SIMBAD references

Spectroscopy of 222 late-type dwarfs in the Praesepe open cluster is used to measure the strengths of molecular and atomic features and to examine their sensitivity to chromospheric activity as measured by the strength of Hα emission. For this cluster we confirm the results of Hawley and coworkers that some TiO features are selectively weakened among active field stars. However, a similar effect for CaOH is only weakly confirmed. Using differential measures of the strengths of features with respect to the median in narrow color bins, we also find that absorption in the Ca II IR triplet is selectively weakened in more active stars, while absorption at K I and Na I is strengthened. A comparison of the distribution of the Hα strengths with V-I color in Praesepe and the similarly aged Hyades cluster indicates that the onset of M dwarf activity occurs at a significantly redder color in Praesepe than in the Hyades. Analysis of similar data on eight clusters from the literature is used to explore the relationship between age and activity in open clusters. We quantify the dependence on cluster age of the location along the main sequence for the onset of activity, confirming that the critical color at which activity becomes prominent is a nearly linear function of the logarithm of the age.