All-Moscow Seminar of Astrophysicists
(ASA)
The seminar is supported by the grant of Federal Programm "Integration"
annotation of talk
Bisnovatyi-Kogan G.S. (SRI)
ADAF is dead (?), but Advection is alive
Recent papers discussing advection-dominated accretion flows (ADAF) as a
solution for astrophysical accretion
problems should be treated with some caution because of their uncertain
physical basis. The suggestions
underlying ADAF involve ignoring the magnetic field reconnection in heating
of the plasma flow, assuming
electron heating due only to binary Coulomb collisions with ions. Here we
analyze the physical processes in
optically thin accretion flows at low accretion rates including the
influence of an equipartition random magnetic
field and heating of electrons due to magnetic field reconnection. The
important role of the magnetic field pointed
out by Shvartsman comes about because the magnetic energy density,
Em,
increases more rapidly with decreasing
distance than the kinetic energy density, Ek
(or thermal energy density).
Once Em grows to a value of order
Ek,
further accretion to smaller distances is possible only if magnetic flux is
destroyed by reconnection. For the smaller
distances it is likely that there is approximate equipartition,
Em=Ek.
Dissipation of magnetic energy is associated
with the destruction of magnetic flux. We discuss reasons for believing that
the field annihilation leads to
appreciable electron heating. Such heating significantly restricts the
applicability of ADAF solutions, and it leads
to a radiative efficiency of the flows of 25% of the standard accretion
disk value.