Statistics
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aas...193.9901h&link_type=abstract
American Astronomical Society, 193rd AAS Meeting, #99.01; Bulletin of the American Astronomical Society, Vol. 30, p.1395
Statistics
Scientific paper
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 Angstroms) and (possibly) Fe XXI (1354 Angstroms); analysis of density sensitive lines (O IV) imply an electron density of n_e ~ 10(12) cm(-3) in the transition region (TR) at T~10(5) 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.
Daw Adrian
Herczeg Gregory J.
Saar Steven H.
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