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**Session 113 - Neutron Stars & Pulsars.**

*Oral session, Saturday, January 10*

*International Ballroom East, *

## [113.02] Highly sub-Eddington Spherical Accretion onto Isolated Neutron Stars

*J. C. L. Wang (U. Maryland), R. S. Sutherland (ANU/MSSSO)*
Highly sub-Eddington accretion may be a commonly
occurring process among isolated old neutron stars
which have spun down sufficiently to start accreting
interstellar gas. We study this type of accretion
in a spherically symmetric setting, which is
applicable to stars moving at very subsonic speeds
(\ll 10 km/s) with respect to the ambient medium.
We devised an iterative scheme to couple the accretion
flow dynamics with the requisite nonequilibrium
atomic processes. The dynamics are treated using
the standard spherical flow equations while the
energetics and ionization are treated using the
MAPPINGS II photoionization code. Adopting a neutron
star with M=1.4 M_ødot and R=10 km, we attempt
to find self-consistent solutions for given choices
of the mass accretion rate (\dot M) and ionizing
spectrum.

We have two main results:

Steady state solutions are possible only for
10^9 < \dot M\ (g/s) < 10^10, or, medium
densities 0.05 < n_\infty\ (cm^-3) < 0.5.
For reference, \dot M_Edd=10^18 g/s. Outside
this regime, variability is expected on timescales
\sim few years (\sim flow time from the accretion
radius at \sim 10^13--10^14 cm).

(2) For the steady state solutions, \dot M obeys
the Bondi formula (for adiabatic accretion) to a good
approximation.

We explore the implications of our results for the
two candidate accreting isolated neutron stars,
RX J1856-3754 and RX J0720-3125.

**Program
listing for Saturday**