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

TL;DR

This study addresses how early narrow high-ionisation features in Type II supernovae can arise without strong ejecta–CSM interaction. The authors analyze SN 2024bch with a two-component ejecta model (bulk ejecta and a low-mass extended envelope) plus gamma-ray leakage, and they interpret the early lines as Bowen fluorescence from a distant CSM. They derive a nickel mass of $M_{^{56}{\rm Ni}} \approx 0.048\,M_{\odot}$ and an ejecta mass of $M_{\rm ej} \approx 4.5$–$5\,M_{\odot}$, with an inner CSM radius $R_{\rm in} \approx 4.4\times10^{15}$ cm, reproducing the bolometric light curve without requiring energy from ejecta–CSM shocks. The results imply a ZAMS progenitor mass of $M_{\rm ZAMS} \approx 15$–$20\,M_{\odot}$ and suggest that early flash-like features do not universally indicate strong interaction, informing models of CC SNe and their environments.

Abstract

In this paper we analyse the spectro-photometric properties of the Type II supernova 2024bch, exploded in NGC 3206 at a distance of $19.9\,\rm{Mpc}$. Its early spectra are characterised by narrow high-ionisation emission lines, 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 different than radioactive decays and H recombination. Our model can reproduce the bolometric light curve of SN 2024bch adopting an ejected mass of $M_{bulk}\simeq5\,\rm{M_{\odot}}$ surrounded by an extended envelope of only $0.2\,\rm{M_{\odot}}$ with an outer radius $R_{env}=7.0\times10^{13}\,\rm{cm}$. An accurate modelling focused on the radioactive part of the light curve, which accounts for incomplete $γ-$ray trapping, gives a $^{56}\rm{Ni}$ mass of $0.048\,\rm{M_{\odot}}$. We propose narrow lines to be powered by Bowen fluorescence induced by scattering of He II Ly$α$ photons, resulting in the emission of high-ionisation resonance lines. 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 $\simeq4.4\times10^{15}\,\rm{cm}$, compatible with an absence of ejecta-CSM interaction during the first weeks of evolution. Possible signatures of interaction appear only $\sim69\,\rm{days}$ after explosion, although the resulting conversion of kinetic energy into radiation does not seem to contribute significantly to the total luminosity of the transient.

Figures (14)

• Figure 1: Colour image of SN 2024bch and its host galaxy NGC 3206, obtained combining $u-$, $g-$, $r-$ and $i-$band data obtained on 2024 February 5 with the $1.82\,\rm{m}$ Copernico telescope with AFOSC. The transient is the bright source in the middle of the inset.
• Figure 2: UVOT ($w2,\,m2\,w1\,u\,b\,v$; left) and ( right) optical $ugBVriz$ light curves of SN 2024bch. Magnitudes were not corrected for extinction. UV and $BV$ magnitudes were calibrated in the Vega and $griz$ magnitudes in the AB photometric systems.
• Figure 3: Colour evolution of SN 2024bch compared to the ones of selected transients showing high-ionisation features in their early spectra. Objects were selected among those with a similar evolution of the main features (including the duration of the narrow features and SN type) with data available in the literature. In the upper panel, the evolution of the $U-g$ colours for SNe 2017ahn, 2020pni and 2022jox was included, since $u-$band photometry for these objects was not available. $U-$band magnitudes for these objects were converted in the AB photometric system adopting the Vega - AB Magnitude conversions reported in 2007AJ....133..734B. In the same panel, the inset includes a zoom-in of the $u/U-g$ early evolution, showing a rapid decrease in the colours within the first $\sim3\,\rm{days}$ after explosion for all the selected objects (see the main text). Phases refer to the estimated explosion epochs reported in the literature. Light curves of SN 2023ixf were collected from 2024Natur.627..754L and 2024Natur.627..759Z.
• Figure 4: Evolution of the radius and temperature inferred from the blacbody fit to the early SED of SN 2024bch compared to the ones of SN 2012aw 2014ApJ...787..139D computed using the same approach.
• Figure 5: Evolution of the bolometric luminosity of SN 2024bch along with the model used to reproduce the observed data. Dashed lines represent the luminosity contribution from both the envelope and the bulk of the ejecta. The green solid line is the sum of the two components, while the light blue solid line shows the contribution of the extra diffusion time introduced by the CSM.