263 research outputs found
Enigmas from the Sloan Digital Sky Survey DR7 Kleinman White Dwarf Catalog
We report results from a continuation of our searches for high field magnetic
white dwarfs paired in a detached binary with non degenerate companions. We
made use of the Sloan Digital Sky Survey DR7 catalog of Kleinman et al. (2013)
with 19,712 spectroscopically-identified white dwarfs. These include 1,735
white dwarf plus M dwarf detached pairs (almost 10\% of the Kleinman at al.'s
list). No new pairs were found, although we did recover the polar (AM~Herculis
system) ST\,LMi in a low state of accretion. With the larger sample the
original situation reported ten years ago remains intact now at a much higher
level of statistical significance: in the selected SDSS sample, high field
magnetic white dwarfs are not found in an apparently-detached pairing with an M
dwarf, unless they are a magnetic CV in a low state of accretion. This finding
strengthens the case that the fields in the isolated high field magnetic white
dwarfs are generated by stellar mergers but also raises questions on the nature
of the progenitors of the magnetic CVs.Comment: 12 pages, accepted for publication in the Astrophysical Journa
Rare White dwarf stars with carbon atmospheres
White dwarfs represent the endpoint of stellar evolution for stars with
initial masses between approximately 0.07 msun and 8-10 msun, where msun is the
mass of the Sun (more massive stars end their life as either black holes or
neutron stars). The theory of stellar evolution predicts that the majority of
white dwarfs have a core made of carbon and oxygen, which itself is surrounded
by a helium layer and, for ~80 per cent of known white dwarfs, by an additional
hydrogen layer. All white dwarfs therefore have been traditionally found to
belong to one of two categories: those with a hydrogen-rich atmosphere (the DA
spectral type) and those with a helium-rich atmosphere (the non-DAs). Here we
report the discovery of several white dwarfs with atmospheres primarily
composed of carbon, with little or no trace of hydrogen or helium. Our analysis
shows that the atmospheric parameters found for these stars do not fit
satisfactorily in any of the currently known theories of post-asymptotic giant
branch evolution, although these objects might be the cooler counterpart of the
unique and extensively studied PG1159 star H1504+65. These stars, together with
H1504+65, might accordingly form a new evolutionary sequence that follow the
asymptotic giant branch.Comment: 7 pages, 1 figure, to appear in Nov 22nd 2007 edition of Natur
The True Incidence of Magnetism among Field White Dwarfs
We study the incidence of magnetism in white dwarfs from three large and
well-observed samples of hot, cool, and nearby white dwarfs in order to test
whether the fraction of magnetic degenerates is biased, and whether it varies
with effective temperature, cooling age, or distance. The magnetic fraction is
considerably higher for the cool sample of Bergeron, Ruiz, and Leggett, and the
Holberg, Oswalt, and Sion sample of local white dwarfs that it is for the
generally-hotter white dwarfs of the Palomar Green Survey. We show that the
mean mass of magnetic white dwarfs in this survey is 0.93 solar masses or more,
so there may be a strong bias against their selection in the magnitude-limited
Palomar Green Survey. We argue that this bias is not as important in the
samples of cool and nearby white dwarfs. However, this bias may not account for
all of the difference in the magnetic fractions of these samples.
It is not clear that the magnetic white dwarfs in the cool and local samples
are drawn from the same population as the hotter PG stars. In particular, two
or threee of the cool sample are low-mass white dwarfs in unresolved binary
systems. Moreover, there is a suggestion from the local sample that the
fractional incidence may increase with decreasing temperature, luminosity,
and/or cooling age. Overall, the true incidence of magnetism at the level of 2
megagauss or greater is at least 10%, and could be higher. Limited studies
capable of detecting lower field strengths down to 10 kilogauss suggest by
implication that the total fraction may be substantially higher than 10%.Comment: 16 pages, 2 figures, Astronomical Journal in press -- Jan 2003 issu
On the Nature of the Unique H-Emitting T Dwarf 2MASS J12373919+6526148
We explore and discount the hypothesis that the strong, continual
H-emitting T dwarf 2MASS J12373919+6526148 can be explained as a young,
low gravity, very low mass brown dwarf. The source is already known to have a
marginally-fainter absolute magnitude than similar T dwarfs with trigonometric
parallax measurements, and has a tangential velocity consistent with old disk
kinematics. Applying the technique of Burgasser, Burrows & Kirkpatrick on new
near infrared spectroscopy for this source, estimates of its {\teff},
and metallicity ([M/H]) are obtained. 2M 1237+6526 has a {\teff}
800-850 K. If [M/H] is solar, is as high as 5.5 (cgs) and this
source is older than 10 Gyr. We find a more plausible scenario to be a modestly
subsolar metallicity ([M/H] = -0.2) and moderate 5.0, implying
an age older than 2 Gyr and a mass greater than 0.035 M_{\sun}. The
alternative explanation of the unique emission of this source, involving an
interacting, close, double degenerate system, should be investigated further.
Indeed, there is some evidence of a {\teff} 500 K companion to 2M 1237+6526
on the basis of a possible [3.6]--[4.5] color excess. This
excess may, however, be caused by a subsolar metallicity.Comment: Astrophysical Journal, in press 15 pages, 5 figure
SDSS J142625.71+575218.3: A Prototype for A New Class of Variable White Dwarf
We present the results of a search for pulsations in six of the recently discovered carbon-atmosphere white dwarf ("hot DQ") stars. On the basis of our theoretical calculations, the star SDSS J142625.71 + 575218.3 is the only object expected to pulsate. We observe this star to be variable, with significant power at 417.7 s and 208.8 s ( first harmonic), making it a strong candidate as the first member of a new class of pulsating white dwarf stars, the DQVs. Its folded pulse shape, however, is quite different from that of other white dwarf variables and shows similarities with that of the cataclysmic variable AM CVn, raising the possibility that this star may be a carbon-transferring analog of AM CVn stars. In either case, these observations represent the discovery of a new and exciting class of object.NSF AST-0507639, AST-0602288, AST-0607480, AST-0307321Astronom
On the Nature of the Peculiar Hot Star in the Young LMC Cluster NGC1818
The blue star reported in the field of the young LMC cluster NGC1818 by Elson
et al. (1998) has the wrong luminosity and radius to be a "luminous white
dwarf" member of the cluster. In addition, unless the effective temperature
quoted by the authors is a drastic underestimate, the luminosity is much too
low for it to be a cluster member in the post-AGB phase. Other possibilities,
including that of binary evolution, are briefly discussed. However, the
implication that the massive main sequence turnoff stars in this cluster can
produce white dwarfs (instead of neutron stars) from single-star evolution
needs to be reconsidered.Comment: 5 pages, no figures, Ap J Letters in pres
Discovery of a Magnetic DZ White Dwarf with Zeeman-Split Lines of Heavy Elements
A spectroscopic survey of previously-unstudied Luyten Half Second proper
motion stars has resulted in the discoveries of two new cool magnetic white
dwarfs. One (LHS 2273) is a routine DA star, T= 6,500K, with Zeeman-split H
alpha and H beta, for which a simple model suggests a polar field strength of
18.5 MG viewed close to equator-on. However, the white dwarf LHS 2534 proves to
be the first magnetic DZ showing Zeeman-split Na I and Mg I components, as well
as Ca I and Ca II lines for which Zeeman components are blended. The Na I
splittings result in a mean surface field strength estimate of 1.92 MG. Apart
from the magnetic field, LHS 2534 is one of the most heavily-blanketed and
coolest DZ white dwarfs at T ~ 6,000K.Comment: 7 pages, Astrophysical Journal (Letters), in pres
The outer atmospheric layers of the early M dwarf Gliese 1
Using infrared and high-resolution optical observations of the M dwarf Gliese 1, we investigated the temperatures in the upper atmospheric layers of this star with low atmospheric activity. To fit the Hα and metal line profiles, the normal radiative equ
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