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

Abstract

We present an analysis of the X-ray emission of the rapidly rotating giant star YY Mensae observed by Chandra HETGS and XMM-Newton. The high-resolution spectra display numerous emission lines of highly ionized species; Fe XVII to Fe XXV lines are detected, together with H-like and He-like transitions of lower Z elements. Although no obvious flare was detected, the X-ray luminosity changed by a factor of two between the XMM-Newton and Chandra observations taken 4 months apart (from log L X ≈ 32.2 to 32.5 erg s −1 , respectively). The coronal abundances and the emission measure distribution have been derived from three different methods using optically thin collisional ionization equilibrium models, which is justified by the absence of opacity effects in YY Men as measured from line ratios of Fe XVII transitions. The abundances show a distinct pattern as a function of the first ionization potential (FIP) suggestive of an inverse FIP effect as seen in several active RS CVn binaries. The low-FIP elements (< 10 eV) are depleted relative to the high-FIP elements; when compared to its photospheric abundance, the coronal Fe abundance also appears depleted. We find a high N abundance in YY Men's corona which we interpret as a signature of material processed in the CNO cycle and dredged-up in the giant phase. The corona is dominated by a very high temperature (20 − 40 MK) plasma, which places YY Men among the magnetically active stars with the hottest coronae. Lower temperature plasma also coexists, albeit with much lower emission measure. Line broadening is reported in some lines, with a particularly strong significance in Ne X Lyα. We interpret such a broadening as Doppler thermal broadening, although rotational broadening due to X-ray emitting material high above the surface could be present as well. We use two different formalisms to discuss the shape of the emission measure distribution. The first one infers the properties of coronal loops, whereas the second formalism uses flares as a statistical ensemble. We find that most of the loops in the corona of YY Men have their maximum temperature equal to or slightly larger than about 30 MK. We also find that small 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 to detect flares with peak luminosities L X ≤ 10 31 erg s −1 with current detectors.

X-ray properties of active M dwarfs as observed by XMM-Newton

“…While the abundances determined with medium resolution spectroscopy are apparently often low, the results obtained with the global fits of high resolution data in our work or in Besselaar et al (2003) appear more reliable. We also point out that our joint fit approach, which uses a data selection comparable to Audard et al (2004), who obtained consistent results on the abundances with line-based and global models applied to high resolution X-ray spectra, results in slightly increased but still significant subsolar abundance values.…”

The X-ray spectra of the flaring and quiescent states of YZ CMi observed by XMM-Newton

“…Various approaches have been developed (see discussion in Güdel 2004), although the basic results appear to be less dependent on the methodology than on the still significant systematic uncertainties in, and the incompleteness of, the atomic physics tabulations. In particular, Audard et al (2004) and Telleschi et al (2005) have studied several cases of coronal spectra using approaches that make use of the binned spectra, on the one hand, and of a list of extracted line fluxes on the other. In both cases, satisfactory agreement was obtained for the thermal emission measure (EM) structure and the coronal abundances.…”

“…Here we apply a multi‐temperature fit with 10 different thermal components on a grid. This method is somewhat similar to one of the approaches discussed by Telleschi et al (2005) and Audard et al (2004). For this case, the temperatures were fixed on a grid between 0.02 and 10.24 keV with a binwidth of 0.3 dex, that is, in steps of a factor of 2.0 in temperature.…”

X‐Ray Spectroscopic Signatures of the Extended Corona of FK Comae

“…Photoexcitation of the intercombination lines of He-like O by the radiation field of Algol A also failed to exhibit the phase dependence that would be expected from a compact corona. Finally, we note that Audard et al (2004) detected line broadening in excess of the instrumental profile in Chandra spectra of the active single giant YY Men (v sin i ¼ 45 km s À1 ; Piskunov et al 1990). While these authors found the measured line widths reasonably consistent with thermal broadening in the extremely hot (up to $10 8 K) corona of this star, they also suggest that some contribution from rotational broadening due to X-rayYemitting material high above the surface could be present as well.…”