On the secular evolution of the ratio between gas and dust radii in protoplanetary discs

ABSTRACT A key problem in protoplanetary disc evolution is understanding the efficiency of dust radial drift. This process makes the observed dust disc sizes shrink on relatively short time-scales, implying that discs started much larger than what we see now. In this paper, we use an independent constraint, the gas radius (as probed by CO rotational emission), to test disc evolution models. In particular, we consider the ratio between the dust and gas radius, RCO/Rdust. We model the time evolution of protoplanetary discs under the influence of viscous evolution, grain growth, and radial drift. Then, using the radiative transfer code radmc with approximate chemistry, we compute the dust and gas radii of the models and investigate how RCO/Rdust evolves. Our main finding is that, for a broad range of values of disc mass, initial radius, and viscosity, RCO/Rdust becomes large (>5) after only a short time (