The Massive Star Content of NGC 6822: Ground-based and[ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Photometry

Abstract

We investigate the massive star population of NGC 6822 with ground-basedUBV photometry covering the whole of the galaxy and HST WFPC2 photometrywith filters F255W, F336W, F439W, and F555W of two fields containingvery rich and crowded OB associations. The four-band WFPC2 photometry isused to derive T_{eff} and E(B-V). H-R diagrams are constructedfor the OB associations included in our fields. These show that 10 Myrold populations are present in OB 9 and OB 6, while more recent starformation have occurred in OB 8, OB 13, OB 15, and OB 7. Twoparticularly interesting H II regions, Hubble V and Hubble X, areincluded in our fields. The luminous massive stars that power thesebright H II regions (H{α} luminosity several times that of the Orionnebula), could be resolved even in their dense cores thanks to the HSTspatial resolution. Our data reveal very young (a few million years),apparently coeval populations, with several massive star candidates. Theluminosity function for the UV-selected stars in the two WFPC2 fieldsshows two components: a population with age of a few million years inthe regions of Hubble V and Hubble X, and a 10 million year oldbackground population. The main-sequence stellar luminosity function,including all stars measured in the V filter, is similar to thecanonical composite luminosity function for main-sequence Galactic fieldstars. The reddening (foreground plus internal) is found to vary betweenE(B-V)=0.25 and 0.45.