2017MNRAS.465.1807M

Massive OB stars are critical to the ecology of galaxies and yet our knowledge of OB stars in the Milky Way, fainter than V ∼ 12, remains patchy. Data from the VST Photometric Hα Survey (VPHAS+) permit the construction of the first deep catalogues of blue excess-selected OB stars, without neglecting the stellar field. A total of 14 900 candidates with 2MASS cross-matches are blue-selected from a 42 deg² region in the Galactic plane, capturing the Carina Arm over the Galactic longitude range 282° <= l <= 293°. Spectral energy distribution fitting is performed on these candidates' combined VPHAS+ u, g, r, i and 2MASS J, H, K magnitudes. This delivers effective temperature constraints, statistically separating O from early-B stars and high-quality extinction parameters, A₀ and R_V (random errors typically ∼0.1). The high-confidence O-B2 candidates number 5915 and a further 5170 fit to later B spectral type. Spectroscopy of 276 of the former confirms 97 per cent of them. The fraction of emission-line stars among all candidate B stars is 7-8 per cent. Greyer (R_V > 3.5) extinction laws are ubiquitous in the region, over the distance range 2.5-3 to ∼10 kpc. Near prominent massive clusters, R_V tends to rise, with particularly large and chaotic excursions to R_V ∼ 5 seen in the Carina Nebula. The data reveal a hitherto unnoticed association of 108 O-B2 stars around the O5If+ star LSS 2063 (l = 289 _.^○77, b = -1 _.^○22). Treating the OB star scaleheight as a constant within the thin disc, we find an orderly mean relation between extinction (A₀) and distance in the Galactic longitude range, 287 _.^○6 < l < 293 _.^○5, and infer the subtle onset of thin-disc warping. A halo around NGC 3603, roughly a degree in diameter, of ∼500 O-B2 stars with 4 < A₀(mag) < 7 is noted.