{"id":117,"date":"2026-02-16T21:40:07","date_gmt":"2026-02-16T21:40:07","guid":{"rendered":"https:\/\/teams.issibern.ch\/nuclearspintemperatures\/?page_id=117"},"modified":"2026-02-16T21:47:34","modified_gmt":"2026-02-16T21:47:34","slug":"first-meeting","status":"publish","type":"page","link":"https:\/\/teams.issibern.ch\/nuclearspintemperatures\/meetings\/first-meeting\/","title":{"rendered":"First Meeting"},"content":{"rendered":"

First ISSI Teams Meeting:<\/strong><\/span>\u00a0February 9 – 13, 2026, Bern, Swiss<\/strong><\/span><\/span><\/h2>\n

Motivations<\/b><\/span><\/h3>\n

The aim of this ISSI International Teams is to brings together observers, modellers, and laboratory\/theory experts to reassess what nuclear-spin isomer ratios \u2014 most prominently ortho-to-para ratios (OPRs) \u2014 actually tell us in astronomical environments. The central motivation is that OPRs have long been treated as \u201ccosmogonic thermometers\u201d (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. <\/span>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 H<\/span>2<\/span><\/sub>O, NH<\/span>3<\/span><\/sub>, NH2<\/sub><\/span>, H2<\/sub><\/span>CO, CH<\/span>4<\/span><\/sub>, C2<\/sub><\/span>H<\/span>6<\/span><\/sub>, c-C3<\/sub><\/span>H2<\/sub><\/span>, and ions such as H2<\/sub><\/span>O<\/span>+<\/span><\/sup>, H3<\/sub><\/span>O+<\/sup><\/span>, NH3<\/sub>+<\/sup><\/span>, NH<\/span>4<\/sub>+<\/sup><\/span>.\u00a0<\/span><\/span><\/p>\n

Key scientific themes and takeaways<\/b><\/span><\/h3>\n

Does OPRs measure formation temperature? <\/b><\/span>A repeated theme across talks and discussions is that the historical \u201cspin temperature\u201d interpretation \u2014 particularly the ~ 30 K narrative for cometary water \u2014 may be biased by how OPRs map onto theoretical curves that approach statistical equilibrium (SE) asymptotically. Many cometary measurements cluster near the SE value (H<\/span>2<\/span><\/sub>O OPR \u2248 3), and small analysis\/modelling systematics can produce apparently meaningful \u201ctemperatures\u201d even when the underlying ratio is simply near its asymptote. <\/span>The group\u2019s emerging consensus is that <\/span>OPR measurements may not preserve primitive formation conditions,<\/strong> but rather diagnose local processing (coma physics\/chemistry, opacity, phase-transition physics, and\/or state-dependent formation\/destruction chemistry). <\/span><\/span><\/p>\n

Nuclear spin conversion and phase transitions: <\/b><\/span>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) <\/span>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?<\/span><\/p>\n

Gas-phase chemistry: \u201cformation mechanism drives OPR\u201d (Quack’s selection rules): <\/b><\/span>Another cornerstone is that gas-phase reactive formation pathways<\/b> can imprint non-statistical OPRs under certain selection rules. The group discussed p<\/strong>roton-hop<\/b> vs <\/span>proton-scrambling<\/b> pathways producing different branching ratios, <\/span>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 th<\/span><\/span>e coupling of OPR behaviour to the <\/span>OPR of H2<\/sub> in cold environments. <\/span>This theme became especially important for interpreting ISM and disk results (NH3<\/sub><\/span>, H2<\/sub><\/span>O, H2<\/sub><\/span>CO), and it motivates ongoing laboratory work on state-dependent reactivity (e.g., ortho vs para water reactions).<\/span><\/span><\/p>\n

Comets: water and organics, large datasets, variability, and JWST-era complexity: <\/b>Several discussions stressed that continuum placement, temperature assumptions, and retrieval systematics can strongly affect derived OPRs,<\/strong> so the community needs clearer communication of limitations and uncertainty propagation. Now available large database can help to identify these biases. <\/span>Opacity<\/span><\/strong> emerged as an important problem: in active comets, fundamental bands can saturate, making \u201capparent OPR\u201d artificially low. Studies on NH2<\/sub> and H2<\/sub>O+<\/sup> suggest that the observed ratios are heavily influenced by dissociation\/ionisation pathways and molecular structure rather than just reflecting a single “spin temperature.”\u00a0<\/span><\/span><\/p>\n

ISM and diffuse clouds: puzzling low OPRs and chemistry-driven explanations:\u00a0<\/b><\/span>From PRISMAS\/Herschel and related work, typical values\u00a0in the diffuse and translucent ISM\u00a0show water OPRs near statistical equilibrium, but the coldest and densest clouds show <\/span>lower OPRs<\/b>. Also in this case, <\/span>OPR may primarily probe chemistry and local conditions<\/b>, not formation temperature.<\/span><\/span><\/p>\n

Photodissociation regions and protoplanetary discs: <\/b><\/span>For H2<\/sub>CO, early interpretations tied OPR to grain-surface formation vs gas formation, but newer understanding complicates this picture. <\/span>Chemical modelling suggests H2<\/sub>CO OPR can track <\/span>kinetic temperature and the OPR of H2<\/sub><\/b>, with elemental abundances (C\/O ratio) and cosmic ray ionisation affecting abundances but less so the OPR itself.\u00a0<\/span><\/span><\/p>\n

List of talks presented at the meeting:\u00a0<\/span><\/strong><\/span><\/p>\n