Structure and Stability of Steady Protostellar Accretion Flows - Part Three - Nonlinear Instabilities

Details Structure and Stability of Steady Protostellar Accretion Flows - Part Three - Nonlinear Instabilities Structure and Stability of Steady Protostellar Accretion Flows - Part Three - Nonlinear Instabilities

Mar 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991a%26a...243..205b&link_type=abstract

Astronomy and Astrophysics, Vol.243, NO. 1/MAR, P. 205, 1991

Astronomy and Astrophysics

Astrophysics

11

Star Formation, Protostellar Accretion, Radiation Hydrodynamics

Scientific paper

Recently, a global, linear stability analysis of the structure of spherically symmetric steady protostellar accretion flows with a shock discontinuity has been made (Balluch 1990b). A detailed non-linear time-dependent radiation hydrodynamics calculation is presented to complement this study.
In the `ideal-case' with constant opacity, the shock front around the second, inner core appears unstable with respect to oscillation due to critical cooling, starting at the instant, when a cooling region occurs in the calculation (due to its resolution in the late stages of accretion) and lasting as long as the mass flow rate is larger than M ≥ 2.8 10-6 Msun yr-1. This is in best agreement with the results of the linear analysis.
Next, a detailed calculation of the formation of the first, outer core using quasimolecular artificial viscosity length scales, is presented. In about twice the e-folding time of the unstable mode in the linear analysis, a significant growth of a disturbance can be seen. It appears first in the velocity and the radiation flux in the settling zone, accompanied by an oscillation of the radiation flux in the region upstream from the shock up to r = 1014 cm. Some time later, the shock front starts to move. Again, these characteristics are in best concordance with the linear results. In addition, the calculation shows the growth of these oscillations deep in the non-linear regime until the beginning of a rapid expansion of the whole protostellar core.
At last, a calculation of the global evolution of this expansion of the first, outer core is presented. It is shown that the expansion is stopped when about twice the core mass is involved. Then another collapse follows, and the whole scenario of formation and expansion of the outer core starts anew. During this evolution, up to 3 shock fronts were present at the same time in the flow. The largest expansion leads to central physical quantities comparable to the initial ones of the interstellar medium. At least 90% of the total mass (i.e. 1 Msun) continuously falls in the direction of the center, leading to a global time evolution of these large scale oscillations. After 8 oscillations, the disturbance in the outer core develops as usual, but the core collapses to an inner core before another expansion can occur.

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