The molecular chemistry of Type Ibc supernovae, and diagnostic potential with the James Webb Space Telescope

S. Liljegren, A. Jerkstrand, P. S. Barklem, G. Nyman, R. Brady, S. N. Yurchenko

Published 2022-03-14Version 1

A currently unsolved question in supernova research is the origin of stripped-envelope supernovae (SESNe). Such SNe lack spectral signatures of hydrogen (Type Ib), or hydrogen and helium (Type Ic), indicating that the outer stellar layers have been stripped during their evolution. The mechanism for this is not well understood, and to disentangle the different scenarios determination of nucleosynthesis yields from observed spectra can be attempted. However, the interpretation of observations depends on the adopted spectral models. A previously missing ingredient in these is the inclusion of molecular effects, which can be significant. We aim to investigate how the molecular chemistry in stripped-envelope supernovae affects physical conditions and optical spectra and produces ro-vibrational emission in the mid-infrared (MIR). We also aim to assess the diagnostic potential of observations of such MIR emission with JWST. We couple a chemical kinetic network including carbon, oxygen, silicon, and sulfur-bearing molecules into the NLTE spectral synthesis code SUMO. We let four species - CO, SiO, SiS and SO - participate in the NLTE cooling of the gas to achieve self-consistency between the molecule formation and the temperature. We apply the new framework to model the spectrum of a Type Ic supernova in the 100-600d time range. Molecules are predicted to form in SESN ejecta in significant quantities (typical mass $10^{-3}$ $M_\odot$) throughout the 100-600d interval. The impact on the temperature and optical emission depends on the density of the oxygen zones and varies with epoch. For example, the [O I] 6300, 6364 feature can be quenched by molecules from 200 to 450d depending on density. The MIR predictions show strong emission in the fundamental bands of CO, SiO, and SiS, and in the CO and SiO overtones.