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EPoS |
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EPoS Contribution
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Theoretical Considerations of Massive Star Formation
Harold Yorke JPL - Caltech, Pasadena, CA, USA | |
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Sufficiently massive molecular clumps will collapse to produce high mass stars, but the evolution is not a straightforward scaled-up version of low mass star formation. Outflows, pressure and radiative effects strongly influence but do not prevent the growth of massive stars via accretion from an initially low mass object. While accreting, stars evolve toward the main sequence and - for stars more massive than about 15 Msun - begin core hydrogen burning while still accreting material. A massive main sequence star - whether accreting or not - simultaneously photoevaporates its circumstellar disk and nearby disks on a timescale of ~100,000 yr. Photoevaporating disks are first observable in the radio even while the ionizing star accretes material through its disk as hypercompact HII regions (HCHII). The final mass of the central star and nearby neighboring systems is determined by the interplay between radiation acceleration, UV photoevaporation, stellar winds and outflows, and details of the accretion process.
The evolutionary sequence for high mass star formation is: from 1) cold molecular core to 2) hot molecular core to 3) deeply embedded HCHIIs. 4) Subsequently, the HCHIIs evolve and perhaps merge into ultra-compact HII (UCHII) and compact HII regions. The final result is 5) a cluster of OB stars, an OB association, or possibly an "isolated" massive star, depending on the detailed initial conditions. | |