8 research outputs found
A Two-Dimensional Hydrostatically Equilibrium Atmosphere of a Neutron Star with Given Differential Rotation
An analytic solution has been found in the Roche approximation for the
axially symmetric structure of a hydrostatically equilibrium atmosphere of a
neutron star produced by collapse. A hydrodynamic (quasione-dimensional) model
for the collapse of a rotating iron core in a massive star gives rise to a
heterogeneous rotating protoneutron star with an extended atmosphere composed
of matter from the outer part of the iron core with differential rotation
(Imshennik and Nadyozhin, 1992). The equation of state of a completely
degenerate iron gas with an arbitrary degree of relativity is taken for the
atmospheric matter. We construct a family of toroidal model atmospheres with
total masses and total angular momenta , which are acceptable for the
outer part of the collapsed iron core, in accordance with the hydrodynamic
model, as a function of constant parameters of the
specified differential rotation law in spherical
coordinates. The assumed rotation law is also qualitatively consistent with the
hydrodynamic model for the collapse of an iron core.Comment: 9 pages, 6 figures, 1 tabl
The Possibility of Emersion of the Outer Layers in a Massive Star Simultaneously with Iron-Core Collapse: A Hydrodynamic Model
We analyze the behavior of the outer envelope in a massive star during and
after the collapse of its iron core into a protoneutron star (PNS) in terms of
the equations of one-dimensional spherically symmetric ideal hydrodynamics. The
profiles obtained in the studies of the evolution of massive stars up to the
final stages of their existence, immediately before a supernova explosion
(Boyes et al. 1999), are used as the initial data for the distribution of
thermodynamic quantities in the envelope.We use a complex equation of state for
matter with allowances made for arbitrary electron degeneracy and relativity,
the appearance of electron-positron pairs, the presence of radiation, and the
possibility of iron nuclei dissociating into free nucleons and helium nuclei.
We performed calculations with the help of a numerical scheme based on
Godunov's method. These calculations allowed us to ascertain whether the
emersion of the outer envelope in a massive star is possible through the
following two mechanisms: first, the decrease in the gravitational mass of the
central PNS through neutrino-signal emission and, second, the effect of hot
nucleon bubbles, which are most likely formed in the PNS corona, on the
envelope emersion. We show that the second mechanism is highly efficient in the
range of acceptable masses of the nucleon bubbles ()
simulated in our hydrodynamic calculations in a rough, spherically symmetric
approximation.Comment: 14 pages, 11 figure
A hydrodynamic model for asymmetric explosions of rapidly rotating collapsing supernovae with a toroidal atmosphere
We numerically solved the two-dimensional axisymmetric hydrodynamic problem
of the explosion of a low-mass neutron star in a circular orbit. In the initial
conditions, we assumed a nonuniform density distribution in the space
surrounding the collapsed iron core in the form of a stationary toroidal
atmosphere that was previously predicted analytically and computed numerically.
The configuration of the exploded neutron star itself was modeled by a
torus with a circular cross section whose central line almost coincided with
its circular orbit. Using an equation of state for the stellar matter and the
toroidal atmosphere in which the nuclear statistical equilibrium conditions
were satisfied, we performed a series of numerical calculations that showed the
propagation of a strong divergent shock wave with a total energy of 0.2x10^51
erg at initial explosion energy release of 1.0x10^51 erg. In our calculations,
we rigorously took into account the gravitational interaction, including the
attraction from a higher-mass (1.9M_solar) neutron star located at the
coordinate origin, in accordance with the rotational explosion mechanism for
collapsing supernovae.W e compared in detail our results with previous similar
results of asymmetric supernova explosion simulations and concluded that we
found a lower limit for the total explosion energy.Comment: 13 pages, 5 figures, 2 table
The Toroidal Iron Atmosphere of a Protoneutron Star: Numerical Solution
A numerical method presented by Imshennik et al. (2002) is used to solve the
two dimensional axisymmetric hydrodynamic problem on the formation of a
toroidal atmosphere during the collapse of an iron stellar core and outer
stellar layers. An evolutionary model from Boyes et al. (1999) with a total
mass of is used as the initial data for the distribution of
thermodynamic quantities in the outer shells of a high-mass star. We analyze in
detail the results of three calculations in which the difference mesh and the
location of the inner boundary of the computational region are varied. In the
initial data, we roughly specify an angular velocity distribution that is
actually justified by the final result - the formation of a hydrostatic
equilibrium toroidal atmosphere with reasonable total mass, , and total angular momentum, , for the two main calculations. We compare the
numerical solution with our previous analytical solution in the form of
toroidal atmospheres (Imshennik and Manukovskii 2000). This comparison
indicates that they are identical if we take into account the more general and
complex equation of state with a nonzero temperature and self-gravitation
effects in the atmosphere. Our numerical calculations, first, prove the
stability of toroidal atmospheres on characteristic hydrodynamic time scales
and, second, show the possibility of sporadic fragmentation of these
atmospheres even after a hydrodynamic equilibrium is established. The
calculations were carried out under the assumption of equatorial symmetry of
the problem and up to relatively long time scales .Comment: 15 pages, 12 figures, 3 table