Determine CH3OH parent scale length on dates with only low-resolution data

Ascertain the parent scale length L_p for methanol (CH3OH) in interstellar comet 3I/ATLAS on 2025 August 28, September 22, and October 1, for which the authors could not calculate an assumed L_p due to gas–dust velocity decoupling at small nucleocentric distances.

Background

High-resolution CH3OH spectra on September 18 enabled retrieval of a non-zero L_p suggestive of coma production. However, on other dates only low-resolution spectra were available. Because the CH3OH precursor likely originates from icy grains, the usual scaling of a gas-phase precursor’s photodissociation rate could not be applied.

The authors therefore state they could not calculate an assumed L_p for August 28, September 22, and October 1, and defaulted to parent models for those dates, leaving L_p undetermined.

References

Since the velocities of the gas and dust decouple at small nucleocentric distances \citep{Bodewits2024}, we cannot calculate an assumed L_p on August 28, September 22, and October 1.

— CH$_3$OH and HCN in Interstellar Comet 3I/ATLAS Mapped with the ALMA Atacama Compact Array: Distinct Outgassing Behaviors and a Remarkably High CH$_3$OH/HCN Production Rate Ratio  (2511.20845 - Roth et al., 25 Nov 2025) in Appendix C, Section C.3 (Application to Low Resolution CH3OH Spectra)