Constraining the properties of spots on Pleiades very low mass stars

¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany e-mail: aleks@astro.utoronto.ca
² University of Toronto, Department of Astronomy & Astrophysics, 60 St. George Street, Toronto, M5S 3H8, Canada
³ Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland

Received: 20 December 2004
Accepted: 13 April 2005

Abstract

We present results of a multi-filter monitoring campaign for very low mass (VLM) stars in the Pleiades. Simultaneous to our I-band time series (Scholz & Eislöffel 2004b), which delivered photometric periods for nine VLM stars, we obtained light curves in the J- and H-band. One VLM star with (BPL129) shows a period in all three wavelength bands. The amplitudes in I, J, and H are 0.035, 0.035, and 0.032 mag. These values are compared to simulations, in which we compute the photometric amplitude as a function of spot temperature and filling factor. The best agreement between observations and models is found for cool spots with a temperature contrast of 18–31% and a very low surface filling factor of 4–5%. We suggest that compared to more massive stars VLM objects may have either very few spots or a rather symmetric spot distribution. This difference might be explained with a change from a shell to a distributed dynamo in the VLM regime.