1,795 research outputs found

### The rp Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star

We calculate the nucleosynthesis during stable nuclear burning on an
accreting neutron star. This is appropriate for weakly magnetic neutron stars
accreting at near-Eddington rates in low mass X-ray binaries, and for most
accreting X-ray pulsars. We show that the nuclear burning proceeds via the
rapid proton capture process (rp process), and makes nuclei far beyond the iron
group. The final mixture of nuclei consists of elements with a range of masses
between approximately A=60 and A=100. The average nuclear mass of the ashes is
set by the extent of helium burning via (alpha,p) reactions, and depends on the
local accretion rate.
Our results imply that the crust of these accreting neutron stars is made
from a complex mixture of heavy nuclei, with important implications for its
thermal, electrical and structural properties. A crustal lattice as impure as
our results suggest will have a conductivity set mostly by impurity scattering,
allowing more rapid Ohmic diffusion of magnetic fields than previously
estimated.Comment: To appear in the Astrophysical Journal (33 pages, LaTeX, including 11
postscript figures

### Critical phenomena in Newtonian gravity

We investigate the stability of self-similar solutions for a gravitationally
collapsing isothermal sphere in Newtonian gravity by means of a normal mode
analysis. It is found that the Hunter series of solutions are highly unstable,
while neither the Larson-Penston solution nor the homogeneous collapse one have
an analytic unstable mode. Since the homogeneous collapse solution is known to
suffer the kink instability, the present result and recent numerical
simulations strongly support a proposition that the Larson-Penston solution
will be realized in astrophysical situations. It is also found that the Hunter
(A) solution has a single unstable mode, which implies that it is a critical
solution associated with some critical phenomena which are analogous to those
in general relativity. The critical exponent $\gamma$ is calculated as
$\gamma\simeq 0.10567$. In contrast to the general relativistic case, the order
parameter will be the collapsed mass. In order to obtain a complete picture of
the Newtonian critical phenomena, full numerical simulations will be needed.Comment: 25 pages, 7 figures, accepted for publication in Physical Review

### Low Temperature Symmetry of Pyrochlore Oxide Cd2Re2O7

We report the X-ray study for the pyrochlore oxide Cd2Re2O7. Two
symmetry-lowering structural transitions were observed at Ts1=200K and
Ts2=120K. The former is of the second order from the ideal cubic pyrochlore
structure with space group Fd-3m to a tetragonally distorted structure with
I-4m2, while the latter is of the first order likely to another tetragonal
space group I4122. We discuss the feature of the lattice deformation.Comment: 4 pages, 4 figure

### Dust-cooling--induced Fragmentation of Low-metallicity Clouds

Dynamical collapse and fragmentation of low-metallicity cloud cores is
studied using three-dimensional hydrodynamical calculations, with particular
attention devoted whether the cores fragment in the dust-cooling phase or not.
The cores become elongated in this phase, being unstable to non-spherical
perturbation due to the sudden temperature decrease. In the metallicity range
of 10^{-6}-10^{-5}Z_sun, cores with an initial axis ratio >2 reach a critical
value of the axis ratio (>30) and fragment into multiple small clumps. This
provides a possible mechanism to produce low-mass stars in ultra-metal-poor
environments.Comment: 4 pages, 3 figures, ApJ Letters in pres

### No Go Theorem for Kinematic Self-Similarity with A Polytropic Equation of State

We have investigated spherically symmetric spacetimes which contain a perfect
fluid obeying the polytropic equation of state and admit a kinematic
self-similar vector of the second kind which is neither parallel nor orthogonal
to the fluid flow. We have assumed two kinds of polytropic equations of state
and shown in general relativity that such spacetimes must be vacuum.Comment: 5 pages, no figures. Revtex. One word added to the title. Final
version to appear in Physical Review D as a Brief Repor

### Surface r Modes and Burst Oscillations of Neutron Stars

We study the $r$-modes propagating in steadily mass accreting, nuclear
burning, and geometrically thin envelopes on the surface of rotating neutron
stars. For the modal analysis, we construct the envelope models which are fully
radiaitive or have a convective region. As the angular rotation frequency
$\Omega$ is increased, the oscillation frequency $\omega$ of the $r$-modes in
the thin envelopes deviates appreciably from the asymptotic frequency
$\omega=2m\Omega/l^\prime(l^\prime+1)$ defined in the limit of $\Omega\to 0$,
where $\omega$ is the frequency observed in the corotating frame of the star,
and $m$ and $l^\prime$ are the indices of the spherical harmonic function
$Y_{l^\prime}^m$ representing the angular dependence of the modes. We find that
the fundamental $r$-modes in the convective models are destabilized by strong
nuclear burning in the convective region. Because of excessive heating by
nuclear buring, the corotating-frame oscillation frequency $\omega$ of the
$r$-modes in the convective models becomes larger, and hence the inertial-frame
oscillation frequency $|\sigma|$ becomes smaller, than those of the
corresopnding $r$-modes in the radiative models, where $\sigma=\omega-m\Omega$
is negative for the $r$-modes of positive $m$. We find that the relative
frequency change $f=-(\sigma_{conv}-\sigma_{rad})/\sigma_{rad}$ is always
positive and becomes less than $\sim$0.01 for the fundamental $r$-modes of
$l^\prime>|m|+1$ at $|\sigma_{rad}|/2\pi\sim$300Hz for $m=1$ or at
$|\sigma_{rad}|/2\pi\sim$600Hz for $m=2$, where $\sigma_{conv}$ and
$\sigma_{rad}$ denote the oscillation frequencies for the convective and the
radiative envelope models, respectively.Comment: 20 pages, 12 figure

### Rotational Evolution During Type I X-Ray Bursts

The rotation rates of six weakly-magnetic neutron stars accreting in low-mass
X-ray binaries have most likely been measured by Type I X-ray burst
observations with RXTE. The nearly coherent oscillations detected during the
few seconds of thermonuclear burning are most simply understood as rotational
modulation of brightness asymmetries on the neutron star surface. We show that,
as suggested by Strohmayer and colleagues, the frequency changes of 1-2 Hz
observed during bursts are consistent with angular momentum conservation as the
burning shell hydrostatically expands and contracts. We calculate how vertical
heat propagation through the radiative outer layers of the atmosphere and
convection affect the coherence of the oscillation. We show that the evolution
of the rotational profile depends strongly on whether the burning layers are
composed of pure helium or mixed hydrogen/helium. Our results help explain the
absence (presence) of oscillations from hydrogen-burning (helium-rich) bursts
that was found by Muno and collaborators.
We investigate angular momentum transport within the burning layers and the
recoupling of the burning layers with the star. We show that the
Kelvin-Helmholtz instability is quenched by the strong stratification, and that
mixing between the burning fuel and underlying ashes by the baroclinic
instability does not occur. However, the baroclinic instability may have time
to operate within the differentially rotating burning layer, potentially
bringing it into rigid rotation.Comment: To appear in The Astrophysical Journal; minor corrections made to
tables and figure

### Hall effect in superconducting Fe(Se0.5Te0.5) thin films

The Hall effect is investigated for eight superconducting Fe(Se_0.5_Te_0.5_)
thin films grown on MgO and LaSrAlO_4_ substrates with different transition
temperatures (T_c_). The normal Hall coefficients (R_H_) have positive values
with magnitude of 1 - 1.5 x 10^-3^ cm^3^/C at room temperature for the all
samples. With decreasing temperature, we find two characteristic types of
behavior in R_H_(T) depending on T_c_. For thin films with lower T_c_
(typically T_c_ < 5 K), R_H_ start decreasing approximately below T = 250 K
toward a negative side, some of which shows sign reversal at T = 50 - 60 K, but
turns positive toward T = 0 K. On the other hand for the films with higher T_c_
(typically T_c_ > 9 K), R_ H_ leaves almost unchanged down to T = 100 K, and
then starts decreasing toward a negative side. Around the temperatures when
R_H_ changes its sign from positive to negative, obvious nonlinearity is
observed in the field-dependence of Hall resistance as to keep the low-field
R_H_ positive while the high-field R_H_ negative. Thus the electronic state
just above T_c_ is characterized by n_e_ (electron density) > n_h_ (hole
density) with keeping \mu_e_ < \mu_h_. These results suggest the dominance of
electron density to the hole density is an essential factor for the occurence
of superconductivity in Fe-chalcogenide superconductors.Comment: 11 pages, 4 figures, revised version for Physical Review B. accepted
for publication in Physical Review

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