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Session 44 - Double Stars.
Display session, Thursday, January 08
Exhibit Hall,
The RS CVn-type binary UX Ari was observed for 14 hours with all four detectors onboard the ASCA satellite. After 12 hours of constant, quiescent X-ray emission, ASCA observed a powerful flare with a peak luminosity of 1.4\times 10^32 ergs s^-1.
We present a spectral and temporal analysis of the observations using a two-ribbon flare model. A time-dependent reconstruction of the emission measure (EM) distribution shows that two separate plasma components evolve initially. One is identified as the quiescent EM, and the other (flare) EM reaches temperatures between 50 MK and more than 100 MK. The maximum arcade length is estimated to be about 2R_\star, with a width and height about 1R_\star. Lower limits to the flare density obtained with the maximum flux model drop from about 8.5\times 10^10 cm^-3 at flare start to 3\times 10^10 cm^-3 at flare peak. The flare parameters imply a (conductive and radiative) cooling loss time of about one hour at flare peak for the maximum volume model, or less for more compact flares. The elemental abundances increase significantly during the flare rise, with the abundances of the low-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\pm4)% of the solar photospheric value during quiescence up to (89\pm18)% at flare peak. Either fractionation occurs during the chromospheric evaporation phase to selectively enrich low-FIP elements in the corona, or the evaporation itself brings metal-rich plasma into the metal-poor corona.
This work is supported by NASA grant NAG5-2750.
