The luminosity function of galactic ultra-compact HII regions and the IMF for massive stars

Abstract

The population of newly formed massive stars, while still embedded in their parent molecular clouds, is studied on the galactic disk scale. We analyse the luminosity function of IRAS point-like sources, with far-infrared (FIR) colours of ultra-compact HII regions, that have been detected in the CS(2-1) line - a tracer of high density molecular gas. The FIR luminosities of 555 massive star forming regions (MSFRs), 413 of which lie within the solar circle, are inferred from their fluxes in the four IRAS bands and from their kinematic distances, derived using the CS(2-1) velocity profiles. The luminosity function (LF) for the UCHII region candidates shows a peak well above the completeness limit, and is different within and outside the solar circle (96% confidence level). While within the solar circle the LF has a maximum for 2E5 Lo, outside the solar circle the maximum is at 5E4 Lo. We model the LF using three free parameters: -alpha, the exponent for the initial mass function (IMF) expressed in log(M/Mo); -beta, the exponent for a power law distribution in N*, the number of stars per MSFR; and N*max, an upper limit for N*. While alpha has a value of \~ 2.0 throughout the Galaxy, beta changes from ~ 0.5 inside the solar circle to \~ 0.7 outside, with a maximum for the number of stars per MSFR of ~650 and \~450 (with 1 <M/Mo< 120). While the IMF appears not to vary, the average number of stars per MSFR within the solar circle is higher than for the outer Galaxy.