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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents

ID: 731766.0, MPI für Astronomie / Publikationen_mpia
Measuring turbulence in TW Hydrae with ALMA: methods and limitations
Authors:Teague, R.; Guilloteau, S.; Semenov, D.; Henning, T.; Dutrey, A.; Piétu, V.; Birnstiel, T.; Chapillon, E.; Hollenbach, D.; Gorti, U.
Date of Publication (YYYY-MM-DD):2016
Title of Journal:Astronomy and Astrophysics
Start Page:id. A49 (14 pp)
Audience:Not Specified
Abstract / Description:<BR /> Aims: We aim to obtain a spatially resolved measurement of velocity dispersions in the disk of TW Hya. <BR /> Methods: We obtained images with high spatial and spectral resolution 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 was derived with two direct methods and one modelling approach. The first method requires a single transition and derives Tex directly from the line profile, yielding a vturb. The second method assumes that two different molecules are co-spatial, which allows using their relative line widths for calculating Tkin and vturb. Finally we fitted a parametric disk model in which the physical properties of the disk are described by power laws, to compare our direct methods with previous values. <BR /> Results: The two direct methods were limited to the outer r > 40 au disk because of beam smear. The direct method found vturb to range from ≈130 m s-1 at 40 au, and to drop to ≈50 m s-1 in the outer disk, which is qualitatively recovered with the parametric model fitting. This corresponds to roughly 0.2-0.4 cs. CN was found to exhibit strong non-local thermal equilibrium effects outside r ≈ 140 au, so that vturb was limited to within this radius. The assumption that CN and CS are co-spatial is consistent with observed line widths only within r ≲ 100 au, within which vturb was found to drop from 100 m s-1 (≈0.4 cs) to zero at 100 au. The parametric model yielded a nearly constant 50 m s-1 for CS (0.2-0.4 cs). We demonstrate that absolute flux calibration is and will be the limiting factor in all studies of turbulence using a single molecule. <BR /> Conclusions: The magnitude of the dispersion is comparable with or below that predicted by the magneto-rotational instability theory. A more precise comparison would require reaching an absolute calibration precision of about 3%, or finding a suitable combination of light and heavy molecules that are co-located in the disk. The reduced datacubes (FITS files) are only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A> (<A href=""></A>) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/592/A49">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/592/A49</A>
Free Keywords:techniques: interferometric; turbulence; methods: observational; ISM: kinematics and dynamics; submillimeter: ISM
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
Identifiers:ISSN:0004-6361 %R 10.1051/0004-6361/201628550
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