Distance scale for high-luminosity stars in OB associations and in field with Gaia DR2. Spurious systematic motions

TL;DR

This study assesses Gaia DR2 parallaxes for OB associations, finds a zero-point offset leading to overestimated distances, and highlights the impact of uncorrected parallaxes on perceived stellar motions.

Contribution

It quantifies the Gaia DR2 parallax zero-point offset and demonstrates the need to revise the OB star distance scale and correct for systematic motions.

Findings

Gaia DR2 parallaxes are underestimated by about 0.11 mas.

The OB star distance scale should be reduced by 10-20%.

Uncorrected parallaxes cause spurious systematic motions of 10-20 km/s.

Abstract

We calculated the median parallaxes for 47 OB associations including at least 10 stars with known Gaia DR2 parallaxes. A comparison between trigonometric and photometric parallaxes of OB associations reveals a zero-point offset of delta_pi=-0.11 +\- 0.04 mas indicating that Gaia DR2 parallaxes are, on average, underestimated and the distances derived from them are overestimated. The correction of delta_pi=-0.11 mas is consistent with the estimate that Arenou et al. (2018) obtained for bright stars. An analysis of parallaxes of OB associations and high-luminosity field stars confirms our previous conclusion (Dambis et al. 2001) that the distance scale for OB stars established by Blaha and Humphreys (1989) must be reduced by 10--20%. Spurious systematic motions of 10--20 km s-1 at the distances of 2--3 kpc from the Sun are found to arise from the use of the uncorrected Gaia DR2 parallaxes.