Contribution of Sun-like faculae to the light-curve modulation of young active dwarfs^*

European Space Agency, ESTEC - Postbus 299, 2200 AG Noordwijk, The Netherlands e-mail: pgondoin@rssd.esa.int

Received: 9 July 2007
Accepted: 5 November 2007

Abstract

Aims.The time variability of the broadband solar irradiance depends not only on the intrinsic evolution and visibility modulation of sunspots but also on that of faculae that become brighter near the limb during the solar rotation. Sun-like faculae around spots could also play a significant role in modulating the broadband visible flux of active dwarfs. It is the aim of the present study to test this hypothesis.

Methods.I analyzed high accuracy light-curves of two active dwarfs obtained with the MOST satellite during several stellar rotation periods. The observed time series were fitted using a model that takes into account not only starspot contributions but also the areas of faculae in active regions and their bolometric contrast.

Results.A low value of the mean ratio of faculae to cool spot areas in active regions provides the best description of ϵ Eri and κ Ceti light curves.

Conclusions.Although not conclusive, this result suggests that the ratio of faculae to cool spot areas decreases in stars somewhat more active than the Sun.