Cm-wavelength observations of MWC 758: resolved dust trapping in a vortex

Abstract

The large crescents imaged by ALMA in transition disks suggest that azimuthal dust trapping concentrates the larger grains, but centimetre-wavelengths continuum observations are required to map the distribution of the largest observable grains. A previous detection at ∼1 cm of an unresolved clump along the outer ring of MWC 758 (Clump 1), and buried inside more extended sub-mm continuum, motivates followup VLA observations. Deep multiconfiguration integrations reveal the morphology of Clump 1 and additional cm-wave components which we characterize via comparison with a deconvolution of recent 342 GHz data (∼1 mm). Clump 1, which concentrates ∼ 1/3 of the whole disk flux density at ∼1 cm, is resolved as a narrow arc with a deprojected aspect ratio χ > 5.6, and with half the azimuthal width than at 342 GHz. The spectral trends in the morphology of Clump 1 are quantitatively consistent with the Lyra-Lin prescriptions for dust trapping in an anticyclonic vortex, provided with porous grains ( f ∼ 0.2 ± 0.2) in a very elongated ( χ ∼ 14 ± 3) and cold (T ∼ 23 ± 2 K) vortex. The same prescriptions constrain the turbulence parameter α and the gas surface density Σ g through log 10 α × Σ g /g cm −2 ∼ −2.3 ± 0.4, thus requiring values for Σ g larger than a factor of a few compared to that reported in the literature from the CO isotopologues, if α 10 −3 . Such physical conditions imply an appreciably optically thick continuum even at cm-wavelengths (τ 33 GHz ∼ 0.2). A secondary and shallower peak at 342 GHz is about twice fainter relative to Clump 1 at 33 GHz. Clump 2 appears to be less efficient at trapping large grains.

Dust traps in the protoplanetary disc MWC 758: two vortices produced by two giant planets?

“…4) have been interpreted as dust traps at local pressure maxima arising from two large-scale vortices . The cmwavelength VLA observations presented in Casassus et al (2019) support the dust trapping scenario for Clump 1, and suggest marginal trapping for Clump 2. Still, the mechanism behind the formation of the possible dust-trapping vortices in the MWC 758 disc remains elusive and is the subject of this paper.…”

“…Vortices may not be long-lived structures, however, and dust particles can progressively lose the azimuthal trapping of their vortex when the latter decays on account of the disc's turbulent viscosity. Based on the ALMA and VLA continuum observations of Dong et al (2018) and Casassus et al (2019), which are displayed in the right-hand panels of Figs. 4 and 5, we argue that Clump 1 is consistent with azimuthal trapping in a vortex, while Clump 2 is more consistent with loss of azimuthal trapping in a decaying vortex.…”

“…for the maps at 9 mm, since the peak intensities at Clumps 1 and 2 in the VLA image of Casassus et al (2019), which combines data sets in the A, B and C array configurations, are only about 15 and 7 times higher than the rms noise level, respectively. Instead of simulating the exact same uv coverage and thermal noise as in the observed datasets, we adopt a much simpler strategy, which is to add white noise to the raw maps of continuum emission.…”

“…This is done by adding at each pixel of the raw maps a random number that follows a Gaussian probability distribution with zero mean and standard deviation consistent with the rms noise in the observations. For the synthetic maps at 9 mm, the standard deviation is set to 2µJy/beam, which is the rms noise level in Casassus et al (2019)'s VLA image. Similarly, the noise standard deviation in the synthetic maps at 0.9 mm is 20µJy/beam, which is the rms noise level in Dong et al (2018)'s ALMA image.…”

“…32 as well as a compact crescent-shaped structure at ∼0. 53 in the (sub)millimetre emission Boehler et al 2018;Dong et al 2018;Casassus et al 2019). The asymmetries in the (sub)millimetre emission, which we will refer to as Clump 1 (crescent at ∼0.…”