STRUCTURE AND EVOLUTION OF PRE-MAIN-SEQUENCE CIRCUMSTELLAR DISKS
- 21 July 2009
- journal article
- research article
- Published by American Astronomical Society in The Astrophysical Journal
- Vol. 701 (1), 260-282
- https://doi.org/10.1088/0004-637x/701/1/260
Abstract
We present new subarcsecond (similar to 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 similar to 20 AU to similar 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 alpha-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-circle dot, 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 Sigma(R) with the radial profile of the disk viscosity nu(R) alpha R-gamma. We find values of gamma 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 alpha parameterization for the viscosity, we argue that alpha 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) x 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 gamma < 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.Other Versions
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