508 research outputs found
High resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia Progenitor Survey III. DA white dwarfs
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO) DOI: 10.1051/0004-6361/200912531Context. The ESO Supernova Ia Progenitor Survey (SPY) took high-resolution spectra of more than 1000 white dwarfs and pre-white dwarfs. About two thirds of the stars observed are hydrogen-dominated DA white dwarfs. Here we present a catalog and detailed spectroscopic analysis of the DA stars in the SPY. Aims. Atmospheric parameters effective temperature and surface gravity are determined for normal DAs. Double-degenerate binaries, DAs with magnetic fields or dM companions, are classified and discussed. Methods. The spectra are compared with theoretical model atmospheres using a fitting technique. Results. Our final sample contains 615 DAs, which show only hydrogen features in their spectra, although some are double-degenerate binaries. 187 are new detections or classifications. We also find 10 magnetic DAs (4 new) and 46 DA+dM pairs (10 new).Peer reviewe
White Dwarfs in Globular Clusters: HST Observations of M4
Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258
white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial
distances from the cluster center were observed and sizeable WD populations
were found in all three. The location of these WDs in the color-magnitude
diagram, their mean mass of 0.51()M, and their luminosity
function confirm basic tenets of stellar evolution theory and support the
results from current WD cooling theory. The WDs are used to extend the cluster
main-sequence mass function upward to stars that have already completed their
nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and
found to be at most a few percent of all the main-sequence stars. The most
ancient WDs found are about 9 Gyr old, a level which is set solely by the
photometric limits of our data. Even though this is less than the age of M4, we
discuss how these cooling WDs can eventually be used to check the turnoff ages
of globular clusters and hence constrain the age of the Universe.Comment: 46 pages, latex, no figures included, figures available at
ftp://ftp.astro.ubc.ca/pub/richer/wdfig.uu size 2.7Mb. To be published in the
Astrophysical Journa
Photometric and Spectroscopic Analysis of Cool White Dwarfs with Trigonometric Parallax Measurements
A photometric and spectroscopic analysis of 152 cool white dwarf stars is
presented. The discovery of 7 new DA white dwarfs, 2 new DQ white dwarfs, 1 new
magnetic white dwarf, and 3 weak magnetic white dwarf candidates, is reported,
as well as 19 known or suspected double degenerates. The photometric energy
distributions, the Halpha line profiles, and the trigonometric parallax
measurements are combined and compared to model atmosphere calculations to
determine the effective temperature and the radius of each object, and also to
constrain the atmospheric composition. New evolutionary sequences with C/O
cores with thin and thick hydrogen layers are used to derive masses and ages.
We confirm the existence of a range in Teff between 5000 and 6000K where almost
all white dwarfs have H-rich atmospheres. There is little evidence for mixed
H/He dwarfs, with the exception of 2 He-rich DA stars, and 5 C2H white dwarfs
which possibly have mixed H/He/C atmospheres. The DQ sequence terminates near
6500K, below which they are believed to turn into C2H stars. True DC stars
slightly above this temperature are found to exhibit H-like energy
distributions despite the lack of Halpha absorption. Attempts to interpret the
chemical evolution show the problem to be complex. Convective mixing is
necessary to account for the non-DA to DA ratio as a function of temperature.
The presence of helium in cool DA stars, the existence of the non-DA gap, and
the peculiar DC stars are also explained in terms of convective mixing,
although our understanding of how this mechanism works needs to be revised. The
oldest object in our sample is about 7.9 Gyr or 9.7 Gyr old depending on
whether thin or thick hydrogen layer models are used. The mean mass of our
sample is 0.65 +/- 0.20 Msun.Comment: Accepted by ApJ Suppl (~April 2001); 79 pages incl. 25 figure
Experimental study of Radiation induced DNA damage by internal Auger electron cascade compared to external Îł-rays
Discovery of magnetic fields in hot subdwarfs
We present initial results of a project to measure mean longitudinal magnetic
fields in a group of sdB/OB/O stars. The project was inspired by the discovery
of three super-metal-rich sdOB stars, each having metals (e.g. Ti, V) enhanced
by factors of 10^3 to 10^5. Similar behaviour is observed in chemically
peculiar A stars, where strong magnetic fields are responsible for the
enrichment. With this in mind, we obtained circularly polarised spectra of two
of the super-metal-rich sdOBs, two "normal" sdBs and two sdOs using FORS1 on
the ESO/VLT. By examining circular polarisation in the hydrogen Balmer lines
and in helium lines, we have detected magnetic fields with strengths of 1-2 kG
in most of our targets. This suggests that such fields are relatively common in
hot subdwarfs.Comment: 4 pages, to appear in White Dwarfs, eds. D. Koester, S. Moehler, ASP
Conf. serie
Discovery of Spectroscopic Variations in the DAB White Dwarf GD 323
We report the discovery of spectroscopic variations in GD 323, the
prototypical DAB white dwarf. Simultaneous optical spectroscopic observations
over five consecutive nights of GD 323 and of PG 1234+482, a non-variable
comparison DA white dwarf of similar brightness, are used to reveal
quasi-periodic variations in both the hydrogen and helium absorption lines over
a timescale of hours. The amplitude of the variation of the equivalent width of
Hbeta is ~30 %. Moreover, the strength of the hydrogen lines is shown to vary
in opposite phase from that of He I 4471. These results suggest that the model
currently thought to be the most viable to account for the simultaneous
presence of hydrogen and helium lines in GD 323, namely a static stratified
atmosphere, may need to be reexamined. Instead, a model with an inhomogeneous
surface composition, resulting perhaps from the dilution of a thin hydrogen
atmosphere with the underlying helium convection zone, may be a better
representation of GD 323. The observed variation timescale of ~3.5 hours is
consistent with the slow rotation rate of white dwarf stars.Comment: 23 pages, 8 figures, accepted for publication in the Astrophysical
Journa
The mass of the sdB primary of the binary HS 2333+3927
Short period sdB binaries with cool companions are crucial to understand
pre-CV evolution, because they will evolve into cataclysmic variables, when the
sdB will have left the extended horizontal branch. Recently we discovered the
sixth such system, HS 2333+3927, consisting of an sdB star and an M dwarf
(period: 0.172 d) with a very strong reflection effect, but no eclipses. The
reflection is stronger than in any of the other similar systems which renders a
quantitative spectral analysis very difficult because the Balmer line profiles
may be disturbed by the reflected light. A spectroscopic analysis results in
Teff = 36500 K, log g = 5.70, and log (n_He/n_H) = -2.15. Mass-radius relations
were derived from the results of the analysis of light and radial-velocity
curves. Comparison with the mass-radius relation derived from the surface
gravity of the sdB star favours a rather low mass of 0.38 Msun for the primary.
The mass of the companion is 0.29 M_sun. HS 2333+3927 is the only known sdB+dM
system with a period above the CV period gap.Comment: 6 pages, 3 Fig., to appear in 14th European Workshop on White Dwarfs,
ASP Conference Series, eds. D. Koester, S. Moehle
Partly burnt runaway stellar remnants from peculiar thermonuclear supernovae
We report the discovery of three stars that, along with the prototype
LP40-365, form a distinct class of chemically peculiar runaway stars that are
the survivors of thermonuclear explosions. Spectroscopy of the four confirmed
LP 40-365 stars finds ONe-dominated atmospheres enriched with remarkably
similar amounts of nuclear ashes of partial O- and Si-burning. Kinematic
evidence is consistent with ejection from a binary supernova progenitor; at
least two stars have rest-frame velocities indicating they are unbound to the
Galaxy. With masses and radii ranging between 0.20-0.28 Msun and 0.16-0.60
Rsun, respectively, we speculate these inflated white dwarfs are the partly
burnt remnants of either peculiar Type Iax or electron-capture supernovae.
Adopting supernova rates from the literature, we estimate that ~20 LP40-365
stars brighter than 19 mag should be detectable within 2 kpc from the Sun at
the end of the Gaia mission. We suggest that as they cool, these stars will
evolve in their spectroscopic appearance, and eventually become peculiar O-rich
white dwarfs. Finally, we stress that the discovery of new LP40-365 stars will
be useful to further constrain their evolution, supplying key boundary
conditions to the modelling of explosion mechanisms, supernova rates, and
nucleosynthetic yields of peculiar thermonuclear explosions.Comment: 22 pages, 14 figures, 6 tables. Accepted for publication on MNRA
Identification of the slow E3 transition 136mCs -> 136Cs with conversion electrons
We performed at ISOLDE the spectroscopy of the decay of the 8- isomer in
136Cs by and conversion-electron detection. For the first time the excitation
energy of the isomer and the multipolarity of its decay have been measured. The
half-life of the isomeric state was remeasured to T1/2 = 17.5(2) s. This isomer
decays via a very slow 518 keV E3 transition to the ground state. In addition
to this, a much weaker decay branch via a 413 keV M4 and a subsequent 105 keV
E2 transition has been found. Thus we have found a new level at 105 keV with
spin 4+ between the isomeric and the ground state. The results are discussed in
comparison to shell model calculations.Comment: Phys. Rev. C accepted for publicatio
The ESO supernovae type Ia progenitor survey (SPY) : The radial velocities of 644 DA white dwarfs
Accepted for publication in A&A. 59 pages.Close double degenerate binaries are one of the favoured progenitor channels for type Ia supernovae, but it is unclear how many suitable systems there are in the Galaxy. We report results of a large radial velocity survey for double degenerate (DD) binaries using the UVES spectrograph at the ESO VLT (ESO SN Ia Progenitor surveY - SPY). Exposures taken at different epochs are checked for radial velocity shifts indicating close binary systems. We observed 689 targets classified as DA (displaying hydrogen-rich atmospheres), of which 46 turned out to possess a cool companion. We measured radial velocities (RV) of the remaining 643 DA white dwarfs. We managed to secure observations at two or more epochs for 625 targets, supplemented by eleven objects meeting our selection criteria from literature. The data reduction and analysis methods applied to the survey data are described in detail. The sample contains 39 double degenerate binaries, only four of which were previously known. 20 are double-lined systems, in which features from both components are visible, the other 19 are single-lined binaries. We provide absolute RVs transformed to the heliocentric system suitable for kinematic studies. Our sample is large enough to sub-divide by mass: 16 out of 44 low mass targets (0.45 Msun are double. Although the detected fraction amongst the low mass objects (36.4 +/- 7.3%) is significantly higher than for the higher-mass, carbon/oxygen-core dominated part of the sample (3.9 +/- 0.8%), it is lower than the detection efficiency based upon companion star masses >= 0.05 Msun. This suggests either companion stars of mass <0.05 Msun, or that some of the low mass white dwarfs are single.Peer reviewe
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