Photospheric and chromospheric activity in V405 Andromedae

An M dwarf binary with components on the two sides of the full convection limit

¹ Eötvös Loránd University, Department of Astronomy, 1518 Budapest, PO Box 32, Hungary e-mail: vidakris@elte.hu
² Konkoly Observatory of the Hungarian Academy of Sciences, 1525 Budapest, PO Box 67, Hungary
³ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
⁴ Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁵ AEKI, KFKI Atomic Energy Research Institute, Thermohydraulic Department, 1525 Budapest 114, PO Box 49, Hungary

Received: 14 April 2009
Accepted: 30 June 2009

Abstract

We investigate the fast-rotating (), active dwarf binary V405 And (M0V+M5V) using photometric and optical spectroscopic data. The light variation is caused by the combined effect of spottedness and binarity with a small eclipse. We estimate the system parameters from the available light and radial velocity curves. Three flare events occurred during the observations: two were found in the spectroscopic data and one was observed photometrically in colours. An interesting eruptive phenomenon emerged from the photometric measurements that can be interpreted as a series of post-flare eruptions lasting for at least 3 orbits (rotations) of the system, originating from trans-equatorial magnetic loops, which connect the active regions in the two hemispheres. The two components of V405 And have masses well over and below the theoretical limit of full convection. This rare property makes the binary an ideal target for observing and testing models for stellar dynamo action.