Some Like It Hot: The X‐Ray Emission of the Giant Star YY Mensae

(Abridged abstract) We present an analysis of the X-ray emission of therapidly rotating giant star YY Mensae observed by Chandra HETGS and XMM-Newton.Although no obvious flare was detected, the X-ray luminosity changed by afactor of two between the XMM-Newton and Chandra observations taken 4 monthsapart. The coronal abundances and the emission measure distribution have beenderived from three different methods using optically thin collisionalionization equilibrium models. The abundances show an inverse first ionizationpotential (FIP) effect. We further find a high N abundance which we interpretas a signature of material processed in the CNO cycle. The corona is dominatedby a very high temperature (20-40 MK) plasma, which places YY Men among themagnetically active stars with the hottest coronae. Lower temperature plasmaalso coexists, albeit with much lower emission measure. Line broadening isreported, which we interpret as Doppler thermal broadening, although rotationalbroadening due to X-ray emitting material high above the surface could bepresent as well. We use two different formalisms to discuss the shape of theemission measure distribution. The first one infers the properties of coronalloops, whereas the second formalism uses flares as a statistical ensemble. Wefind that most of the loops in the corona of YY Men have their maximumtemperature equal to or slightly larger than about 30 MK. We also find thatsmall flares could contribute significantly to the coronal heating in YY Men.Although there is no evidence of flare variability in the X-ray light curves,we argue that YY Men's distance and X-ray brightness does not allow us todetect flares with peak luminosities Lx <= 10^{31} erg/s with currentdetectors.Comment: Accepted paper to appear in Astrophysical Journal, issue Nov 10, 2004 (v615). This a revised version. Small typos are corrected. Figure 7 and its caption and some related text in Sct 7.2 are changed, without incidence for the conclusion