Hydrodynamic Model of H$\alpha$ Emission from Accretion Shocks of Proto-Giant Planet and Circumplanetary Disk

TL;DR

This study combines hydrodynamic simulations to model Hα emission from accretion shocks on proto-giant planets and their circumplanetary disks, revealing localized emission regions and variable accretion rates.

Contribution

It introduces a combined 2D and 1D simulation approach to understand Hα emission and accretion processes in proto-planet systems, addressing the scale gap problem.

Findings

Hα emission mainly from localized surface areas of the protoplanet.

Accretion shocks above the circumplanetary disk produce weaker Hα emission.

Accretion rates are steady on 10-day timescales but variable on shorter timescales.

Abstract

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 CPD produce much weaker H$\alpha$ emission (approximately 1-2 orders of magnitude smaller in luminosity). Nevertheless, the accretion shocks above 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 reveal the property of the accretion process.