Initial state of the collapsing medium

Determine the initial state of the medium undergoing collapse before the Stage 1 chemical evolution of the NGC 253 central molecular zone.

Background

The UCLCHEM calculations require an assumed initial chemical and irradiation state for the collapsing cloud. Because the initial state of the medium in the central molecular zone of NGC 253 is not observationally established, the authors adopt a Galactic cosmic-ray ionization rate during Stage 1 so that sulphur-bearing species can form under comparatively normal conditions. Constraining the actual initial state would test the sensitivity of the subsequent shock and static-cloud predictions to this assumption.

References

Since we do not know the initial state of the medium and to allow species to form under normal conditions, we therefore used a Galactic CRIR for Stage 1.

— Sulphur within the extreme environment of the central molecular zone of NGC 253: a chemical modelling approach  (2609.01162 - Bouvier et al., 1 Sep 2026) in Section 2, Stage 1 – Collapsing cloud