Constrain HCN parent scale length on 2025 September 12 in 3I/ATLAS

Determine the parent scale length L_p for hydrogen cyanide (HCN) in interstellar comet 3I/ATLAS during the 2025 September 12 ALMA ACA observations (r_H ≈ 2.17 au), for which three-dimensional SUBLIME radiative transfer modeling of the HCN J=4–3 line did not yield a well-constrained value.

Background

The authors modeled high-resolution ALMA ACA observations of the HCN J=4–3 transition using both 1D and 3D versions of the SUBLIME coma radiative transfer code. While they obtained constraints on the parent scale length on September 15, they report that the 3D modeling could not provide a well-constrained L_p for September 12.

This uncertainty is tied to the highly asymmetric line profile and likely projection effects related to changing nucleus rotational phases. Clarifying the HCN production mechanism on September 12 requires establishing L_p (with L_p = 0 km indicating direct nucleus release).

References

We were unable to retrieve a well-constrained L_p from the 3D model on September 12, but for September 15 the 3D model gives L_p<1430 km.

— 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 Section 4.1 (Complex Line Profile and Production Mechanisms of HCN)