Carbon monoxide formation and cooling in supernovae

Abstract

Context. The inclusion of molecular physics is an important piece that tends to be missing from the puzzle when modeling the spectra of supernovae (SNe). Molecules have both a direct impact on the spectra, particularly in the infrared, and an indirect one as a result of their influence on certain physical conditions, such as temperature. Aims. In this paper, we aim to investigate molecular formation and non-local thermodynamic equilibrium (NLTE) cooling, with a particular focus on CO, the most commonly detected molecule in supernovae. We also aim to determine the dependency of supernova chemistry on physical parameters and the relative sensitivity to rate uncertainties. Methods. We implemented a chemical kinetic description of the destruction and formation of molecules into the SN spectral synthesis code SUMO. In addition, selected molecules were coupled into the full NLTE level population framework and, thus, we incorporated molecular NLTE cooling into the temperature equation. We produced a test model of the CO formation in SN 1987A between 150 and 600 days and investigated the sensitivity of the resulting molecular masses to the input parameters. Results. We find that there is a close inter-dependency between the thermal evolution and the amount of CO formed, mainly through an important temperature-sensitive CO destruction process with O+. After a few hundred days, CO completely dominates the cooling of the oxygen-carbon zone of the supernova which, therefore, contributes little optical emission. The uncertainty of the calculated CO mass scales approximately linearly with the typical uncertainty factor for individual rates. We demonstrate how molecular masses can potentially be used to constrain various physical parameters of the supernova.

The explosion of 9–29M⊙stars as Type II supernovae: Results from radiative-transfer modeling at one year after explosion

“…CO fundamental (day 117) and first overtone emission (day 112) was first detected in SN 1987A by Spyromilio et al (1988). Modeling by these authors suggested a CO mass of about 5 × 10 −5 M at day 285, while Liljegren et al (2020) estimated a much larger mass of 4 × 10 −3 M . Because of the reduction in temperature in the C/O region, Liljegren et al (2020) indicates that the O/C region will not be a significant contribution to the [O I] λλ 6300, 6364 in the optical region.…”

“…Modeling by these authors suggested a CO mass of about 5 × 10 −5 M at day 285, while Liljegren et al (2020) estimated a much larger mass of 4 × 10 −3 M . Because of the reduction in temperature in the C/O region, Liljegren et al (2020) indicates that the O/C region will not be a significant contribution to the [O I] λλ 6300, 6364 in the optical region. SiO was also present from ∼160 d, with an estimated mass of about 4 ± 2 × 10 −6 M near day 500 (Roche et al 1991).…”

Photometric, polarimetric, and spectroscopic studies of the luminous, slow-decaying Type Ib SN 2012au

“…the carbon monoxide (CO) features in the K-band region) and is crucial to investigate dust formation (Morgan & Edmunds 2003). Due to the large number of collisionally excitable energy levels of CO emission, it acts as a coolant and can shed light on the dust production rate in SN ejecta (Gearhart, Wheeler & Swartz 1999;Morgan & Edmunds 2003;Liljegren, Jerkstrand & Grumer 2020). The NIR spectra of SNe Ib exhibit strong features of He I λ10 800 and C I, which give an idea about the extent of envelope stripping in these objects.…”

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

“…Both from the observed strength of the molecular emission (e.g. Meikle et al 1989;Kotak 2009), and the only two studies calculating molecular cooling so far (Liu & Dalgarno 1995;Liljegren et al 2020) molecules appear capable of cooling the ejecta significantly, perhaps by several thousand degrees. This would strongly affect the optical spectrum which for many SNe is the only spectral region observed.…”