Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997aas...19111302w&link_type=abstract
American Astronomical Society, 191st AAS Meeting, #113.02; Bulletin of the American Astronomical Society, Vol. 29, p.1395
Astronomy and Astrophysics
Astronomy
Scientific paper
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 (<< 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_sun 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: (1) 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 ~ few years ( ~ flow time from the accretion radius at ~ 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.
Sutherland Ralph S.
Wang John C. L.
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