Dust in discs around T Tauri stars: grain growth?

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Abstract
We have made multiband millimeter/submillimeter continuum observations of T Tauri systems in Taurus-Auriga to investigate the frequency dependence of the dust grain opacity and the properties of the discs around pre-main-sequence low-mass stars. Observations within the wavelength interval 350 to 2000 μm were made of the T Tauri systems DG Tau, Haro 6-13, DO Tau, DR Tau, DL Tau and RY Tau. Their long-wavelength emission is thought to arise principally in the cool outer regions of extensive dusty circumstellar discs. We fit simple disc models and quantify both the quality of fit and the uncertainty on the model parameters by examining the variation of reduced |$\chi^2$| over parameter space. The discs in DL Tau and RY Tau appear to be optically thick out to the longest wavelengths at which we observed them. However, for the remaining four discs we find optically thin emission, and we can use our new long-wavelength measurements to examine the frequency variation of dust grain opacity. The opacity coefficient ($\kappa_\text v$) of dust grains is generally assumed to vary as a power law at long wavelengths, and we find that the index of the opacity function β is ≈ 0.6. This value is significantly smaller than the observationally determined values often quoted for interstellar grains, and is smaller than that expected for crystalline materials at long wavelengths, where |$2\pi\ a/\lambda \lt\lt 1$|⁠. This confirms the low values for opacity indices reported by others for the same sample of T Tauri systems. Although these low indices could be indicative of non-crystalline (amorphous) or fractal dust grains, they would also be expected for grains significantly larger than those in the general interstellar medium. This may indicate the growth and accumulation of grains prior to the formation of planetesimals. We estimate upper limits to mass accretion rates of |$\sim 10^{-6}\enspace\text M_\odot\enspace\text {yr}^{-1}$|⁠. We also find that the outer radii of the discs in our sample are in the approximate range 25 to 60 au and the total mass of material (gas + dust) in each of the discs is greater than or equal to that estimated for the early solar nebula. Taken together, our results support the conclusion that many classical T Tauri stars are attended by protoplanetary discs.

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