THE EVOLUTION OF ACCRETION IN YOUNG STELLAR OBJECTS: STRONG ACCRETORS AT 3-10 Myr

Abstract

While the rate of accretion onto T Tauri stars is predicted to decline with age, objects with strong accretion have been detected up to ages of 10 Myr. We analyze a sample of these old accretors identified by having a significant U band excess and infrared emission from a circumstellar disk. Objects were selected from the ∼3 Myr σ Ori, 4-6 Myr Orion OB1b and 7-10 Myr Orion OB1a star forming associations. We use high resolution spectra from the Magellan Inamori Kyocera Echelle to estimate the veiling of absorption lines and calculate extinction for our T Tauri sample. We also use observations, obtained with the Magellan Echellette and in a few cases the SWIFT Ultraviolet and Optical Telescope, to estimate the excess produced in the accretion shock, which is then fit with accretion shock models to estimate the accretion rate. We find that even objects as old as 10 Myr may have high accretion rates, up to ∼10 −8 M ⊙ yr −1 . These objects cannot be explained by viscous evolution models, which would deplete the disk in shorter timescales, unless the initial disk mass is very high, a situation which is unstable. We show that the infrared spectral energy distribution of one object, CVSO 206, does not reveal evidence of significant dust evolution, which would be expected during the 10 Myr lifetime. We compare this object to predictions from photoevaporation and planet formation models and suggest that neither of these processes have had a strong impact on the disk of CVSO 206.

Rocky Planet Formation: Quick and Neat

“…The ultraviolet excess produced by accretion onto the central star declines on a similar time scale (Hartmann et al 1998;Kennedy & Kenyon 2009;Sicilia-Aguilar et al 2010;Ingleby et al 2014). …”

“…CO fundamental emission (e.g., Najita et al 2003;Salyk et al 2011, Doppmann, Najita, & Carr 2016 and UV transitions of molecular hydrogen (e.g., Ingleby et al 2011a) probe molecular gas within an AU of young stars. Among A-type stars with debris disks, several have neutral or ionized gas close to the star (e.g., Hobbs et al 1985;Ferlet et al 1987;Welsh et al 1998;Redfield 2007;Montgomery & Welsh 2012;Kiefer et al 2014;Cataldi et al 2014, and references therein).…”

X-shooter spectroscopy of young stellar objects in Lupus

“…Continuum excess luminosity has been measured in a number of objects from spectroscopy at different resolutions (e.g. Gullbring et al 1998;Herczeg & Hillenbrand 2008;Rigliaco et al 2012;Ingleby et al 2013;Alcalá et al 2014;Ingleby et al 2014;Manara et al 2014Manara et al , 2016a. More often, L acc has been computed from empirical relations between line luminosity, L line , and L acc (e.g.…”

X-Shooter study of accretion in Chamaeleon I

“…Modeling the excess continuum emission at these wavelengths yields direct measurements of L acc . This method has been used in the past on a substantial number of objects (e.g., Gullbring et al 1998;Herczeg & Hillenbrand 2008;Ingleby et al 2014), but often using non-simultaneous and low-resolution spectra at optical wavelengths to determine the photospheric properties of the target. On the other hand, the indirect method of measuring the luminosity of various emission lines (L line ) and converting this Article published by EDP Sciences A136, page 1 of 18 A&A 585, A136 (2016) into L acc through calibrated L acc -L line relations (e.g., Muzerolle et al 1998;Fang et al 2009) can be efficiently used on spectra covering a relatively narrow wavelength range, such as those offered by multi-object spectrographs.…”