Spectroscopic confirmation of M-dwarf candidate members of the Beta Pictoris and AB Doradus Moving Groups

TL;DR

This study spectroscopically confirms M-dwarf members of the Beta Pictoris and AB Doradus groups, revises their ages considering magnetic activity, and emphasizes the importance of magnetic effects in age estimation.

Contribution

It provides spectroscopic confirmation of candidate members and revises the age estimates of the groups by incorporating magnetic activity effects.

Findings

Confirmed 10 BPMG and 6 ABDMG members using spectroscopic criteria.

Revised the LDB age of BPMG to 24 +/- 4 Myr considering magnetic effects.

Estimated the ABDMG age range as 35-150 Myr with current data.

Abstract

Optical spectroscopic observations are reported for 24 and 23, nearby, proper-motion-selected M-dwarf candidate members of the Beta Pictoris and AB Doradus moving groups (BPMG and ABDMG). Using kinematic criteria, the presence of both Halpha emission and high X-ray-to-bolometric luminosity, and position in absolute colour-magnitude diagrams, 10 and 6 of these candidates are confirmed as likely members of the BPMG and ABDMG respectively. Equivalent widths or upper limits for the Li I 6708A line are reported and the lithium depletion boundary (LDB) age of the BPMG is revisited. Whilst non-magnetic evolutionary models still yield an estimated age of 21 +/- 4 Myr, models that incorporate magnetic inhibition of convection imply an older age of 24 +/- 4 Myr. A similar systematic increase would be inferred if the stars were 25 per cent covered by dark magnetic starspots. Since young, convective M-dwarfs are magnetically active and do have starspots, we suggest that the original LDB age estimate is a lower limit. The LDB age of the ABDMG is still poorly constrained -- non-magnetic evolutionary models suggest an age in the range 35-150 Myr, which could be significantly tightened by new measurements for existing candidate members.