Asymmetries and Circumstellar Interaction in the Type II SN 2024bch

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 ⁵⁶Ni 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 Hα 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.