Observations of Circumplanetary Disks with ngVLA
Bochao Hu, 1,2 Hideko Nomura, 2 Takashi Tsukagoshi, 2 Seongjoong Kim, 2 Ryohei Kawabe 21Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2–12–1 Ookayama, Meguro-ku, Tokyo 152–8551
2National Astoronomical Observatory of Japan, 2–21–1 Osawa, Mitaka-shi, Tokyo 181–8588
hubochao2018@outlook.com

Abstract
Planets are formed in protoplanetary disks and gas giant planets are surrounded by circumplanetary disks (CPDs),
which are the birth place of moons. Candidate CPDs have been reported at infrared wavelength, and more recently
at millimeter wavelength by ALMA. Model calculations suggest that ngVLA will be able to detect dust continuum
emission from CPDs at wavelength of 3 mm. In addition, CO J = 1 − 0 line will be detectable if CPDs are brightenough. Detection of dust continuum emission at ngVLA Bands together with ALMA Bands will make constraint
on the properties of dust grains: temperature, optical depth and frequency dependence of opacity, which will be
useful to understand the moon formation in the CPDs.
Key words: circumplanetary disks — dust grains — satellite formation

1. Introduction
Planets are formed in protoplanetary disks, and gas giant planets are surrounded by circumplanetary disks (CPDs)
when they are formed, according to the results of hydrodynamical simulations (e.g., Lubow et al. 1999; Machida et al.
2010; Tanigawa et al. 2012). In the CPDs, regular moons, suchas the Galilean moons of Jupiter in our Solar system, are expected to be formed. It is known from observations of theGalilean moons that compositions of moons, such as ice massfractions, are very different, depending on the distance fromthe central planets, and they will be controlled by the physicalconditions of the CPDs (e.g., Canup & Ward 2002; Canup &Ward 2006; Shibaike et al. 2019).
Formation processes of CPDs and moons are theoreticallywell studied, but these theories are hardly tested by observations. Candidate CPDs of infrared point sources have beenreported so far (e.g., Sallum et al. 2015; Müller et al. 2018).
Meanwhile, Atacama Large Millimeter/submillimeter Array(ALMA) has now revealed that most of the spatially resolved
protoplanetary disks have ring and gap structures in their dust
continuum images, which are often linked with the structurescaused by gravitational interaction with planets formed in thedisks (e.g., Fig. 1, ALMA Partnership et al. 2015; Andrews
et al. 2016; Tsukagoshi et al. 2016; Andrews et al. 2018).
However, since the ring and gap structures can be formed not
only by planets, but also by other mechanisms, such as dust accumulation near the snowlines of molecules (e.g., Zhang et al.
2015; Okuzumi et al. 2016), more direct evidences are required
for further understanding of planet and CPD formation in theprotoplanetary disks. More recently, high spatial resolution andhigh sensitivity observations by ALMA have succeeded to detect au-scale millimeter point sources in dust continuum emission from protoplanetary disks, which are candidates of CPDs
(Fig. 1, Tsukagoshi et al. 2019; Isella et al. 2019).
Observations of CPDs with ngVLA are useful to make constraint on the properties of dust grains in CPDs, together withthe multiwavelength observations by ALMA. In addition, theFig. 1. ALMA observations of a planet forming disk with a signatureof a circumplanetary disk (Tsukagoshi et al. 2019).

CO J = 1−0 line will be detectable from CPDs with ngVLA if
the disks are bright enough. These observations will be helpful
to understand the formation process of moons in CPDs.

2[Vol. ,10-6MJ yr-1

・M =10-8M yr-1
p10-7MJ yr-1

J

10-5MJ yr-1

Fig. 2. Model prediction for fluxes of dust continuum emission
and fluxes at line peaks of CO transition lines from CPDs
with various mass accretion rates as a function of disk mass.

2. Detectability of Dust and Gas Emission fromCircumplanetary Disks
Fluxes of dust continuum emission and CO line emission
from CPDs are calculated based on the viscous accretion disk
model in Zhu et al. (2018) (see also Shakura & Sunyaev
1973). A planet with Jupiter mass and Jupiter radius is usedin the model. The mass accretion rates of Ṁp = 10−8 −10−5 MJ yr−1 , the turbulent viscous parameter of α = 0.001,
and the outer disk radius of 3 au are adopted for the CPD
model. Fig. 2 shows the calculated fluxes of CO J = 7 − 6,6 −
5,4−3,3−2,2−1 and 1−0 lines and dust continuum emission
at the corresponding wavelengths as a function of the disk mass(see also Rab et al. 2019). The CO J = 1 − 0 line and the dust
continuum emission at the corresponding wavelength is observable at ngVLA Band 6. If the CPD is located at a distanceof 150 pc from the Earth, the angular size of the disk diameter is 40 mas. The dust continuum emission is optically thinand the fluxes are proportional to the disk mass. For the modelwith Ṁp = 10−7 MJ yr−1 , the dust continuum emission from anunresolved disk is detectable with ∼ 8σ level with ∼ 1hr integration time. If we resolve it with a beam size of 20 mas, ∼ 5hr
integration time is needed in order to detect it with ∼ 5σ level.
For the model with Ṁp = 10−8 MJ yr−1 , we need more than
25 hr for ∼ 5σ detection. The CO 1 − 0 line is optically thickand the line flux is not sensitive to the disk mass, but affectedby the disk temperature. The detection of the line emission isrelatively difficult and requires ∼ 5hr integration time even in
the case of the model with Ṁp = 10−5 MJ yr−1 . Such highaccretion rate could be achieved according to episodic massaccretion scenario of CPDs (Brittain et al. 2020).
3. Dust Properties Constrained by Multi-Wavelength
Observations
Detection of dust continuum emission at the ngVLA Bands
is useful for constraining the properties of dust grains in
the CPDs. Multi-wavelength observations of dust continuum emission, including both optically thin and thick wavelengths, give information of dust temperature, dust opticalFig. 3. Deviation of frequency-dependent dust opacity power-law index, β, obtained by synthesized multi-wavelength observations fromthe model value. The ngVLA Band 6 (= ALMA Band 1) is included
for the results in the top-right, while only ALMA Bands 3-10 are used
for the results in the bottom-left. (See Kim et al. 2019 for details).

depth, and frequency dependence of dust opacity (κ ∝ ν β , ν
is the frequency). Observations of dust continuum emission atngVLA Bands together with ALMA Bands give a wider wavelength range, and thus, better constraint on the dust properties.
Fig. 3 shows that if we include ngVLA Bands for the multiwavelength observations, the constraint on β becomes betterby roughly three times than those with ALMA Bands 3-10 only
for a certain model (Kim et al. 2019). Since moons will beformed as a result of dust growth, the obtained dust properties
will be useful to understand the moon formation in the CPDs.
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