V347 Aurigae: from Peak to Quiescence

We present an analysis of eight epochs of high-resolution optical spectra of the YSO (young stellar object) V347 Aur. The source exhibits periodic brightness variations spanning about 2 mag over several months, interpreted as cyclic accretion-burst and quiescent phases, making V347 Aur a useful probe of the broad range of accretion and outflow variability in YSOs. When faint, V347 Aur shows an M-type photospheric absorption spectrum; when bright, it exhibits a strong emission-line spectrum. Our Keck/HIRES spectroscopy was obtained during the decay of an accretion burst. The emission spectrum includes the strong H I, Ca II, and He I transitions commonly seen in YSOs, as well as numerous weaker metal lines, for which we provide a complete inventory. As the star fades, photospheric absorption lines deepen, consistent with decreasing veiling from the accretion continuum. H I and He I emission, which trace accretion, strengthen relative to the continuum as it fades, but show the expected decline in strength with decreasing accretion after correcting for the strong early veiling. Ca II behaves similarly, with line profiles that alternate between red- and blueshifted dominance, which we interpret as the rotation of the magnetospheric accretion flows. Narrow, slightly blueshifted absorption in Ca II, Na I, and K I indicates disk winds with localized density enhancements. Forbidden [O I] and [S II] emission is blueshifted, with broad shoulders and low- and high-velocity narrow peaks, consistent with a disk geometry that obscures much of the receding outflow. Their behavior as the continuum fades is similar to that of the broad accretion lines. The V347 Aur system is a useful proxy both for the ubiquitous short-term accretion variability observed in YSOs, and for their secular evolution on longer time scales.