90 research outputs found
In-medium enhancement of the modified Urca neutrino reaction rates
We calculate modified Urca neutrino emission rates in the dense nuclear
matter in neutron star cores. We find that these rates are strongly enhanced in
the beta-stable matter in regions of the core close to the direct Urca process
threshold. This enhancement can be tracked to the use of the in-medium nucleon
spectrum in the virtual nucleon propagator. We describe the in-medium nucleon
scattering in the non-relativistic Bruckner-Hartree-Fock framework taking into
account two-body as well as the effective three-body forces, although the
proposed enhancement does not rely on a particular way of the nucleon
interaction treatment. Finally we suggest a simple approximate expression for
the emissivity of the n-branch of the modified Urca process that can be used in
the neutron stars cooling simulations with any nucleon equation of state of
dense matter.Comment: 8 pages, 3 figures; accepted for publication in PLB. In v.2 misprint
in eq.(9) corrected and discussion of cooling curves expande
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
Shear viscosity in neutron star cores
We calculate the shear viscosity in a neutron
star core composed of nucleons, electrons and muons ( being the
electron-muon viscosity, mediated by collisions of electrons and muons with
charged particles, and the neutron viscosity, mediated by
neutron-neutron and neutron-proton collisions). Deriving , we take
into account the Landau damping in collisions of electrons and muons with
charged particles via the exchange of transverse plasmons. It lowers
and leads to the non-standard temperature behavior
. The viscosity is calculated taking
into account that in-medium effects modify nucleon effective masses in dense
matter. Both viscosities, and , can be important, and
both are calculated including the effects of proton superfluidity. They are
presented in the form valid for any equation of state of nucleon dense matter.
We analyze the density and temperature dependence of for different
equations of state in neutron star cores, and compare with the bulk
viscosity in the core and with the shear viscosity in the crust.Comment: 22 pages, 7 figures, Phys. Rev. D., accepted. In v.2 typos and two
refs. correcte
Fusion of neutron rich oxygen isotopes in the crust of accreting neutron stars
Fusion reactions in the crust of an accreting neutron star are an important
source of heat, and the depth at which these reactions occur is important for
determining the temperature profile of the star. Fusion reactions depend
strongly on the nuclear charge . Nuclei with can fuse at low
densities in a liquid ocean. However, nuclei with Z=8 or 10 may not burn until
higher densities where the crust is solid and electron capture has made the
nuclei neutron rich. We calculate the factor for fusion reactions of
neutron rich nuclei including O + O and Ne + Ne. We
use a simple barrier penetration model. The factor could be further
enhanced by dynamical effects involving the neutron rich skin. This possible
enhancement in should be studied in the laboratory with neutron rich
radioactive beams. We model the structure of the crust with molecular dynamics
simulations. We find that the crust of accreting neutron stars may contain
micro-crystals or regions of phase separation. Nevertheless, the screening
factors that we determine for the enhancement of the rate of thermonuclear
reactions are insensitive to these features. Finally, we calculate the rate of
thermonuclear O + O fusion and find that O should burn at
densities near g/cm. The energy released from this and similar
reactions may be important for the temperature profile of the star.Comment: 7 pages, 4 figs, minor changes, to be published in Phys. Rev.
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