Observability of Forming Planets and their Circumplanetary Disks I. -- Parameter Study for ALMA

TL;DR

This study uses simulated ALMA observations to explore the detectability of circumplanetary disks around forming planets, highlighting optimal wavelengths and the effects of observational parameters on gap and disk detection.

Contribution

The paper provides a detailed parameter study on the observability of CPDs with ALMA, including effects of wavelength, planet mass, and orbital separation, which is novel in simulating realistic observational scenarios.

Findings

CPDs are hot and detectable at ALMA Band 9 despite being unresolved.

Optically thin CPDs can be detected in Band 7 with longer integration times.

Gap profiles vary significantly across ALMA bands, affecting planet mass estimates.

Abstract

We present mock observations of forming planets with ALMA. The possible detections of circumplanetary disks (CPDs) were investigated around planets of Saturn, 1, 3, 5, and 10 Jupiter-masses that are placed at 5.2 AU from their star. The radiative, three dimensional hydrodynamic simulations were then post-processed with RADMC3D and the ALMA Observation Simulator. We found that even though the CPDs are too small to be resolved, they are hot due to the accreting planet in the optically thick limit, therefore the best chance to detect them with continuum observations in this case is at the shortest ALMA wavelengths, such as Band 9 (440 microns). Similar fluxes were found in the case of Saturn and Jupiter-mass planets, as for the 10 $\mathrm{M_{Jup}}$ gas-giant, due to temperature weighted optical depth effects: when no deep gap is carved, the planet region is blanketed by the optically thick circumstellar disk leading to a less efficient cooling there. A test was made for a 52 AU orbital separation, showed that optically thin CPDs are also detectable in band 7 but they need longer integration times ($>$5hrs). Comparing the gap profiles of the same simulation at various ALMA bands and the hydro simulation confirmed that they change significantly, first because the gap is wider at longer wavelengths due to decreasing optical depth; second, the beam convolution makes the gap shallower and at least 25% narrower. Therefore, caution has to be made when estimating planet masses based on ALMA continuum observations of gaps.