# Gaia-DR2 confirms VLBA parallaxes in Ophiuchus, Serpens and Aquila

## TL;DR

This study confirms Gaia-DR2 astrometry results with VLBA measurements for star-forming regions in Ophiuchus, Serpens, and Aquila, establishing accurate distances and revealing small parallax offsets and gradients across clouds.

## Contribution

It provides the first direct comparison between Gaia-DR2 and VLBA parallaxes in these regions, confirming Gaia's accuracy and refining the distances to key molecular clouds.

## Key findings

- Gaia-DR2 and VLBA parallaxes are consistent within small offsets.
- Serpens Main, Serpens South, and W40 are at the same distance (~436 pc).
- Distance gradients are observed across Ophiuchus, from 144.2 to 138.4 pc.

## Abstract

We present Gaia-DR2 astrometry of a sample of YSO candidates in Ophiuchus, Serpens Main and Serpens South/W40 in the Aquila Rift, which had been mainly identified by their infrared excess with Spitzer. We compare the Gaia-DR2 parallaxes against published and new parallaxes obtained from our Very Long Baseline Array (VLBA) program GOBELINS. We obtain consistent results between Gaia and the VLBA for the mean parallaxes in each of the regions analyzed here. We see small offsets, when comparing mean values, of a few tens of micro-arcseconds in the parallaxes, which are either introduced by the Gaia zero-point error or due to a selection effect by Gaia toward the brightest, less obscured stars. Gaia-DR2 data alone conclusively places Serpens Main and Serpens South at the same distance, as we first inferred from VLBA data alone in a previous publication. Thus, Serpens Main, Serpens South and W40 are all part of the same complex of molecular clouds, located at a mean distance of 436+/-9 pc. In Ophiuchus, both Gaia and VLBA suggest a small parallax gradient across the cloud, and the distance changes from 144.2+/-1.3 pc to 138.4+/-2.6 pc when going from L1689 to L1688.