Distances to Galactic OB-stars: Photometry vs. Parallax
  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 IUE and FUSE 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 $2.47\pm0.57$ kpc (Gaia
parallax) and $2.99\pm0.14$ kpc (photometric) using a standard grid of absolute
magnitudes (Bowen et al. 2008). For 29 O-stars in Car OB1 associated with
Trumpler-16, Trumpler-14, Trumpler-15, and Collinder-228 star clusters, we find
$2.87\pm0.73$ kpc (Gaia parallax) and $2.60\pm0.28$ kpc (photometric). Using an
alternative grid of O-star absolute magnitudes (Martins et al. 2005) 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.
