Hydrodynamic Model of Hα Emission from Accretion Shocks of a Proto-giant Planet and Circumplanetary Disk

Recent observations have detected excess H alpha emission from young stellar systems with an age of several Myr such as PDS 70. One-dimensional radiation-hydrodynamic models of shock-heated flows that we developed previously demonstrate that planetary accretion flows of >a few ten km s(-1) can produce H alpha emission. It is, however, a challenge to understand the accretion process of proto-giant planets from observations of such shock-originated emission because of a huge gap in scale between the circumplanetary disk (CPD) and the microscopic accretion shock. To overcome the scale gap problem, we combine two-dimensional, high-spatial-resolution global hydrodynamic simulations and the one-dimensional local radiation-hydrodynamic model of the shock-heated flow. From such combined simulations for the protoplanet-CPD system, we find that the H alpha emission is mainly produced in localized areas on the protoplanetary surface. The accretion shocks above the CPD produce much weaker H alpha emission (approximately one to two orders of magnitude smaller in luminosity). Nevertheless, the accretion shocks above the CPD significantly affect the accretion process onto the protoplanet. The accretion occurs at a quasi-steady rate if averaged on a 10 day timescale, but its rate shows variability on shorter timescales. The disk surface accretion layers including the CPD shocks largely fluctuate, which results in the time-variable accretion rate and H alpha luminosity of the protoplanet. We also model the spectral emission profile of the H alpha line and find that the line profile is less time-variable despite the large variability in luminosity. High-spectral-resolution spectroscopic observation and monitoring will be key to revealing the property of the accretion process.

Automated summary

Observations have shown excess H alpha emission from young stellar systems like PDS 70. Models suggest that planetary accretion flows can produce this emission, but understanding the accretion process from shock-originated emission is challenging due to scale gaps. Combined simulations show that H alpha emission mainly occurs on localized areas on the protoplanetary surface, with weaker emission from accretion shocks above the CPD. The variability in accretion rate and H alpha luminosity of the protoplanet is influenced by fluctuating disk surface accretion layers.