LOOP MODELING OF CORONAL X-RAY SPECTRA. IV. ONE- AND TWO-LOOP MODEL FITTING OF ROSAT PSPC SPECTRA : THREE TEST CASES

We present a detailed application of coronal hydrostatic loop models to fit ROSAT/PSPC X-ray spectra, using three late-type stars, Procyon (F5 IV V), ι Vir (F6 III), and HR 3625 (F9 V), selected as test cases showing different activity levels. We present results of the spectral 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 χ fitting method as accurate as in the case of the two-temperature modeling, including evaluation of statistical confidence regions in the model parameter space. We show that the observed spectra of Procyon and ι Vir can be successfully fitted with one-loop models, while 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 that a coronal density estimate – based on emission line ratios – is consistent with the results of our one-loop model fitting. The case of HR 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 stars: Procyon emission is dominated by relatively short (L ∼ 10 cm), low pressure (p0 ∼ 2 dyn cm ) and low temperature (Tmax = 1.7× 10 6 K) loops, covering few tens percent of the stellar surface; the corona of ι Vir appears composed of relatively higher pressure (p0 = 210 dyn cm), higher temperature (Tmax = 1 − 2 × 10 7 K) loops with height (L = 1 − 6 × 10 cm) comparable with the pressure scale height; finally, the coronal emission of HR 3625 can be modelled with low-temperature (Tmax = 12× 10 K), short (L 6 dyn cm , possibly covering a large fraction of the stellar surface, plus higher temperature (Tmax > 7 × 10 6 K) loops which may range from compact, very active region loops (L 10 2 dyn cm) covering less than 0.5% of the surface, to larger, relatively less active region loops (L ∼ 10 cm, p0 = 5-10 dyn cm ) covering at most 10% of the surface. Send offprint requests to: A. Maggio