{"arxiv_id":"2611.17224","title":"The origin of Potassium in Galactic chemical evolution - The effect of C-O shell mergers in massive stars","abstract":"The Galactic chemical evolution (GCE) of potassium (K) and of most other odd-Z elements remains poorly understood: models generally underproduce the observed [K/Fe] over a wide metallicity range, depending on the adopted massive star yields. A proposed site for efficient K nucleosynthesis is the merger between the oxygen and carbon shells (C-O shell merger) that can occur in massive stars a few hours before core collapse. Some GCE models with specific yield sets have been shown to reconcile predictions with observations. Here we re-analyse these yield sets to quantify the contribution of C-O shell mergers to K production and what the Galactic evolution of K implies about the occurrence of these events. We analyse 194 pre-supernova models from three independent yield sets, covering a wide range of initial masses, metallicities and rotational velocities, and including both single and binary-stripped stars. We identify the progenitors undergoing a C-O shell merger and compute GCE models with and without their contribution. In all three sets, the enhanced K production can be traced back to a small number of C-O merger progenitors, which eject up to ~10-50 times more K than non-merger progenitors of similar mass. Without them, no model reproduces the observations and [K/Fe] is sub-solar over most of the metallicity range. With them, [K/Fe] is enhanced only within the metallicity window where mergers occur in each grid. Binarity itself plays a minor role, as single stars alone produce similar amounts of K. Since the K synthesised in a C-O merger is essentially primary, the observed nearly linear increase of [K/H] with [Fe/H] can be reproduced only if C-O shell mergers occur with a similar frequency at all metallicities. K could therefore be used to infer the occurrence rate of C-O shell mergers in massive stars as a function of time.","primary_cat":"astro-ph.GA","cross_cats":["astro-ph.HE","astro-ph.SR"],"submitted_at":"2026-11-12T18:19:37+00:00","latest_version":1,"source":"arxiv","source_label":"arXiv","source_url":null,"has_pdf":true,"has_source":false,"verdict":"CONDITIONAL","confidence":"HIGH","one_line_summary":"C-O shell mergers in massive stars are the dominant source of galactic potassium, and the observed [K/H] vs [Fe/H] trend requires these mergers to occur at all metallicities.","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","upvotes":0,"downvotes":0,"pith_score":0,"inbound_count":0,"_links":{"html":"https://pith.science/paper/2611.17224","verdict":"/api/v1/verdicts/2611.17224"}}