Prediction of stellar mass in star formation: Theory and its application to the orion a cloud
We have developed a way to estimate the mass of a star from the physical quantities of the cloud core in which it forms. Because matter falls onto a stellar core mainly through an accretion disk, a significant fraction of the mass outflow and ultraviolet radiation escapes from the inner region without interacting with the infalling matter and disturbs the cloud core matter which has not yet contracted much. A highvelocity mass outflow gradually pushes out the surrounding matter forming a thin dense shell. On the other hand, as the central star grows, a compact HII region develops and pushes out the surrounding matter also forming a thin dense shell. A considerable fraction of the initial core matter is blown off when the radius of the bubble or the HII region inside the shell grows to the initial radius of the cloud core, and the remaining matter disperses because it is no longer gravitationally bound. In this way the supply of matter from the cloud core to the disk stops. Consequently accretion onto the star also stops, and the mass of the forming star is fixed. The stellar mass determined in this way is a function of the core density n c, the mass inflow rate M I, M o/M I, and M c/M j, or a function of n c, M c, M o/M I, and M c/M J, where M o is the mass outflow rate, M c is the cloud core mass, and M j is the generalized Jeans mass for the core. When the mass outflow is dominant, we have M * α M c 7/6 n c 1/12 with the proportionality coefficient dependent on M o/M I, and thus the stellar mass is almost independent of n c. Therefore, the star formation efficiency M */M c is mainly determined by M o/M I and is only weakly dependent on the core parameters M c and n c; we obtain M */M c≈0.04 for M o/M I=0.1 around M c≈100M ⊙. Applying this result to the observed cloud cores in the Orion A molecular cloud by assuming that each core does not contain subclumps we estimate the stellar mass and the initial mass function of stars (IMF) expected in this cloud. As long as M o/M I≳0.02, the mass outflow is more efficient than the HII region in determining the stellar mass for all the Orion A cores. The IMF at M *≳4M ⊙ can be approximated by a power law dN */dlogM *αM * −1.7 for M o/M I=0.1, which is in reasonable agreement with the IMF of field stars α M * −1.5 at M *≳3M ⊙.
KeywordsStar Formation Accretion Disk Molecular Cloud Stellar Mass Core Mass
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