The secrets of dust grains unveiled in a triple-star system

Young stars are often surrounded by a disc made of gas and dust. In that disc, small dust grains grow bigger and bigger, constituting the first step towards the formation of planets by core accretion. However, young stars are rarely alone: they are usually part of multiple systems, where they orbit alongside other stars. In this context, it is important to understand how planets form in multiple-star systems. This can be done by characterising dust grains, their evolution, and their ability to grow in multiple-star systems.

Thanks to observations of thermal emission of dust grains with the ALMA and VLA interferometers, a team of scientists unveiled the properties of dust grains in the V892 Tau system. V892 Tau harbours three stars, the two closest of which are surrounded by a circumbinary disc. The observations reveal this circumbinary disc, which takes the form of a dust ring orbiting the central binary. The cavity at the centre of the ring is believed to be dynamically carved by the binary star. The cavity is not empty, and one can still notice emission at its centre. The authors associate this emission to the magnetic activity of the binary components.

The circumbinary ring was modelled by using the radiative transfer code MCFOST. It revealed an important concentration of dust at the inner rim of the disc. At this location, dust grains close to the disc’s mid-plane can reach temperatures of 80 K and sizes of 2 mm. The authors have shown that the size of dust grains in the disc of V892 Tau is likely limited by collisions with other dust grains. Given the low dust sizes found in V892 Tau compared to other systems, the binary star likely accelerates the collision speed between dust grains and hinder their collisional growth.

This work is one of the first to make a link between dust grain properties and stellar orbits in an observational way. It paves the way for a systematic characterisation of dust grain properties in multiple-star systems, in an attempt to conclude on the impact of stellar multiplicity on planet formation.