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Astron. Astrophys. 356, 627-642 (2000)

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Twin X-ray flares and the active corona of AB Dor observed with Beppo SAX

A. Maggio 1, R. Pallavicini 1, F. Reale 2 and G. Tagliaferri 3

1 Osservatorio Astronomico di Palermo, Palazzo dei Normanni, 90134 Palermo, Italy
2 Dipartimento Scienze Fisiche e Astronomiche, Università Palermo, Sezione di Astronomia, Piazza del Parlamento 1, 90134 Palermo, Italy
3 Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy

Received 25 October 1999 / Accepted 25 January 2000


We present two X-ray observations of the young, active star AB Dor, performed with Beppo SAX on Nov 9 and Nov 29 1997, and lasting 50 ksec and 140 ksec, respectively. Two, quite similar, large flares occurred during these observations, characterized by a 100-fold increase of the X-ray flux, peak temperatures of about [FORMULA] K, and peak emission measures of [FORMULA] cm-3.

We have performed a detailed, time-resolved analysis of the LECS and MECS X-ray spectra during the flares, and also in the subsequent quiescent phases. We have derived a metal abundance of the coronal plasma about half the photospheric one, with no significant increase during the flaring events. However, abundance variations of less than a factor [FORMULA] cannot be excluded, within our statistical uncertainties. The exponential decay of the X-ray light curves, and the time evolution of plasma temperature and emission measure are similar to those observed in compact solar flares. The analysis of the flare decay, following the method of Reale et al. (1997), indicates the presence of sustained heating, and yields coronal loop maximum heights smaller than (but comparable with) the stellar radius, in both cases.

The flares occurred at different rotation phases, and in one case, the decay was observed for one entire rotation period with no evidence of self-eclipse by the star. We conclude that the flaring loops may be located in the circumpolar region of AB Dor, but they do not appear to be related in any way with the large magnetic structures which sustain the well-known prominences of AB Dor, at several stellar radii above the surface.

During the quiescent phase, lasting two rotation periods after the second flare, we have found evidence of significant low-level variability but no indication of rotational modulation. This variability, together with the high plasma temperature ([FORMULA] MK) and large emission measure ([FORMULA] cm-3), are interpreted in the framework of a saturated corona.

Key words: stars: coronae – stars: late-type – stars: abundances – stars: activity – X-rays: stars – stars: individual: AB Dor

Send offprint requests to: (maggio@oapa.astropa.unipa.it)

© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000