Photometric evidence for two-temperature photospheric inhomogeneities on magnetically active K dwarf stars

¹ INAF - Catania Astrophysical Observatory, via S. Sofia 78, 95123 Catania, Italy e-mail: [sme;gcutispoto;nlanza]@oact.inaf.it
² Dept. of Astronomy and Astrophysics, Villanova University, Villanova 19085, PA, USA e-mail: edward.guinan@villanova.edu
³ Dept. of Physics and Astronomy, Catania University, via S. Sofia 78, 95123 Catania, Italy e-mail: mrodono@oact.inaf.it

Received: 7 August 2005
Accepted: 29 September 2005

Abstract

We derive the properties of spotted regions in late-type active stars from V- and B-band photometric data. Specifically, we compare the amplitudes of spot-induced light and color variations with model amplitudes, which depend on the area, temperature and distribution of spotted regions over the stellar surface. Our analysis is applied to three well-known chromospherically active K dwarf stars: DX Leonis, AB Doradus and LQ Hydrae, and indicates that all three stars show significant variations of the average spot temperature from epoch to epoch. The observed temporal variations of the starspot temperatures are best explained as arising from the contemporary presence on the stellar photosphere of two-component (two-temperature) active regions. These consist of either dark spots and hot faculae and/or dark umbrae and penumbrae and the variations over time arise from change of their areal ratio.