The Circumbinary Disc of HD 34700A II. Analysis of a strong dust asymmetry

Abstract

ALMA observations have shown that substructures are ubiquitous in protoplanetary discs. A sub-group, the transition discs, shows large cavities and rings in dust continuum. Among these, some present very high contrast asymmetries possibly due to the presence of vortices. HD 34700A is a binary system featuring a cavity, a ring, and multiple spiral arms detected in scattered light, a prominent crescent in the ALMA continuum and a complex gas morphology possibly connected with ongoing infall. We present new ALMA band 6 (1.3 mm) continuum images of the circumbinary disc around HD 34700A and compare them with two other systems showcasing high ($\gtrsim30$, measured as the peak-to-azimuthal-average ratio) contrast continuum asymmetries, IRS 48 and HD 142527. We aim to characterise the crescent morphology and discuss their possible origin. We perform visibility modelling of the new high resolution (0.''11x0.''09) ALMA band 6 continuum data of HD 34700A, together with improved visibility modelling of the other two targets. Our visibility model is in remarkable agreement with the HD 34700A data, featuring only localised residuals in the region of the disc corresponding to the tail of the asymmetry. We reproduce the double-peaked emission in HD 142527, and recover the crescent shape in IRS 48. We then run a hydrodynamic model of a vortex with different dust fluids, reproducing the general asymmetric crescent morphology observed in the HD 34700A and IRS 48 systems. With a combination of visibility, dust evolution and hydrodynamical models, we have constrained the morphology of the dust continuum emission of HD 34700A for the first time, and improved existing models for IRS 48 and HD 142527. The high azimuthal contrast of the asymmetries rules out the orbit clustering of eccentric cavities scenario, while the dust evolution models we consider suggest that the vortex scenario is a plausible option.

Figures (7)

• Figure 1: Continuum images of HD 34700 A (left), IRS 48 (middle) and HD 142527 (right). The white/black contours are taken at $3\sigma$, $5\sigma$ and $2^n\sigma$, with integer numbers $n\geq3$. The white plus sign marks the centre of the ring in the galario model, the ellipse in the bottom left corner represents the synthesised beam and the arrow in the bottom right corner shows the direction of the gas rotation. We apply an asinh stretch from $\{0.007, 0.007, 0.000\}~\rm mJy beam^{-1}$ to a factor $\{0.2, 0.4, 0.7\}$ of the peak intensity, using a stretch parameter of $\{0.1, 0.01, 0.1\}$ to the colour scale to visually enhance the fainter emission.
• Figure 2: Left column: de-projected continuum images of HD 34700 A (top), IRS 48 (middle) and HD 142527 (bottom) in polar coordinates. Contours are the same as in Figure \ref{['fig:Continuum gallery']}. In the bottom left corner we show the rotation direction of the gas, while in the bottom right corner we show the beam size in the radial direction. Right column: Azimuthal profiles of the de-projected continuum images from the left column, taken at different radii around the peak location, corresponding to the dashed lines with the same colours in the left column. We use the savgol_filter function of the scipyVirtanen_2020 module to smooth the oscillations introduced by the interpolation on the polar grid. We also show the ALMA beam size associated to the azimuthal profile evaluated at the peak radius. The plots have been shifted azimuthally so that the image peak is in the centre and the $-180\degree$ azimuth coincide with the $180\degree$ azimuth.
• Figure 3: Top to bottom: Images of HD 34700 A; IRS 48; HD 142527. Left to right: Continuum images, same as in Figure \ref{['fig:Continuum gallery']}; CLEANed galario model images; CLEANed residual images; In the left and middle columns, contours are the same as in Figure \ref{['fig:Continuum gallery']}. In the right column, white solid (positive) and dashed (negative) contours start at $3\sigma$ and $5\sigma$, and increase by $5\sigma$, while black contours correspond to the $3\sigma$ emission from the CLEANed model.
• Figure 4: Comparison between azimuthal profile of the observed dust emission at the radial location of the peak intensity (purple line) and model intensity azimuthal profile (blue line) for the HD 34700 A system. The black dashed line corresponds to the azimuthal average of the observed intensity at $170~\rm au$.
• Figure 5: Synthetic observation of an HD 34700 A analogue using a multi-fluid hydrodynamical simulation of a vortex. The emission is optically thin with the exception of the centre of the vortex (black circle after convolving). A slight tail is visible on the simulated vortex, similar to the observations of HD 34700 A and IRS 48.