1 Introduction

The brightness variability of late-type stars, on the time scale of the star's rotation period, is attributed to the combined light contribution of dark and bright inhomogeneities on the stellar photosphere, such as dark starspot and bright plages respectively, which are carried in and out-of-view as the star rotates (see e.g. Lanza & Rodonò 1999; Strassmeier & Linsky 1996). In ultra fast rotating stars, such as the active members of young open star clusters, on which the present program is focused, short- and long-term brightness variability is mainly dominated by dark spots, the plages contribution being negligible. The latter dominates the long-term variability in slow rotating stars, such as the Sun (e.g. Radick et al. 1989, 1998; Foukal & Lean 1988). The presence of dark spots is determined by the emergence into the photosphere of bundles of intense magnetic flux tubes, whose generation and intensification is attributed to the action of a hydromagnetic dynamo (Parker 1979; Schüssler 1983).

Therefore, the fraction of stellar photosphere covered by spots represents a measure of the photospheric magnetic filling factor by spots ($f_{\rm S}$), which may be used to probe the dependence of that fraction of photospheric magnetic activity confined to spots on global stellar parameters. However, the value of the spot coverage and, consequently, of the magnetic filling factor ($f_{\rm S}$) must be considered as a lower limit when it is inferred from the rotational brightness variability amplitude. In fact, several factors such as the inclination (i) of the star's rotation axis and the latitude where the spots or spot groups are centred, the total fractional area covered by spots and the asymmetric component of their longitudinal distribution, all play a key role in reducing the visibility modulation of the total spots area projected onto the stellar disk.

As predicted by the dynamo theory, which expects the magnetic flux density to decrease towards slow rotation rates, the maximum light curve amplitude from extended time series ( $A_{\rm max}$) is found to be a decreasing function of the rotation period ( $P_{\rm rot}$). However, analysing a large sample of single lower main-sequence stars of known rotation period and with well determined light curve amplitude, Messina et al. (2001) have noted that the data upper envelope in the rotation-amplitude relations displays different behaviours when considering the following different rotation period ranges:

$P\leq 1.10$ (d)	( ultra fast rotators)
P > 1.10 (d)	( fast rotators).

The existence of these rotation ranges is better seen considering K-type stars, which are the most numerous in the studied sample. In particular, the maximum light curve amplitude shows different decreasing slopes when passing from ultra fast to fast rotators. In order to better determine the maximum amplitude upper envelope position and its dependence on rotation period on a more statistically robust ground, a larger number of light curves is needed for each star. In fact, only from a large number of light curves covering a sufficiently extended time interval the amplitudes can reliably approach their maximum value. Therefore, a program of photometric monitoring of ultra fast rotating active members of the Pleiades and Alpha Persei star clusters was undertaken in the fall of 1999.

 

 

Table 1: Journal of the observations

HII	HJD-2450000	Date (1999)
number		October	November
250	1462-1492	10-13	8-10
324	1461-1492	9-13	9-10
335	1457-1492	5-13	10
345	1458-1492	6-13	8-10
625	1457-1492	5-13	8-10
686	1457-1492	6-13	8-10
738	1457-1492	6-13	8-9
739	1457-1492	6-13	8-10
882	1457-1492	5-11	8-10
1039	1457-1492	6-13	8-10
1532	1457-1492	5-13	8-10
1653	1462-1492	11-13	8-10
2244	1461-1490	10-13	8
3063	1457-1492	6-13	8-9
AP	HJD-2450000	December (1999)
number
15	1515-1517	2-4
19	1515-1517	2-4
32	1515-1517	2-4
43	1515-1517	2-4
93	1515-1516	2-3
193	1515-1516	2-3
244	1515-1517	2-4