Accretion‐induced Lithium Line Enhancements in Classical T Tauri Stars: RW Aurigae

Abstract

It is widely accepted that much of the stochastic variability of T Tauri stars is due to accretion by a circumstellar disk. The emission-line spectrum as well as the excess continuum emission are common probes of this process. In this communication, we present additional probes of the circumstellar environment in the form of resonance lines of low ionization potential elements. Using a set of 14 high-resolution echelle observations of the classical T Tauri star (CTTS) RW Aur, taken between 1986 and 1996, we carefully measure the continuum veiling at each epoch by comparing more than 500 absorption lines with those of an appropriate template. This allows us to accurately subtract out the continuum emission and to recover the underlying photospheric spectrum. In doing so, we find that selected photospheric lines are enhanced by the accretion process, namely, the resonance lines of Li I and K I. A resonance line of Ti I and a low excitation potential line of Ca I also show weak enhancements. Simple slab models and computed line bisectors lead us to propose that these line enhancements are markers of cool gas at the beginning of the accretion flow which provides an additional source of line opacity. These results suggest that published values of surface lithium abundances of classical T Tauri stars are likely to be overestimated. This would account for the various reports of surface lithium abundances in excess of meteoritic values among the extreme CTTSs. Computing LTE lithium abundances of RW Aur in a low and then high accretion state yields abundances which vary by 1 order of magnitude. The low accretion state lithium abundance is consistent with theoretical predictions for a star of this age and mass, while the high accretion state spectrum yields a supermeteoritic lithium abundance.