2012AJ....144..142B

We studied the young stellar populations of 22 star-forming regions in the Andromeda galaxy (M31), with Hubble Space Telescope (HST) multi-band imaging from far-UV to I. The regions were selected from Galaxy Evolution Explorer (GALEX) wide-field far-UV imaging; they sample different environments and galactocentric distances from 6 to 22 kpc. They were imaged with 30 HST fields (360 distinct images, in six bandpasses), with a pixel scale of 0.38 pc projected on the sky, at the distance of M31. This study is part of HST treasury survey program HST-GO-11079, which includes star-forming regions in eight Local Group galaxies. We provide a merged catalog of six-band stellar photometry in the 30 M31 fields, containing 118,036 sources brighter than V and B ∼ 23 mag. Each HST field covers about 0.3 kpc² in M31, and contains up to ∼7000 stars, of which the number varies by a factor of >7 among the target regions; a large fraction of the sample are hot massive stars, due to our choice of filters and exposures. We derived stellar physical parameters and interstellar extinction for individual sources by spectral energy distribution analysis with model-atmosphere colors, and used the results to infer ages, massive stars content, and extinction of the star-forming regions. Reddening is up to E(B - V) ≲ 0.6 mag in some OB associations, and lowest in the outermost regions (average of ≲ 0.12 mag in OB184 at 21.9 kpc). We examined the spatial distribution (clustering) of the hot massive stars, and defined OB associations on various spatial scales from compact to wider, more spread out ones. A hierarchical structuring is observed, with small compact groups arranged within large complexes. Their areas vary from less than 10 to 10⁵ pc², and masses are up to ~10⁵ M_☉, in the scales sampled by our analysis. Their cumulative mass distribution follows a power law, at least in part of the sampled regime. Hot-star counts in the young regions compare very well with integrated measurements of UV flux from GALEX.