The Palomar/MSU Nearby Star Spectroscopic Survey.II.The Southern M Dwarfs and Investigation of Magnetic Activity

Abstract

Spectroscopic data for the southern stars (δ<-30) in the PMSU survey are presented. The data were combined with the data from Paper I [Reid, Hawley & Gizis, AJ, 110, 1838 (1995)] to obtain a list of all the magnetically active dMe stars in the survey. The incidence of activity increases monotonically toward later spectral type, reaching a level of 60% at spectral types >M5. This is not a selection effect, since the chromospheric activity level (LHα/Lbol) remains high throughout the spectral type range where the incidence is increasing. At very late types, there is evidence that the mean activity level drops in both chromospheric and coronal emission. The ratio between the chromospheric and coronal emission levels remains constant, indicating that the heating mechanism is not changing. There is little or no dependence of the chromospheric activity level on rotational velocity (υ sin i). The color-magnitude diagrams in both MV and MK show strong evidence for a break between spectral types M3-MS, in both the dMe and dM stars. The dMe stars at earlier types than M3 are about 0.5 magnitudes brighter than dM stars of the same spectral type in both MV and MK. They also are slightly (≤0. 1 magnitude) redder. Both the Hyades and IC 2602 clusters show the same absolute magnitude effect for the early type dMe stars. The Balmer decrement varies widely among the dMe field stars, in contrast to the Hyades stars. The photospheric TiO bands show detailed structure which depends on the chromospheric activity level of the star. These spectroscopic signatures will provide useful constraints on atmospheric models. A rigorous maximum likelihood analysis of the kinematic properties of a complete subs ample of the survey shows that the dMe stars are, as a whole, kinematically younger than the dM stars. Subsets of the dM sample also show that the early type dM stars are younger than the late dM stars, suggesting that the dMe phenomenon lasts longer in later type stars. This provides a natural explanation for the increase in the incidence of activity toward later spectral types.