First ISSI Teams Meeting: February 9 – 13, 2026, Bern, Swiss
Motivations
The aim of this ISSI International Teams is to brings together observers, modellers, and laboratory/theory experts to reassess what nuclear-spin isomer ratios — most prominently ortho-to-para ratios (OPRs) — actually tell us in astronomical environments. The central motivation is that OPRs have long been treated as “cosmogonic thermometers” (i.e., formation-temperature tracers), especially in comets, but recent laboratory and theoretical work suggests spin-state populations may be altered by ice physics, phase transitions, chemistry, and radiative-transfer effects. The group is composed of members of communities studying comets, the Interstellar medium (ISM, clouds diffuse to dense), star-forming regions, and protoplanetary discs (PPDs), with attention to different molecules including H2O, NH3, NH2, H2CO, CH4, C2H6, c-C3H2, and ions such as H2O+, H3O+, NH3+, NH4+.
Key scientific themes and takeaways
Does OPRs measure formation temperature? A repeated theme across talks and discussions is that the historical “spin temperature” interpretation — particularly the ~ 30 K narrative for cometary water — may be biased by how OPRs map onto theoretical curves that approach statistical equilibrium (SE) asymptotically. Many cometary measurements cluster near the SE value (H2O OPR ≈ 3), and small analysis/modelling systematics can produce apparently meaningful “temperatures” even when the underlying ratio is simply near its asymptote. The group’s emerging consensus is that OPR measurements may not preserve primitive formation conditions, but rather diagnose local processing (coma physics/chemistry, opacity, phase-transition physics, and/or state-dependent formation/destruction chemistry).
Nuclear spin conversion and phase transitions: A major thread focused on when and how NSC occurs and whether OPRs can be constant over long timescales. Laboratory studies were discussed where isolated molecules prepared in a given spin state show conversion upon warming, and where desorption from ice can project populations toward the statistical 3:1 ratio. Open questions emphasised: (1) Does OPR have a robust meaning in amorphous vs. crystalline ice? (2) Is OPR altered during long-term storage in ice, at sublimation, or in the coma after release? (3) How representative are laboratory conditions of cometary, ISM, PPD environments, and which physics is truly transferable?
Gas-phase chemistry: “formation mechanism drives OPR” (Quack’s selection rules): Another cornerstone is that gas-phase reactive formation pathways can imprint non-statistical OPRs under certain selection rules. The group discussed proton-hop vs proton-scrambling pathways producing different branching ratios, the need to identify compelling observational evidence for non-statistical OPRs and to assess whether state-dependent destruction reactions can reshape observed ratios. The discussion also highlighted the coupling of OPR behaviour to the OPR of H2 in cold environments. This theme became especially important for interpreting ISM and disk results (NH3, H2O, H2CO), and it motivates ongoing laboratory work on state-dependent reactivity (e.g., ortho vs para water reactions).
Comets: water and organics, large datasets, variability, and JWST-era complexity: Several discussions stressed that continuum placement, temperature assumptions, and retrieval systematics can strongly affect derived OPRs, so the community needs clearer communication of limitations and uncertainty propagation. Now available large database can help to identify these biases. Opacity emerged as an important problem: in active comets, fundamental bands can saturate, making “apparent OPR” artificially low. Studies on NH2 and H2O+ suggest that the observed ratios are heavily influenced by dissociation/ionisation pathways and molecular structure rather than just reflecting a single “spin temperature.”
ISM and diffuse clouds: puzzling low OPRs and chemistry-driven explanations: From PRISMAS/Herschel and related work, typical values in the diffuse and translucent ISM show water OPRs near statistical equilibrium, but the coldest and densest clouds show lower OPRs. Also in this case, OPR may primarily probe chemistry and local conditions, not formation temperature.
Photodissociation regions and protoplanetary discs: For H2CO, early interpretations tied OPR to grain-surface formation vs gas formation, but newer understanding complicates this picture. Chemical modelling suggests H2CO OPR can track kinetic temperature and the OPR of H2, with elemental abundances (C/O ratio) and cosmic ray ionisation affecting abundances but less so the OPR itself.
List of talks presented at the meeting:
- “Interpretations of spin ratios in comets”, by B. P. Bonev
- “Nuclear-Spin astrochemistry: from laboratory to observations (and vice-versa)”, by A. Faure
- “Water spin temperature in comets: Cosmogonic indicator?”, by S. Faggi
- “Ortho-to-para ratio statistics in CH3OH, C2H6 and CH4“, by M. Lippi
- “Ortho-to-Para Ratios in Comets: NH2 and H2O+“, by H. Kawakita
- “H2CO OPRs in PDRs and PPD”, by V. Guzman
- “OPR in H2CO ALMA observations of C/2017 K2 (PanSTAARS)”, by D. Bockelee-Morvan
- “The ortho-para chemistry of H2CO in protoplanetary disks”, by M. Gaillard,
- “Water ortho-para ratio in the coma of 67P/Churuymov-Gerasimenko”, by D. Bockelee-Morvan
- “Testing the cosmogonic significance of spin temperatures across the solar system“, by G. L. Villanueva
* The agenda was adjusted day by day depending on the outcome of the discussions