The Luminosity Functions and Timescales of MYSOs and Compact HII regions

Joseph~C.~Mottram[1,2], Melvin~G.~Hoare[2], Ben~Davies[5,2], Stuart~L.~Lumsden[2], Rene~D.~Oudmaijer[2], James~S.~Urquhart[3], Toby~J.~T.~Moore[4], Heather~D.B.~Cooper[2], Joseph~J.~Stead[2]

1: School of Physics, University of Exeter, Exeter, Devon, EX4 4QL, UK
2: School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
3: Australia Telescope National Facility, CSIRO Astronomy and Space Science, Sydney, NSW 2052, Australia
4: Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK
5: Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623, USA

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 $Lgtrsim 10^{5}$Lsol, whilst the CHII regions are detected up to ~10$^{6}Lsol. The lifetimes of these phases are also calculated as a function of luminosity by comparison with the luminosity function for local main-sequence OB stars. These indicate that the MYSO phase has a duration ranging from 4x10$^{5}$ yrs for 10$^{4}$Lsol to ~7x10$^{4}$ yrs at 10$^{5}$Lsol, 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.

Reference: accepted to ApJL
Status: Manuscript has been accepted


Comments: 16 pages in pre-print format, 4 figures, 1 table