Brown Dwarfs in the Hyades and Beyond?

We have used both the Low-Resolution Imaging Spectrograph and the HIRESechelle spectrograph on the Keck telescopes to obtain spectra of twelvecandidate members of the Hyades cluster identified by Leggett and Hawkins(1988, 1989). All of the objects are chromospherically-active, late-typeM-dwarfs, with H$\alpha$ equivalent widths varying from 1 to 30\AA. Based onour measured radial velocities, the level of stellar activity and otherspectroscopic features, only one of the twelve stars has properties consistentwith cluster membership. We consider how this result affects estimates of theluminosity and mass function of the Hyades cluster. Five of the eleven fieldstars have weak K I 7665/7699\AA and CaH absorption as compared with M-dwarfstandards of the same spectral type, suggesting a lower surface gravity. Two ofthese sources, LH0416+14 and LH0419+15, exhibit significant lithium 6708 \AAabsorption. Based partly on parallax measurements by the US Naval Observatory(Harris et al, 1998), we identify all five as likely to be young, pre-mainsequence objects in or near the Taurus-Auriga association at distances ofbetween 150 and 250 parsecs. A comparison with theoretical models of pre-mainsequence stars indicates masses of less than 0.05 M$_\odot$.Comment: to appear in AJ, January 1999; 34 pages, (Latex format), including 10 embedded postscript figures and two table