366 research outputs found
Cygnus X-2: the Descendant of an Intermediate-Mass X-Ray Binary
The X-ray binary Cygnus X-2 (Cyg X-2) has recently been shown to contain a
secondary that is much more luminous and hotter than is appropriate for a
low-mass subgiant. We present detailed binary-evolution calculations which
demonstrate that the present evolutionary state of Cyg X-2 can be understood if
the secondary had an initial mass of around 3.5 M_sun and started to transfer
mass near the end of its main-sequence phase (or, somewhat less likely, just
after leaving the main sequence). Most of the mass of the secondary must have
been ejected from the system during an earlier rapid mass-transfer phase. In
the present phase, the secondary has a mass of around 0.5 M_sun with a
non-degenerate helium core. It is burning hydrogen in a shell, and mass
transfer is driven by the advancement of the burning shell. Cyg X-2 therefore
is related to a previously little studied class of intermediate-mass X-ray
binaries (IMXBs). We suggest that perhaps a significant fraction of X-ray
binaries presently classified as low-mass X-ray binaries may be descendants of
IMXBs and discuss some of the implications
Observing Lense-Thirring Precession in Tidal Disruption Flares
When a star is tidally disrupted by a supermassive black hole (SMBH), the
streams of liberated gas form an accretion disk after their return to
pericenter. We demonstrate that Lense-Thirring precession in the spacetime
around a rotating SMBH can produce significant time evolution of the disk
angular momentum vector, due to both the periodic precession of the disk and
the nonperiodic, differential precession of the bound debris streams. Jet
precession and periodic modulation of disk luminosity are possible
consequences. The persistence of the jetted X-ray emission in the Swift
J164449.3+573451 flare suggests that the jet axis was aligned with the spin
axis of the SMBH during this event.Comment: 4 pages, 4 figures. Accepted for publication in Physical Review
Letters. Minor changes made to match proof
Mass loss out of close binaries
In a liberal evolutionary scenario, mass can escape from a binary during eras
of fast mass transfer. We calculate the mass lost by binaries with a B-type
primary at birth where mass transfer starts during hydrogen core burning of the
donor. We simulate the distribution of mass-ratios and orbital periods for
those interacting binaries. The amount of time the binary shows Algol
characteristics within different values of mass-ratio and orbital period has
been fixed from conservative and liberal evolutionary calculations. We use
these data to simulate the distribution of mass-ratios and orbital periods of
Algols with the conservative as well as the liberal model. We compare
mass-ratios and orbital periods of Algols obtained by conservative evolution
with those obtained by our liberal model. Since binaries with a late B-type
primary evolve almost conservatively, the overall distribution of mass-ratios
will only yield a few Algols more with high mass-ratios than conservative
calculations do. Whereas the simulated distribution of orbital periods of
Algols fits the observations well, the simulated distribution of mass-ratios
produces always too few systems with large values.Comment: 6 pages, 6 figures, accepted for publication in A&A; accepted versio
A New Evolutionary Path to Type Ia Supernovae: Helium-Rich Super-Soft X-Ray Source Channel
We have found a new evolutionary path to Type Ia supernovae (SNe Ia) which
has been overlooked in previous work. In this scenario, a carbon-oxygen white
dwarf (C+O WD) is originated, not from an asymptotic giant branch star with a
C+O core, but from a red-giant star with a helium core of . The helium star, which is formed after the first common envelope
evolution, evolves to form a C+O WD of with transferring
a part of the helium envelope onto the secondary main-sequence star. This new
evolutionary path, together with the optically thick wind from mass-accreting
white dwarf, provides a much wider channel to SNe Ia than previous scenarios. A
part of the progenitor systems are identified as the luminous supersoft X-ray
sources or the recurrent novae like U Sco, which are characterized by the
accretion of helium-rich matter. The white dwarf accretes hydrogen-rich,
helium-enhanced matter from a lobe-filling, slightly evolved companion at a
critical rate and blows excess matter in the wind. The white dwarf grows in
mass to the Chandrasekhar mass limit and explodes as an SN Ia. A theoretical
estimate indicates that this channel contributes a considerable part of the
inferred rate of SNe Ia in our Galaxy, i.e., the rate is about ten times larger
than the previous theoretical estimates for white dwarfs with slightly evolved
companions.Comment: 19 pages including 12 figures, to be published in ApJ, 519, No.
The Dynamic Formation of Prominence Condensations
We present simulations of a model for the formation of a prominence
condensation in a coronal loop. The key idea behind the model is that the
spatial localization of loop heating near the chromosphere leads to a
catastrophic cooling in the corona (Antiochos & Klimchuk 1991). Using a new
adaptive grid code, we simulate the complete growth of a condensation, and find
that after approx. 5,000 s it reaches a quasi-steady state. We show that the
size and the growth time of the condensation are in good agreement with data,
and discuss the implications of the model for coronal heating and SOHO/TRACE
observations.Comment: Astrophysical Journal latex file, 20 pages, 7 b-w figures (gif files
Supernova Hosts for Gamma-Ray Burst Jets: Dynamical Constraints
I constrain a possible supernova origin for gamma-ray bursts by modeling the
dynamical interaction between a relativistic jet and a stellar envelope
surrounding it. The delay in observer's time introduced by the jet traversing
the envelope should not be long compared to the duration of gamma-ray emission;
also, the jet should not be swallowed by a spherical explosion it powers. The
only stellar progenitors that comfortably satisfy these constraints, if one
assumes that jets move ballistically within their host stars, are compact
carbon-oxygen or helium post-Wolf-Rayet stars (type Ic or Ib supernovae); type
II supernovae are ruled out. Notably, very massive stars do not appear capable
of producing the observed bursts at any redshift unless the stellar envelope is
stripped prior to collapse. The presence of a dense stellar wind places an
upper limit on the Lorentz factor of the jet in the internal shock model;
however, this constraint may be evaded if the wind is swept forward by a photon
precursor. Shock breakout and cocoon blowout are considered individually;
neither presents a likely source of precursors for cosmological GRBs.
These envelope constraints could conceivably be circumvented if jets are
laterally pressure-confined while traversing the outer stellar envelope. If so,
jets responsible for observed GRBs must either have been launched from a region
several hundred kilometers wide, or have mixed with envelope material as they
travel. A phase of pressure confinement and mixing would imprint correlations
among jets that may explain observed GRB variability-luminosity and
lag-luminosity correlations.Comment: 17 pages, MNRAS, accepted. Contains new analysis of pressure-confined
jets, of jets that experience oblique shocks or mix with their cocoons, and
of cocoons after breakou
Magnetohydrostatic solar prominences in near-potential coronal magnetic fields
We present numerical magnetohydrostatic solutions describing the
gravitationally stratified, bulk equilibrium of cool, dense prominence plasma
embedded in a near-potential coronal field. These solutions are calculated
using the FINESSE magnetohydrodynamics equilibrium solver and describe the
morphologies of magnetic field distributions in and around prominences and the
cool prominence plasma that these fields support. The equilibrium condition for
this class of problem is usually different in distinct subdomains, separated by
free boundaries, across which solutions are matched by suitable continuity or
jump conditions describing force balance. We employ our precise finite element
elliptic solver to calculate solutions not accessible by previous analytical
techniques with temperature or entropy prescribed as free functions of the
magnetic flux function, including a range of values of the polytropic index,
temperature variations mainly across magnetic field lines and photospheric
field profiles sheared close to the polarity inversion line. Out of the many
examples computed here, perhaps the most noteworthy is one which reproduces
precisely the three-part structure often encountered in observations: a cool
dense prominence within a cavity/flux rope embedded in a hot corona. The
stability properties of these new equilibria, which may be relevant to solar
eruptions, can be determined in the form of a full resistive MHD spectrum using
a companion hyperbolic stability solver.Comment: To appear in ApJ August 200
Formation and evolution of a 0.242 Msun helium white dwarf in presence of element diffusion
A 0.242 Msun object that finally becomes a helium white dwarf is evolved from
Roche lobe detachment down to very low luminosities. In doing so, we employ our
stellar code to which we have added a set of routines that compute the effects
due to gravitational settling, and chemical and thermal diffusion. Initial
model is constructed by abstracting mass to a 1 Msun red giant branch model up
to the moment at which the model begins to evolve bluewards. We find that
element diffusion introduces noticeable changes in the internal structure of
the star. In particular, models undergo three thermonuclear flashes instead of
one flash as we found with the standard treatment. This fact has a large impact
on the total mass fraction of hydrogen left in the star at entering the final
cooling track. As a result, at late stages of evolution models with diffusion
are characterized by a much smaller nuclear energy release, and they evolve
significantly faster compared to those found with the standard treatment.
We find that models in which diffusion is considered predict evolutionary
ages for the white dwarf companion to the millisecond pulsar PSR B1855+09 in
good agreement with the spin-down age of the pulsar.Comment: 6 pages, 3 figures, 12th European Workshop on White Dwarf
Off-Center Carbon Ignition in Rapidly Rotating, Accreting Carbon-Oxygen White Dwarfs
We study the effect of stellar rotation on the carbon ignition in a
carbon-oxygen white dwarf accreting CO-rich matter. Including the effect of the
centrifugal force of rotation, we have calculated evolutionary models up to the
carbon ignition for various accretion rates. The rotation velocity at the
stellar surface is set to be the Keplerian velocity. The angular velocity in
the stellar interior has been determined by taking into account the transport
of angular momentum due to turbulent viscosity. We have found that an
off-center carbon ignition occurs even when the effect of stellar rotation is
included if the accretion rate is sufficiently high; the critical accretion
rate for the off-center ignition is hardly changed by the effect of rotation.
Rotation, however, delays the ignition;i.e., the mass coordinate of the
ignition layer and the total mass at the ignition are larger than those for the
corresponding no-rotating model. The result supports our previous conclusion
that a double-white dwarf merger would not be a progenitor of a SN Ia.Comment: 18 pages, 6 figures; To appear in the Astrophysical Journal, 1
November 2004, V615 issu
Asymmetric Dark Matter May Alter the Evolution of Low-mass Stars and Brown Dwarfs
We study energy transport by asymmetric dark matter in the interiors of very
low-mass stars and brown dwarfs. Our motivation is to explore astrophysical
signatures of asymmetric dark matter, which otherwise may not be amenable to
conventional indirect dark matter searches. In viable models, the additional
cooling of very-low mass stellar cores can alter stellar properties. Asymmetric
dark matter with mass 4 < Mx/GeV < 10 and a spin-dependent (spin-independent)
cross sections of sigma \sim 10^{-37} cm^2 (sigma \sim 10^{-40} cm^2) can
increase the minimum mass of main sequence hydrogen burning, partly determining
whether or not the object is a star at all. Similar dark matter candidates
reduce the luminosities of low-mass stars and accelerate the cooling of brown
dwarfs. Such light dark matter is of particular interest given results from the
DAMA, CoGeNT, and CRESST dark matter searches. We discuss possibilities for
observing dark matter effects in stars in the solar neighborhood, globular
clusters, and, of particular promise, local dwarf galaxies, among other
environments, as well as exploiting these effects to constrain dark matter
properties.Comment: 6 Pages, 4 Figures. Accepted for Publication in Phys. Rev. D Rapid
Communication
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