On the X-ray emission from M-type giants *
Matthias Hünsch 1, 2,
Jürgen H.M.M. Schmitt 2,
Klaus-Peter Schröder 3 and
Franz-Josef Zickgraf 2, 4
Received 10 January 1997 / Accepted 13 October 1997
We have searched for X-ray emission from M-type giants and supergiants listed in the Bright Star Catalogue (BSC) in the data of the ROSAT all-sky survey (RASS). These stars lie to the far right of the X-ray dividing line and are thus not expected to be X-ray sources. Any X-ray detection would therefore violate the common paradigm of X-ray dark M-type giants beyond the X-ray dividing line. We found 11 BSC M-type giants and supergiants to coincide with RASS X-ray sources. While for 4 stars the X-ray emission is very likely related to their cataclysmic or symbiotic nature or can be attributed to a visual G-type companion, the other 7 stars are candidates for intrinsic X-ray emission. Of these objects, 3 have a rather large offset between optical and X-ray position, so their proper identification with late-type giants is at least questionable. For the remaining four stars, we obtained optical low-dispersion spectra in order to search for emission lines indicative of a possible symbiotic nature. None of these stars shows any bright emission lines, so they are probably quite normal M-type giants. We discuss possible origins of X-ray emission in these stars and the importance of evolutionary aspects.
Key words: stars: activity stars: coronae stars: late-type X-rays: stars
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© European Southern Observatory (ESO) 1998
Online publication: January 8, 1998