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