STARS APPROACHING THE SUBSTELLAR LIMIT IN THE ALPHA-PERSEI OPEN CLUSTER

We present intermediate dispersion optical spectroscopy for seven very low-mass stars in the alpha Per open cluster with spectral types betw een M3 and M6, including the brown dwarf candidate of Rebolo et al. (1 992). Our radial velocity measurements are found to be generally consi stent with the mean cluster velocity to within the measurement errors. H alpha equivalent widths have been measured and compared to previous published data for other known members of the cluster. A turnover in chromospheric activity around spectral type M3-M4 is observed. The bro wn dwarf candidate, located in the cool side of the turnover, is confi rmed to exhibit a strong H alpha variability. It is the coolest alpha Per candidate member for which spectra are available and for which chr omospheric activity has been measured. Using the Li I doublet at lambd a 6707.8 Angstrom, we derive upper limits to the atmospheric Li abunda nce in the sample and discuss them in the context of the most recent s tellar evolutionary tracks. The non-detection of the Li I line in the spectrum of the brown dwarf candidate implies a mass greater than 0.08 M(circle dot), and therefore it is not a substellar object. However, its position in the HR diagram implies that this star is indeed very c lose to the substellar limit.