Investigating the Upper Scorpius OB association with HERMES. I. The spectroscopic sample and 6D kinematics

TL;DR

This study uses spectroscopic and astrometric data to analyze the 6D kinematics of the Upper Scorpius OB association, revealing asymmetric expansion patterns and insights into its star formation history.

Contribution

It provides the first comprehensive 6D kinematic analysis of Upper Scorpius members, identifying distinct groups and their expansion and rotation trends, and discusses implications for star formation scenarios.

Findings

Evidence of asymmetric expansion in multiple groups

Estimated expansion timescales for subgroups

Star formation likely proceeded independently in subgroups

Abstract

We present results from a large spectroscopic survey of the nearest young association to the Sun, Upper Scorpius, conducted using 2dF/HERMES on the Anglo-Australian Telescope. We use spectroscopic youth criteria such as Li-equivalent widths to identify >1000 pre-main sequence (PMS) members across the region and measure radial velocities, combining these with Gaia EDR3 5-parameter astrometry to obtain 6D kinematic information. We separate confirmed PMS association members into distinct kinematic groups and measure expansion and rotation trends in each. We find strong evidence for asymmetric expansion in several groups and derive expansion timescales from the greatest rates of expansion in each group. We also trace the past motion of these groups using an epicycle approximation and estimate the time since their most compact configuration. These kinematic properties are compared to literature ages and the star formation history of Upper Scorpius is discussed. We find evidence that a scenario in which star formation in the subgroups of Upper Scorpius proceeded independently, either by self-instability or external feedback from Upper Centaurus-Lupus, is more likely than a recently proposed "cluster chain" scenario in which these subgroups have triggered each other.