Synthetic Observations of Gravitationally Unstable Protoplanetary Discs with ALMA & the SKA

It is well known that protoplanetary discs (PPDs) are the birthplace of extrasolar planetary systems. These discs are home to a wide array of complex physical processes, many of which require further investigation to be understood. Amongst the phenomena worth studying in PPDs is gravitational instability (GI), which occurs in discs of sufficiently high mass. When a disc is gravitationally unstable the relatively laminar, Keplerian flow of orbiting gas is disrupted by self-gravitational interactions, causing spiral structures to form. The arms of these spirals are regions of high local density, which trap dust grains at low relative velocities – making them potential hotbeds for early planetesimal formation. Observations of these structures up to ~millimeter wavelengths with the Atacama Large Millimeter/Submillimeter Array (ALMA) have provided great insight into GI, but observations at longer wavelengths are still needed to form a more complete picture. Enter the Square Kilometer Array (SKA): a long baseline interferometric radio telescope which is currently under construction and projected to begin scientific operations in 2028. The SKA will provide unprecedented resolution and sensitivity at ~centimeter wavelengths, allowing astronomers to probe the effects of dust trapping for larger grains than previously possible. This work aims to predict how the SKA might be used to advance our understanding of dust trapping caused by GI in PPDs. Hydrodynamic simulations of PPDs are used in conjunction with ray tracing software to develop idealized images of gravitationally unstable discs at 1mm (ALMA band 7) and 2.4 cm (SKA band 5b). These images show the discs as they would be seen with “perfect” telescopes, unencumbered by realistic limitations such as Airy diffraction, atmospheric seeing, and attenuation from foreground objects. Noise is then added to the idealized images in accordance with ALMA’s current capabilities, the SKA’s projected capabilities, and standard models of atmospheric seeing. The relative abundances of dust grains that thermally emit at these wavelengths can be constrained via measures of the relative fluxes. Differences in the relative abundances inside and outside of spiral arms can then provide information about the efficiency of dust trapping for different grain sizes.