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تسجيل مستخدم جديدMeasuring Turbulence in TW Hya with ALMA: Methods and Limitations
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We obtain high spatial and spectral resolution images of the CO J=2-1, CN N=2-1 and CS J=5-4 emission with ALMA in Cycle~2. The radial distribution of the turbulent broadening is derived with three approaches: two `direct and one modelling. The first requires a single transition and derives Tex{} directly from the line profile, yielding a vturb{}. The second assumes two different molecules are co-spatial thus their relative linewidths allow for a calculation of Tkin{} and vturb{}. Finally we fit a parametric disk model where physical properties of the disk are described by power laws, to compare our `direct methods with previous values. The two direct methods were limited to the outer $r > 40$~au disk due to beam smear. The direct method found vturb{} ranging from $approx$~vel{130} at 40~au, dropping to $approx$~vel{50} in the outer disk, qualitatively recovered with the parametric model fitting. This corresponds to roughly $0.2 - 0.4~c_s$. CN was found to exhibit strong non-LTE effects outside $r approx 140$~au, so vturb{} was limited to within this radius. The assumption that CN and CS are co-spatial is consistent with observed linewidths only within $r lesssim 100$~au, within which vturb{} was found to drop from vel{100} ($approx~0.4~c_s$) to nothing at 100~au. The parametric model yielded a near constant vel{50} for CS ($0.2 - 0.4~c_s$). We demonstrate that absolute flux calibration is and will be the limiting factor in all studies of turbulence using a single molecule. The magnitude of the dispersion is comparable with or below that predicted by the magneto-rotational instability theory. A more precise comparison would require to reach an absolute calibration precision of order 3%, or to find a suitable combination of light and heavy molecules which are co-located in the disk.
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