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The Accretion of Infalling Gas by a Protobinary System

[Accretion Image]

Bate & Bonnell (1997)

Matthew Bate studies star formation. One aspect of star formation is the formation of binary and higher-order multiple stellar systems. Most stars in the galaxy are not alone, like the Sun, but instead are members of binary stellar systems. These are thought to form via the fragmentation of gaseous molecular cloud cores as they collapse under their own gravity1.

When a collapsing molecular cloud core first fragments to form a multiple system, the fraction of the total gas mass that is contained in the protostars is very low (typically from 1-10 percent). The protostars grow to their final mass by accreting the gas that falls on to the system from the rest of the cloud core. To follow the formation of such a multiple stellar system from the beginning of the collapse until all of the infalling gas has been accreted is exceedingly difficult because of the enormous range in densities and the high resolution that is required to get accurate results. However, if the evolution of the system as it accretes is not followed and calculations are stopped as a protostellar system is formed, then it is impossible to compare the results of such calculations to observed stellar systems. This is because the accretion of so much gas radically changes the masses of the stars and the orbits they had when they first formed.

Rather than attempting to calculate the whole problem at once, in the past Matthew Bate has investigated the process of the accretion of the infalling gas on to a binary protostellar system2,3 using the Smoothed Particle Hydrodynamics (SPH)4,5 technique. For a given protobinary system, it is essential to know which of the two protostars accretes most of the material, how the orbit of the binary is altered, and what type of discs are formed around the protostars (since these are essential if planets are to be formed).

The accretion of infalling gas by a protobinary system with mass ratio q=0.6 is shown in the 6 figures above. In each figure the binary system is the same (with the most massive star, the primary, on the right), but the specific angular momentum of the infalling gas differs (increasing to the right and downwards). The destination of the gas depends on its specific angular momentum relative to the binary. For gas with low specific angular momentum, most is captured by the primary in a small circumstellar disc. There is no circumstellar disc formed around the secondary, and no circumbinary disc. For gas with higher specific angular momentum, the radius of the primary's disc is larger and a circumsecondary disc may also be formed. For gas with even higher specific angular momentum, not all of the gas is captured by the individual components; some also forms a circumbinary disc. The secondary also captures more gas than the primary, equalising the masses of the protostars. Finally, if the infalling gas has too much angular momentum it cannot form circumstellar discs and all goes into a circumbinary disc. For the orbit of the binary, when low specific angular momentum gas is accreted, the two stars get closer, while the accretion of high specific angular momentum gas increases the binary's separation.

The goal of this work was to use this knowledge about how a protobinary system evolves under accretion to help constrain models of star formation. The results were published in 2,3,6.

1) Burkert A., Bate, M.R., Bodenheimer P. 1997, MNRAS, 289, 497-504
2) Bate, M.R. 1997, MNRAS, 285, 16-32
3) Bate, M.R., Bonnell I.A. 1997, MNRAS, 285, 33-48
4) Benz, W.: 1990, in "The Numerical Modeling of Nonlinear Stellar Pulsations", ed. J.R. Buchler, Kluwer, Dordrecht, p.269
5) Bate, M.R., Bonnell, I.A, Price, N.M. 1995, MNRAS, 277, 362-376
6) Bate, M.R. 2000, MNRAS, 314, 33

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