Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...525..386l&link_type=abstract
The Astrophysical Journal, Volume 525, Issue 1, pp. 386-398.
Astronomy and Astrophysics
Astronomy
1
Accretion, Accretion Disks, Stars: Magnetic Fields, Stars: Neutron
Scientific paper
We compute steady and transonic disk accretion flows (slim disks) around a weakly magnetized neutron star, where the effects of the magnetic stress and the Joule heating due to the dipole field are included in the angular momentum conservation and the energy conservation equations, respectively. In this calculation, we also included the magnetic pressure effect in the vertical hydrostatic equation. A nondimensional parameter ξ=ζ(B0/107 G)2(M/Mc)-1, although the method of normalizing B0 and M is somewhat arbitrary, is found to be a good parameter to determine the angular momentum distribution in the disk flow near the star, where M and B0 are the mass accretion rate (in grams per second) and the magnetic dipole field strength (in gauss) at the surface of the star, Mc=3.11x1018 g s-1 is the critical mass accretion rate, and ζ is the magnetic Reynolds number, assumed to be of the order of unity. In general, the critical point of the disk flow moves outward, as ξ increases, as a result of the angular momentum extraction from the disk in a strong gravitational field. This outward shift of the critical point as ξ increases is limited by the corotation radius, which depends on the spin rate of the central star. The Joule heating becomes effective to keep the disk temperature inside the critical point high for large values of ξ. The region inside the critical point becomes optically thin for small mass accretion rates M~0.01Mc when ξ>~104. We find that a net amount of angular momentum can be given to or removed from the spinning neutron star, depending on the value of the parameter ξ, as a result of the magnetic interaction between the slim disk and the star.
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