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Coronae on solar-like stars
Jürgen H.M.M. Schmitt
Received 14 March 1996 / Accepted 27 June 1996
The results of a complete and sensitive X-ray survey of all known stars of spectral type A, F, and G in the immediate solar vicinity with distances less than 13 pc are presented. The X-ray data were obtained primarily from the ROSAT all-sky survey (RASS); those program stars not detected in the RASS data, were subsequently studied in the ROSAT pointed observation program. The detection rate among the F stars in the sample is 95 %, that of the G stars 83 %, the non-detections being due to survey observations. On the other hand, none of the A-stars with spectral type earlier than A7 could be detected even in sensitive pointings. I conclude from this that coronal formation in stars with surface convection zones is universal. The X-ray luminosities range over about four orders of magnitude, and can be well described with a log-normal distribution. Large X-ray outputs are correlated with kinematic age as assessed from space motions. I show the existence of a correlation between the total emitted X-ray surface flux and spectral hardness, such that more luminous objects tend to have larger spectral hardness, thus implying higher coronal temperatures. The mean X-ray surface fluxes span the same range as is observed for various solar coronal features, with a rather well-defined minimum X-ray flux being present; this minimum X-ray surface flux agrees very well with the X-ray surface flux of solar coronal holes. It therefore appears that a coronal hole represents the minimum state of "activity" not only for the Sun but also for other stars. I discuss a few implications of this finding especially with regard to properties of stars in Maunder minima states.
Key words: stars: X-rays coronae nearby stars
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Online publication: July 8, 1998