Gravitational instabilities in a protosolar-like disc III: molecular line detection and sensitivities

TL;DR

This study uses radiative transfer simulations to assess the detectability of gravitational instabilities in young, embedded protoplanetary discs via molecular line observations, highlighting observational strategies with ALMA.

Contribution

It demonstrates the feasibility of detecting GI-induced spiral structures in protosolar-like discs using molecular line observations and provides specific observational parameters for ALMA.

Findings

Spiral features are visible in absorption due to GI heating and high extinction.

High spatial resolution and specific molecular lines are needed for resolving spirals.

PV diagrams can reveal finger-like features indicative of GI activity with feasible observation times.

Abstract

At the time of formation, protoplanetary discs likely contain a comparable mass to their host protostars. As a result, gravitational instabilities (GIs) are expected to play a pivotal role in the early phases of disc evolution. However, as these young objects are heavily embedded, confirmation of GIs has remained elusive. Therefore, we use the radiative transfer code LIME to produce line images of a $0.17\,\mathrm{M}_{\odot}$ self-gravitating protosolar-like disc. We note the limitations of using LIME and explore methods to improve upon the default gridding. We synthesise noiseless observations to determine the sensitivities required to detect the spiral flux, and find that the line flux distribution does not necessarily correlate to the abundance density distribution; hence performing radiative transfer calculations is imperative. Moreover, the spiral features are seen in absorption, due to the GI-heated midplane and high extinction, which could be indicative of GI activity. If a small beamsize and appropriate molecular line are used then spatially resolving spirals in a protosolar-like disc should be possible with ALMA for an on-source time of 30 hr. Spectrally resolving non-axisymmetric structure takes only a tenth as long for a reasonable noise level, but attributing this structure to GI-induced activity would be tentative. Finally, we find that identifying finger-like features in PV diagrams of nearly edge-on discs, which are a direct indicator of spirals, is feasible with an on-source time of 19 hr, and hence likely offers the most promising means of confirming GI-driven spiral structure in young, embedded protoplanetary discs.