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

Abstract

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≳105 L☉, while the CH ii regions are detected up to ∼106 L☉. 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 4×105 yr for 104 L☉ to ∼7×104 yr at 105 L☉, while the CH ii region phase lasts of order 3×105 yr or ∼3%–10% of the exciting star's main-sequence lifetime. MYSOs between 104 L☉ and ∼105 L☉ 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 ∼105 L☉ the MYSO phase lifetime becomes comparable to the main-sequence Kelvin–Helmholtz 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.