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

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 H II 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 sola r circle, are inferred from their fluxes in the four IRAS bands and from th eir kinematic distances, derived using the CS(2-1) velocity profiles. The l uminosity function (LF) for the UCH II 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 210(5) L., outside the solar circle the maximum is at 5 10(4) L .. We model the LF using three free parameters: - alpha, the exponent for t he initial mass function (IMF) expressed in log(M/M.); -beta, the exponent for a power law distribution in N*, the number of stars per MSFR; and N*(ma x), an upper limit for N*. While alpha has a value of similar to 2.0 throug hout the Galaxy, beta changes from similar to 0.5 inside the solar circle t o similar to 0.7 outside, with a maximum for the number of stars per MSFR o f similar to 650 and similar to 450 (with 1 less than or equal to M/M. less than or equal to 120). Thus the IMF appears not to vary, although the aver age number of stars per MSFR within the solar circle is higher than for the outer Galaxy.