Intensity of Hydrogen Line Emission from Accreting Gas-Giant Planets

Planets have been thought to form in circumstellar gaseous disks. Indeed, a number of young stars surrounded by such disks have been already detected. Recently, there are some reports on detection of gap-like structure in circumstellar disks, which suggests that there are forming massive protoplanets embedded in the disks. A challenging issue is how to find forming planets in circumstellar disks directly. In this study, we investigate whether detectable emission occurs from accreting gas-giant planets. In a circumstellar disk, once a solid core becomes massive enough, it captures the surrounding disk gas gravitationally in a runaway manner. Since the disk gas accretion occurs much faster than angular momentum loss, a circumplanetary disk is formed in the mid-plane of the circumstellar disk. Recent three-dimensional hydrodynamic simulations by Tanigawa et al. (2012) revealed that the gas flowed from the cicumstellar disk to the circumplanetary disk not horizontally but vertically. According to those simulations, the disk gas falls onto the circumplanetary disk at a speed comparable to the free fall speed, and the local gas temperature reaches up to tens of thousands of kelvin because of shock heating near the planet. Thus, the presence of an accreting gas giant planet may be found by observation of the radiative emission from such hot gas in the circumplanetary disk, which we quantify in this study. In particular, we focus on the intensity of line emission from hydrogen. We have simulated the post-shock gas flow with non-equilibrium chemical reaction and electron transition. Then, we have found that the intensity of some hydrogen lines is proportional to the number density of the surrounding circumstellar disk gas and square of the planet mass, so the protoplanet’s hydrogen-line emission is less intense by a few orders of magnitude relative to the protostar’s emission under some realistic conditions. Also, the duration time is comparable to the dissipation time of the disk gas (i.e., several hundred thousand years). Thus, we conclude that an accreting gas giant planet is detectable via hydrogen-line emission observation, provided the planet is massive enough or the density of the disk gas is high enough.