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Astron. Astrophys. 330, 225-231 (1998)

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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

1 Institut für Astronomie und Astrophysik, Universität Kiel, Olshausenstrasse 40, D-24118 Kiel, Germany
2 Max-Planck-Institut für Extraterrestrische Physik, D-85748 Garching, Germany
3 Institut für Astronomie und Astrophysik, TU-Berlin, Hardenbergstrasse 36, D-10623 Berlin, Germany
4 Observatoire Astronomique de Strasbourg, 11, rue de l'Universite, F-67000 Strasbourg, France

Received 10 January 1997 / Accepted 13 October 1997

Abstract

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

* Based on observations by the ROSAT and Calar Alto Observatories

Send offprint requests to: M. Hünsch
(e-mail: huensch@astrophysik.uni-kiel.de)

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This article was processed by the author using Springer-Verlag LATEX A&A style file L-AA version 3.

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© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998
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