What is known about the masses of
main-sequence stars from the analysis of binary
orbits? Double-lined eclipsing binaries are the
main source of very precise stellar masses and
radii (e.g. Andersen 1997), contributing more than
100 determinations with better than 2% precision
over the range 0.6 to 20 Mʘ.
For lower-mass stars we are forced to turn to
nearby systems with astrometric orbits (e.g. Henry
et al. 1993). Not only is the number of good mass
determinations from such systems smaller, but also
the precision is generally poorer. We are
approaching an era when interferometers should
have a major impact by supplying good astrometric
orbits for dozens of double-lined systems. Already
we are beginning to see the sorts of results to
expect from this (e.g. Torres et al.
1997).
Figure 1. Mass
vs. absolute V magnitude for eclipsing binaries
(circles) and nearby astrometric binaries
(squares)
Figure 1 is an updated version of a
diagram presented by Henry et al. (1993, their
Figure 2). It shows the general run of mass
determinations from about 10
Mʘ down to the substellar
limit near 0.075
Mʘ.
Ninety of the points in Figure 1 are for eclipsing
binary masses from Andersen’s review (1991) and
are plotted as open circles. The results for
eclipsing binaries published since 1991 are
plotted as 30 filled circles, adopting the same
limit of 2% for the mass precision. In most cases
the uncertainties are similar to the size of the
symbols. Especially noteworthy is the pair of new
points for CM Draconis (Metcalfe et al. 1996) with
masses near 0.25 Mʘ. Together
with the points for YY Geminorum near 0.6
Mʘ, these are the only M
dwarfs that have precise mass determinations. For
the most part we are forced to rely on nearby
stars with astrometric orbits, to fill in the M
dwarf region of the diagram. We have used filled
squares in Figure 1 for 29 such systems from Henry
et al. (1993), updated using 14 new parallaxes
from Hipparcos and 4 from the new Yale Parallax
Catalog (1995). Gliese 508 is not included,
because it is now known to be a triple, while
Gliese 67AB, 570BC, and 623AB are not included
because there are not yet any direct measurements
of the V magnitude
difference for these systems.
The Mass-Luminosity Relation From
Binaries