HOW VARIABLE IS ACCRETION IN YOUNG STARS?

We analyze the variability in accretion-related emission lines for 40 Classical T Tauri stars to probe the extent of accretion variations in young stellar objects. Our analysis is based on multi-epoch high-resolution spectra for young stars in Taurus-Auriga and Chamaeleon I. For all stars, we typically obtain four spectra, covering timescales from hours to months. As proxies for the accretion rate, we use the H alpha 10% width and the Ca II-lambda 8662 line flux. We find that while the two quantities are correlated, their variability amplitude is not. Converted to accretion rates, the Ca II fluxes indicate typical accretion rate changes of 0.35 dex, with 32% exceeding 0.5 dex, while Ha 10% width suggests changes of 0.65 dex, with 66% exceeding 0.5 dex. We conclude that Ca II fluxes are a more robust quantitative indicator of accretion than Ha 10% width, and that intrinsic accretion rate changes typically do not exceed 0.5 dex on timescales of days to months. The maximum extent of the variability is reached after a few days, suggesting that rotation is the dominant cause of variability. We see a decline of the inferred accretion rates toward later spectral types, reflecting the (M) over dot versus M relationship. There is a gap between accretors and nonaccretors, pointing to a rapid shutdown of accretion. We conclude that the similar to two orders of magnitude scatter in the (M) over dot versus M relationship is dominated by object-to-object scatter instead of intrinsic source variability.