224 research outputs found
Spectral Decomposition of the Tent Map with Varying Height
The generalized spectral decomposition of the Frobenius-Perron operator of
the tent map with varying height is determined at the band-splitting points.
The decomposition includes both decay onto the attracting set and the approach
to the asymptotically periodic state on the attractor. Explicit compact
expressions for the polynomial eigenstates are obtained using algebraic
techniques.Comment: 39 pages, 7 figures, in LATeX with embedded PS figure
White Dwarfs in NGC 6791: Avoiding the Helium Flash
We propose that the anomalously bright white dwarf luminosity function
observed in NGC 6791 (Bedin et al 2005) is the consequence of the formation of
0.5 Msun white dwarfs with Helium cores instead of Carbon cores. This may
happen if mass loss during the ascent of the Red Giant Branch is strong enough
to prevent a star from reaching the Helium flash. Such a model can explain the
slower white dwarf cooling (relative to standard models) and fits naturally
with scenarios advanced to explain Extreme Horizontal Branch stars, a
population of which are also found in this cluster.Comment: 4 pages, 4 postscript figures, submitted to Ap
The Temperature and Cooling Age of the White-Dwarf Companion to the Millisecond Pulsar PSR B1855+09
We report on Keck and {\em Hubble Space Telescope} observations of the binary
millisecond pulsar PSR B1855+09. We detect its white-dwarf companion and
measure \mv=25.90\pm0.12 and \mi=24.19\pm0.11 (Vega system). From the
reddening-corrected color, (\mv-\mi)_0=1.06\pm0.21, we infer a temperature
\Teff=4800\pm800 K. The white-dwarf mass is known accurately from
measurements of the Shapiro delay of the pulsar signal,
\Mcomp=0.258^{+0.028}_{-0.016} \Msun. Hence, given a cooling model, one can
use the measured temperature to determine the cooling age. The main uncertainty
in the cooling models for such low-mass white dwarfs is the amount of residual
nuclear burning, which is set by the thickness of the hydrogen layer
surrounding the helium core. From the properties of similar systems, it has
been inferred that helium white dwarfs form with thick hydrogen layers, with
mass \simgt3\times10^{-3} \Msun, which leads to significant additional
heating. This is consistent with expectations from simple evolutionary models
of the preceding binary evolution. For PSR B1855+09, though, such models lead
to a cooling age of Gyr, which is twice the spin-down age of the
pulsar. It could be that the spin-down age were incorrect, which would call the
standard vacuum dipole braking model into question. For two other pulsar
companions, however, ages well over 10 Gyr are inferred, indicating that the
problem may lie with the cooling models. There is no age discrepancy for models
in which the white dwarfs are formed with thinner hydrogen layers
(\simlt3\times10^{-4} \Msun).Comment: 7 pages, 1 figure, aas4pp2.sty. Accepted for publication in ApJ
Optical Identification of Close White Dwarf Binaries in the LISA Era
The Laser Interferometer Space Antenna (LISA) is expected to detect close
white dwarf binaries (CWDBs) through their gravitational radiation. Around 3000
binaries will be spectrally resolved at frequencies > 3 mHz, and their
positions on the sky will be determined to an accuracy ranging from a few tens
of arcminutes to a degree or more. Due to the small binary separation, the
optical light curves of >~ 30% of these CWDBs are expected to show eclipses,
giving a unique signature for identification in follow-up studies of the LISA
error boxes. While the precise optical location improves binary parameter
determination with LISA data, the optical light curve captures additional
physics of the binary, including the individual sizes of the stars in terms of
the orbital separation. To optically identify a substantial fraction of CWDBs
and thus localize them very accurately, a rapid monitoring campaign is
required, capable of imaging a square degree or more in a reasonable time, at
intervals of 10--100 seconds, to magnitudes between 20 and 25. While the
detectable fraction can be up to many tens of percent of the total resolved
LISA CWDBs, the exact fraction is uncertain due to unknowns related to the
white dwarf spatial distribution, and potentially interesting physics, such as
induced tidal heating of the WDs due to their small orbital separation.Comment: 4 pages, 2 figure
White Dwarf Donors in Ultracompact Binaries: The Stellar Structure of Finite Entropy Objects
We discuss the mass-radius (M-R) relations for low-mass (M<0.1 Msun) white
dwarfs (WDs) of arbitrary degeneracy and evolved (He, C, O) composition. We do
so with both a simple analytical model and models calculated by integration of
hydrostatic balance using a modern equation of state valid for fully ionized
plasmas. The M-R plane is divided into three regions where either Coulomb
physics, degenerate electrons or a classical gas dominate the WD structure. For
a given M and central temperature, T_c, the M-R relation has two branches
differentiated by the model's entropy content. We present the M-R relations for
a sequence of constant entropy WDs of arbitrary degeneracy parameterized by M
and T_c for pure He, C, and O. We discuss the applications of these models to
the recently discovered accreting millisecond pulsars. We show the relationship
between the orbital inclination for these binaries and the donor's composition
and T_c. In particular we find from orbital inclination constraints that the
probability XTE J1807-294 can accommodate a He donor is approximately 15% while
for XTE J0929-304, it is approximately 35%. We argue that if the donors in
ultracompact systems evolve adiabatically, there should be 60-160 more systems
at orbital periods of 40 min than at orbital periods of 10 min, depending on
the donor's composition.Comment: emulateapj style, 11 pages, 12 figures. Accepted to the Astrophysical
Journal. Tables with interpolation routines of the M-R relations are
available at http://www.physics.ucsb.edu/~cjdeloye/research.htm
The Clusters AgeS Experiment (CASE). I. V209 omega Cen - An Eclipsing Post-Common Envelope Binary in the Globular Cluster omega Cen
We use photometric and spectroscopic observations of the detached eclipsing
binary V209 omega Cen to derive the masses, radii, and luminosities of the
component stars. The system exhibits total eclipses and, based on the measured
systemic velocity and the derived distance, is a member of the globular cluster
omega Cen. We obtain 0.945 +/- 0.043 Msun, 0.983 +/- 0.015 Rsun and 6.68 +/-
0.88 Lsun for the cooler, but larger and more luminous primary component. The
secondary component has 0.144 +/- 0.008 Msun, 0.425 +/- 0.008 Rsun and 2.26 +/-
0.28 Lsun. The effective temperatures are estimated at 9370 K for the primary
and at 10866 K for the secondary. On the color-magnitude diagram of the
cluster, the primary component occupies a position between the tip of the blue
straggler region and the extended horizontal branch while the secondary
component is located close to the red border of the area occupied by hot
subdwarfs. However, its radius is too large and its effective temperature is
too low for it to be an sdB star. We propose a scenario leading to the
formation of a system with such unusual properties with the primary component
``re-born'' from a former white dwarf which accreted a new envelope through
mass transfer from its companion. The secondary star has lost most of its
envelope while starting its ascent onto the sub-giant branch. It failed to
ignite helium in its core and is currently powered by a hydrogen burning shell.Comment: 24 pages, 9 figures, AJ, in pres
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