Trigonometric Parallaxes and a Kinematically Adjusted Distance Scale for OB Associations

By directly comparing the photometric distances of Blaha and Humphreys (1989) (BH) to OB associations and field stars with the corresponding Hipparcos trigonometric parallaxes, we show that the BH distance scale is overestimated, on average, by 10-20%. This result is independently corroborated by applying the rigorous statistical-parallax method and its simplified analog (finding a kinematically adjusted rotation-curve solution from radial velocities and proper motions) to a sample of OB associations. These two methods lead us to conclude that the BH distance scale for OB associations should be shrunk, on average, by 11 +/- 6 and 24 +/- 10 %, respectively. Kinematical parameters have been determined for the system of OB associations: u_0 = 8.2 +/- 1.3 km/s, v_0= 11.9 +/- 1.1 km/s, w_0 = 9.5 +/- 0.9 km/s, sigma_u = 8.2 +/- 1.1 km/s, sigma_v = 5.8 +/- 0.8 km/s, sigma_w = 5.0 +/- 0.8 km/s, Omega_0 = 29.1 +/- 1.0 km/s/kpc, (Omega_0)' = -4.57 +/- 0.20 km/s/kpc^2, and (Omega_0)'' = 1.32 +/- 0.14 km/s/kpc^3. The distance scale for OB associations reduced by 20% matches the short Cepheid distance scale (Berdnikov and Efremov 1985; Sitnik and Mel'nik 1996). Our results are a further argument for the short distance scale in the Universe.