215 research outputs found
White Dwarfs: Cosmological and Galactic Probes
The emphasis on white dwarf stars and cosmology arises from the most recent advances in cosmological and galactic structure research in which white dwarf stars are playing a very prominent role. Examples are Type Ia supernovae (i.e. white dwarf supernovae), the origin and evolution of the universe, the age of the galactic disk, cosmochronology using white dwarfs in globular clusters and galactic clusters, and the physics of accretion onto compact (very dense) stars. As an assisting guide to the reader, we have included, by invitation, comprehensive review articles in each of the four major areas of the book, white dwarf supernovae, cosmology, accretion physics and galactic structure. The reviews include introductory material that they build upon. The book is suitable and most useful to advanced undergraduates, graduate students and scientific professionals (e.g. astronomers, astrophysicists, cosmologists, physicists)
LP 400-22, A very low-mass and high-velocity white dwarf
We report the identification of LP 400-22 (WD 2234+222) as a very low-mass
and high-velocity white dwarf. The ultraviolet GALEX and optical photometric
colors and a spectral line analysis of LP 400-22 show this star to have an
effective temperature of 11080+/-140 K and a surface gravity of log g =
6.32+/-0.08. Therefore, this is a helium core white dwarf with a mass of 0.17
M_solar. The tangential velocity of this white dwarf is 414+/-43 km/s, making
it one of the fastest moving white dwarfs known. We discuss probable
evolutionary scenarios for this remarkable object.Comment: accepted for publication in ApJ Letters, made minor correction
Theoretical UBVRI colors of iron core white dwarfs
We explore photometric properties of hypothetical iron core white dwarfs and
compute their expected colors in UBVRI Johnson broadband system. Atmospheres of
iron core WDs in this paper consist of pure iron covered by a pure hydrogen
layer of an arbitrary column mass. LTE model atmospheres and theoretical
spectra are calculated on the basis of Los Alamos TOPS opacities and the
equation of state from the OPAL project, suitable for nonideal Fe and H gases.
We have also computed UBVRI colors of the models and determined an area on the
B-V vs. U-B and U-B vs. V-I planes, occupied by both pure Fe, and pure H model
atmospheres of WD stars. Finally, we search for iron core white dwarf
candidates in the available literature.Comment: 13 pages, 12 figures, Astronomy & Astrophysics (2003) in prin
High ions towards white dwarfs: circumstellar line shifts and stellar temperature
Based on a compilation of OVI, CIV, SiIV and NV data from IUE, FUSE, GHRS,
STIS, and COS, we derive an anti- correlation between the stellar temperature
and the high ion velocity shift w.r.t. to the photosphere, with positive (resp.
negative) velocity shifts for the cooler (resp. hotter) white dwarfs. This
trend probably reflects more than a single process, however such a dependence
on the WD's temperature again favors a CS origin for a very large fraction of
those ion absorptions, previously observed with IUE, HST-STIS, HST-GHRS, FUSE,
and now COS, selecting objects for which absorption line radial velocities,
stellar effective temperature and photospheric velocity can be found in the
literature. Interestingly, and gas in near-equilibrium in the star vicinity. It
is also probably significant that the temperature that corresponds to a null
radial velocity, i.e. \simeq 50,000K, also corresponds to the threshold below
which there is a dichotomy between pure or heavy elements atmospheres as well
as some temperature estimates for and a form of balance between radiation
pressure and gravitation. This is consistent with ubiquitous evaporation of
orbiting dusty material. Together with the fact that the fraction of stars with
(red-or blue-) shifted lines and the fraction of stars known to possess heavy
species in their atmosphere are of the same order, such a velocity-temperature
relationship is consistent with quasi-continuous evaporation of orbiting CS
dusty material, followed by accretion and settling down in the photosphere. In
view of these results, ion measurements close to the photospheric or the IS
velocity should be interpreted with caution, especially for stars at
intermediate temperatures. While tracing CS gas, they may be erroneously
attributed to photospheric material or to the ISM, explaining the difficulty of
finding a coherent pattern of the high ions in the local IS 3D distribution.Comment: Accepted by A&A. Body of paper identical to v1. This submission has a
more appropriate truncation of the original abstrac
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.Comment: Accepted for publication in Ap
Mass-Radius Relation for Magnetic White Dwarfs
Recently, several white dwarfs with very strong surface magnetic fields have
been observed. In this paper we explore the possibility that such stars could
have sufficiently strong internal fields to alter their structure. We obtain a
revised white dwarf mass-radius relation in the presence of strong internal
magnetic fields. We first derive the equation of state for a fully degenerate
ideal electron gas in a magnetic field using an Euler-MacLaurin expansion. We
use this to obtain the mass-radius relation for magnetic He, C,
and Fe white dwarfs of uniform composition.Comment: 7 pages, 7 figures and 1 table, To appear in Ap
A new EUVE-detected flare star (EUVE J0613-23.9B)
Extreme Ultraviolet Explorer (EUVE) observations have provided detailed spectroscopic and timing studies of several flare stars, including AU Mic (Cully et al., 1993), AD Leo (Hawley et al., 1995) and EQ Peg (Monsignori-Fossi et al., 1995).
In this bulletin, we present the EUVE and optical follow-up observations of a newly detected flare star. This star was serendipitously discovered during an EUVE observation of the G star HD 43162 as part of the analysis for the 3rd EUVE Right Angle Program Catalog (Christian, 2002). Analysis of the EUVE spectra obtained during the largest flare (Fe XIX-XXIV emission and a strong 300-650 Å continuum) have been presented elsewhere (Christian et al., 2003). We present optical spectroscopy and results obtained at MT Stromlo Observatory to identify the optical counterpart in § 1, and the long-term EUVE Deep Survey light-curves in § 2
LP 133-373: A New Chromospherically Active Eclipsing dMe Binary with a Distant, Cool White Dwarf Companion
We report the discovery of the partially eclipsing binary LP 133-373. Nearly identical eclipses along with observed photometric colors and spectroscopy indicate that it is a pair of chromospherically active dM4 stars in a circular 1.6 day orbit. Light and velocity curve modeling to our differential photometry and velocity data show that each star has a mass and radius of 0:340 ± 0:014 MꙨ and 0:33 ± 0:02 RꙨ. The binary is itself part of a common proper motion pair with LP 133-374, a cool DC or possible DA white dwarf with a mass of 0.49Y0.82 MꙨ, which would make the system at least 3 Gyr old
Constraints on the origin of the massive, hot, and rapidly rotating magnetic white dwarf RE J 0317-853 from an HST parallax measurement
We use the parallax measurements of RE J 0317-853 to determine its mass,
radius, and cooling age and thereby constrain its evolutionary origins. We
observed RE J 0317-853 with the the Hubble Space Telescope's Fine Guidance
System to measure the parallax of RE J 0317-853 and its binary companion, the
non-magnetic white dwarf LB 9802. In addition, we acquired spectra of
comparison stars with the Boller & Chivens spectrograph of the SMARTS telescope
to correct the parallax zero point. For the corrected parallax, we determine
the radius, mass, and the cooling age with the help of evolutionary models from
the literature. The properties of RE J 0317-853 are constrained using the
parallax information. We discuss the different cases of the core composition
and the uncertain effective temperature. We confirm that RE J 0317-853 is close
to the Chandrasekhar's mass limit in all cases and almost as old as its
companion LB 9802. The precise evolutionary history of RE J 0317-853 depends on
our knowledge of its effective temperature. It is possible that it had a single
star progenitor possible if we assume that the effective temperature is at the
cooler end of the possible range from 30000 to 50000; if T_eff is instead at
the hotter end, a binary-merger scenario for RE J 0317-853 becomes more
plausible.Comment: 11 pages, 6 figures; language revised edition with added references.
Accepted for publication in Astronomy and Astrophysic
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