Identifying and characterising young, nearby, low-mass members of stellar moving groups

Abstract

Since the early 1990s, several groups of comoving, coeval stars younger than 100Myr and within 100 pc have been revealed. Studying and identifying members in these ‘Nearby Young Moving Groups’ (MGs) is vital because they provide: well characterised samples to test pre-main sequence evolution; ideal targets for direct imaging of exoplanets, discs and brown dwarfs; observational evidence for the birthsites of stars in the Solar neighbourhood.

Spectroscopy is used to perform tests of membership for 24 M-dwarf candidates of both the Beta Pictoris MG (BPMG) and AB Doradus MG, confirming 8 and 6 new members, respectively. Measurements of lithium provide a precise age for BPMG, using the ‘Lithium Depletion Boundary’ (LDB) technique. This represents the most accurate age yet determined for this important MG and is about double what has been commonly assumed in the literature from other methods.

A kinematically unbiased sample of 146 X-ray emitting FGK stars in the Northern hemisphere with short rotation periods chosen from the SuperWASP All-Sky Survey were spectroscopically investigated to assess their ages and kinematics. The search identified 26 stars younger than 200Myr based on their photospheric lithium. Whilst most of these were not associated with any MG, seven are comoving with the sparse (mostly Southern) Octans-Near MG.

Infrared photometry is used to identify debris discs amongst M-dwarfs in MGs and their debris disc fractions are compared as a function of mass and age. Eight percent of the sample younger than 40Myr were identified as debris disc objects, although some may have remained undetected because the sensitivity limits for detecting debris discs around M-dwarfs is lower than for higher-mass stars. No debris discs were observed in MGs older than 40Myr, suggesting the timescale for disc removal is more rapid than for higher-mass stars.