Stellar Activity in Coeval Open Clusters: Praesepe and the Hyades

Abstract

Randich & Schmitt found that the coronal activity of solar-type and low-mass stars in Praesepe is significantly lower than that of stars in the Hyades cluster. This result is quite surprising, since the Hyades and Praesepe have approximately the same age and metallicity and are often thought to have originated in the same giant molecular cloud complex. We have carried out several tests in order to find a possible explanation for this result. We have measured radial velocities of two groups of Praesepe stars (a dF-dK sample and a dM sample) and have measured H? as a chromospheric activity index for the dM sample. Based on analyses of these data, we conclude that the Praesepe catalog used in the X-ray analysis does not contain a significant number of nonmembers and, thus, that membership problems do not seem to be the cause of the Randich & Schmitt result. The comparison of the H? equivalent widths for the M dwarfs in Praesepe with those in the Hyades indicates that, at least for stars in this mass range, the Praesepe stars are as active or more active than their Hyades counterparts. The similarity of chromospheric emission allows us to reject differences in the rotational velocity distribution as the origin of the dissimilar Lx luminosity functions. We have also analyzed a few ROSAT PSPC pointings of Praesepe in order to obtain a new and independent estimate of the X-ray luminosities and upper limits for a small sample of Praesepe members. This analysis suggests that the previous ROSAT PSPC analysis produced slightly optimistic X-ray upper limits; however, the differences between the old and new upper limits are not large enough to explain the dichotomy in the X-ray properties of Praesepe and the Hyades. Therefore, our examination of the available data does not provide a clear reason to explain why the X-ray luminosity functions of the two clusters are different. Part of the explanation could be found in the binaries. Speculatively, these clusters could have different orbital period distributions, with more short-period binaries among the Hyades, which would show larger coronal activity.