Flaring and Quiescent Coronae of UX Arietis: Results fromASCAandEUVECampaigns

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Abstract
The RS CVn binary star UX Ari was observed for 14 hr with all four detectors onboard the Advanced Satellite for Cosmology and Astrophysics (ASCA), and for 135 ks with the spectrometers onboard the Extreme-Ultraviolet Explorer (EUVE). During the ASCA observations, the X-ray emission was at a constant, quiescent level during the first 12 hr, after which time a powerful flare with a peak luminosity of 1.4×1032 ergs s-1 started. The flare was observed until shortly after its peak. The EUVE observations were obtained on two different days when the star was in a quiescent phase. We present a spectral and temporal analysis of the UX Ari observations and interpret the ASCA flare data with a two-ribbon flare model including estimates for cooling losses. The quiescent emission measure (EM) distributions derived independently from ASCA and EUVE data agree remarkably. The distribution increases up to a peak around 25 MK. We find elemental abundances that are significantly subsolar, in particular for Fe (≈17%). A time-dependent reconstruction of the flare EM distribution shows that two separate plasma components evolve during the flare (one being identified with the quiescent EM). Most of the flare EM reaches temperatures between 50 and 100 MK or more. Magnetic confinement requires the loop arcade to be geometrically large, with length scales on the order of one stellar radius. The electron densities inferred from the model decrease from initial values around 1012 cm-3 early in the flare to about 1011 cm-3 at the flare peak. The best-fit models require surface magnetic field strengths of a few hundred G, compatible with the maximum photospheric fields expected from equipartition. The flare parameters imply a (conductive and radiative) cooling loss time of less than 1 hr at flare peak. The elemental abundances increase significantly during the flare rise, with the abundances of the low first ionization potential (FIP) elements Fe, Mg, Si, and Ni typically increasing to higher levels than the high-FIP elements, such as S or Ne. The Fe abundance increases from 17%±4% of the solar photospheric value during quiescence up to 89%±18% at flare peak. A fractionation process that occurs during the chromospheric evaporation phase may selectively enrich low-FIP elements as in the solar corona; alternatively, the chromospheric evaporation may itself bring metal-rich chromospheric plasma into the metal-poor corona.