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