The stellar content of LH 9 and 10 (N11) in the LMC - A case for sequential star formation

Abstract

The young OB associations Lucke-Hodge 9 and 10 are studied with UBVphotometry that is independent of reddening to determine the IMFdirectly from star counts. The temperature and reddening of the starsare determined which, in conjunction with the spectroscopicclassification of the earliest stars, is employed to place the stellargroups on the theoretical H-R diagram. Observations are also presentedof the highly compact H II region/knot N11A and the multiple system HD32228, and LH 9 and 10 are compared. The Lyman ionizing flux calculatedat 4.9-7.2 x 10 exp 50/s agrees well with flux required to generate theH-alpha luminosity of the H II region. LH 10 has a much flatter slope, ahigher ratio of higher-mass to lower-mass stars, and greater reddeningthan LH 9, and LH 10 contains all of the O stars earlier than O6. It isconcluded that LH 9 is older than LH 10 and probably contributed to theinitiation of star formation in LH 10.