In situ accretion of gaseous envelopes on to planetary cores embedded in evolving protoplanetary discs

The core accretion hypothesis posits that planets with significant gaseous envelopes accreted them from their protoplanetary discs after the formation of rocky/icy cores. Observations indicate that such exoplanets exist at a broad range of orbital radii, but it is not known whether they accreted their envelopes in situ, or originated elsewhere and migrated to their current locations. We consider the evolution of solid cores embedded in evolving viscous discs that undergo gaseous envelope accretion in situ with orbital radii in the range 0.1-10 au. Additionally, we determine the long-term evolution of the planets that had no runaway gas accretion phase after disc dispersal. We find the following. (i) Planets with 5M(circle plus) cores never undergo runaway accretion. The most massive envelope contained 2.8M(circle plus) with the planet orbiting at 10 au. (ii) Accretion is more efficient on to 10M. and 15M(circle plus) cores. For orbital radii ap = 0.5 au, 15M(circle plus) cores always experienced runaway gas accretion. For a(p) = 5 au, all but one of the 10M(circle plus) cores experienced runaway gas accretion. No planets experienced runaway growth at a(p) = 0.1 au. (iii) We find that, after disc dispersal, planets with significant gaseous envelopes cool and contract on Gyr time-scales, the contraction time being sensitive to the opacity assumed. Our results indicate that Hot Jupiters with core masses less than or similar to 15M(circle plus) at less than or similar to 0.1 au likely accreted their gaseous envelopes at larger distances and migrated inwards. Consistently with the known exoplanet population, super-Earths and mini-Neptunes at small radii during the disc lifetime, accrete only modest gaseous envelopes.