Paper: astro-ph/0508208
Date: Tue, 9 Aug 2005 15:59:16 GMT (108kb)

Title: A deeper understanding of white dwarf interiors

Authors: Travis S. Metcalfe (High Altitude Observatory, NCAR)

Comments: 5 pages, 2 figures, 1 table, Accepted for MNRAS Letters
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A detailed record of the physical processes that operate during
post-main-sequence evolution is contained in the internal chemical structure of
white dwarfs. Global pulsations allow us to probe the stellar interior through
asteroseismology, revealing the signatures of prior nuclear burning, mixing,
and diffusion in these stars. I review the rapid evolution of structural models
for helium-atmosphere variable (DBV) white dwarfs over the past five years, and
I present a new series of model-fits using recent observations to illustrate
the relative importance of various interior structures. By incorporating
physically motivated C/O profiles into double-layered envelope models for the
first time, I finally identify an optimal asteroseismic model that agrees with
both diffusion theory and the expected nuclear burning history of the
progenitor. I discuss the implications of this fundamental result, and I
evaluate the prospects for continued progress in the future.

\\ ( http://arXiv.org/abs/astro-ph/0508208 , 108kb)

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Paper: astro-ph/0508213
Date: Tue, 9 Aug 2005 17:47:17 GMT (10kb)

Title: GCRT J1745-3009 as a Transient White Dwarf Pulsar

Authors: Bing Zhang (UNLV), Janusz Gil (Zielona Gora Univ. & UNLV)

Comments: 4 pages, submitted to ApJL
\\
A transient radio source in the direction of the Galactic Center, GCRT
J1745-3009, exhibited 5 peculiar consecutive outbursts at 0.33 GHz with a
period of 77.13 minutes and a duration of ~10 minutes for each outburst. It has
been claimed to be the prototype of a hitherto unknown class of transient radio
sources. We interpret it as a transient white dwarf pulsar with a period of
77.13 minutes. The ~10-minute flaring duration corresponds to the epoch when
the radio beam sweeps our line of sight. The bursting epoch corresponds to the
episodes when stronger sunspot-like magnetic fields emerge into the white dwarf
polar cap region during which the pair production condition is satisfied and
the white dwarf behaves like a radio pulsar. It switches off as the pair
production condition breaks down.

\\ ( http://arXiv.org/abs/astro-ph/0508213 , 10kb)

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Paper: astro-ph/0508233
Date: Wed, 10 Aug 2005 16:28:52 GMT (22kb)

Title: Period Changes in Ultra-compact Double White Dwarfs

Authors: T. R. Marsh, G. Nelemans

Comments: 6 pages, 1 figure, accepted for publication in MNRAS
\\
In recent years there has been much interest in the nature of two stars, V407
Vul and RXJ0806+1527, which are widely thought to be binary white dwarfs of
very short orbital period, 570 and 321 seconds respectively. As such they
should be strong sources of gravitational waves and possible ancestors of the
accreting AM CVn stars. Monitoring at X-ray and optical wavelengths has
established that the period of each star is decreasing, at rates compatible
with that expected from gravitational radiation. This has been taken to support
the ``unipolar inductor'' model in which the white dwarfs are detached and the
X-rays produced by the dissipation of magnetically-induced electric currents.
In this paper we show that this interpretation is incorrect because it ignores
associated torques which transfer angular momentum between the spin of the
magnetic white dwarf and the orbit. We show that this torque is 10^5 times
larger than the GR term in the case of V407 Vul, and 10 times larger for
RXJ0806+1527. For V407 Vul, the unipolar inductor model can only survive if the
white dwarf spins 100 times faster than the orbit. Since this could only come
about through accretion, the validity of the unipolar inductor appears
questionable for this star. We also consider whether accretion models can fit
the observed spin-up, concluding that they can, provided that a mechanism
exists for driving the mass transfer rate away from its equilibrium value.

\\ ( http://arXiv.org/abs/astro-ph/0508233 , 22kb)