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Session 64 - Disks and Bipolar Outflows from Hot Stars - II.
Oral session, Wednesday, June 12
Union Theater,
The process for the formation of a hot massive star is likely to be quite different from that of a low mass star, because both the luminosity and wind momentum are much larger for the high mass star. In the case of the massive stars the core is shrouded by the thick protostellar cloud so we cannot directly observe the central object during the relatively brief accretion phase. However, theoretically we expect the core to be undergoing nuclear burning during the late phases of the accretion and to have radiation and wind properties like those of observed hot stars. The radiation radiation field should evaporate dust in a central region giving rise to a cocoon with an inner radius of order 10^16 cm. In one-dimensional spherical models the stellar radiation is degraded to IR radiation at the inner boundary of the cocoon. The transfer of this light through the infalling material can impede the inflow of dust grains, and because of the coupling between the grains and gas, halt the accretion process. Such studies indicate that there is a theoretical maximal mass limit, which is very sensitive to the opacity and metal abundance of the protostellar cloud. The upper mass limit is larger for clouds with lower metal abundance. Do the most massive stars in galaxies tend to lie in the metal poor regions? or are the spherical star formation models misleading in this regard? Two-dimensional models account for rotational effects and predict that the infalling material forms a equatorial disk at distances beyond the cocoon inner boundary. Significant mass accretion onto the star may then occur via the disk. However, matter does not enter such a disk is is likely to be inhomogeneous, and it may penetrate the cocoon boundary in the form of infalling blobs. The inward motion of these can then be affected by the loss of angular momentum arising from interaction with the stellar wind. That being the case, we ask whether accretion disks are necessary during the final growth phases of massive star formation? The winds and ionizing radiation may also play an important role in the outward appearance of a newly formed massive star through the production of an ultra-compact HII region.
