The massive star content of NGC 6822:: Ground-based and Hubble Space Telescope photometry

We investigate the massive star population of NGC 6822 with ground-based UBV photometry covering the whole of the galaxy and HST WFPC2 photometry with filters F255W, F336W, F439W, and F555W of two fields containing very rich and crowded OB associations. The four-band WFPC2 photometry is used to derive T-eff and E(B-V). H-R diagrams are constructed for the OB associations included in our fields. These show that 10 Myr old populations are present in OB 9 and OB 6, while more recent star formation have occurred in OB 8, OB 13, OB 15, and OB 7. Two particularly interesting H II regions, Hubble V and Hubble X, are included in our fields. The luminous massive stars that power these bright H II regions (H alpha luminosity several times that of the Orion nebula), could be resolved even in their dense cores thanks to the HST spatial resolution. Our data reveal very young (a few million years), apparently coeval populations, with several massive star candidates. The luminosity function for the UV-selected stars in the two WFPC2 fields shows two components: a population with age of a few million years in the regions of Hubble V and Hubble X, and a 10 million year old background population. The main-sequence stellar luminosity function, including all stars measured in the V filter, is similar to the canonical composite luminosity function for main-sequence Galactic field stars. The reddening (foreground plus internal) is found to vary between E(B-V) = 0.25 and 0.45.