Formation and early evolution of gas giants: Shocks and planetary spins (Part II)

Recent observational efforts have started to reveal a population of young and bright gas giants at large distances from their stars through the technique of direct imaging. Because of their youth, the brightness of these objects still reflects the conditions of their formation and the physical processes that shape them. Also, rotation rates are starting to be measured for planets of ever smaller mass. Even more recently, a few systems have been discovered in which we are witnessing the ongoing formation of gas giants, as revealed by accretion signatures. The issue with these exciting detections, however, is that models are necessary to infer masses from the luminosities but that these models are currently unconstrained at early times. What is thought to be the single most important element setting a planet’s luminosity is the radiative accretion shock at the surface of the planet. The extreme outcomes lead to hot and cold starts, respectively, in which the planet begins its post-formation cooling with a high or low luminosity. These differences in luminosity are colossal and, critically, have been hampering a reliable determination of the masses. Therefore, we propose to conduct a study of the planet accretion shock by performing zoom three-dimensional radiation-hydrodynamical simulations. We will focus on the accretion region at the surface of the planet to determine the post-shock conditions as well as the efficiency of the shock, i.e., the fraction of the incoming kinetic energy which is lost as radiation. These results will enable us to predict the luminosity of gas giants after their formation and as well as to provide input to simulations of the observational appearance of (embedded) accreting planets. Furthermore, we will predict the distribution of the spins of planets as a function of time, and predict their spectral appearance. This research is designed as a series of smaller tasks which extend and combine existing tools. In view of near-future direct detections, this makes the proposed research a low-risk, high-impact project.