180 research outputs found
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
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
The Two Micron All-Sky Survey: Removing the Infrared Foreground
We introduce the properties of the Two Micron All-Sky Survey (2MASS) survey for IAU Symposium 204. 2MASS is a near-infrared survey of the entire sky characterized by high reliability and completeness. Catalogs and images for 47% of the sky are now available online. This data release has been used by Wright (2000) and Cambr´esy et al. (2000) to subtract the stellar foreground at 1.25 and 2.2 microns from COBE DIRBE data, revealing the cosmological near-infrared background
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