Astro Seminar: Accreting gas giants in smoothing-free, 2.5 radiation-hydrodynamical simulations: Low H alpha emission efficiency

Surveys have looked for H alpha emission from accreting gas giants but found very few objects. Analyses of the detections and non-detections have assumed that the entire gas flow feeding the planet is in radial free-fall. However, hydrodynamical simulations suggest that this is far from reality. We present high-resolution 2.5D (that is, azimuthally-symmetric) radiation-hydrodynamical simulations of accretion onto a gas giant in the limiting case of a negligibly small magnetic field. We follow the gas flow from Hill-sphere scales down to the circumplanetary disc (CPD) and the planetary surface. Importantly, we do not smooth the gravitational potential of the planet, which is crucial to obtain an accurate preshock velocity. Most of the gas lands further away from the planet, spread over the CPD, leading to emission less strong than simple estimates suggest. We calculate the resulting H alpha emission and find that the production efficiency is orders of magnitude smaller than usually assumed, which must be taken into account when analysing (non-)detection statistics.

Time permitting, we also show how JWST can be used to study accretion at planetary-mass objects even at low accretion rates by discussing our recent analysis of the JWST/NIRSpec observations of TWA27B (2M1207b). This gives tantalising clues about the mass reservoir that must be present but is yet unobserved. We put this in context of the various, not mutually exclusive accretion frameworks.