Large grains in the disk of CQ Tau
L. Testi, A. Natta, D.S. Shepherd, D.J. Wilner
We present 7mm observations of the dusty disk surrounding the 10 Myr old
1.5 Msun pre-main-sequence star CQ Tauri obtained at the Very Large Array
with 0.8 arcsecond resolution and 0.1 mJy rms sensitivity.
These observations resolve the 7mm emission in approximately the north-south
direction, confirming previous results obtained with lower resolution.
We use a two-layer flared disk model to interpret the observed fluxes
from 7mm to 1.3mm together with the resolved 7mm structure.
We find that the disk radius is constrained to the range 100 to 300 AU,
depending on the steepness of the disk surface density distribution.
The power law index of the dust opacity coefficient, beta,
is constrained to be 0.5 to 0.7.
Since the models indicate that the disk is optically thin at millimeter
wavelengths for radii greater than 8 AU, the contribution of
an optically thick region to the emission is less than 10%.
This implies that high optical depth or complex disk geometry cannot be
the cause of the observed shallow millimeter spectral index.
Instead, the new analysis supports the earlier suggestion that dust
particles in the disk have grown to sizes as large as a few centimeters.
The dust in the CQ Tauri system appears to be evolved much like that
in the TW Hydra system, a well-studied pre-main-sequence star of similar age
and lower mass.
The survival of gas-rich disks with incomplete grain evolution at such old ages
deserves further investigations.