Probing dust and grain growth in the optically thick circumbinary ring of V892 Tau

TL;DR

This study investigates dust and grain growth in the optically thick circumbinary ring of the triple system V892 Tau using archival ALMA data from $0.9$ to $3.1$ mm and VLA data up to $9.8$ mm. Through spectral-index analysis and 3D radiative-transfer modelling with fixed geometry, it constrains the ring’s inner/outer radii ($R_{\rm in}=25^{+4}_{-2}$ au, $R_{\rm out}=51\pm3$ au), inclination ($i=55.5^{\circ}$) and position angle ($PA=52.7^{\circ}$), and derives radial profiles for dust surface density $\Sigma_d$, temperature $T_d$, maximum grain size $a_{\max}\approx0.2$ cm, and size-distribution exponent $q=-3.5$. The ring remains optically thick out to $\sim3$ mm, with $\tau>1$ across that range, while longer-wavelength emission becomes progressively optically thinner. The inferred fragmentation velocity is $v_{\rm frag}\approx5$ m s$^{-1}$ inside $\sim32$ au and $\approx8$ m s$^{-1}$ beyond, with results suggesting dust-trapping at the cavity edge but tidal perturbations from the central binary limit grain growth; these findings underscore the importance of studying planet formation in multiple systems and demonstrate a methodology that can be extended to a broader sample of circumbinary discs.

Abstract

A considerable proportion of young stars belong to multiple star systems. Constraining the planet formation processes in multiple stellar systems is then key to understand the global exoplanet population. This study focuses on investigating the dust reservoir within the triple system V892 Tau. Our objective is to establish constraints on the properties and characteristics of the dust present in the system's circumbinary ring. Based on archival ALMA and VLA data from 0.9 mm to 9.8 mm, we present a multi-wavelength analysis of the ring of V892 Tau. We first studied the spatial variation of the spectral index, before employing 3D full radiative transfer calculations to constrain the ring's geometry and the radial dependence of the dust grain properties. Spectral indices are consistent with non-dust emission in the vicinity of the central binary, and with dust emission in the ring likely remaining optically thick up to 3.0 mm. Our radiative transfer analysis supports these interpretations, yielding a model that reproduces the observed intensities within the 1-sigma uncertainties across all wavelengths. The resulting dust surface density and temperature profiles both decrease with increasing radius, and are in agreement with values reported in the literature. Maximum grain sizes are constrained to 0.2 cm, with a size distribution power-law index -3.5. These results imply that the dust grain fragmentation velocity does not exceed 8 m/s. Whilst our results suggest dust trapping at the cavity edge, they also suggest that tidal perturbations triggered by the central binary limit grain growth within the ring. This highlights the need to further constrain planet formation efficiency in multiple stellar systems, a goal that may be advanced by applying the methodology of this work to a wider sample of systems.

Figures (16)

• Figure 1: From left to right : Continuum maps of V892 Tau from ALMA observations at $0.9$ mm, $1.3$ mm, and $3.1$ mm, and from combined VLA observations at $8.0$ mm and $9.8$ mm. The size of the synthesized beam is represented by the grey ellipse at the bottom left of each image. Orientation of the sky plane is indicated in the leftmost panel.
• Figure 2: Deprojected radial profiles computed from the images shown in Figure \ref{['fig:obs_gallery']}, respectively at $0.9$ mm, $1.3$ mm, $3.0$ mm, and combined observations of effective wavelength $8.9$ mm from left to right. The shaded area in each panel indicates the uncertainty computed following Equation \ref{['eq:rad_error']}. At $8.9$ mm the radial profile of the binary-subtracted image is also shown. The average geometrical size of the beam is plotted in the top right of each panel.
• Figure 3: SED of V892 Tau measured from ALMA and VLA observations. The fluxes of the inner and outer discs are represented by the blue and green points, respectively. Our best-fit power-laws and their corresponding spectral index are represented in plain lines in orange between $0.9$ mm and $3.0$ mm (i.e.$100-333$ GHz), and in black between $3.0$ mm and $9.8$ mm (i.e.$30.5-100$ GHz). $\alpha_{\text{in}}$ and $\alpha_{\text{out}}$ are the spectral indices of the inner and outer discs respectively. The error-bars for the outer disc are smaller than the markers at wavelengths shorter than $3.0$ mm.
• Figure 4: Spectral index maps of V892 Tau. Before the calculation of the maps, a $3\sigma$ clipping has been applied to each image, which were then convolved to a common resolution. For a spectral index map between two wavelengths, the resolution was chosen accordingly to the largest beam of the two images, which is depicted at the bottom left of each panel. The black contour indicates the $\alpha=2$ threshold.
• Figure 5: Reduced $\chi^2_{\text{geo}}$ map of the geometric models tested for V892 Tau. The best-fit model is highlighted by the blue square and consists of an inner radius of $R_{in}=25$ au and an outer radius $R_{out}=51$ au. It is found with $\Hat{\chi}^2_{\text{geo}}=33$.