Accretion dynamics of EX Lupi in quiescence

Abstract

Context. EX Lupi is a young, accreting M0 star and the prototype of EXor variable stars. Its spectrum is very rich in emission lines, including many metallic lines with narrow and broad components. The presence of a close companion has also been proposed, based on radial velocity signatures. Aims. We use the metallic emission lines to study the accretion structures and to test the companion hypothesis. Methods. We analyse 54 spectra obtained during five years of quiescence time. We study the line profile variability and the radial velocity of the narrow and broad metallic emission lines. We use the velocity signatures of different species with various excitation conditions and their time dependency to track the dynamics associated with accretion. Results. We observe periodic velocity variations in the broad and the narrow line components, consistent with rotational modulation. The modulation is stronger for lines with higher excitation potentials (e.g. He II), which are likely produced in a confined area very close to the accretion shock. Conclusions. 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 in the accretion column. 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. We demonstrate that a rotationally modulated line-dependent veiling can explain the radial velocity signature of the photospheric absorption lines, making the close-in companion hypothesis unnecessary. The accretion structure is locked to the star and very stable during the five years of observations. Not all stars with similar spectral types and accretion rates show the same metallic emission lines, which could be related to differences in temperature and density in their accretion structure(s). The contamination of photospheric signatures by accretion-related processes can be turned into a very useful tool for determining the innermost details of the accretion channels in the proximity of the star. The presence of 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, given the similarity of the accretion-related signatures with those produced by a companion.

X-shooter spectroscopy of young stellar objects in Lupus

“…OurṀ acc determination of 3.6×10 −8 M /yr is much higher than the one estimated by Sicilia-Aguilar et al (2015): these authors used the same X-Shooter data as us, but they analysed the reduced 1-D spectra gathered from the ESO Phase-3 data release and to our knowledge did not correct for slit losses, despite the narrow slits used during the observations. Two main reasons may explain the large discrepancy.…”

“…The spectrum shows narrower emission lines in comparison with other spectra of the same object acquired during burst (e.g. Sicilia-Aguilar et al 2015). Thus, it is most likely that the object was not in burst during the X-Shooter observation.…”

“…Therefore, we think that the flux of the emission lines, measured by Sicilia-Aguilar et al (2015) in the ESO Phase-3 1D spectrum to calculate L acc , was underestimated.…”

“…Two main reasons may explain the large discrepancy. First, Sicilia-Aguilar et al (2015) adopt A V = 0 mag, while our best fit to the spectrum yields A V = 1.1 mag. Second, we have applied a factor of 2.5 to correct for slit losses, and since the object is quite variable, we may overestimate the absolute flux of the spectrum.…”

Redshifted X-rays from the material accreting onto TW Hydrae: Evidence of a low-latitude accretion spot

“…Moreover, despite the lower significance level, the same radial velocity also emerges from the TW Hya (2) observation, indicating that the geometry of these low-latitude accretion spots could likely have remained stable over a timescale of a few years. Evidence of long-term stability of the accretion configuration was observed in a few other cases (e.g., Hamaguchi et al 2012;Sicilia-Aguilar et al 2015), although accretion more often appears as a highly variable phenomenon (e.g., Sousa et al 2016;Rigon et al 2017).…”