231 research outputs found
A Census of White Dwarfs Within 40 Parsecs of the Sun
Our aim is to compile a catalog of white dwarfs within 40 parsecs of the Sun,
in which newly discovered objects would significantly increase the completeness
of the current census. White dwarf candidates are identified from the
SUPERBLINK proper motion database (Lepine & Shara 2005), which allows us to
investigate stars down to a proper motion limit as low as 40 mas yr-1. The
selection criteria and distance estimates are based on a combination of
color-magnitude and reduced proper motion diagrams. Candidates with distances
less than 50 parsecs are selected for spectroscopic follow-up. We present our
preliminary sample of spectroscopically confirmed white dwarfs, as well as
their atmospheric parameters. These parameters are obtained using the
spectroscopic technique developed in Bergeron et al.(1992) for DA stars. DB,
DQ, and DZ stars are also analyzed spectroscopically. For featureless spectra
as well as those showing only Halpha, we perform a detailed photometric
analysis of their energy distribution.Comment: 4 pages, 3 figures, to appear in AIP Conference Proceedings for the
17th European White Dwarf Worksho
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
High-speed Photometric Observations of ZZ Ceti White Dwarf Candidates
We present high-speed photometric observations of ZZ Ceti white dwarf
candidates drawn from the spectroscopic survey of bright DA stars from the
Villanova White Dwarf Catalog by Gianninas et al., and from the recent
spectroscopic survey of white dwarfs within 40 parsecs of the Sun by Limoges et
al. We report the discovery of six new ZZ Ceti pulsators from these surveys,
and several photometrically constant DA white dwarfs, which we then use to
refine the location of the ZZ Ceti instability strip.Comment: 4 pages, 1 table, 2 figures, to appear in "19th European White Dwarf
Workshop" in the ASP Conference Serie
A Gravitational Redshift Determination of the Mean Mass of White Dwarfs. DBA and DB Stars
We measure apparent velocities (v_app) of absorption lines for 36 white
dwarfs (WDs) with helium-dominated atmospheres -- 16 DBAs and 20 DBs -- using
optical spectra taken for the European Southern Observatory SN Ia progenitor
survey (SPY). We find a difference of 6.9+/-6.9 km/s in the average apparent
velocity of the H-alpha lines versus that of the HeI 5876AA for our DBAs. This
is a measure of the blueshift of this He line due to pressure effects. By using
this as a correction, we extend the gravitational redshift method employed by
Falcon et al. (2010) to use the apparent velocity of the HeI 5876AA line and
conduct the first gravitational redshift investigation of a group of WDs
without visible hydrogen lines. We use biweight estimators to find an average
apparent velocity, _BI, (and hence average gravitational redshift,
_BI) for our WDs; from that we derive an average mass, _BI. For the
DBAs, we find _BI = 40.8+/-4.7 km/s and derive _BI = 0.71 +0.04 -0.05
Msun. Though different from of DAs (32.57 km/s) at the 91% confidence
level and suggestive of a larger DBA mean mass than that for normal DAs derived
using the same method (0.647 +0.013 -0.014 Msun; Falcon et al. 2010), we do not
claim this as a stringent detection. Rather, we emphasize that the difference
between _BI of the DBAs and of normal DAs is no larger than 9.2
km/s, at the 95% confidence level; this corresponds to roughly 0.10 Msun. For
the DBs, we find ^He_BI = 42.9+/-8.49 km/s after applying the blueshift
correction and determine _BI = 0.74 +0.08 -0.09 Msun. The difference between
^He_BI of the DBs and of DAs is less than or equal to 11.5 km/s
(~0.12 Msun), at the 95% confidence level. The gravitational redshift method
indicates much larger mean masses than the spectroscopic determinations of the
same sample by Voss et al. (2007)...Comment: Accepted to the Astrophysical Journal, 10 pages double-column, 3
figures, 5 table
Multiwavelength Observations of the Hot DB Star PG 0112+104
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 Halpha,
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 Msun), and a hydrogen abundance of log N(H)/N(He) <
-4.0. The FUSE spectra reveal the presence of CII and CIII lines that
complement the previous detection of CII 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.Comment: 10 pages in emulateapj, 9 figures, accepted for publication in Ap
The Discovery of the Most Metal-Rich White Dwarf: Composition of a Tidally Disrupted Extrasolar Dwarf Planet
Cool white dwarf stars are usually found to have an outer atmosphere that is
practically pure in hydrogen or helium. However, a small fraction have traces
of heavy elements that must originate from the accretion of extrinsic material,
most probably circumstellar matter. Upon examining thousands of Sloan Digital
Sky Survey spectra, we discovered that the helium-atmosphere white dwarf SDSS
J073842.56+183509.6 shows the most severe metal pollution ever seen in the
outermost layers of such stars. We present here a quantitative analysis of this
exciting star by combining high S/N follow-up spectroscopic and photometric
observations with model atmospheres and evolutionary models. We determine the
global structural properties of our target star, as well as the abundances of
the most significant pollutants in its atmosphere, i.e., H, O, Na, Mg, Si, Ca,
and Fe. The relative abundances of these elements imply that the source of the
accreted material has a composition similar to that of Bulk Earth. We also
report the signature of a circumstellar disk revealed through a large infrared
excess in JHK photometry. Combined with our inferred estimate of the mass of
the accreted material, this strongly suggests that we are witnessing the
remains of a tidally disrupted extrasolar body that was as large as Ceres.Comment: 7 pages in emulateapj, 5 figures, accepted for publication in Ap
Ancient planetary systems are orbiting a large fraction of white dwarf stars
Infrared studies have revealed debris likely related to planet formation in
orbit around ~30% of youthful, intermediate mass, main sequence stars. We
present evidence, based on atmospheric pollution by various elements heavier
than helium, that a comparable fraction of the white dwarf descendants of such
main sequence stars are orbited by planetary systems. These systems have
survived, at least in part, through all stages of stellar evolution that
precede the white dwarf. During the time interval (~200 million years) that a
typical polluted white dwarf in our sample has been cooling it has accreted
from its planetary system the mass of one of the largest asteroids in our solar
system (e.g., Vesta or Ceres). Usually, this accreted mass will be only a
fraction of the total mass of rocky material that orbits these white dwarfs;
for plausible planetary system configurations we estimate that this total mass
is likely to be at least equal to that of the Sun's asteroid belt, and perhaps
much larger. We report abundances of a suite of 8 elements detected in the
little studied star G241-6 that we find to be among the most heavily polluted
of all moderately bright white dwarfs.Comment: 31 pages, 4 figures, 5 tables. Accepted for the Astrophysical
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