New Formation Models for the Kepler-36 System

TL;DR

This paper presents advanced formation models for the Kepler-36 system, incorporating heavy element enrichment and stellar radiation effects, successfully matching observed planetary masses and radii with fewer hydrogen-helium layers.

Contribution

It introduces updated formation models that include heavy element dissolution and stellar radiation effects, providing a better fit to Kepler-36 observations compared to standard models.

Findings

Models fit Kepler-36 c's mass and radius with less H/He.

Kepler-36 b's H/He envelope is fully lost early on.

Updated models are hotter and less dense than standard models.

Abstract

Formation of the planets in the Kepler-36 system is modeled by detailed numerical simulations according to the core-nucleated accretion scenario. The standard model is updated to include the dissolution of accreting rocky planetesimals in the gaseous envelope of the planet, leading to substantial enrichment of the envelope mass in heavy elements and a non-uniform composition with depth. For Kepler-36 c, models involving in situ formation and models involving orbital migration are considered. The results are compared with standard formation models. The calculations include the formation (accretion) phase, as well as the subsequent cooling phase, up to the age of Kepler-36 (7 Gyr). During the latter phase, mass loss induced by stellar XUV radiation is included. In all cases, the results fit the measured mass, 7.84 M$_\oplus$, and radius, 3.68 R$_\oplus$, of Kepler-36 c. Two parameters are varied to obtain these fits: the disk solid surface density at the formation location, and the "efficiency" factor in the XUV mass loss rate. The updated models are hotter and therefore less dense in the silicate portion of the planet and in the overlying layers of H/He, as compared with standard models. The lower densities mean that only about half as much H/He is needed to be accreted to fit the present-day mass and radius constraints. For Kepler-36 b, an updated in situ calculation shows that the entire H/He envelope is lost, early in the cooling phase, in agreement with observation.