Explain the unresolved multi-aperture CO flux mismatch of DFK 1

Determine the origin of the inability of one-dimensional radiative-transfer models to reproduce the increase in observed CO flux density with increasing aperture for DFK 1, including whether the discrepancy arises from data processing or from an incorrect cluster-membership distance.

Background

The paper models CO J=2J=2–1 emission from red supergiants in RSGC2 using one-dimensional non-LTE radiative transfer and compares synthetic spectra extracted with multiple aperture sizes. For DFK 1, none of the tested models reproduces the observed flux behavior across apertures: the observed flux increases with aperture, but the models do not reproduce this trend satisfactorily.

The authors identify two unresolved possibilities—a problem introduced during data cleaning or processing, or an incorrect assumed distance caused by DFK 1 not being a cluster member. Alternative distances were tested but did not resolve the discrepancy, so the physical origin remains undetermined.

References

We note that none of our models can reproduce the spectra toward DFK\,1 across multiple apertures in a satisfactory way (Fig.~\ref{fig:comodels-dfk1}). This mismatch, where we are unable to simulate the increase of the observed flux densities with increasing apertures, could have its origin in the processing (cleaning) of the data set, or perhaps indicate that this star is not a cluster member and needs a revised distance.

— The Molecular Circumstellar Environments of Red Supergiants in Galactic Open Clusters  (2609.31362 - Siebert et al., 25 Sep 2026) in Section 3.5.2, paragraph “RSGC2”