|Authors||A. Marino(1), G. Micela(2), G. Peres(1), S. Sciortino(2)|
|Affiliation||(1) Dipartimento di Scienze Fisiche e Astronomiche, Sez. di Astronomia, UniversitÓ di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy|
(2) INAF - Osservatorio Astronomico di Palermo G.S. Vaiana, Piazza del Parlamento 1, I-90134 Palermo, Italy
|Accepted by||Astronomy & Astrophysics|
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.