VV Puppis in a low state: secondary-star irradiation or stellar activity?^*

¹ European Southern Observatory (ESO) Alonso de Cordova 3107, Vitacura, Santiago, Chile e-mail: emason@eso.org
² NOAO, 950 N. Cherry Ave., Tucson, AZ, USA e-mail: howell@noao.edu
³ Lowell Observatory, Planetary Research Center, 1400 W. Mars Hill Rd., Flagstaff, AZ, USA e-mail: barman@lowell.edu
⁴ Dept. of Astronomy, University of Washington, Seattle, WA, USA e-mail: szkody@astro.washington.edu
⁵ Australian National University, Australia e-mail: dayal@maths.anu.edu.au

Received: 6 October 2007
Accepted: 6 August 2008

Abstract

Aims. Emission lines in polars show complex profiles with multiple components that are typically ascribed to the accretion stream, threading region, accretion spot, and the irradiated secondary-star. In low-state polars the fractional contribution by the accretion stream, and the accretion spot is greatly reduced offering an opportunity to study the effect of the secondary-star irradiation or stellar activity. We observed VV Pup during an exceptional low-state to study and constrain the properties of the line-forming regions and to search for evidence of chromospheric activity and/or irradiation.

Methods. We obtained phase-resolved optical spectra at the ESO VLT+FORS1 with the aim of analyzing the emission line profile and radial velocity as a function of the orbital period. We also tailored irradiated secondary-star models to compare the predicted and the observed emission lines and to establish the nature of the line-forming regions.

Results. Our observations and data analysis, when combined with models of the irradiated secondary-star, show that, while the weak low ionization metal lines (FeI and MgI) may be consistent with irradiation processes, the dominant Balmer H emission lines, as well as NaI and HeI, cannot be reproduced by the irradiated secondary-star models. We favor the secondary-star chromospheric activity as the main forming region and cause of the observed H, NaI, and He emission lines, though a threading region very close to the L1 point cannot be excluded.