Sunday, October 25, 2015

New Members of the TW Hya Association and 2 Accreting M Dwarfs in Sco-Cen

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


Murphy et al


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 λ6708 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 μm excesses and strong, variable Hα 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.

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