On the Distance to the Ophiuchus Star-Forming Region

TL;DR

This paper refines the distance to the Ophiuchus star-forming region to approximately 139 parsecs using Hipparcos data, aligning it closely with the Upper Sco subgroup and providing insights into its motion and formation history.

Contribution

It provides a new, precise distance estimate to Ophiuchus and compares its motion to Upper Sco, challenging previous formation scenarios.

Findings

Distance to Ophiuchus is about 139 pc with 4% error.

Ophiuchus and Upper Sco have similar velocity vectors.

Both regions show negligible vertical motion in the Galaxy.

Abstract

The Ophiuchus molecular cloud complex has produced in Lynds 1688 the richest known embedded cluster within ~300 pc of the Sun. Unfortunately, distance estimates to the Oph complex vary by nearly ~40% (~120-165 pc). Here I calculate a new independent distance estimate of 135 +- 8 pc to this benchmark star-forming region based on Hipparcos trigonometric parallaxes to stars illuminating reflection nebulosity in close proximity to Lynds 1688. Combining this value with recent distance estimates from reddening studies suggests a consensus distance of 139 +- 6 pc (4% error), situating it within ~11 pc of the centroid of the ~5 Myr old Upper Sco OB subgroup of Sco OB2 (145 pc). The velocity vectors for Oph and Upper Sco are statistically indistinguishable within ~1 km/s in each vector component. Both Oph and Upper Sco have negligible motion (<1 km/s) in the Galactic vertical direction with respect to the Local Standard of Rest, which is inconsistent with the young stellar groups having formed via the high velocity cloud impact scenario.