Distances to Galactic OB Stars: Photometry versus Parallax

Abstract

Abstract For application to surveys of interstellar matter and Galactic structure, we compute new spectrophotometric distances to 139 OB stars frequently used as background targets for UV spectroscopy. Many of these stars have updated spectral types and digital photometry with reddening corrections from the Galactic O-Star (GOS) spectroscopic survey. We compare our new photometric distances to values used in previous International Ultraviolet Explorer and Far Ultraviolet Spectroscopic Explorer surveys and to parallax distances derived from Gaia-DR2, after applying a standard (0.03 mas) offset from the quasar celestial reference frame. We find substantial differences between photometric and parallax distances at d > 1.5 kpc, with increasing dispersion when parallax errors exceed 8%. Differences from previous surveys arise from new GOS stellar classifications, especially luminosity classes, and from reddening corrections. We apply our methods to two OB associations. For Perseus OB1 (nine O stars) we find mean distances of 2.47 ± 0.57 kpc (Gaia parallax) and 2.99 ± 0.14 kpc (photometric) using a standard grid of absolute magnitudes. For 29 O stars in Car OB1 associated with Trumpler 16, Trumpler 14, Trumpler 15, and Collinder 228 star clusters, we find 2.87 ± 0.73 kpc (Gaia) and 2.60 ± 0.28 kpc (photometric). Using an alternative grid of O-star absolute magnitudes shifts these photometric distances ∼7% closer. Improving the distances to OB stars will require attention to spectral types, photometry, reddening, binarity, and the grid of absolute magnitudes. We anticipate that future measurements in Gaia-DR3 will improve the precision of distances to massive star-forming regions in the Milky Way.