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Astron. Astrophys. 325, 237-248

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Loop modeling of coronal X-ray spectra

IV. One- and two-loop model fitting of ROSAT PSPC spectra: three test cases

A. Maggio and G. Peres

Istituto e Osservatorio Astronomico, Palazzo dei Normanni, I-90134 Palermo, Italy
(maggio@oapa.astropa.unipa.it, gperes@oapa.astropa.unipa.it)

Received 16 January 1997 / Accepted 6 March 1997


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), FORMULA 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 FORMULA 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 FORMULA 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 ( FORMULA cm), low pressure (p0 FORMULA 2 dyn cm-2) and low temperature ( FORMULA = 1.7 FORMULA K) loops, covering few tens percent of the stellar surface; the corona of FORMULA Vir appears composed of relatively higher pressure (p0 = 2-10 dyn cm-2), higher temperature ( FORMULA FORMULA K) loops with height ( FORMULA cm) comparable with the pressure scale height; finally, the coronal emission of HR 3625 can be modelled with low-temperature ( FORMULA = 1-2 FORMULA K), short ( FORMULA cm) loops, with base pressure p0 > 6 dyn cm-2, possibly covering a large fraction of the stellar surface, plus higher temperature ( FORMULA FORMULA K) loops which may range from compact, very active region loops ( FORMULA cm, p0 FORMULA dyn cm-2) covering less than 0.5% of the surface, to larger, relatively less active region loops ( FORMULA cm, p0 = 5-10 dyn cm-2) covering at most 10% of the surface.

Key words: stars: coronae - stars: late-type - X-rays: stars - methods: data analysis

Send offprint requests to: A. Maggio

© European Southern Observatory (ESO) 1997