Accretion-Induced Lithium Line Enhancements in Classical T Tauri Stars: RW Aur

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 LiI and KI. A resonance line of TiI and a low excitation potential line of CaI 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 CTTS. Computing LTE lithium abundances of RW Aur in a low and then high accretion state yields abundances which vary by one 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 super-meteoritic lithium abundance.Comment: 28 pages, 8 figures, accepted by Ap

Effect of Si on the hydrogen-based direct reduction of Fe2O3 studied by XPS of sputter-deposited thin-film model systems

Understanding the effect of gangue elements is of critical importance to optimize the efficiency of hydrogen -based direct reduction (HyDR) of iron ore, as one of the key steps towards climate-neutral steel production. Here, we demonstrate on the example of Si-doped Fe2O3, how thin films can be effectively utilized as a model system to facilitate systematic investigation of the solid-state reduction behavior. In-vacuo X-ray photoelectron spectroscopy (XPS) is used to probe the reduction kinetics by analyzing the chemical state of iron oxide thin films before and after annealing at 700 degrees C in an Ar+5%H2 atmosphere. It is demonstrated that even low Si concen-trations of 3.7 at.% inhibit the HyDR of Fe2O3 by the formation of a SiOx-enriched reduction barrier in the surface-near region