How Variable is Accretion in Young Stars?

Details How Variable is Accretion in Young Stars? How Variable is Accretion in Young Stars?

Jan 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21331802n&link_type=abstract

American Astronomical Society, AAS Meeting #213, #318.02; Bulletin of the American Astronomical Society, Vol. 41, p.296

Astronomy and Astrophysics

Astronomy

Scientific paper

We analyze the variability in accretion-related emission lines for 50 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 and Chamaeleon I. For all stars, we obtain typically four spectra, covering timescales from hours to months. As accretion indicators, we use Hα 10% width and CaII- line flux, both known to be correlated with the accretion rate. While the two quantities are correlated, their variability is not. Converted to accretion rates, the CaII fluxes indicate typical accretion rate changes of 0.35dex, with 33% exceeding 0.5dex, while Hα 10% width variability correspond to changes of 0.78dex, with 65% exceeding 0.5dex. We conclude that CaII fluxes are a more robust quantitative indicator of accretion than Hα 10% width. Our results demonstrate that intrinsic accretion rate changes typically do not exceed 0.5dex on timescales of days to months. Thus, accretion rates in young stars are mostly stable. The maximum extent of the variability is reached after a few days, indicating that rotation may be the dominant cause of variability. We see a clear decline of the accretion indicators towards later spectral types, reflecting the Mdot vs. M relationship. There is a gap between accretors and non-accretors, pointing to a rapid shutdown of accretion and a `two timescale' disk evolution. We conclude that the 2 orders of magnitude scatter in the Mdot vs. M relationship is dominated by object-to-object scatter instead of intrinsic source variability. This favors scenarios where the appearance of the Mdot vs. mass plot is determined primarily by evolutionary stage, environmental factors such as disk parameters, or both. This work was supported in part by NSERC grants to RJ and MHvK and an Early Researcher Award from Ontario to RJ.

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