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The collapse of a suciently massive molecular clump can in principal
produce one or more high mass stars, but compared to the total number
of stars produced high mass stars are the rare exception and not the
rule. Nevertheless, when one or more high mass stars form, they dominate
the evolution of the parent molecular cloud and control subsequent
star formation through their winds, ionizing radiation and, ultimately,
supernova explosions. In spite of their importance to star formation, the
production of heavy elements and the overall evolution of galaxies, our
understanding of high mass star formation is rather sketchy. The process
of forming a massive star is not a straightforward scaled-up version
of low mass star formation. Massive stars seldom, if ever, form individually;
multiple systems, clusters and associations are the general rule.
Outflows, pressure and radiative e cts from multiple sources strongly
influence but do not prevent the formation of massive stars via accretion.
Accretion growth of an initially low mass object up to high masses is
possible through a circumstellar disk. This requires high accretion rates
onto the disk and through the disk onto central star of the order of or
greater than 10^(-4) M_sun/yr. Central hydrogen burning begins while the
young massive star continues to accrete material and it simultaneously
photoevaporates its circumstellar disk and nearby disks on a timescale
of 10^5 yr. 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.
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