819 research outputs found
Theta Hya: Spectroscopic identification of a second B star + white dwarf binary
We report the identification, in an Extreme Ultraviolet Explorer (EUVE)
spectrum, of a hot white dwarf companion to the 3rd magnitude late-B star Theta
Hya (HR3665, HD79469). This is the second B star + white dwarf binary to be
conclusively identified; Vennes, Berghofer and Christian (1997), and Burleigh
and Barstow (1998) had previously reported the spectroscopic discovery of a hot
white dwarf companion to the B5V star y Pup (HR2875). Since these two
degenerate stars must have evolved from main sequence progenitors more massive
than their B star companions, they can be used to place observational lower
limits on the maximum mass for white dwarf progenitors, and to investigate the
upper end of the initial-final mass relation. Assuming a pure hydrogen
composition, we constrain the temperature of the white dwarf companion to Theta
Hya to lie between 25,000K and 31,000K. We also predict that a third bright B
star, 16 Dra (B9.5V), might also be hiding an unresolved hot white dwarf
companion.Comment: 4 pages, 1 figure. Accepted for publication in Astronomy and
Astrophysic
A search for hidden white dwarfs in the ROSAT EUV survey II: Discovery of a distant DA+F6/7V binary system in a direction of low density neutral hydrogen
We report the results of our final search for hot white dwarfs in unresolved,
Sirius-type, binary systems with IUE. One new system, RE J0500-364 (DA+F6/7V),
has been identified. This star appears to lie at a distance of between
500-1000pc, making it one of the most distant white dwarfs, if not the most
distant, to be detected in the EUV surveys. The very low line-of-sight neutral
hydrogen volume density to this object could place a lower limit on the length
of the Beta CMa interstellar tunnel of diffuse gas, which stretches away from
the Local Bubble in a similar direction to RE J0500-364.Comment: 1 LaTex file plus 15 figures; accepted for publication in Monthly
Notices of the Royal Astronomical Societ
Near-infrared spectroscopy of the very low mass companion to the hot DA white dwarf PG1234+482
We present a near-infrared spectrum of the hot (
55,000 K) DA white dwarf PG 1234+482. We confirm that a very low mass companion
is responsible for the previously recognised infrared photometric excess. We
compare spectra of M and L dwarfs, combined with an appropriate white dwarf
model, to the data to constrain the spectral type of the secondary. We find
that uncertainties in the 2MASS photometry of the white dwarf prevent us
from distinguishing whether the secondary is stellar or substellar, and assign
a spectral type of L01 (M9-L1).Therefore, this is the hottest and youngest
( yr) DA white dwarf with a possible brown dwarf companion.Comment: 5 pages, 2 figures, accepted by MNRA
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
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
Hubble Space Telescope Imaging and Spectroscopy of the Sirius-Like Triple Star System HD 217411
We present Hubble Space Telescope imaging and spectroscopy of HD 217411, a G3
V star associated with the extreme ultraviolet excess source (EUV 2RE
J2300-07.0). This star is revealed to be a triple system with a G 3V primary
(HD 217411 A) separated by ~1.1" from a secondary that is in turn composed of
an unresolved K0 V star (HD 217411 Ba) and a hot DA white dwarf (HD 217411 Bb).
The hot white dwarf dominates the UV flux of the system. However; it is in turn
dominated by the K0 V component beyond 3000 {\AA}. A revised distance of 143 pc
is estimated for the system. A low level photometric modulation having a period
of 0.61 days has also been observed in this system along with a rotational
velocity on the order of 60 km s-1 in the K0 V star. Together both observations
point to a possible wind induced spin up of the K0 V star during the AGB phase
of the white dwarf. The nature of all three components is discussed as are
constraints on the orbits, system age and evolution.Comment: 11 pages, 6 figure
A comparison of DA white dwarf temperatures and gravities from FUSE Lyman line and ground-based Balmer line observations
Observation of the strengths and profiles of the hydrogen Balmer absorption
series is an established technique for determining the effective temperature
and surface gravity of hot H-rich white dwarf stars. In principle, the Lyman
series lines should be equally useful but, lying in the far-UV, are only
accessible from space. Nevertheless, there are situations (for example, where
the optical white dwarf spectrum is highly contaminated by the presence of a
companion) in which use of the Lyman series may be essential. Therefore, it is
important to establish whether or not the Lyman lines provide an equally valid
means of measurement. We have already made a first attempt to study this
problem, comparing Lyman line measurements from a variety of far-UV instruments
with ground-based Balmer line studies. Within the measurement uncertainties we
found the results from each line series to be broadly in agreement. However, we
noted a number of potential systematic effects that could bias either
measurement. With the availability of the FUSE data archive and observations
from our own Guest Observer programmes, we now have an opportunity of examining
the use of the Lyman series in more detail from observations of 16 DA white
dwarfs. The new results partially reproduce the earlier study, showing that
Balmer and Lyman line determined temperatures are in good agreement up to
\~50000K. However, above this value there is an increasing systematic
difference between the Lyman and Balmer line results, the former yielding the
higher temperature.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society: 15 pages (inc 10 figures
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