Oligarchic growth and migration scenarios for short-period neptune and super-earth formation
Meeting Room 2, CMS
The discovery of short-period Neptune-mass objects, now including the remarkable system HD69380 with three Neptune analogues, presents challenges to current formation models. Several formation scenarios have been proposed, where most combine the canonical oligarchic picture of core accretion with type I migration and planetary atmosphere physics (e.g. Terquem & Papaloizou 2007; Alibert et al. 2006). These consider only a very small number of progenitors at late times, raising questions about the earlier evolution. Using global N-body simulations, we ask whether the standard model of oligarchic core accretion with embryos experiencing type I migration can generate a population of hot Neptune systems. This problem is investigated using both traditional semianalytic methods for modelling oligarchic growth as well as a new code designed specifically for treating formation problems with large dynamic range (McNeil & Nelson 2009). We consider a wide range of plausible disc parameters, and find that it is difficult for oligarchic migration models to reproduce the observed distribution. By comparison, it is relatively straightforward to form short-period icy super-Earths. We conclude that either the conditions in discs which produce hot Neptunes differ significantly from those of our simple disc models, or we are missing important physics that modifies the migratory behaviour of forming planets.