Accretion variability in RU Lup^⋆,⋆⋆

¹ Dublin Institute for Advanced Studies (DIAS), School of Cosmic Physics, Astronomy and Astrophysics Section, 31 Fitzwilliam Place, Dublin 2, Ireland
e-mail: cstock@cp.dias.ie
² Trinity College Dublin, School of Physics, College Green, Dublin 2, Ireland
³ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy

Received: 21 June 2022
Accepted: 22 September 2022

Abstract

Context. The process of accretion in classical T Tauri stars (CTTSs) has been observed to vary on different timescales. Studying this variability is vital to understanding a star’s evolution and provides insight into the complex processes at work within, including sources of the veiling present. Understanding the dichotomy between continuum veiling and emission line veiling is integral to accurately measuring the amount of veiling present in stellar spectra.

Aims. Here, 15 roughly consecutive nights of optical spectroscopic data from the spectropolarimeter ESPaDOnS are utilised to characterise the short-term accretion activity in the CTTS, RU Lup, and investigate its relationship with the veiling in the Li I 6707 Å absorption line.

Methods. The accretion-tracing H I Balmer series emission lines were studied and used to obtain the accretion luminosity (L_acc) and mass accretion rate (Ṁ_acc) for each night, which vary by a factor of ∼2 between the brightest and dimmest nights. We also measured the veiling using multiple photospheric absorption lines (Na I 5688 Å, Mn I 6021 Å, and Li I 6707 Å) for each night.

Results. We find the Li I 6707 Å line provides measurements of veiling that produce a strong, positive correlation with L_acc in the star. When corrected for Li depletion, the average veiling measured in the Li I 6707 Å line is r_{Li I, avg} ∼ 3.25 ± 0.20, which is consistent with the other photospheric lines studied (r_avg ∼ 3.28 ± 0.65).

Conclusions. We measured short timescale variability in the accretion luminosity and mass accretion rate that are intrinsic and not due to geometric effects. As the forbidden line emission we observe ([O I] 6300 Å and [S II] 6730 Å) remains remarkably constant over our epochs, it is clear that the variations in the mass accretion rate are too short to have an effect on these outflow tracers. Upon comparing the changes in veiling and accretion luminosity, we find a strong, positive correlation. This study provides an example of how this correlation can be used as a tool to determine whether a measured variability is due to extinction or an intrinsic change in accretion. As the determination of veiling is an independent process from measuring L_acc, their relationship allows further exploration of accretion phenomena in young stars.