Asymmetries and Circumstellar Interaction in the Type II SN 2024bch

Abstract We present a comprehensive multi-epoch photometric and spectroscopic study of SN 2024bch, a nearby (19.9 Mpc) Type II supernova (SN) with prominent early high-ionization emission lines. Optical spectra from 2.8 days after the estimated explosion reveal narrow lines of H i , He ii , C iv , and N iv that disappear by day 6. High-cadence photometry from the ground and Transiting Exoplanet Survey Satellite show that the SN brightened quickly and reached a peak M V ~ −17.8 mag within a week of explosion, and late-time photometry suggests a 56 Ni mass of 0.050 M ⊙ . High-resolution spectra from days 7.9 and 43 trace the unshocked circumstellar medium (CSM) and indicate a wind velocity of 30–40 km s −1 , a value consistent with a red supergiant (RSG) progenitor. Comparisons between models and the early spectra suggest a pre-SN mass-loss rate of M ̇ ~ 1 0 − 3 – 1 0 − 2 M ⊙ yr − 1 , which is too high to be explained by quiescent mass loss from RSGs, but is consistent with some recent measurements of similar SNe. Persistent blueshifted H i and O i emission lines seen in the optical and near-IR spectra could be produced by asymmetries in the SN ejecta, while the multicomponent H α may indicate continued interaction with an asymmetric CSM well into the nebular phase. SN 2024bch provides another clue to the complex environments and mass-loss histories around massive stars.