Structure and evolution of FK Comae corona

Abstract

Abstract. FK Comae (HD 117555) is a rapidly rotating single G giant whose distinctive characteristics include a quasisinusoidal optical light curve and high X-ray luminosity. FK Comae was observed twice at two weeks interval in January 2001 by the XMM-Newton space observatory. Analysis results suggest a scenario where the corona of FK Comae is dominated by large magnetic structures similar in size to interconnecting loops between solar active regions but significantly hotter. The interaction of these structures themselves could explain the permanent flaring activity on large scales that is responsible for heating FK Comae plasma to high temperatures. During our observations, these flares were not randomly distributed on the star surface but were partly grouped within a large compact region of about 30 degree extent in longitude reminiscent of a large photospheric spot. We argue that the α − Ω dynamo driven activity on FK Comae will disappear in the future with the effect of suppressing large scale magnetic structures in its corona.

The corona of V390 Aurigae (HD 33798)

“…Figure 6 confirms that the X-ray luminosity of V390 Aurigae is among the highest within this sample of single nearby F, G and K giants, thus supporting the above evolution scenario. The coronal structure and evolutionary status of V390 Aurigae would thus be similar to that of FK Comae (Gondoin et al 2002). This justifies the classification of V390 Aurigae as an FK Comae-type star (Fekel & Marshall 1991).…”

“…Endal & Sofia (1979) and Gray & Endal (1982) pointed out that the expansion of the stars on the red giant branch together with the rearrangement of angular momentum due to the increasing depth of the convection zones may well explain the decrease of v sin i for cool giants. Gondoin et al (2002) calculated the equatorial velocity evolution of 1.7 M and 2.5 M giants using Schaller et al (1992) evolutionary models and assuming angular momentum conservation and v sin i = 150 km s −1 at T eff = 7000 K on the evolutionary tracks out of the main sequence. Comparisons of v sin i measurements with the theoretical model (see Fig.…”

Investigations of stellar coronae with XMM-Newton

“…A near-photospheric mixture of elements is brought up into the corona. Increases in the Fe abundance (or in the average metallicity) has also been observed in flares in YY Gem Stelzer et al, 2002), AB Dor , and possibly FK Com (Gondoin et al, 2002).…”

“…These lengths are about 60% of the stellar radius. Gondoin et al (2002) finds an upper limit of § ¥ cm for a flare on FK Com. The chromospheric evaporation scenario has been tested in flares in the M-type dwarf Proxima Centauri (Güdel et al, 2002b) and in the RS CVn binary Gem (Güdel et al, 2002c).…”

H$\mathsf{_{\alpha}}$ observations of the star FK Com

“…The X-ray observations of Gondoin et al (2002) reveal that the corona of FK Com is dominated by large magnetic structures similar in size to interconnecting loops between solar active regions but significantly hotter. The ratio L x /L bol of FK Com is quite lower than that of the RS CVn star AR Lac near the same period (White et al 1987;Bopp & Stencel 1981).…”