The deep OB star population in Carina from the VST Photometric Hα Survey (VPHAS

Massive OB stars are critical to the ecology of galaxies and yet our knowledge of OB stars in the Milky Way, fainter than V similar to 12, remains patchy. Data from the VST Photometric Ha 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 deg(2) region in the Galactic plane, capturing the Carina Arm over the Galactic longitude range 282 degrees less than or similar to l less than or similar to 293 degrees. 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, A(0) and RV (random errors typically similar to 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 similar to 10 kpc. Near prominent massive clusters, RV tends to rise, with particularly large and chaotic excursions to R-V similar to 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.degrees 77, b = -1.degrees 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.degrees 6 < l < 293.degrees 5, and infer the subtle onset of thin-disc warping. A halo around NGC 3603, roughly a degree in diameter, of similar to 500 O-B2 stars with 4 < A(0)(mag) < 7 is noted.