Hertzsprung gap stars in nearby galaxies and the quest for luminous red nova progenitors

Context. After the main sequence phase, stars more massive than 2.5 M ⊙ rapidly evolve through the Hertzsprung gap as yellow giants and yellow supergiants (YSGs) before settling into the red giant branch. Identifying Hertzsprung gap stars in nearby galaxies is crucial for pinpointing progenitors of luminous red novae (LRNe) – astrophysical transients attributed to stellar mergers. In the era of extensive transient surveys like the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST), this approach offers a new way to predict and select common envelope transients. Aims. This study investigates potential progenitors and precursors of LRNe by analysing Hubble Space Telescope (HST) photometry of stellar populations in galaxies within ∼20 Mpc to identify YSG candidates. Additionally, we use the Zwicky Transient Facility and MeerLICHT/BlackGEM to identify possible precursors, preparing for future observations by the LSST. Methods. We compiled a sample of 369 galaxies with HST exposures in the F 475 W , F 555 W , F 606 W , and F 814 W filters. We identified YSG candidates using MESA stellar evolution tracks and statistical analysis of colour–magnitude diagrams. Results. Our sample includes 154 494 YSG candidates with masses between 3 M ⊙ and 20 M ⊙ and is affected by various contaminants, notably foreground stars and extinguished main sequence stars. After excluding foreground stars using Gaia proper motions, contamination is estimated at 1% from foreground stars (based on TRILEGAL simulations) and ∼20% from extinction affecting main sequence stars. Combining our YSG candidates with time-domain catalogues yielded several interesting candidates. In particular, we identified 12 LRN precursor candidates for which follow-up is encouraged. Conclusions. We highlight the importance of monitoring future transients that match YSG candidates to avoid missing potential LRNe and other rare transients. LSST will be a game changer in the search for LRN progenitors and precursors; it is predicted to discover over 300 000 new YSG candidates and 100 LRN precursors within 20 Mpc.