Formation of super-Earths around low-mass stars: evolution of an icy dead zone

Abstract

Exoplanet observations show that close-in super-Earths are more common around M-dwarfs than around solar mass stars. Since the snow line in a protoplanetary disc plays a crucial role in determining the amount of solid material available for planet formation, we explore the icy regions of protoplanetary discs around stars with masses 0.1, 0.5 and 1 $\rm M_\odot$. In a protoplanetary disc, a dead zone, where the magneto-rotational instability (MRI) is suppressed, provides a quiescent region for solids to settle to the mid-plane and planets to form. Viscosity may be driven in the dead zone by gravitational instability if enough material builds up. Heating from the gravitational instability can trigger the MRI and an accretion outburst onto the star. There may be two icy regions in a disc: (1) far from the star and (2) in the dead zone close to the star. We solve the 1D disc equations to find steady state solutions and time-dependent evolution with different values for the critical surface density in the MRI-active surface layers. Larger surface density in the MRI-active surface layers reduces the extent and lifetime of the inner icy region. The inner icy region in the dead zone around a solar mass star is small and short-lived. Around M-dwarfs, the size of the inner icy region is more persistent and oscillates between the accretion outbursts in the region 0.1-1 au. An extended icy region within the dead zone of a disc around M-dwarfs may promote the formation of more numerous and massive close-in super-Earths.