The Luminosity Functions and Timescales of MYSOs and Compact HII regions

Abstract

We present a determination of the luminosity functions of massive young stellar objects (MYSOs) and compact (C)HII regions within the Milky Way Galaxy using the large, well-selected sample of these sources identified by the Red MSX Source (RMS) survey. The MYSO luminosity function decreases monotonically such that there are few with Lgtrsim 10 5Lsol, whilst the CHII regions are detected up to ~10 6Lsol. 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 5 yrs for 10 4Lsol to ~7x10 4 yrs at 10 5Lsol, whilst the CHII region phase lasts of order 3x10 5 yrs or ~3-10% of the exciting star's main-sequence lifetime. MYSOs between 10 4 Lsol and ~10 5 Lsol are massive but do not display the radio continuum or near-IR HI recombination line emission indicative of an HII region, consistent with being swollen due to high ongoing or recent accretion rates. Above ~10 5 Lsol 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 ionise a CHII region, thus explaining why few highly luminous MYSOs are observed.