# Measuring Turbulence in TW Hya with ALMA: Methods and Limitations

1 AMOR 2016
LAB - Laboratoire d'Astrophysique de Bordeaux [Pessac]
Abstract : 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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Article dans une revue
Astronomy and Astrophysics - A&A, EDP Sciences, 2016, 592, pp.id.A49. 〈10.1051/0004-6361/201628550〉

https://hal.archives-ouvertes.fr/hal-01325420
Contributeur : Marie-Paule Pomies <>
Soumis le : jeudi 2 juin 2016 - 11:17:56
Dernière modification le : mardi 29 mai 2018 - 12:51:08

### Citation

Richard, Teague, S. Guilloteau, Dmitry, Semenov, Thomas, Henning, Anne Dutrey, et al.. Measuring Turbulence in TW Hya with ALMA: Methods and Limitations. Astronomy and Astrophysics - A&A, EDP Sciences, 2016, 592, pp.id.A49. 〈10.1051/0004-6361/201628550〉. 〈hal-01325420〉

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