Stellar Activity in Coeval Open Clusters: Praesepe and the Hyades

Randich and Schmitt [1995, A&A 298, 115] found that the coronal activity ofsolar-type and low mass stars in Praesepe is significantly lower than that ofstars in the Hyades cluster. We have carried out several tests in order to finda possible explanation for this result. We have measured radial velocities oftwo groups of Praesepe stars (a dF-dK sample and a dM sample) and have measuredH$\alpha$ as a chromospheric activity index for the dM sample. We conclude thatthe Praesepe catalog used in the X-ray analysis does not contain a significantnumber of non-members. The comparison of the H$\alpha$ equivalent widths forthe M dwarfs in Praesepe with those in the Hyades indicates that, at least forstars in this mass range, the Praesepe stars are as active or more active thantheir Hyades counterparts. We have also analyzed a few ROSAT PSPC pointings ofPraesepe in order to obtain a new and independent estimate of the X-rayluminosities and upper limits for a small sample of Praesepe members concludingthat the small differences between the old and new upper limits are not largeenough to explain the dichotomy in the X-ray properties of Praesepe and theHyades. Therefore, our examination of the available data does not provide aclear reason to explain why the X-ray luminosity functions of the two clustersare different. Part of the explanation could be found in the binaries.Speculatively, these clusters could have different orbital perioddistributions, with more short period binaries among the Hyades, which wouldshow larger coronal activity.Comment: Accepted for publiction in Ap