High velocities and cocoon stars

Abstract

LARGE scale gas movements with velocities of the order of 50 km s⁻¹ giving rise to split, asymmetric or broadened emission line profiles have been observed in many H II regions^1–4. Proposed explanations for these motions include supernova explosions within or near to the H II regions⁵, and the acceleration of their ionised material by relativistic particles⁶ or by massive stellar winds^7–9.I suggest that the breakup of the neutral gas and dust shell around a cocoon star could conceivably also produce high velocities. The existence of cocoon stars seems fairly well established from observations of infrared point sources^10,11 and these are frequently associated with compact H II regions¹² and OH masers. Kahn¹³ investigated the evolution of a cocoon star and found that a star which accretes matter will form a false photosphere with a radius of the order 10¹³ cm and no appreciable H II region, provided that the rate of accretion exceeds approximately 10²⁰ g s⁻¹. Around the false photosphere is a dust-free region with a radius of about 10¹⁶ cm, which corresponds to the radius at which refractory particles melt. At accretion rates of less than 10²² g s⁻¹ the radiation pressure on the dust becomes powerful enough to stop the infall of matter and to drive a shock wave into the accreting gas flow outside a radius of 10¹⁶ cm. This situation is Rayleigh–Taylor unstable, and the shell will break up into globules. Consider a portion of neutral material with a density of M g cm⁻², at a distance, R, from a star with mass, M_*, and luminosity L_* (M_* ≈ 6 × 10³⁴ g, L_* ≈ 6 × 10³⁸ erg s⁻¹). If the radiation force just balances the gravitational force exerted by the star, then Thus, M ≈ 0.7 g cm⁻² independent of distance. This is very close to the surface density required by the models of Dyson¹⁴ (M ≃ 2 g cm⁻²) for which a great deal of observational evidence^15,16 has been collected, particularly in the case of M8 and M42. It is thus possible that some of the globules represent the material that was just balanced by the radiation—gravitational forces, at the time of the breakup of the cocoon.