EXOSAT observations of active stellar coronae in the Cygnus Loop, T Tau & NGC 2264 regions

Abstract

A highly variable point X-ray source, first seen by the Einstein IPC, has been positioned with the EXOSAT CMA and identified with a bright (V = 8.5) K0 star. Although in the direction of the southern half of the Cygnus Loop, this star is almost certainly a foreground object and typical of other active cool stars that are related to RS CVn systems. An EXOSAT program to study T Tauri stars failed to detect T Tau itself. However, a strong X-ray source was observed 15' from T Tau, which in its turn had not been seen by Einstein. This new source has been identified with a hitherto unstudied 13 mag star which is likely to be a dMe flare star. The young star cluster NGC 2264 was observed with the EXOSAT CMA in an attempt to identify the sources found during an Einstein IPC study of S Mon. Apart from S Mon itself, only UV-bright objects were seen, but several of these are considered likely counterparts of the Einstein sources.