Signatures of anti-social mass loss in the ordinary Type II SN 2024bch

In this paper we analyse the spectro-photometric properties of the Type II supernova SN 2024bch, which exploded in NGC 3206 at a distance of 19.9 Mpc. Its early spectra are characterised by narrow high-ionisation emission lines, which are often interpreted as signatures of ongoing interaction between rapidly expanding ejecta and a confined dense circumstellar medium. However, we provide a model for the bolometric light curve of the transient that does not require sources of energy other than radioactive decays and H recombination. Our model can reproduce the bolometric light curve of SN 2024bch if we adopt an ejected mass of Mbulk ≃ 5 M⊙ surrounded by an extended envelope of only 0.2 M⊙ with an outer radius Renv = 7.0 × 1013 cm. Accurate modelling focused on the radioactive part of the light curve, which accounts for incomplete γ-ray trapping, gives a 56Ni mass of 0.048 M⊙. We propose that narrow lines are powered by Bowen fluorescence that is induced by scattering of He <ns0:sc>II</ns0:sc> Lyα photons. Simple light travel time calculations based on the maximum phase of the narrow emission lines place the inner radius of the H-rich, un-shocked shell at a radius ≃ 4.4 × 1015 cm, consistent with no interaction occurring between the ejecta and the surrounding circumstellar material during the initial weeks of evolution. Possible signatures of interaction appear only ∼ 69 days after the explosion, although the resulting conversion of kinetic energy into radiation does not seem to contribute significantly to the total luminosity of the transient.