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Published August 10, 2009 | Published
Journal Article Open

Structure and Evolution of Pre-main-sequence Circumstellar Disks


We present new subarcsecond (~0".7) Combined Array for Research in Millimeter-wave Astronomy (CARMA) observations of the 1.3 mm continuum emission from circumstellar disks around 11 low- and intermediate-mass pre-main-sequence stars. High-resolution observations for three additional sources were obtained from the literature. In all cases the disk emission is spatially resolved. We adopt a self-consistent accretion disk model based on the similarity solution for the disk surface density and constrain the dust radial density distribution on spatial scales of about 40 AU. Disk surface densities appear to be correlated with the stellar ages where the characteristic disk radius increases from ~20 AU to ~100 AU over about 5 Myr. This disk expansion is accompanied by a decrease in the mass accretion rate, suggesting that our sample disks form an evolutionary sequence. Interpreting our results in terms of the temporal evolution of a viscous α-disk, we estimate (1) that at the beginning of the disk evolution about 60% of the circumstellar material was located inside radii of 25-40 AU, (2) that disks formed with masses from 0.05 to 0.4 M⊙ , and (3) that the viscous timescale at the disk initial radius is about 0.1-0.3 Myr. Viscous disk models tightly link the surface density Σ(R) with the radial profile of the disk viscosity ν(R) ∝ R^γ. We find values of γ ranging from –0.8 to 0.8, suggesting that the viscosity dependence on the orbital radius can be very different in the observed disks. Adopting the α parameterization for the viscosity, we argue that α must decrease with the orbital radius and that it may vary between 0.5 and 10^(–4). From the inferred disk initial radii we derive specific angular momenta, j, for parent cores of (0.8 – 4) × 10^(–4) km s^(–1) pc. Comparison with the values of j in dense cores suggests that about 10% of core angular momentum and 30% of the core mass are conserved in the formation of the star/disk system. We demonstrate that the similarity solution for the surface density for γ < 0 can explain the properties of some "transitional disks" without requiring discontinuities in the disk surface density. In the case of LkCa 15, a smooth distribution of material from few stellar radii to about 240 AU can produce both the observed spectral energy distribution and the spatially resolved continuum emission at millimeter wavelengths. Finally we show that among the observed sample, TW Hya is the only object that has a disk radius comparable with the early solar nebula.

Additional Information

© 2009. The American Astronomical Society. Received 2009 February 16; accepted 2009 June 15; published 2009 July 21. Print publication: Issue 1 (2009 August 10). We are indebted to Meredith Hughes and David Wilner for providing the SMA data of GM Aur and TW Hya. We thank Antonella Natta, Sean Andrews, and Leonardo Testi for the useful discussions and the referee for the very useful comments. We thank the OVRO/CARMA staff and the CARMA observers for their assistance in obtaining the data. We acknowledge support from the Owens Valley Radio Observatory, which is supported by the National Science Foundation through grant AST 05-40399. This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Michelson Fellowship Program. JPL is managed for NASA by the California Institute of Technology.

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