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

Wright

Abstract

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 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 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 RV (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 (RV > 3.5) extinction laws are ubiquitous in the region, over the distance range 2.5–3 to ~10 kpc. Near prominent massive clusters, RV tends to rise, with particularly large and chaotic excursions to RV ~ 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.

• Acceptance Date: Oct 14, 2016
• Publication Date: Oct 16, 2016
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Publisher: Oxford University Press
• Pages: 1807 - 1830
• DOI: https://doi.org/10.1093/mnras/stw2751
• Keywords: surveys, stars: early-type, dust, extinction, Galaxy: disc, open clusters and associations: general
• Publisher URL: https://academic.oup.com/mnras/article/465/2/1807/2417471