Variable structure in the PDS70 disc and uncertainties in radio-interferometric image restoration

TL;DR

This study revisits ALMA observations of the PDS70 disc, highlighting the impact of data processing choices on detecting the circum-planetary disc PDS70c, and reveals its variability and inner disc structure through alternative imaging strategies.

Contribution

It introduces a new procedure for joint multi-epoch imaging and analyzes the effects of the JvM correction, revealing PDS70c's variability and inner disc features.

Findings

JvM correction inflates signal-to-noise by up to 10 times.

PDS70c detected at 8sigma in 2019 data, but not in 2017, indicating variability.

Inner disc shows offset and variability consistent with Keplerian rotation.

Abstract

The compact mm-wavelength signal in the central cavity of the PDS70 disc, revealed by deep ALMA observations, is aligned with unresolved Halpha emission, and is thought to stem from a circum-planetary disc (CPD) around PDS70c. We revisit the available ALMA data on PDS70c with alternative imaging strategies, and with special attention to uncertainties and to the impact of the so-called "JvM correction", which is thought to improve the dynamic range of restored images. We also propose a procedure for the alignment and joint imaging of multi-epoch visibility data. We find that the JvM correction exaggerates the peak signal-to-noise of the data, by up to a factor of 10. In the case of PDS70, we recover the detection of PDS70c from the July 2019 data, but only at 8sigma. However, its non-detection in Dec. 2017 suggests that PDS70c is variable by at least 42%+-13% over a 1.75yr time-span, so similar to models of the Halpha variability. We also pick up fine structure in the inner disc, such that its peak is offset by ~ 0.04" from the disc centre. The inner disc is variable too, which we tentatively ascribe to Keplerian rotation as well as intrinsic morphological changes.