IDENTIFYING THE YOUNG LOW-MASS STARS WITHIN 25 pc. II. DISTANCES, KINEMATICS, AND GROUP MEMBERSHIP

Abstract

We have conducted a kinematic study of 165 young M dwarfs with ages of <300\nMyr. Our sample is composed of stars and brown dwarfs with spectral types\nranging from K7 to L0, detected by ROSAT and with photometric distances of <25\npc assuming the stars are single and on the main-sequence. In order to find\nstars kinematically linked to known young moving groups (YMGs), we measured\nradial velocities for the complete sample with Keck and CFHT optical\nspectroscopy and trigonometric parallaxes for 75 of the M dwarfs with the\nCAPSCam instrument on the du Pont 2.5-m Telescope. Due to their youthful\noverluminosity and unresolved binarity, the original photometric distances for\nour sample underestimated the distances by 70% on average, excluding two\nextremely young (<3 Myr) objects found to have distances beyond a few hundred\nparsecs. We searched for kinematic matches to 14 reported YMGs and identified 9\nnew members of the AB Dor YMG and 2 of the Ursa Majoris group. Additional\npossible candidates include 6 Castor, 4 Ursa Majoris, 2 AB Dor members, and 1\nmember each of the Her-Lyr and beta Pic groups. Our sample also contains 27\nyoung low-mass stars and 4 brown dwarfs with ages <150 Myr which are not\nassociated with any known YMG. We identified an additional 15 stars which are\nkinematic matches to one of the YMGs, but the ages from spectroscopic\ndiagnostics and/or the positions on the sky do not match. These warn against\ngrouping stars together based only on kinematics and that a confluence of\nevidence is required to claim that a group of stars originated from the same\nstar-forming event.\n