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 $L\gtrsim 10^5$Lsol, whilst the CHII regions are detected up to ~10${}^{6}Lsol.Thelifetimesofthesephasesarealsocalculatedasafunctionofluminositybycomparisonwiththeluminosityfunctionforlocalmain-sequenceOBstars.TheseindicatethattheMYSOphasehasadurationrangingfrom4x10$^{5}$yrsfor10$^{4}$Lsolto7x10$^{4}$yrsat10$^{5}$Lsol,whilsttheCHIIregionphaselastsoforder3x10$^{5}$yrsor3-10$^{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.

16 pages in pre-print format, 4 figures, 1 table, accepted to ApJL

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