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G.J. Herczeg (Notre Dame), S.H. Saar, A. Daw (SAO)
We have analyzed two orbits of HST GHRS rapid-readout data of the active RS CVn subgiant II Peg. About 30 min of data were taken at medium resolution centered at C IV apparently during the late decay of a flare. These show strong symmetric C IV lines composed of a narrow (50 km/s FWHM) and a broad (150 km/s FWHM) component. About 45 min of data taken at low resolution show both quiescent emission and a flare.
In the quiescent state we detect a wide range of lines including O V (1371 Å) and (possibly) Fe XXI (1354 Å); analysis of density sensitive lines (O IV) imply an electron density of ne \approx 1012 cm-3 in the transition region (TR) at T\approx105 K. Analysis of the time-arrival distribution of photons in the quiescent data suggests an excess at high counts over that expected by Poisson statistics; thus the data can be modeled by a steady-state quiescent (Poisson) process, plus a stochastic process probably due to weak flares and microflares. By fitting the maximum possible Poisson component, we estimate that in the ``quiescent'' state, at least 11% of the flux in TR lines and at least 5% in the chromospheric lines comes from microflare heating.
Analysis of the main flare shows it is dominated by continuum emission, which reaches a blackbody temperature of 22,000 K some 2.5 min into the flare. The continuum emission does not appear to be consistent with Si I recombination suggested previously; rather, we suggest reprocessed X-rays are a more likely source. The flare generated red-shifted line emission which rapidly reached +120 km/s and remained constant thereafter. Higher T lines (C IV and Si IV) were more enhanced and changed more rapidly than chromospheric lines (C II). There is some evidence for compression followed by explosive evaporation in the TR during the initial phases of the flare, but the density diagnostics give conflicting results.
The author(s) of this abstract have provided an email address for comments about the abstract: gherczeg@darwin.cc.nd.edu