Constraints on PDS 70 b and c from the dust continuum emission of the circumplanetary discs considering in situ dust evolution

TL;DR

This study models dust evolution in circumplanetary discs around PDS 70 b and c to constrain their masses and accretion rates using ALMA dust emission data, providing first such constraints from dust continuum flux.

Contribution

Introduces a novel dust evolution model to derive constraints on planet properties from dust continuum emission in circumplanetary discs.

Findings

PDS 70 c has a dust flux density indicating a mass >5MJ and accretion rate >0.02 MJ/Myr.

The MMdot parameter for PDS 70 c exceeds 0.4 MJ^2/Myr.

Non-detection of PDS 70 b's dust emission suggests it has lower mass or accretion rate, or different dust properties.

Abstract

The young T Tauri star PDS 70 has two gas accreting planets sharing one large gap in a pre-transitional disc. Dust continuum emission from PDS 70 c has been detected by Atacama Large Millimeter/submillimeter Array (ALMA) Band 7, considered as the evidence of a circumplanetary disc, but the emission from PDS 70 b has not. We constrain the planet mass and the gas accretion rate of the planets by introducing a model of dust evolution in the CPDs and reproducing the detection and non-detection. We first develop a 1D steady gas disc model of the CPDs reflecting the planet properties. We then calculate the radial distribution of the dust profiles considering the dust evolution in the gas disc and calculate the total flux density of dust thermal emission from the CPDs. We find positive correlations between the flux density and three planet properties, the planet mass, gas accretion rate, and their product called "MMdot". We then find that the MMdot of PDS 70 c is larger than 0.4 MJ^2/Myr, and the planet mass and the gas accretion rate are larger than 5MJ and 0.02 MJ/Myr, respectively. This is the first case to succeed in obtaining constraints on planet properties from the flux density of dust continuum from a CPD. We also find some loose constraints on the properties of PDS 70 b from the non-detection. We propose possible scenarios for the planets explaining the non-detection respectively detection of the dust emission. The first explanation is that planet c has larger planet mass and/or larger gas accretion rate than planet b. The other possibility is that the CPD of planet c has a larger amount of dust supply and/or weaker turbulence than that of planet b. If the dust supply to planet c is larger than b due to its closeness to the outer dust ring, it is also quantitatively consistent with that planet c has weaker H-alpha line emission than planet b considering the dust extinction effect.