Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...450..262f&link_type=abstract
Astrophysical Journal v.450, p.262
Astronomy and Astrophysics
Astronomy
3
Accretion, Accretion Disks, Stars: Mass Loss, Stars: Pre-Main-Sequence, Stars: Rotation, Stars: White Dwarfs
Scientific paper
The mechanical and thermal structure of accretion flows interacting with the central stars via viscous stress is studied by modeling the spatial variation of dynamical viscosity with a simple one-parameter function. The latter assumption has the advantage of yielding the analytical solution of rotation laws, and the thermal balance near the disk-star interface is analyzed by means of a one-zone approximation. Based on these results, a general picture of the evolutionary changes in the accretion process is presented with the spin history of central stars taken into account. Under a constant accretion rate, there exists the maximum rotation rate of the central star which allows the structure of accretion flow in hydrostatic and thermal equilibrium; it is smaller for larger accretion rates and occurs when the inflow rate of angular momentum decreases to the critical value Jdottrn, which takes the values around zero or larger, depending on the physical conditions in the flows. The equilibrium configurations with both the positive and negative inflow rate of angular momentum, as discussed by Paczynski and by Popham & Narayan are possible, in which the stellar rotation rate is no longer accelerated, and hence, the structure of accretion flow remains the same. It is shown, however, that these configurations exchange their thermal stability at this maximum rotation rate; in particular, the structures of mass accretion with the inflow rate of angular momentum below Jdottrn is secularly unstable, and hence, they cannot be realized in the course of the evolution.
The evolution of the systems differs according to the stellar response to the mass accretion; corresponding to Jdottrn, there exists the critical value, Scri (≃ -3 or larger), for the changing rate of the stellar radius 5 = d log R*(t)/d log M*(t)(M* and R* being the mass and equator radius of the central star, respectively). If the central star expands or shrinks moderately during accreting mass as 5 ≥ Scri, the system will settle in the equilibrium configuration before the maximum rotation rate is reached and continue to accrete any amount of matter with the inflow rate of angular momentum greater than Jdottrn. If on the other hand, the radius shrinks so rapidly that S < Scri, the star will be spun-up through this maximum rotation rate; beyond this stage, the thermal imbalance persists near the interface under the hydrostatic equilibrium, owing mainly to the dissipation of kinetic energy, extracted from the shrinking central star. The latter is the case for the massive white dwarfs and possibly for the protostars contracting along and off the Hayashi phase, and may be relevant to the origin of collimated outflows, as observed from the systems including these central stars.
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