New members of the TW Hydrae Association and two accreting M-dwarfs in Scorpius-Centaurus

TL;DR

This study reports the discovery of new young M-dwarfs in the TW Hydrae and Scorpius-Centaurus associations, including accreting stars with circumstellar disks, suggesting longer planet formation timescales around low-mass stars.

Contribution

It identifies new association members and provides a comprehensive catalog of Hipparcos early-type stars with radial velocities, highlighting potential age spreads and disk longevity.

Findings

Two M-dwarfs are consistent with TW Hydrae membership.

Two accreting M-dwarfs exhibit large infrared excesses and H-alpha emission.

Discovery of accreting stars suggests longer disk lifetimes or age spread in Sco-Cen.

Abstract

We report the serendipitous discovery of several young mid-M stars found during a search for new members of the 30-40 Myr-old Octans Association. Only one of the stars may be considered a possible Octans(-Near) member. However, two stars have proper motions, kinematic distances, radial velocities, photometry and Li I 6708AA measurements consistent with membership in the 8-10 Myr-old TW Hydrae Association. Another may be an outlying member of TW Hydrae but has a velocity similar to that predicted by membership in Octans. We also identify two new lithium-rich members of the neighbouring Scorpius-Centaurus OB Association (Sco-Cen). Both exhibit large 12 and 22 micron excesses and strong, variable H-alpha emission which we attribute to accretion from circumstellar discs. Such stars are thought to be incredibly rare at the ~16 Myr median age of Sco-Cen and they join only one other confirmed M-type and three higher-mass accretors outside of Upper Scorpius. The serendipitous discovery of two accreting stars hosting large quantities of circumstellar material may be indicative of a sizeable age spread in Sco-Cen, or further evidence that disc dispersal and planet formation time-scales are longer around lower-mass stars. To aid future studies of Sco-Cen we also provide a newly-compiled catalogue of 305 early-type Hipparcos members with spectroscopic radial velocities sourced from the literature.