256 research outputs found

### A Wide Symbiotic Channel to Type Ia Supernovae

As a promising channel to Type Ia supernovae (SNe Ia), we have proposed a
symbiotic binary system consisting of a white dwarf (WD) and a low mass
red-giant (RG), where strong winds from the accreting WD play a key role to
increase the WD mass to the Chandrasekhar mass limit. Here we propose two new
evolutionary processes which make the symbiotic channel to SNe Ia much wider.
(1) We first show that the WD + RG close binary can form from a wide binary
even with such a large initial separation as $a_i \lesssim 40000 R_\odot$. Such
a binary consists of an AGB star and a low mass main-sequence (MS) star, where
the AGB star is undergoing superwind before becoming a WD. If the superwind at
the end of AGB evolution is as fast as or slower than the orbital velocity, the
wind outflowing from the system takes away the orbital angular momentum
effectively. As a result the wide binary shrinks greatly to become a close
binary. Therefore, the WD + RG binary can form from much wider binaries than
our earlier estimate. (2) When the RG fills its inner critical Roche lobe, the
WD undergoes rapid mass accretion and blows a strong optically thick wind. Our
earlier analysis has shown that the mass transfer is stabilized by this wind
only when the mass ratio of RG/WD is smaller than 1.15. Our new finding is that
the WD wind can strip mass from the RG envelope, which could be efficient
enough to stabilize the mass transfer even if the RG/WD mass ratio exceeds
1.15. With the above two new effects (1) and (2), the symbiotic channel can
account for the inferred rate of SNe Ia in our Galaxy.Comment: 29 pages including 14 firgures, to be published in ApJ, 521, No.

### The Sensitivity of Multidimensional Nova Calculations to the Outer Boundary Conditions

Multidimensional reactive flow models of accreted hydrogen rich envelopes on
top of degenerate cold white dwarfs are very effective tools for the study of
critical, non spherically symmetric, behaviors during the early stages of nova
outbursts. Such models can shed light both on the mechanism responsible for the
heavy element enrichment observed to characterize nova envelope matter and on
the role of perturbations during the early stages of ignition of the runaway.
The complexity of convective reactive flow in multi-dimensions makes the
computational model itself complex and sensitive to the details of the
numerics. In this study, we demonstrate that the imposed outer boundary
condition can have a dramatic effect on the solution. Several commonly used
choices for the outer boundary conditions are examined. It is shown that the
solutions obtained from Lagrangian simulations, where the envelope is allowed
to expand and mass is being conserved, are consistent with spherically
symmetric solutions. In Eulerian schemes which utilize an outer boundary
condition of free outflow, the outburst can be artificially quenched.Comment: 12 Pages 3 figures; Accepted for publication in the Astrophysical
Journa

### Regular black holes with flux tube core

We consider a class of black holes for which the area of the two-dimensional
spatial cross-section has a minimum on the horizon with respect to a
quasiglobal (Krusckal-like) coordinate. If the horizon is regular, one can
generate a tubelike counterpart of such a metric and smoothly glue it to a
black hole region. The resulting composite space-time is globally regular, so
all potential singuilarities under the horizon of the original metrics are
removed. Such a space-time represents a black hole without an apparent horizon.
It is essential that the matter should be non-vacuum in the outer region but
vacuumlike in the inner one. As an example we consider the noninteracting
mixture of vacuum fluid and matter with a linear equation of state and scalar
phantom fields. This approach is extended to distorted metrics, with the
requirement of spherical symmetry relaxed.Comment: 15 pages. 2 references adde

### Spherically symmetric black holes in minimally modified self-dual gravity

We discuss spherically symmetric black holes in the modified self-dual theory
of gravity recently studied by Krasnov, obtained adding a Weyl-curvature
dependent `cosmological term' to the Plebanski lagrangian for general
relativity. This type of modified gravity admits two different types of
singularities: one is a true singularity for the theory where the fundamental
fields of the theory, as well as the (auxiliary) spacetime metric, become
singular, and the other one is a milder "non-metric singularity" where the
metric description of the spacetime breaks down but the fundamental fields
themselves are regular. We first generalise this modified self-dual gravity to
include Maxwell's field and then study basic features of spherically symmetric,
charged black holes, with particular focus on whether these two types of
singularities are hidden or naked. We restrict our attention to minimal forms
of the modification, and find that the theory exhibits `screening' effects of
the electric charge (or `anti-screening', depending upon the sign of the
modification term), in the sense that it leads to the possibility of charging
the black hole more (or less) than it would be possible in general relativity
without exposing a naked singularity. We also find that for any (even
arbitrarily large) value of charge, true singularities of the theory appear to
be either achronal (non-timelike) covered by the hypersurface of a harmless
non-metric singularity, or simply hidden inside at least one Killing horizon.Comment: 42 pages, many colour figures. v2: discussion of the conformal
ambiguity improved, references added. v3: amended to match published versio

### Solutions of Minimal Four Dimensional de Sitter Supergravity

Pseudo-supersymmetric solutions of minimal $N=2$, $D=4$ de Sitter
supergravity are classified using spinorial geometry techniques. We find three
classes of solutions. The first class of solution consists of geometries which
are fibrations over a 3-dimensional manifold equipped with a Gauduchon-Tod
structure. The second class of solution is the cosmological Majumdar-Papapetrou
solution of Kastor and Traschen, and the third corresponds to gravitational
waves propagating in the Nariai cosmology.Comment: 17 Pages. Minor correction to section 4; equation (4.21) corrected
and (old) equation (4.26) deleted; the final result is unchange

### A Modeling of the Super-Eddington Luminosity in Nova Outbursts: V1974 Cygni

We have modeled nova light curves exceeding the Eddington luminosity. It has
been suggested that a porous structure develops in nova envelopes during the
super Eddington phase and the effective opacity is much reduced for such a
porous atmosphere. Based on this reduced opacity model, we have calculated
envelope structures and light curves of novae. The optically thick wind model
is used to simulate nova winds. We find that the photospheric luminosity and
the wind mass-loss rate increase inversely proportional to the reducing factor
of opacities, but the wind velocity hardly changes. We also reproduce the
optical light curve of V1974 Cygni (Nova Cygni 1992) in the super-Eddington
phase, which lasts 13 days from the optical peak 1.7 mag above the Eddington
luminosity.Comment: 8 pages, 4 figures, to appear in ApJ

### Cosmological solutions from fake N=2 EYM supergravity

We characterise the (fake) supersymmetric solutions of Wick-rotated N=2 d=4
gauged supergravity coupled to non-Abelian vector multiplets. In the time-like
case we obtain generalisations of Kastor & Traschen's cosmological black holes:
they have a specific time-dependence and the base-space must be 3-dimensional
hyperCR/Gauduchon-Tod space. In the null-case, we find that the metric has a
holonomy contained in Sim(2), give a general characterisation of the solutions,
and give some examples. Finally, we point out that in some cases the solutions
we found are non-BPS solutions to N=2 d=4 supergravity coupled to vector
multiplets.Comment: 30 pages. Comments and references added, typos correcte

### Intersecting black branes in strong gravitational waves

We consider intersecting black branes with strong gravitational waves
propagating along their worldvolume in the context of supergravity theories.
Both near-horizon and space-filling gravitational wave modes are included in
our ansatz. The equations of motion (originally, partial differential
equations) are shown to reduce to ordinary differential equations, which
include a Toda-like system. For special arrangements of intersecting black
branes, the Toda-like system becomes integrable, permitting a more thorough
analysis of the gravitational equations of motion.Comment: 17 pages; v2: cosmetic improvements, published versio

### The Numerical Solution of Scalar Field for Nariai Case in 5D Ricci-flat SdS Black String Space with Polynomial Approximation

As one exact candidate of the higher dimensional black hole, the 5D
Ricci-flat Schwarzschild-de Sitter black string space presents something
interesting. In this paper, we give a numerical solution to the real scalar
field around the Nariai black hole by the polynomial approximation. Unlike the
previous tangent approximation, this fitting function makes a perfect match in
the leading intermediate region and gives a good description near both the
event and the cosmological horizons. We can read from our results that the wave
is close to a harmonic one with the tortoise coordinate. Furthermore, with the
actual radial coordinate the waves pile up almost equally near the both
horizons.Comment: 8 pages, 4 figure

### Reheating and turbulence

We show that the ''turbulent'' particle spectra found in numerical
simulations of the behavior of matter fields during reheating admit a simple
interpretation in terms of hydrodynamic models of the reheating period. We
predict a particle number spectrum $n_{k}\propto k^{-\alpha}$ with $\alpha \sim
2$ for $k\to 0.$Comment: 10 pages, one figure included in tex

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