On dust evolution in planet-forming discs in binary systems

The  vast  majority  of  stars  are  in  binaries  or  higher-order  multiple  stellar  systems. Although  in  recent years  a  large  number  of  binaries  have  been  proven  to  host  exoplanets,  how  planet  formation  proceeds  in multiple stellar systems has not been studied yet with the necessary insight from the theoretical standpoint. Our aim is to fill this gap, focusing on the evolution of the dust grains in planet-forming discs in binaries. We take into account the dynamics of the gas and the dust in discs around each component of a binary system under the hypothesis that the evolution of the circumprimary and the circumsecondary discs is independent. It is known from previous theoretical and numerical studies that the secular evolution of the gas in binary discs is hastened due to the tidal interactions with their hosting stars. Here we prove that binarity affects dust dynamics too, possibly in a more dramatic way than in the case of the gas. In particular, the presence of a stellar companion significantly reduces the amount of grains retained in binary discs because of a faster, more efficient radial drift, ultimately  shortening their lifetime. We prove that how rapidly discs  disperse depends both on the binary separation, with discs in wider binaries living longer, and on the disc viscosity. Although the less-viscous discs lose high amounts of solids in the earliest stages of their evolution, they are dissipated slowly, while those with higher viscosities show an opposite behaviour. The faster radial migration of solids in binary discs has a striking impact on planet formation, which seems to be inhibited in this hostile environment, unless other disc substructures halt radial drift further in. We conclude that planet formation in multiple stellar systems is likely to take place on very short time scales. As a post-processing step, we compute disc dust sizes from our models at ALMA wavelengths and compare them with the results of the multiple stellar disc surveys in Taurus and ? Ophiuchus. We show that radial drift naturally explains the observed disc dust sizes without invoking very high eccentricities as previously assumed.