The Stellar Content of LH 9 and 10 (N11) in the LMC: A Case for Sequential Star Formation

We present CCD photometry and spectroscopy for stars in Lucke-Hodge 9 and 10, two adjacent OB associations in the northwest corner of the LMC. Our catalog contains UBV photometry (complete to ~18^m^ in all three filters) for 795 stars and BV-only photometry (complete to ~ 19^m^ in both filters) for an additional 434 stars. We have obtained spectra for and classified 43 O stars, 17 of which are earlier than O7, and 30 early B stars. LH 10 contains at least three and possibly as many as six O3 stars, and six stars whose ages appear to be quite young and are located near the ZAMS. We also include observations of the unusual objects HD 32228-a multiple system containing a W-R + O composite-and N11A, which is an extremely compact H II region/knot surrounding an early O star. We calculate the Lyman ionizing flux of the earliest OB stars to be N_Ly_ = 4.9-7.2 X 10^50^ s^-1^, in good agreement with the flux needed to produce the observed Hα luminosity of the surrounding H II region. We discover significant differences between the two associations that appear so close together on the sky. The IMF slope for LH 10 of {GAMMA} = - 1.1+/-0.1 is significantly flatter than the slope for LH 9 of {GAMMA} = - 1.6+/-0.1, indicating that the stellar content of LH 10 contains a higher ratio of higher-mass to lower-mass stars than does that of LH 9. All seven O stars earlier than O6 are located in LH 10, as are at least five of the six "ZAMS" O stars. The reddening of LH 10 is greater than that of LH 9: E(B - V) = 0.17 and 0.05, respectively. These last two points suggest that LH 10 is the younger of the two associations, and star formation there could possibly have been triggered by the evolution of massive stars in LH 9.