From: LARRY KLAES (ljk4_at_msn.com)
Date: Sat Sep 10 2005 - 03:14:19 UTC
Paper: astro-ph/0509165
Date: Wed, 7 Sep 2005 12:17:56 GMT (55kb)
Title: Low Mass Companions to White Dwarfs
Authors: J. Farihi, B. Zuckerman, & E.E. Becklin
Comments: 4 pages with figures, accepted as part of a special issue of AN
dedicated to the proceedings of the workshop "Ultralow Mass Star Formation
and Evolution" held in La Palma, Spain during June/July 2005
\\
This paper summarizes the results of over 17 years of work searching for low
mass stellar and substellar companions to more than 370 nearby white dwarfs.
Roughly 60 low mass, unevolved companions were found and studied all together,
with over 20 discovered in the last few years, including the first unambiguous
brown dwarf companion to a white dwarf, GD 1400B. The resulting spectral type
distributions for companions to white dwarfs and nearby cool field dwarfs are
compared, and the implications for binary star formation are discussed. A brief
analysis of GD 1400B, including new data, is also presented
\\ ( http://arXiv.org/abs/astro-ph/0509165 , 55kb)
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Paper: astro-ph/0509188
Date: Wed, 7 Sep 2005 18:10:45 GMT (35kb)
Title: Excess Infrared Radiation from a Massive DAZ White Dwarf: GD362 - a
Debris Disk?
Authors: Mukremin Kilic, Ted von Hippel (Texas), S. K. Leggett (JAC), and D. E.
Winget (Texas)
Comments: Accepted for publication in ApJ Letters
\\
We report the discovery of excess K-band radiation from a massive DAZ white
dwarf star, GD362. Combining infrared photometric and spectroscopic
observations, we show that the excess radiation cannot be explained by a
stellar or substellar companion, and is likely to be caused by a debris disk.
This would be only the second such system known, discovered 18 years after
G29-38, the only single white dwarf currently known to be orbited by
circumstellar dust. Both of these systems favor a model with accretion from a
surrounding debris disk to explain the metal abundances observed in DAZ white
dwarfs. Nevertheless, observations of more DAZs in the mid-infrared are
required to test if this model can explain all DAZs.
\\ ( http://arXiv.org/abs/astro-ph/0509188 , 35kb)
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Paper: astro-ph/0509193
Date: Wed, 7 Sep 2005 18:50:12 GMT (20kb)
Title: A Dusty Disk Around GD 362, a White Dwarf With a Uniquely High
Photospheric Metal Abundance
Authors: E.E. Becklin, J. Farihi, M. Jura, Inseok Song, A. J. Weinberger, B.
Zuckerman
Comments: 12 pages, 2 figures. ApJ Letters, in press
\\
Eighteen years after an infrared excess was discovered associated with the
white dwarf G29-38, we report ground-based measurements (JHKL'N') with
mJy-level sensitivity of GD 362 that show it to be a second single white dwarf
with an infrared excess. As a first approximation, the excess around GD 362,
which amounts to about 3% of the total stellar luminosity, can be explained by
emission from a passive, flat, opaque dust disk that lies within the Roche
radius of the white dwarf. The dust may have been produced by the tidal
disruption of a large parent body such as an asteroid. Accretion from this
circumstellar disk could account for the remarkably high abundance of metals in
the star's photosphere.
\\ ( http://arXiv.org/abs/astro-ph/0509193 , 20kb)
Paper: astro-ph/0509136
Date: Tue, 6 Sep 2005 16:20:23 GMT (162kb)
Title: XMM Spectroscopy of the Transient Supersoft Source RX J0513.9-6951:
probing the dynamic white dwarf photosphere
Authors: Katherine E. McGowan (1,2), Phil A. Charles (3,4), Alexander J.
Blustin (1), Mario Livio (5), Darragh O'Donoghue (3), Bernard Heathcote (6)
((1) MSSL, (2) LANL, (3) SAAO, (4) University of Southampton, (5) STScI, (6)
Barfold Observatory)
Comments: 14 pages, 8 figures, 4 tables, accepted for publication in MNRAS
\\
The highly luminous (> 10^37 erg s^-1) supersoft X-ray sources (SSS) are
believed to be Eddington limited accreting white dwarfs undergoing surface
hydrogen burning. The current paradigm for SSS involves thermally unstable mass
transfer from a 1-2 solar mass companion. However this model has never been
directly confirmed and yet is crucial for the evolution of cataclysmic
variables in general, and for the establishment of SSS as progenitors of type
Ia supernovae in particular. The key SSS is RX J0513.9-6951 which has recurrent
X-ray outbursts every 100-200 d (lasting for ~40 d) during which the optical
declines by 1 mag. We present the first XMM-Newton observations of RX
J0513.9-6951 through one of its optical low states. Our results show that as
the optical low state progresses the temperature and the X-ray luminosity
decrease, behaviour that is anti-correlated with the optical and UV emission.
We find that as the optical (and UV) intensity recover the radius implied by
the spectral fits increases. The high resolution spectra show evidence of deep
absorption features which vary during the optical low state. Our results are
consistent with the predictions of the white dwarf photospheric contraction
model proposed by Southwell et al. 1996.
\\ ( http://arXiv.org/abs/astro-ph/0509136 , 162kb)
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Paper: astro-ph/0509121
Date: Tue, 6 Sep 2005 13:45:42 GMT (42kb)
Title: Rotation velocities of white dwarfs determined from the CaII K line
Authors: L. Berger, D. Koester, R. Napiwotzki, I. N. Reid, B. Zuckerman
Comments: 7 pages, 3 figures, 3 tables
\\
We determine projected rotation velocities v sini in DAZ white dwarfs, for
the first time using the rotational broadening of the CaII K line. The results
confirm previous findings that white dwarfs are very slow rotators, and set
even more stringent upper limits of typically less than 10 km/s. The few
exceptions include 3 stars known or suspected to be variable ZZ Ceti stars,
where the line broadening is very likely not due to rotation. The results
demonstrate that the angular momentum of the core cannot be preserved
completely between main sequence and final stage.
\\ ( http://arXiv.org/abs/astro-ph/0509121 , 42kb)
Paper: astro-ph/0509070
Date: Sun, 4 Sep 2005 14:57:35 GMT (31kb)
Title: High-energy emission of fast rotating white dwarfs
Authors: N.R. Ikhsanov and P.L. Biermann
Comments: 8 pages, accepted for publication in A&A
\\
The process of energy release in the magnetosphere of a fast rotating,
magnetized white dwarf can be explained in terms of the canonical spin-powered
pulsar model. Applying this model to the white dwarf companion of the low mass
close binary AE Aquarii leads us to the following conclusions. First, the
system acts as an accelerator of charged particles whose energy is limited to
E_p < 3 TeV and which are ejected from the magnetosphere of the primary with
the rate L_kin < 10^{32} erg/s. Due to the curvature radiation of the
accelerated primary electrons the system should appear as a source of soft
gamma-rays (~ 100 keV) with the luminosity < 3x10^{27} erg/s. The TeV emission
of the system is dominated by the inverse Compton scattering of optical photons
on the ultrarelativistic electrons. The optical photons are mainly contributed
by the normal companion and the stream of material flowing through the
magnetosphere of the white dwarf. The luminosity of the TeV source depends on
the state of the system (flaring/quiet) and is limited to < 5x10^{29} erg/s.
These results allow us to understand a lack of success in searching for the
high-energy emission of AE Aqr with the Compton Gamma-ray Observatory and the
Whipple Observatory.
\\ ( http://arXiv.org/abs/astro-ph/0509070 , 31kb)
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