configurations, then simulate the resulting
observations, including scattered light from
the central host star (Figure 12). The best-fitting models yield the bright emission results
noted above, indicate an inclination of 50 degrees, and confirm the inner gap’s size.
This Astronomy and Astrophysics paper is
available in electronic form here.
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 low-mass 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.
January 2015
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 13), 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.
Figure 13.
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).
Nancy A. Levenson is Deputy Direc ѽȁ