Origin of the interstellar O/N chemical differentiation

Determine whether the observed differentiation between nitrogen-bearing and oxygen-bearing molecules in interstellar environments reflects a fundamental chemical divide or results from the combined effects of environmental conditions, physical structure, and observational biases.

Background

The paper reviews a persistent spatial and abundance disparity between oxygen-bearing and nitrogen-bearing molecules across cold prestellar cores, protostellar environments, outflows, disks, high-mass star-forming regions, and external galaxies. Although the pattern is widespread, it varies between sources and molecular species, and may be affected by excitation, dust opacity, sublimation temperatures, binding energies, and observational sensitivity.

The authors therefore leave unresolved whether the disparity represents intrinsic chemical differentiation or an apparent pattern produced by multiple physical and observational effects.

References

Whether the observed differentiation reflects a fundamental chemical divide or instead emerges from the interplay of multiple environmental, physical, and observational effects remains an open question.

— Interstellar disparity: N- vs O-bearing molecules: A multidisciplinary perspective across interstellar environments  (2609.20551 - Bouvier et al., 17 Sep 2026) in Section 1, Introduction; Section 2.1, Galactic observations

Whether O-bearing species exhibit similarly distinct spatial distributions in TMC-1 remains an open question.

— Interstellar disparity: N- vs O-bearing molecules: A multidisciplinary perspective across interstellar environments  (2609.20551 - Bouvier et al., 17 Sep 2026) in Section 2.1, Observational evidence for the galactic O- versus N-bearing dichotomy

The presence of a N- vs O-bearing dichotomy has been found in one study, but the cause of this dichotomy and whether the environment plays a role is not clear and still needs to be investigated.

— Interstellar disparity: N- vs O-bearing molecules: A multidisciplinary perspective across interstellar environments  (2609.20551 - Bouvier et al., 17 Sep 2026) in Section 2.2, Environmental effects and the extragalactic perspective

This immediately raises a broader question: where do the oxygen and nitrogen reaction networks intersect and mix? Does this occur primarily in the gas phase, on icy grains, or only with the aid of energetic processing such as cosmic rays?

— Interstellar disparity: N- vs O-bearing molecules: A multidisciplinary perspective across interstellar environments  (2609.20551 - Bouvier et al., 17 Sep 2026) in Section 2.3, Gas–grain chemistry and functional group selectivity

Is this fundamental difference in binding energies responsible, at least in part, for the chemical differentiation observed between some O-bearing and N-bearing molecules?

— Interstellar disparity: N- vs O-bearing molecules: A multidisciplinary perspective across interstellar environments  (2609.20551 - Bouvier et al., 17 Sep 2026) in Section 3.3, Laboratory experiments on iCOMs formation; Section 5, Laboratory experiments and computations