I. the Stellar Initial Mass Function

o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o oo o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o We summarize recent bservati nal and the retical pr gress aimed at understanding the rigin f the stellar initial mass functi n (IMF), with specific f cus n galactic star-f rming regi ns. We synthesize data fr m vari us eff rts t determine the IMF in very y ung, partially embedded stellar clusters and find that (1) n significant variati ns in the l w–mass IMF have been bserved between different star-f rming regi ns, and (2) the mass distributi ns f y ung stars just emerging fr m m lecular cl uds are c nsistent with having been drawn fr m the IMF derived fr m field stars in the s lar neighb rh d. These results apply nly t gr ss characterizati ns f the IMF (e.g., the rati f hight l w-mass stars); present bservati ns d n t rule ut m re subtle regi nal differences. Further studies are required in rder t assess whether r n t there is evidence f r a universal turn ver near the hydr gen-burning limit. We als pr vide a general framew rk f r discussing the ries f the IMF and summarize recent w rk n several physical mechanisms that c uld play a r le in determining the f rm f the stellar initial mass functi n.