New OB star candidates in the Carina Arm around Westerlund 2 from VPHAS+

Abstract

O and early B stars are at the apex of galactic ecology, but in the Milky Way, only a minority of them may yet have been identified. We present the results of a pilot study to select and parametrise OB star candidates in the Southern Galactic plane, down to a limiting magnitude of $g=20$. A 2 square-degree field capturing the Carina Arm around the young massive star cluster, Westerlund 2, is examined. The confirmed OB stars in this cluster are used to validate our identification method, based on selection from the $(u-g,g-r)$ diagram for the region. Our Markov Chain Monte Carlo fitting method combines VPHAS+ $u,g,r,i$ with published $J,H,K$ photometry in order to derive posterior probability distributions of the stellar parameters $\log ({\mathrm{T}}_{\mathrm{eff}})$ and distance modulus, together with the reddening parameters $A_0$ and $R_V$. The stellar parameters are sufficient to confirm OB status while the reddening parameters are determined to a precision of $\sigma (A_0)\sim 0.09$ and $\sigma (R_V)\sim 0.08$. There are 489 objects that fit well as new OB candidates, earlier than $\sim$B2. This total includes 74 probable massive O stars, 5 likely blue supergiants and 32 reddened subdwarfs. This increases the number of previously known and candidate OB stars in the region by nearly a factor of 10. Most of the new objects are likely to be at distances between 3 and 6 kpc. We have confirmed the results of previous studies that, at these longer distances, these sight lines require non-standard reddening laws with $3.5<R_V<4$.

21 Pages, 17 Figures, 8 Tables. Accepted for publication in MNRAS

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Cited by in corpus (11)

• OB associations and their origins
• The Galactic spiral structure as revealed by O- and early B-type stars
• SOFIA FEEDBACK survey: exploring the dynamics of the stellar wind driven shell of RCW 49
• The deep OB star population in Carina from the VST Photometric H$\alpha$ Survey (VPHAS+)
• The O star hinterland of the Galactic starburst, NGC 3603
• A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space Telescope. III. The present-day stellar mass function
• Proper motions of OB stars in the far Carina Arm
• A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space Telescope. II. Mass accretion in the Pre-Main Sequence Population
• Parallax-based Distances to Galactic Hii Regions: Nearby Spiral Structure
• SOFIA FEEDBACK survey: PDR diagnostics of stellar feedback in different regions of RCW 49
• Radial velocities from far red spectra of Carina Arm O and early B stars