Activity and Kinematics of M and L Dwarfs

I discuss observations of two traditional age indicators, chromospheric activity and kinematics, in late-M and L dwarfs near the hydrogen-burning limit. The frequency and strength of chromospheric activity disappears rapidly as a function of temperature over spectral types M8-L4. There is evidence that young late-M and L dwarfs have weaker activity than older ones, the opposite of the traditional stellar age-activity relation. The kinematics of L dwarfs confirm that lithium L dwarfs are younger than non-lithium dwarfs.Comment: to appear in proceedings of Ultracool Dwarf Stars: Surveys, Properties and Spectral Classification (Lecture Notes in Physics), eds. Jones and Steele. 12 page

The distance to NGC 6397 by M-subdwarf main-sequence fitting

Recent years have seen a substantial improvement both in photometry of low luminosity stars in globular clusters and in modelling the stellar atmospheres of late-type dwarfs. We build on these observational and theoretical advances in undertaking the first determination of the distance to a globular cluster by main-sequence fitting using stars on the lower main sequence. The calibrating stars are extreme M subdwarfs, as classified by Gizis (1997), with parallaxes measured to a precision of better than 10%. Matching against King et al's (1998) deep (V, (V-I)) photometry of NGC 6397, and adopting E_{B-V}=0.18 mag, we derive a true distance modulus of 12.13 +- 0.15 mag for the cluster. This compares with (m-M)_0=12.24 +- 0.1 derived through conventional main-sequence fitting in the (V, (B-V)) plane. Allowing for intrinsic differences due to chemical composition, we derive a relative distance modulus of delta (m-M)_0=2.58 mag between NGC 6397 and the fiducial metal-poor cluster M92. We extend this calibration to other metal-poor clusters, and examine the resulting RR Lyrae (M_V, [Fe/H]) relation.Comment: 19 pages, AASTeX, to appear in the December 1998 A

Low-mass Binaries in the Hyades - A scarcity of brown dwarfs

We have obtained HST Planetary Camera observations of a total of fifty-three low-mass (M < 0.3 M_solar) members of the Hyades cluster. Nine of these stars are resolved as binaries, with separations between 0.1 and 3.1 arcseconds, while a further three are probably equal-mass systems at smaller separations. Allowing for observational selection effects, this corresponds to an observed binary fraction of 11.3 plus or minus 4.6% for systems with separations in the range 14 to 825 a.u., consistent with observations of solar neighbourhood M-dwarfs. The mass-ratio distribution is only consistent marginally with the secondary stars being drawn from a Psi(M) proportioal to M^{-1} mass function, with three equal-mass systems amongst the six binaries with observed separations in the regime where our observations extend below the hydrogen-burning limit. Considering the entire sample, the absence of any brown dwarf companions amongst our sample makes it unlikely that the mass function of stellar/brown dwarf companions rises as steeply as M^{-1}. If the Hyades has an age of 600 Myrs, our results are consistent only at the 2 sigma level with a flat (M^0) mass function.Comment: 19 pages, 2 figures, AASTeX, to appear in the December 1997 A

Probing the LHS Catalog. I. New Nearby Stars and the Coolest Subdwarf

We present moderate resolution spectroscopy of 112 cool dwarf stars to supplement the observations we have already presented in the Palomar/MSU Nearby-Star Spectroscopic Survey. The sample consists of 72 suspected nearby stars added to the The Preliminary Third Catalog of Nearby Stars since 1991 as well as 40 faint red stars selected from the LHS catalog. LHS 1826 is more metal-poor and cooler than the coolest previously known extreme subdwarf, LHS 1742a. LHS 2195 is a very late M dwarf of type M8 V, probably at a distance of ten parsecs. LHS 1937 is an M7 V star at 20 parsecs. Three other previously unobserved LHS stars have estimated distances that place them within 25 parsecs.Comment: 15 pages, 1 figure, AASTeX, to appear in the August 1997 PAS