OPTICAL SPECTROSCOPY OF VERY LOW-MASS STARS IN THE PLEIADES

We have obtained optical spectrophotometry for several cool stars rece ntly found by us in a deep multicolor optical survey of the Pleiades. This survey has the distinct advantage of analyzing stars, which if pr oven to be cluster members, are of a known age and metallicity, which cannot be said of local cool stars. In addition, we have also observed several stars studied by Berriman and Reid in order to place the Plei ades candidates relative to these nearby stars. Based upon analysis of the spectra of the nearby stars, we define several molecular band ind ices (four TiO and one VO) and one index which measures the slope of t he virtual continuum in order to segregate and classify stars in terms of effective temperature. Some of these indices are well correlated, in a linear fashion, with log(T(eff)). Included in the analysis is the poorly understood cool star, LHS 2924, which continues to confuse the trends established with this analysis, by remaining an obvious outlie r with one exception. The VO strength, as expected, roughly correlates with log(T(eff)) for the temperature range under consideration and th e large scatter is almost certain to be due to metallicity variations. Only in the coolest of the Pleiades stars reported here is the VO ind ex comparable to that of the local cool stars such as VB 10. However, in the remaining cool Pleiades stars the VO index is substantially low er and similar to that of G1 905. The pseudocontinuum index defined in this paper appears to be the most reliable effective temperature esti mator for optical spectroscopic data even when the problematic LHS 292 4 is included in the analysis. This index appears to be valid at least over the range 2100 K < T(eff) < 3000 K and is defined by flux densit y points which are not overtly influenced by TiO. With the exception o f the strengths of atomic sodium and Halpha, the spectroscopic propert ies of VB 10 and that of the coolest Pleiades star are very similar. H owever, the bolometric magnitude of the coolest Pleiades star is 1-1.5 mag brighter. In this Pleiades star, atomic sodium is nearly twice as strong as that of VB 10 but the emission strength of Ha is only half as large. The brighter intrinsic luminosity is consistent with the sta r still contracting to the main sequence (hence spinning up) as would be expected for a star that is 70 Myr and only 0.1 M.. The other cool Pleiades stars tend to be similar in properties to that of G1 905.