EDP Sciences
Free access
Volume 406, Number 2, August I 2003
Page(s) 629 - 638
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20030689

A&A 406, 629-638 (2003)
DOI: 10.1051/0004-6361:20030689

X-ray variability of Pleiades late-type stars as observed with the ROSAT-PSPC

A. Marino1, G. Micela2, G. Peres1 and S. Sciortino2

1  Dipartimento di Scienze Fisiche e Astronomiche, Sez. di Astronomia, Università di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
    e-mail: marino@oapa.astropa.unipa.it; peres@oapa.astropa.unipa.it
2  INAF - Osservatorio Astronomico di Palermo G.S. Vaiana, Piazza del Parlamento 1, 90134 Palermo, Italy
    e-mail: giusi@oapa.astropa.unipa.it, sciorti@oapa.astropa.unipa.it

(Received 26 February 2003 / Accepted 23 April 2003)

We present a comprehensive analysis of X-ray variability of the late-type (dF7-dM)  Pleiades stars, detected in all ROSAT-PSPC observations; X-ray variations on short (hours) and medium (months) time scales have been explored. We have grouped the stars in two samples: 89 observations of 42 distinct dF7-dK2 stars and 108 observations of 61 dK3-dM stars. The Kolmogorov-Smirnov test applied on all X-ray photon time series show that the percentage of cases of significant  variability is quite similar on both samples, suggesting that the presence of  variability does not depend on mass for the time scales and mass range explored. The comparison between the Time X-ray Amplitude Distribution functions (XAD) of the set of dF7-dK2 and of the dK3-dM show that, on short time scales, dK3-dM stars show larger variations than dF7-dK2. A subsample of eleven dF7-dK2 and eleven dK3-dM  Pleiades stars allows the study of  variability on longer time scales: we found that  variability on medium - long time scales is relatively more common among dF7-dK2 stars than among dK3-dM ones. For both dF7-dK2 Pleiades stars and dF7-dK2 field stars, the  variability on short time scales depends on Lx while this dependence has not been observed among dK3-dM stars. It may be that the variability among dK3-dM stars is dominated by flares that have a similar luminosity distribution for stars of different Lx, while flaring distribution in dF7-dK2 stars may depend on X-ray luminosity. The lowest mass stars show significant rapid variability (flares?) and no evidence of rotation modulation or cycles. On the contrary, dF7-dK2  Pleiades stars show both rapid  variability and variations on longer time scales, likely associated with rotational modulation or cycles.

Key words: open clusters and associations: individual: Pleiades -- stars: coronae -- stars: late-type -- stars: rotation -- X-ray: stars

Offprint request: A. Marino, marino@oapa.astropa.unipa.it

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