3,006 research outputs found
Neutron star cooling after deep crustal heating in the X-ray transient KS 1731-260
We simulate the cooling of the neutron star in the X-ray transient KS
1731-260 after the source returned to quiescence in 2001 from a long (>~ 12.5
yr) outburst state. We show that the cooling can be explained assuming that the
crust underwent deep heating during the outburst stage. In our best theoretical
scenario the neutron star has no enhanced neutrino emission in the core, and
its crust is thin, superfluid, and has the normal thermal conductivity. The
thermal afterburst crust-core relaxation in the star may be not over.Comment: 5 pages, 2 figures, accepted by MNRAS. In v.2, two references added
and typos correcte
Heating in the Accreted Neutron Star Ocean: Implications for Superburst Ignition
We perform a self-consistent calculation of the thermal structure in the
crust of a superbursting neutron star. In particular, we follow the
nucleosynthetic evolution of an accreted fluid element from its deposition into
the atmosphere down to a depth where the electron Fermi energy is 20 MeV. We
include temperature-dependent continuum electron capture rates and realistic
sources of heat loss by thermal neutrino emission from the crust and core. We
show that, in contrast to previous calculations, electron captures to excited
states and subsequent gamma-emission significantly reduce the local heat loss
due to weak-interaction neutrinos. Depending on the initial composition these
reactions release up to a factor of 10 times more heat at densities < 10^{11}
g/cc than obtained previously. This heating reduces the ignition depth of
superbursts. In particular, it reduces the discrepancy noted by Cumming et al.
between the temperatures needed for unstable 12C ignition on timescales
consistent with observations and the reduction in crust temperature from Cooper
pair neutrino emission.Comment: 10 pages, 11 figures, the Astrophysical Journal, in press (scheduled
for v. 662). Revised from v1 in response to referee's comment
Ab-initio study of the relation between electric polarization and electric field gradients in ferroelectrics
The hyperfine interaction between the quadrupole moment of atomic nuclei and
the electric field gradient (EFG) provides information on the electronic charge
distribution close to a given atomic site. In ferroelectric materials, the loss
of inversion symmetry of the electronic charge distribution is necessary for
the appearance of the electric polarization. We present first-principles
density functional theory calculations of ferroelectrics such as BaTiO3, KNbO3,
PbTiO3 and other oxides with perovskite structures, by focusing on both EFG
tensors and polarization. We analyze the EFG tensor properties such as
orientation and correlation between components and their link with electric
polarization. This work supports previous studies of ferroelectric materials
where a relation between EFG tensors and polarization was observed, which may
be exploited to study ferroelectric order when standard techniques to measure
polarization are not easily applied.Comment: 9 pages, 6 figures, 5 tables, corrected typos, as published in Phys.
Rev.
Control of gene conversion and somatic hypermutation by immunoglobulin promoter and enhancer sequences
It is thought that gene conversion (GCV) and somatic hypermutation (SHM) of immunoglobulin (Ig) genes occur in two steps: the generation of uracils in DNA by activation-induced cytidine deaminase, followed by their subsequent repair by various DNA repair pathways to generate sequence-diversified products. It is not known how either of the two steps is targeted specifically to Ig loci. Because of the tight link between transcription and SHM, we have investigated the role of endogenous Ig light chain (IgL) transcriptional control elements in GCV/SHM in the chicken B cell line DT40. Promoter substitution experiments led to identification of a strong RNA polymerase II promoter incapable of supporting efficient GCV/SHM. This surprising finding indicates that high levels of transcription are not sufficient for robust GCV/SHM in Ig loci. Deletion of the IgL enhancer in a context in which high-level transcription was not compromised showed that the enhancer is not necessary for GCV/SHM. Our results indicate that cis-acting elements are important for Ig gene diversification, and we propose that targeting specificity is achieved through the combined action of several Ig locus elements that include the promoter
Microscopic theory of surface-enhanced Raman scattering in noble-metal nanoparticles
We present a microscopic model for surface-enhanced Raman scattering (SERS)
from molecules adsorbed on small noble-metal nanoparticles. In the absence of
direct overlap of molecular orbitals and electronic states in the metal, the
main enhancement source is the strong electric field of the surface plasmon
resonance in a nanoparticle acting on a molecule near the surface. In small
particles, the electromagnetic enhancement is strongly modified by quantum-size
effects. We show that, in nanometer-sized particles, SERS magnitude is
determined by a competition between several quantum-size effects such as the
Landau damping of surface plasmon resonance and reduced screening near the
nanoparticle surface. Using time-dependent local density approximation, we
calculate spatial distribution of local fields near the surface and enhancement
factor for different nanoparticles sizes.Comment: 8 pages, 6 figures. Considerably extended final versio
Models for Type I X-Ray Bursts with Improved Nuclear Physics
Multi-zone models of Type I X-ray bursts are presented that use an adaptive
nuclear reaction network of unprecedented size, up to 1300 isotopes. Sequences
of up to 15 bursts are followed for two choices of accretion rate and
metallicity. At 0.1 Eddington (and 0.02 Eddington for low metallicity),
combined hydrogen-helium flashes occur. The rise times, shapes, and tails of
these light curves are sensitive to the efficiency of nuclear burning at
various waiting points along the rp-process path and these sensitivities are
explored. The bursts show "compositional inertia", in that their properties
depend on the fact that accretion occurs onto the ashes of previous bursts
which contain left-over hydrogen, helium and CNO nuclei. This acts to reduce
the sensitivity of burst properties to metallicity. For the accretion rates
studied, only the first anomalous burst in one model produces nuclei as heavy
as A=100, other bursts make chiefly nuclei with A~64. The amount of carbon
remaining after hydrogen-helium bursts is typically <1% by mass, and decreases
further as the ashes are periodically heated by subsequent bursts. At the lower
accretion rate of 0.02 Eddington and solar metallicity, the bursts ignite in a
hydrogen-free helium layer. At the base of this layer, up to 90% of the helium
has already burned to carbon prior to the unstable ignition. These
helium-ignited bursts have briefer, brighter light curves with shorter tails,
very rapid rise times (<0.1 s), and ashes lighter than the iron group.Comment: Submitted to the Astrophysical Journal (42 pages; 27 figures
Thermal states of coldest and hottest neutron stars in soft X-ray transients
We calculate the thermal structure and quiescent thermal luminosity of
accreting neutron stars (warmed by deep crustal heating in accreted matter) in
soft X-ray transients (SXTs). We consider neutron stars with nucleon and
hyperon cores and with accreted envelopes. It is assumed that an envelope has
an outer helium layer (of variable depth) and deeper layers of heavier
elements, either with iron or with much heavier nuclei (of atomic weight A >
100) on the top (Haensel & Zdunik 1990, 2003, astro-ph/0305220). The relation
between the internal and surface stellar temperatures is obtained and fitted.
The quiescent luminosity of the hottest (low-mass) and coldest (high-mass)
neutron stars is calculated, together with the ranges of its possible
variations due to variable thickness of the helium layer. The results are
compared with observations of SXTs, particularly, containing the coldest (SAX
J1808.4-3658) and the hottest (Aql X-1) neutron stars. The observations of SAX
J1808.4-3658 in a quiescent state on March 24, 2001 (Campana et al. 2002,
astro-ph/0206376) can be explained only if this SXT contains a massive neutron
star with a nucleon/hyperon core; a hyperon core with a not too low fraction of
electrons is preferable. Future observations may discriminate between the
various models of hyperon/nucleon dense matter. The thermal emission of SAX
J1808.4-3658 is also sensitive to the models of plasma ionization in the
outermost surface layers and can serve for testing such models.Comment: 12 pages, 5 figures, 4 tables, LaTeX2e with aa.cls v.5.3 (included).
Accepted by A&
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