Massive OB stars are critical to the ecology of galaxies , and yet our knowledge of OB stars in 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-degree region in the Galactic Plane , capturing the Carina Arm over the Galactic longitude range 282 ^ { \circ } \lesssim \ell \lesssim 293 ^ { \circ } .
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 ; high-quality extinction parameters , A _ { 0 } 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 % of them .
The fraction of emission line stars among all candidate B stars is 7–8 % .
Greyer ( R _ { V } > 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-height as 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-disk warping .
A halo around NGC 3603 , roughly a degree in diameter , of \sim 500 O-B2 stars with 4 < A _ { 0 } ( mag ) < 7 is noted .