A VLBA Survey of radio stars in the Orion Nebula Cluster: II. Astrometry

TL;DR

This study uses VLBA and Gaia data to measure proper motions of radio stars in the Orion Nebula Cluster, revealing the cluster's overall motion and velocity dispersion without signs of expansion, contraction, or rotation.

Contribution

It provides high-precision astrometry for radio stars in the ONC, combining VLBA and Gaia data to analyze cluster kinematics with improved accuracy.

Findings

Measured the global motion of the ONC as (1.20, 0.18) mas/yr.

Determined the velocity dispersion as (0.84, 1.30) mas/yr.

Found no evidence of expansion, contraction, or rotation in the cluster.

Abstract

From Very Long Baseline Array (VLBA) observations we previously identified a population of 123 young stellar systems with nonthermal radio emission toward the core of the Orion Nebula Cluster (ONC). We find optical sources in the Gaia DR2 catalog for 34 of them within 0.2" of the radio positions. Most of the radio sources are likely companions of Gaia detections. However, there are 11 stars whose VLBA position differs from Gaia by $<1.6$ mas, and some of these radio sources probably are the direct counterparts of the optical stars. We are able to obtain radio proper motions for 23 stars. Combining the stellar proper motions derived from the VLBA and Gaia data we find the global motion and velocity dispersion of the ONC to be ($\mu_\alpha*,\mu_\delta$)=($1.20\pm0.09,0.18\pm0.09$) mas yr$^{-1}$ and ($\sigma_{\mu_\alpha*},\sigma_{\mu_\delta}$)=($0.84\pm0.09,1.30\pm0.09$) mas yr$^{-1}$. Finally, we looked for ordered motions by estimating the means of scalar and vectorial products, which results in $\mathbf{\overline{v\cdot\hat{r}}}=-0.61\pm1.00$ km s$^{-1}$ and $\mathbf{\overline{v\times\hat{r}}}=0.57\pm0.95$ km s$^{-1}$. These do not show indications that the young stellar cluster is in expansion, contraction, or rotation.