The accretion dynamics of EX Lupi in quiescence:The star, the spot, and the accretion column

TL;DR

This study analyzes metallic emission lines in EX Lupi during quiescence to understand accretion structures, demonstrating rotational modulation effects that negate the need for a close companion hypothesis.

Contribution

It provides detailed insights into the accretion dynamics and structure of EX Lupi, showing the stability of accretion columns and their impact on radial velocity measurements.

Findings

Emission line profiles show rotational velocity modulation.

Narrow lines originate in post-shock regions, broad lines in pre-shock material.

Stable accretion structures can mimic companion signals in radial velocity data.

Abstract

EX Lupi is a young star, prototype of EXor variables. Its spectrum is very rich in emission lines, including many metallic lines. It has been also proposed to have a close companion. We use the metallic emission lines to study the accretion structures and to test the companion hypothesis. We analyse 54 spectra taken in 5 years of quiescence time. We study the line profile variability and the radial velocity of the metallic emission lines. We use the velocity signatures of different species with various excitation conditions and their time dependency to track the dynamics associated to accretion. We observe periodic velocity variations in the line components consistent with rotational modulation. The modulation is stronger for lines with higher excitation potentials. We propose that the narrow line components are produced in the post-shock region, while the broad components originate in the more extended, pre-shock material. All the emission lines suffer velocity modulation due to the rotation of the star. The broad components are responsible for the line-dependent veiling observed in EX Lupi. Rotationally-modulated line-dependent veiling can explain the radial velocity signatures, making the close-in companion hypothesis unnecessary. The accretion structure is locked to the star and very stable during the 5 years of observations. Not all stars with similar spectral types and accretion rates show the same metallic emission lines, maybe related to differences in temperature and density in their accretion structure(s). The contamination of photospheric lines by accretion processes can be turned into a very useful tool to determine the innermost details of the accretion channels in the proximities of the star. Emission lines from very stable accretion columns will nevertheless be a very strong limitation for the detection of companions by radial velocity in young stars. (Abridged)