VV Pup in a low state: secondary-star irradiation or stellar activity?

TL;DR

This study investigates emission lines in the low-state polar VV Pup, finding that chromospheric activity of the secondary star likely explains the dominant emission lines, challenging the irradiation model.

Contribution

It provides observational evidence and modeling that favor secondary-star chromospheric activity over irradiation as the main source of emission lines in low-state polars.

Findings

Weak metal lines may be due to irradiation.

Dominant Balmer, NaI, and He lines are likely from chromospheric activity.

Close threading region near L1 cannot be ruled out.

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.