Quiescent and flare analysis for the chromospherically active star Gl 355 (LQ Hya)

Abstract

Abstract. We discuss ROSAT and ASCA observations of the young active star Gl 355. During the ROSAT observation a strong flare was detected with a peak flux more than an order of magnitude larger than the quiescent level. Spectral analysis of the data allows us to study the temperature and emission measure distribution, and the coronal metal abundance, for the quiescent phase and, in the case of ROSAT, also during the evolution of the flare. The global coronal metallicity Z/Z ∼ 0.1 derived from both ROSAT and ASCA data is much lower than solar and presumably also much lower than the photospheric abundance expected for this very young star. The temperature structure of the quiescent corona was about the same during the various observations, with a cooler component at T1 ∼ 7 MK and a hotter component (to which only ASCA was sensitive) at T2 ∼ 20 MK. During the flare, the low temperature component remained approximately constant and equal to the quiescent value, while the high-temperature component was the only one that varied. We have modeled the flare with the hydrodynamic-decay sustained-heating approach of Reale at al. (1997) and we have derived a loop semi-length of the order of ∼1.5 stellar radii, i.e. much larger than the dimensions of flares on the Sun, but comparable with the typical dimensions inferred for other stellar flares. We have compared the derived loop size with that estimated with a simpler (but physically inconsistent) approach, finding that for this, as well for several other stellar flares, the two methods give comparable loop sizes. Possible causes and consequences of this result are discussed.

X-ray spectroscopy of the W UMa-type binary VW Cephei

“…comparable with the radius of the VW Cep primary star. Covino et al (2001) showed that the volumes and densities of stellar flares provided by the Pallavicini et al (1990) approach are not dissimilar to those derived from the hydrodynamic decay-sustained heating model of Reale et al (1997). These authors showed by simulation and verified with Yokhoh observations of solar flares that flares decay approximately along a straight line in the log EM 1/2 − log T diagram.…”

Structure and evolution of FK Comae corona

“…significantly smaller than FK Comae diameter (≈8 to 14 × 10 11 cm). Covino et al (2001) showed that the volumes and densities of stellar flares provided by the Pallavicini et al (1990) approach are not too dissimilar from those derived from the hydro-dynamic decay-sustained heating model of Reale et al (1997). These authors showed by simulation and verified with Yokhoh observations of solar flares that flare decay approximately along a straight line in the log EM 1/2 − log T diagram.…”

Chromospheric activity, lithium and radial velocities of single late-type stars possible members of young moving groups

“…It is a very active star, as indicated by emission in several chromospheric and transition region lines, even with occasional flares (see Montes et al 1999 and references therein). EUV and X-ray emission and X-ray flares have been detected in this star (see Covino et al 2001 and references therein). In our four spectra taken at two different epochs we observed strong emission in the Ca ii K line, the Hα line in emission above the continuum with an intensity similar to that observed in the quiescent spectra by Montes et al (1999), a filled-in absorption Hβ line, and the Ca ii IRT lines in emission.…”