Evolved Stars

Stellar structure theory predicts cool giants to have deep convection zones, one of the requirements for a magnetic dynamo to work. On the other hand, single giants tend to be slow rotators, which are not expected to show high levels of activity. Furthermore, because of the complicated evolutionary paths of giants in the HR diagram, without an at least approximate knowledge of mass, the age and hence the main-sequence progenitors of the giant stars and their state of activity during the main-sequence phase are unknown.

Figure 10.5 shows that X-ray emission is frequently found among single giants. An X-ray dividing line (XDL) separates the X-ray emitting G and K giants from apparently X-ray dark M giants [27]. In a complete volume-limited sample of giants within 25 pc around the Sun all giants (to the left of the XDL) were observed and detected in the ROSAT pointing program. This finding led to the conclusion that

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Fig. 10.11 X-ray luminosity of dG stars from the Orion, p Oph, and Cha I star forming regions, the a Per, Pleiades, and Hyades open clusters, the Sun and field stars together with the 1 a equivalent data spread as a function of age; the lines show the X-ray luminosities corresponding to the saturation limit of a 0.8 Me and 1.0 Me star; Fig. 21 from [7]

giants to the left of the XDL, which all have outer convection zones, are ubiquitous X-ray emitters just like cool main-sequence stars.

The XDL occurs across a very narrow region in the HR diagram. Thus, for example, a G giant (to the left of the XDL) can have a rather high X-ray luminosity of up to Lx ^3 x 1030 ergs s^1, while K giants to the right of the XDL can be almost five orders of magnitude fainter. The most sensitive upper limit in terms of X-ray luminosity has been obtained for the nearby K giant Arcturus (Lx < 3 x 1025 ergs s^1). Expressing this upper limit in terms of mean X-ray surface flux, this X-ray nondetection corresponds to a flux value more than 1 000 times fainter than a solar coronal hole. Therefore, coronae of giants to the right of the XDL must be quite different from the solar corona - if at all they exist. Using the Chandra HRC-I camera, T. Ayres and collaborators have recently obtained a tentative X-ray detection of Arcturus. Around the known position of Arcturus 3 photons have been detected with 0.2 counts expected from background [1].

The concept of a dividing line appears to disappear among the brighter giants and supergiants. Among those stars there is a group exhibiting both signatures of transition region material (as inferred from CIV line detections) as well as cool winds (inferred from UV line profiles), the so-called hybrid stars. As a result of

ROSAT Observations of Giants and Hybrids -er

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