THE RMS SURVEY: THE LUMINOSITY FUNCTIONS AND TIMESCALES OF MASSIVE YOUNG STELLAR OBJECTS AND COMPACT H II REGIONS

We present a determination of the luminosity functions of massive young stellar objects (MYSOs) and compact (C) H II regions within the Milky Way using the large, well-selected sample of these sources identified by the Red MSX Source survey. The MYSO luminosity function decreases monotonically such that there are few with L{approx}>10{sup 5} L{sub sun}, while the CH II regions are detected up to {approx}10{sup 6} L{sub sun}. The lifetimes of these phases are also calculated as a function of luminosity by comparison with the luminosity function for local main-sequence OB stars. These indicate that the MYSO phase has a duration ranging from 4x10{sup 5} yr for 10{sup 4} L{sub sun} to {approx}7x10{sup 4} yr at 10{sup 5} L{sub sun}, while the CH II region phase lasts of order 3x10{sup 5} yr or {approx}3%-10% of the exciting star's main-sequence lifetime. MYSOs between 10{sup 4} L{sub sun} and {approx}10{sup 5} L{sub sun} are massive but do not display the radio continuum or near-IR H I recombination line emission indicative of an H II region, consistent with being swollen due to high ongoing or recent accretion rates. Above {approx}10{sup 5} L{sub sun} the MYSO phase lifetime becomes comparable to the main-sequence Kelvin-Helmholtzmore » timescale, at which point the central star can rapidly contract onto the main sequence even if still accreting, and ionize a CH II region, thus explaining why few highly luminous MYSOs are observed.« less