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Astron. Astrophys. 353, 177-185 (2000)

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A systematic analysis of X-ray variability of dM stars

A. Marino 1, G. Micela 1 and G. Peres 2

1 Osservatorio Astronomico G.S. Vaiana, Piazza del Parlamento 1, 90134 Palermo, Italy (marino, gmicela@oapa.astropa.unipa.it)
2 Dipartimento di Scienze Fisiche e Astronomiche, Università di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy (peres@oapa.astropa.unipa.it)

Received 24 November 1998 / Accepted 11 October 1999

Abstract

We have systematically analyzed X-ray variability of dM stars. Our data base is the sample of all dM stars listed in the CNS3 (Gliese & Jahreiss 1991) catalog which have been observed with the ROSAT PSPC.

Our data sample includes 86 pointed observations of 55 distinct stars or multiple systems. A large fraction of stars shows significant variations, regardless of their quiescent flux. Variability is detected on all observable time scales. The amplitudes of these variations are independent of both stellar X-ray and visual luminosity. Compared to solar X-ray variability properties our results suggest that the amplitude distribution of X-ray variability in dM stars is consistent with the analogous distribution for solar flares.

We discuss the effect of variability on the spread observed in the X-ray luminosity function of M stars.

The comparison of our data with those obtained with Einstein IPC shows that variations on time scales shorter than a few month are more common than long term variations comparable to, e.g., the 11 years solar cycle.

Key words: stars: coronae – stars: late-type – X-rays: stars – Galaxy: solar neighbourhood

Send offprint requests to: G. Micela (gmicela@oapa.astropa.unipa.it)

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© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999
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