Measuring turbulence in TW Hydrae with ALMA: methods and limitations

By R. Teague, S. Guilloteau, D. Semenov, Th. Henning, A. Dutrey, V. Pi'etu, T. Birnstiel, E. Chapillon, D. Hollenbach, U. Gorti. 2016

141 citations recorded by the source corpus.

Aims. We aim to obtain a spatially resolved measurement of velocity dispersions in the disk of TW Hya. 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 T ex directly from the line profile, yielding a v turb . The second method assumes that two different molecules are co-spatial, which allows using their relative line widths for calculating T kin and v turb . 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. Results. The two direct methods were limited to the outer r > 40 au disk because of beam smear. The direct method found v turb 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 c s . CN was found to exhibit strong non-local thermal equilibrium effects outside r ≈ 140 au, so that v turb 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 v turb was found to drop from 100 m s -1 (≈0.4 c s ) to zero at 100 au. The parametric model yielded a nearly constant 50 m s -1 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. 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.

Subfields: Astronomy and Astrophysics, Geophysics

View Paper: https://www.semanticscholar.org/paper/357ad61e57f94a826db6735602249830cb3789c7

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In Astronomy and Astrophysics

1. D. Wilner
2. S. Andrews
3. T. Birnstiel
4. Xue-Ning Bai
5. A. Hughes
6. Philip Armitage
7. C. Qi
8. Y. Alibert

In Geophysics

1. Xue-Ning Bai
2. Y. Alibert
3. T. Birnstiel

Citations in the corpus

• ALMA Survey of Lupus Protoplanetary Disks. II. Gas Disk Radii (2018, unknown): “10−3 (Hughes et al. 2011; Flaherty et al. 2015; Teague et al. 2016).”
• A Kinematical Detection of Two Embedded Jupiter-mass Planets in HD 163296 (2018, methodology): “To derive vrot, we exploit the azimuthal symmetry of the system and build upon the spectral deprojection and azimuthally averaging technique used in previous studies of protoplanetary and debris disks (Teague et al. 2016; Yen et al. 2016; Matrà et al. 2017).”
• Planetesimal formation starts at the snow line (2017, background): “Recent estimates range from αt < 10−3 for the outer parts of the disk around HD 163296 (Flaherty et al. 2015) to αt ≈ 10−2 in the outer parts of the TW Hya disk (Teague et al. 2016).”
• Turbulence in the TW Hya Disk (2018, methodology): “Previous efforts to measure turbulence (Carr et al. 2004; Hughes et al. 2011; Guilloteau et al. 2012; Flaherty et al. 2015; Teague et al. 2016; Flaherty et al. 2017), utilize, at minimum, the full spectral line profile in searching for non-thermal motion and can rely on differences in e.g. the line…”
• A Three-dimensional View of Turbulence: Constraints on Turbulent Motions in the HD 163296 Protoplanetary Disk Using DCO+ (2017, methodology): “Teague et al. (2016) use CO(2-1), CN(2-1), and CS(2-1) to measure the turbulence in the disk around TW Hya, finding non-thermal motion from the CO(2-1) emission layer, although the uncertainty in the amplitude calibration severely limits the measurements.”