Signatures of anti-social mass loss in the ordinary Type II SN 2024bch. A non-interacting supernova with early high-ionisation features

In this paper we analyse the spectro-photometric properties of the Type II supernova which exploded in NGC 3206 at a distance of 19. 9 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 surrounded by an extended envelope of only 0. 2 with an outer radius R_ env cm. Accurate modelling focused on the radioactive part of the light curve, which accounts for incomplete γ-ray trapping, gives a ^ Ni mass of 0. 048 We propose that narrow lines are powered by Bowen fluorescence that is induced by scattering of He II 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 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 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.