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 highresolution spectra for young stars in Taurus-Auriga and Chamaeleon I. For all stars, we obtain typically four spectra, covering timescales from hours to months. As proxies for the accretion rate, we use the Hα 10% width and the Ca II-λ8662 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 Hα 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 Hα 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 towards later spectral types, reflecting the Ṁ vs. M relationship. There is a gap between accretors and non-accretors, pointing to a rapid shutdown of accretion. We conclude that the ∼ 2 orders of magnitude scatter in the Ṁ vs. M relationship is dominated by object-to-object scatter instead of intrinsic source variability. Subject headings: accretion, accretion disks — circumstellar matter — stars: formation — stars: low-mass, brown dwarfs — planetary systems