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 M bulk ≃ 5 M ⊙ surrounded by an extended envelope of only 0.2 M ⊙ with an outer radius R env = 7.0 × 10 13 cm. Accurate modelling focused on the radioactive part of the light curve, which accounts for incomplete γ -ray trapping, gives a 56 Ni mass of 0.048 M ⊙ . We propose that narrow lines are powered by Bowen fluorescence that is induced by scattering of He II Ly α 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 × 10 15 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.