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For application to surveys of interstellar matter and Galactic structure, wecompute new spectrophotometric distances to 139 OB stars frequently used asbackground targets for UV spectroscopy. Many of these stars have updatedspectral types and digital photometry with reddening corrections from theGalactic O-Star (GOS) spectroscopic survey. We compare our new photometricdistances to values used in previous IUE and FUSE surveys and to parallaxdistances derived from Gaia-DR2, after applying a standard (0.03 mas) offsetfrom the quasar celestial reference frame. We find substantial differencesbetween photometric and parallax distances (at d > 1.5 kpc) with increasingdispersion when parallax errors exceed 8%. Differences from previous surveysarise from new GOS stellar classifications, especially luminosity classes, andfrom reddening corrections. We apply our methods to two OB associations. ForPerseus OB1 (nine O-stars) we find mean distances of $2.47\pm0.57$ kpc (Gaiaparallax) and $2.99\pm0.14$ kpc (photometric) using a standard grid of absolutemagnitudes (Bowen et al. 2008). For 29 O-stars in Car OB1 associated withTrumpler-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 analternative grid of O-star absolute magnitudes (Martins et al. 2005) shiftsthese photometric distances 7% closer. Improving the distances to OB-stars willrequire attention to spectral types, photometry, reddening, binarity, and thegrid of absolute magnitudes. We anticipate that future measurements in Gaia-DR3will improve the precision of distances to massive star-forming regions in theMilky Way.

Distances to Galactic OB-stars: Photometry vs. Parallax