Determine whether molecular hydrogen accounts for the EON shell mass discrepancy

Determine whether a significant fraction of the hydrogen in the extended Orion nebula shell is molecular, thereby providing sufficient shielding and accounting for the discrepancy between the shell masses inferred from H I and [C II] observations.

Background

The H I observations yield a substantially lower mass for the front hemisphere of the extended Orion nebula (EON) shell than the mass inferred from [C II] emission. The authors discuss whether this discrepancy could arise because a significant fraction of the hydrogen nuclei is in molecular form rather than in atomic H I.

Dust-opacity fluctuations do not conclusively exclude the column densities required for substantial molecular material, but CO observations provide little evidence for a sufficiently large molecular component. The authors therefore identify the molecular content of the EON shell as unresolved and note that direct H2 observations, including ultraviolet absorption and infrared quadrupole emission, are needed to test the possibility.

References

This implies that the integrated dust opacity does not conclusively exclude such column densities in the shell, leaving open the possibility of sufficient shielding for a significant fraction of the hydrogen to remain in molecular form.

— The Neutral Atomic Hydrogen in the solar neighborhood (NeAtHood) project I. Ghost in the shell: Neutral atomic hydrogen in the extended Orion nebula  (2609.02658 - Soler et al., 2 Sep 2026) in Section 3.2.2, “The H2 possibility”