The kinematics of active late-type stars observed by the ROSAT Wide Field Camera

Abstract

A sample of single, main-sequence late-type stars detected during the ROSAT Wide Field Camera EUV all-sky survey is kinematically investigated. It is confirmed that the galactic velocity dispersion of magnetically active objects, as defined by the ratio of EUV to bolometric flux, is substantially less than that of a random sample of late-type stars. This is due to the relationships between rotation and magnetic activity, age and rotation, and velocity dispersion and age. It is found, however, that the choice of only the most active stars does not decrease the velocity dispersions any further, and this may be evidence either for a long spin-down time-scale for the dMe stars that constitute the majority of this sample, or that very young stars are clumped around discrete velocity vectors that are somewhat separated in velocity space. A cluster analysis of the data reveals strong evidence for groups in the kinematic data that have space motions coincident with the Local Association and Sirius supercluster streams. These stellar streams are associated with the Pleiades and Ursa Major open clusters, with ages of |$\sim7\times {10}^{7}$| and |$\sim3\times {10}^{8}$| yr respectively. Supporting evidence for Local Association membership is found in terms of the magnetic activity, rotation rates and lithium abundance for some of the candidates. Assuming that the coronal properties of the Local Association late-type stars are similar to those of Pleiades cluster members, an upper limit of 10–15 is placed on the number of F–K type association members within 25 pc of the Sun. It is also estimated that as many as 103 late-type stars detected within 100 pc by the ROSAT X-ray telescope survey will be Local Association members, and that such stars will form an increasing fraction of the stellar content of deeper X-ray surveys.