Kinematics of OB-associations in Gaia epoch

TL;DR

This study uses Gaia and Hipparcos data to analyze the kinematics of OB-associations, revealing their velocity dispersions, masses, and expansion behaviors, and providing insights into their dynamical states and star formation efficiency.

Contribution

First comprehensive kinematic analysis of OB-associations using Gaia epoch data, comparing with Hipparcos, and examining their dynamical properties and expansion history.

Findings

OB-associations are generally unbound objects.

OB-associations show median star-formation efficiency of 2.1%.

Per OB1 and Car OB1 are expanding with velocities around 6.3 km/s.

Abstract

We use stellar proper motions from the TGAS catalog to study the kinematics of OB-associations. The TGAS proper motions of OB-associations generally agree well with the Hipparcos proper motions. The parameters of the Galactic rotation curve obtained with TGAS and Hipparcos proper motions agree within the errors. The average one-dimensional velocity dispersion inside 18 OB-associations with more than 10 TGAS stars is sigma_v=3.9 km s-1, which is considerably smaller, by a factor of 0.4, than the velocity dispersions derived from Hipparcos data. The effective contribution from orbital motions of binary OB-stars into the velocity dispersion sigma_v inside OB-associations is sigma_b=1.2 km s-1. The median virial and stellar masses of OB-associations are equal to 7.1 10^5 and 9.0 10^3 Ms, respectively. Thus OB-associations must be unbound objects provided they do not include a lot of dense gas. The median star-formation efficiency is epsilon=2.1 percent. Nearly one third of stars of OB-associations must lie outside their tidal radius. We found that the Per OB1 and Car OB1 associations are expanding with the expansion started in a small region of 11--27 pc 7--10 Myr ago. The average expansion velocity is 6.3 km s-1.