Optical spectroscopy of EX Lupi during quiescence and outburst: Infall, wind, and dynamics in the accretion flow

TL;DR

This study analyzes optical spectra of EX Lupi during quiescence and outburst, revealing complex accretion dynamics, variable emission lines, and evidence of infall, rotation, and winds in the star's accretion flow.

Contribution

It provides detailed spectroscopic evidence of accretion processes, including infall and wind features, during different activity phases of EX Lupi, highlighting the variability and structure of the accretion flow.

Findings

Outburst increased emission lines to over a thousand, with variable profiles.

The accretion region is at ~0.1-0.2 AU, consistent with the inner disk rim.

Evidence of infall, rotation, and winds in the accretion flow.

Abstract

We explore the accretion mechanisms in EX Lupi, prototype of EXor variables, during its quiescence and outburst phases. We analyse high-resolution optical spectra taken before, during, and after its 2008 outburst. In quiescence and outburst, the star presents many permitted emission lines, including typical CTTS lines and numerous neutral and ionized metallic lines. During the outburst, the number of emission lines increases to over a thousand, with narrow plus broad component structure (NC+BC). The BC profile is highly variable on short timescales (24-72h). An active chromosphere can explain the metallic lines in quiescence and the outburst NC. The dynamics of the BC line profiles suggest an origin in a hot, dense, non-axisymmetric, and non-uniform accretion column that suffers velocity variations along the line-of-sight on timescales of days. Assuming Keplerian rotation, the emitting region would be located at ~0.1-0.2 AU, consistent with the inner disk rim, but the velocity profiles of the lines reveal a combination of rotation and infall. Line ratios of ions and neutrals can be reproduced with a temperature of T~6500 K for electron densities of a few times 10$^{12}$cm$^{-3}$ in the line-emitting region. The data confirm that the 2008 outburst was an episode of increased accretion, albeit much stronger than previous EX Lupi and typical EXors outbursts. The line profiles are consistent with the infall/rotation of a non-axisymmetric structure that could be produced by clumpy accretion during the outburst phase. A strong inner disk wind appears in the epochs of higher accretion. The rapid recovery of the system after the outburst and the similarity between the pre-outburst and post-outburst states suggest that the accretion channels are similar during the whole period, and only the accretion rate varies, providing a superb environment for studying the accretion processes.