6. Lifetime of UCH II regions
UCH II regions are fairly short lived in comparison with giant H II regions. WC89a estimated the lifetimes of UCH II regions by comparing the number of O stars in the solar neighbourhood (Conti et al. 1983) and the number of IRAS point sources that match the IRAS colours of UCH II regions. They conclude that 10% to 20% of an O star's main sequence lifetime is spent embedded in a molecular cloud, in the UCH II phase. This fraction has been reevaluated to only 0.5% by CT96, and our estimate is (see below). Under the assumption that the luminosity of a MSFR is dominated by the most massive star, CT96 show that if there is a variation of UCH II lifetime with stellar mass, it is not as significant as the uncertainties in the IMF. The crossing time at 10 km s-1 is 104 yrs for a typical UCH II region (i.e. G34.3+0.2, which at a distance of 3.7 kpc is about cm). Thus the margin between the dynamical timescale and the time O stars spend in the embedded phase of yrs is narrow, and can be affected by many parameters other than the mass of the most massive star, such as clumpiness of the molecular clouds and relative motions between the exciting star and the molecular clumps. It is unlikely that the lifetimes of UCH II regions depend strongly on their stellar contents.
From our synthetic population of MSFRs, we find that the average number of O stars per MSFR, with luminosities in excess of ), is 0.55 for and 0.29 for - we will consider 0.5 O stars per IRAS/CS source. Within 2.5 kpc of the Sun, Conti et al. (1983) report 436 O stars with . We have 15 IRAS/CS sources above the luminosity limit ) (counting all sources, outside the cuts in longitude described in Sect. 2 but including the sources with ). The average fraction of lifetime spent in the embedded phase would thus be . Alternatively, counting the IRAS/CS sources with galactocentric radii within 0.9 to , we have 40 sources more luminous than ), distributed over an area of 80.5 kpc2 (BCMN). This would give 9.8 IRAS/CS sources within 2.5 kpc of the Sun. The average fraction of lifetime O stars spend in the embedded phase would thus be . This latter approach has the advantage of not suffering as much from the peculiar velocities of local sources. But some O stars in the transition between the embedded UCH II phase and the field stars could have been missed by Conti et al. (1983). Therefore we estimate that O stars spend of their lifetime in the embedded phase.
© European Southern Observatory (ESO) 2000
Online publication: June 8, 2000