Constraining stellar rotation and planetary atmospheric evolution of a dozen systems hosting sub-Neptunes and super-Earths

Abstract

We constrain the planetary atmospheric mass fraction at the time of the dispersal of the protoplanetary disk and the evolution of the stellar rotation rate for a dozen multi-planet systems that host sub-Neptunes and/or super-Earths. We employ a custom-developed Python code that we have dubbed Pasta (Planetary Atmospheres and Stellar roTation rAtes), which runs within a Bayesian framework to model the atmospheric evolution of exoplanets. The code combines MESA stellar evolutionary tracks, a model describing planetary structures, a model relating stellar rotation and activity level, and a model predicting planetary atmospheric mass-loss rates based on the results of hydrodynamic simulations. Through a MCMC scheme, we retrieved the posterior PDFs of all considered parameters. For ages older than about 2 Gyr, we find a median spin-down (i.e. $P(t)\propto t^y$) of $\bar{y}=0.38_{-0.27}^{+0.38}$, indicating a rotation decay slightly slower than classical literature values ($\approx$0.5), though still within $1σ$. At younger ages, we find a median spin-down (i.e. $P(t)\propto t^x$) of $\bar{x}=0.26_{-0.19}^{+0.42}$, which is below what is observed in young open clusters, though within $1σ$. However, these two results are likely due to a selection bias as the systems suitable to be analysed by Pasta contain at least one planet with a hydrogen-dominated atmosphere, implying that the host star has more likely evolved as a slow rotator. We further look for correlations between the initial atmospheric mass fraction of the considered planets and system parameters, but without finding any. The TESS, CHEOPS, and PLATO missions are going to be instrumental in identifying and precisely measuring systems amenable to Pasta's analysis and can thus potentially constrain planet formation and stellar evolution.