GeminiFocus May 2014 | Page 12

This Astronomy and Astrophysics paper is available in electronic form at: arXiv: 1402.1766. Identifying Low-mass Stars in Young Groups Obtaining a complete census of low-mass stars is an important step in determining the intrinsic distribution of stellar masses (the initial mass function), and these lowmass examples also offer some of the best opportunities to image planets, because the host stars are not overwhelmingly bright. In recent work, Lison Malo (Université de Montréal) and collaborators concentrate on the case of young nearby groups of stars, providing additional evidence to identify 130 more low-mass stars as members of these young moving groups. The original target sample included 920 lowmass stars that exhibited some evidence for youth. Bayesian statistical techniques provided distance estimates and reduced the sample of interest. The new work concentrates on the most likely candidates and adds high spectral resolution observations, obtained primarily using Phoenix on Gemini South. These data enable measurement of the radial velocity, which provides kinematic evidence for group membership, and projected rotational velocity, which indicates age. The age is an important characteristic to confirm group membership. One further result from this work is to identify X-ray luminosity as an additional useful age discriminant for the M dwarfs of interest (Figure 3), which is shown to be even more effective than the ratio of X-ray to bolometric luminosity that has previously been been applied. The X-ray luminosity technique also offers the advantage of extending to a broader (older) age range than some other common methods, such as measurements of lithium line strength. Absolute confirmation of the individual stars as members of these nearby (distance less than 100 parsecs) and young (age less than 100 million years) moving groups still requires measurement of parallax, although the work so far provides a high likelihood that they would be confirmed. The results will be published in The Astrophysical Journal, and a preprint is now available at arXiv:1402.6053. Nancy A. Levenson is Deputy Director and Head of Science at Gemini Observatory and can be reached at: [email protected] Figure 3. Cumulative distribution of the X-ray luminosity demonstrates the utility of this measurement as an age indicator, showing field stars (green circles), confirmed members of the Beta Pictoris moving group (black triangles), and confirmed members of the AB Doradus moving group (blue squares). 10 GeminiFocus April 2014