168 research outputs found
On the quasi-isometric rigidity of graphs of surface groups
Let be a word hyperbolic group with a cyclic JSJ decomposition that
has only rigid vertex groups, which are all fundamental groups of closed
surface groups. We show that any group quasi-isometric to is
abstractly commensurable with .Comment: 54 pages, 10 figures, comments welcom
Thermal Timescale Mass Transfer and the Evolution of White Dwarf Binaries
The evolution of binaries consisting of evolved main sequence stars (1 <
M_d/Msun < 3.5) with white dwarf companions (0.7 < M_wd/Msun < 1.2) is
investigated through the thermal mass transfer phase. Taking into account the
stabilizing effect of a strong, optically thick wind from the accreting white
dwarf surface, we have explored the formation of several evolutionary groups of
systems for progenitors with initial orbital periods of 1 and 2 days. The
numerical results show that CO white dwarfs can accrete sufficient mass to
evolve to a Type Ia supernova and ONeMg white dwarfs can be built up to undergo
accretion induced collapse for donors more massive than about 2 Msun. For
donors less massive than ~2 Msun the system can evolve to form a He and CO or
ONeMg white dwarf pair. In addition, sufficient helium can be accumulated (~0.1
Msun) in systems characterized by 1.6 < M_d/Msun < 1.9 and 0.8 < M_wd/Msun < 1
such that sub Chandrasekhar mass models for Type Ia supernovae, involving off
center helium ignition, are possible for progenitor systems evolving via the
Case A mass transfer phase. For systems characterized by mass ratios > 3 the
system likely merges as a result of the occurrence of a delayed dynamical mass
transfer instability. A semi-analytical model is developed to delineate these
phases which can be easily incorporated in population synthesis studies of
these systems.Comment: 9 pages, 6 figures, Latex, emulateapj style, ApJ accepte
Stability of radiation-pressure dominated disks. I. The dispersion relation for a delayed heating alpha-viscosity prescription
We derive and investigate the dispersion relation for accretion disks with
retarded or advanced heating. We follow the alpha-prescription but allow for a
time offset (\tau) between heating and pressure perturbations, as well as for a
diminished response of heating to pressure variations. We study in detail
solutions of the dispersion relation for disks with radiation-pressure fraction
1 - \beta . For \tau <0 (delayed heating) the number and sign of real solutions
for the growth rate depend on the values of the time lag and the ratio of
heating response to pressure perturbations, \xi . If the delay is larger than a
critical value (e.g., if \Omega \tau <-125 for \alpha =0.1, \beta =0 and \xi
=1) two real solutions exist, which are both negative. These results imply that
retarded heating may stabilize radiation-pressure dominated accretion disks.Comment: 11 pages, 10 figures, to be submitted to A&
The most massive progenitors of neutron stars: CXO J164710.2-455216
The evolution leading to the formation of a neutron star in the very young
Westerlund 1 star cluster is investigated. The turnoff mass has been estimated
to be 35 Msun, indicating a cluster age ~ 3-5 Myr. The brightest X-ray source
in the cluster, CXO J164710.2-455216, is a slowly spinning (10 s) single
neutron star and potentially a magnetar. Since this source was argued to be a
member of the cluster, the neutron star progenitor must have been very massive
(M_zams > 40 Msun) as noted by Muno et al. (2006). Since such massive stars are
generally believed to form black holes (rather than neutron stars), the
existence of this object poses a challenge for understanding massive star
evolution. We point out while single star progenitors below M_zams < 20 Msun
form neutron stars, binary evolution completely changes the progenitor mass
range. In particular, we demonstrate that mass loss in Roche lobe overflow
enables stars as massive as 50-80 Msun, under favorable conditions, to form
neutron stars. If the very high observed binary fraction of massive stars in
Westerlund 1 (> 70 percent) is considered, it is natural that CXO
J164710.2-455216 was formed in a binary which was disrupted in a supernova
explosion such that it is now found as a single neutron star. Hence, the
existence of a neutron star in a given stellar population does not necessarily
place stringent constraints on progenitor mass when binary interactions are
considered. It is concluded that the existence of a neutron star in Westerlund
1 cluster is fully consistent with the generally accepted framework of stellar
evolution.Comment: 5 pages of text and 4 figures (submitted to Astrophysical Journal
Why the braking indices of young pulsars are less than 3?
In this letter we discuss two possible reasons which cause the observed
braking indices n of young radio pulsars to be smaller than 3: (a) the evolving
spin-down model of the magnetic field component increases with
time; (b) the extrinsic braking torque model in which the tidal torques exerted
on the pulsar by the fallback disk, and carries away the spin angular momentum
from the pulsar. Based on some simple assumptions, we derive the expression of
the braking indices, and calculate the spin-down evolutionary tracks of pulsars
for different input parameters.Comment: 4 pages, 3 figures, accepted for publication in A&A Letter
The Evolutionary Status of SS433
We consider possible evolutionary models for SS 433. We assume that
common-envelope evolution is avoided if radiation pressure is able to expel
most of a super-Eddington accretion flow from a region smaller than the
accretor's Roche lobe. This condition is satisfied, at least initially, for
largely radiative donors with masses in the range 4-12 solar masses. For donors
more massive than about 5 solar masses, moderate mass ratios q = M_2/M_1 > 1
are indicated, thus tending to favor black-hole accretors. For lower mass
donors, evolutionary considerations do not distinguish between a neutron star
or black hole accretor. In all cases the mass transfer (and mass loss) rates
are much larger than the likely mass-loss rate in the precessing jets. Almost
all of the transferred mass is expelled at radii considerably larger than the
jet acceleration region, producing the "stationary" H-alpha line, the infrared
luminosity, and accounting for the low X-ray luminosity.Comment: 13 pages, Astrophysical Journal Letters, accepte
On the Origin of X-ray Emission From Millisecond Pulsars in 47 Tuc
The observed spectra and X-ray luminosities of millisecond pulsars in 47 Tuc
can be interpreted in the context of theoretical models based on strong, small
scale multipole fields on the neutron star surface. For multipole fields that
are relatively strong as compared to the large scale dipole field, the emitted
X-rays are thermal and likely result from polar cap heating associated with the
return current from the polar gap. On the other hand, for weak multipole
fields, the emission is nonthermal and results from synchrotron radiation of
pairs created by curvature radiation. The X-ray luminosity, , is
related to the spin down power, , expressed in the form with and for strong and weak
multipole fields respectively. If the polar cap size is of the order of the
length scale of the multipole field, and , the polar cap
temperature is .
A comparison of the X-ray properties of millisecond pulsars in globular
clusters and in the Galactic field suggests that the emergence of relatively
strong small scale multipole fields from the neutron star interior may be
correlated with the age and evolutionary history of the underlying neutron
star.Comment: 25 pages, 2 figures, accepted for publication in Ap
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