The deep OB star population in Carina from the VST Photometric H$\alpha$ Survey (VPHAS+)

Massive OB starsare critical to the ecology of galaxies, and yet our knowledge of OB starsin the Milky Way, fainter than $V \sim 12$, remains patchy. Data from the VST Photometric H$\ alpha$ Survey (VPHAS+) permit the construction of the first deep catalogues of blue excess-selected OB stars, without neglecting the stellar field. A total of 14900 candidates with 2MASS cross-matches are blue-selected from a 42 square-degreeregion in the Galactic Plane, capturing the Carina Arm over the Galactic longitude range $282^{\circ} \lesssim \ell \lesssim 293^{\circ}$. Spectral energy distributionfitting is performed on these candidates' combined VPHAS+ $u,g,r,i$ and 2MASS $J,H,K$ magnitudes. This delivers: effective temperatureconstraints, statistically separating O from early-B stars; high-quality extinction parameters, $A_0$ and $R_V$ ( random errorstypically $ 3.5$) extinction laws are ubiquitous in the region, over the distance range 2.5--3 kpc to $\sim$10~kpc. Near prominent massive clusters, $R_V$ tends to rise, with particularly large and chaotic excursions to $R_V \sim 5$ seen in the Carina Nebula. The data reveal a hitherto unnoticed association of 108 O-B2 stars around the O5If$+$ star LSS 2063 ($\ell = 289.77^{\circ}$, $b = -1.22^{\circ}$). Treating the OB star scale-heightas a constant within the thin disk, we find an orderly mean relation between extinction ($A_0$) and distance in the Galactic longitude range, $287.6^{\circ} < \ell < 293.5^{\circ}$, and infer the subtle onset of thin-diskwarping. A halo around NGC 3603, roughly a degree in diameter, of $\sim$500 O-B2 stars with $4 < A_0 (\rm{mag}) < 7$ is noted.