masses for a sample of 46 hot DA white dwarfs selected from the Extreme Ultraviolet Explorer (EUV E) and ROSAT Wide Field Camera bright source lists in the course of a near-infrared survey for low-mass companions. Our analysis, based on hydrogen non-LTE model atmospheres, provides a map of LTE correction vectors, which allow a thorough comparison with previous LTE studies. We Ðnd that previous studies underestimate both the systematic errors and the observational scatter in the determination of white dwarf parameters obtained via Ðts to model atmospheres. The structure of very hot or low-mass white dwarfs depends sensitively on their history. To compute white dwarf masses, we thus use theoretical mass-radius relations that take into account the complete evolution from the main sequence. We Ðnd a peak mass of our white dwarf sample of 0.59 M in agreement with the results of previous analyses. However, we do _, not conÐrm a trend of peak mass with temperature reported in two previous analyses. Analogous to other EUV-selected samples, we note a lack of low-mass white dwarfs and a large fraction of massive white dwarfs. Only one white dwarf is likely to have a helium core. While the lack of helium white dwarfs in our sample can be easily understood from their high cooling rate, and therefore low detection probability in our temperature range, this is not enough to explain the large fraction of massive white dwarfs. This feature very likely results from a decreased relative sample volume for low-mass white dwarfs caused by interstellar absorption in EUV-selected samples.Peer reviewe