Secondary Atmospheres on HD 219134 b and c

We analyze the interiors of HD 219134 b and c, which are among the coolest super-Earths detected thus far. Without using spectroscopic measurements, we aim at constraining if the possible atmospheres are hydrogen-rich or hydrogen-poor. In the first step, we employ a full probabilistic Bayesian inference analysis to rigorously quantify the degeneracy of interior parameters given the data of mass, radius, refractory element abundances, semimajor axes, and stellar irradiation. We obtain constraints on structure and composition for core, mantle, ice layer, and atmosphere. In the second step, we aim to draw conclusions on the nature of possible atmospheres by considering atmospheric escape. Specifically, we compare the actual possible atmospheres to a threshold thickness above which a primordial (H2-dominated) atmosphere can be retained against evaporation over the planet's lifetime. The best-constrained parameters are the individual layer thicknesses. The maximum radius fraction of possible atmospheres are 0.18 and 0.13 R (radius), for planets b and c, respectively. These values are significantly smaller than the threshold thicknesses of primordial atmospheres: 0.28 and 0.19 R, respectively. Thus, the possible atmospheres of planets b and c are unlikely to be H2-dominated. However, whether possible volatile layers are made of gas or liquid/solid water cannot be uniquely determined. Our main conclusions are (1) the possible atmospheres for planets b and c are enriched and thus possibly secondary in nature, and (2) both planets may contain a gas layer, whereas the layer of HD 219134 b must be larger. HD 219134 c can be rocky.