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Measuring Turbulence in TW Hya with ALMA: Methods and Limitations | | Richard Teague
; Stephane Guilloteau
; Dmitry Semenov
; Thomas Henning
; Anne Dutrey
; Vincent Pietu
; Tilman Birnstiel
; Edwige Chapillon
; David Hollenbach
; Uma Gorti
; | | Date: |
31 May 2016 | | 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. | | Source: | arXiv, 1606.0005 | | Services: | Forum | Review | PDF | Favorites | |
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