LOOP MODELING OF CORONAL X-RAY-SPECTRA .4. ONE-LOOP AND 2-LOOP MODEL-FITTING OF ROSAT PSPC SPECTRA - 3 TEST CASES

We present a detailed application of coronal hydrostatic loop models t o fit ROSAT/PSPC X-ray spectra, using three late-type stars, Procyon ( F5 IV - V), iota Vir (F6 III), and HR 3625 (F9 V), selected as test ca ses showing different activity levels. We present results of the spect ral analysis with models of coronae made of one class of loops or two classes of loops, and compare them with the results obtained with two- component isothermal models. The loop model analysis is performed with a chi(2) fitting method as accurate as in the case of the two-tempera ture modeling, including evaluation of statistical confidence regions in the model parameter space. We show that the observed spectra of Pro cyon and iota Vir can be successfully fitted with one-loop models, whi le two-loop models are required to fit the spectrum of HR 3625. In the case of Procyon, for which EUVE spectra are also available, we find t hat a coronal density estimate - based on emission line ratios - is co nsistent with the results of our one-loop model fitting. The case of H R 3625 is adopted to illustrate how the confidence regions in the two- loop model parameter space can be explored and interpreted. Our final results indicate different coronal conditions for the three selected s tars: Procyon emission is dominated by relatively short (L similar to 10(9) cm), low pressure (p(0) similar to 2 dyn cm(-2)) and low tempera ture (T-max = 1.7 x 10(6) K) loops, covering few tens percent of the s tellar surface; the corona of iota Vir appears composed of relatively higher pressure (p(0) = 2-10 dyn cm(-2)), higher temperature (T-max = 1 - 2 x 10(7) K) loops with height (L = 1 - 6 x 10(10) cm) comparable with the pressure scale height; finally, the coronal emission of HR 36 25 can be modelled with low-temperature (T-max = 1 - 2 x 10(6) K), sho rt (L < 5 x 10(8) cm) loops, with base pressure p(0) > 6 dyn cm(-2), p ossibly covering a large fraction of the stellar surface, plus higher temperature (T-max > 7 x 10(6) K) loops which may range from compact? very active region loops (L < 10(9) cm, p(0) > 10(2) dyn cm(-2)) cover ing less than 0.5% of the surface, to larger, relatively less active r egion loops (L similar to 10(10) cm, p(0) = 5-10 dyn cm(-2)) covering at most 10% of the surface.