Nebular phase properties of supernova Ibc from He-star explosions

Abstract

Following our recent work on Type II supernovae (SNe), we present a set of 1D nonlocal thermodynamic equilibrium radiative transfer calculations for nebular-phase Type Ibc SNe starting from state-of-the-art explosion models with detailed nucleosynthesis. Our grid of progenitor models is derived from He stars that were subsequently evolved under the influence of wind mass loss. These He stars, which most likely form through binary mass exchange, synthesize less oxygen than their single-star counterparts with the same zero-age main sequence (ZAMS) mass. This reduction is greater in He-star models evolved with an enhanced mass loss rate. We obtain a wide range of spectral properties at 200 d. In models from He stars with an initial mass > 6 M⊙, the [O I] λλ 6300, 6364 is of a comparable or greater strength than [Ca II] λλ 7291, 7323 – the strength of [O I] λλ 6300, 6364 increases with the He-star initial mass. In contrast, models from lower mass He stars exhibit a weak [O I] λλ 6300, 6364, strong [Ca II] λλ 7291, 7323, and also strong N II lines and Fe II emission below 5500 Å. The ejecta density, which is modulated by the ejecta mass, the explosion energy, and clumping, has a critical impact on gas ionization, line cooling, and spectral properties. We note that Fe II dominates the emission below 5500 Å and is stronger at earlier nebular epochs. It ebbs as the SN ages, while the fractional flux in [O I] λλ 6300, 6364 and [Ca II] λλ 7291, 7323 increases with a similar rate as the ejecta recombine. Although the results depend on the adopted wind mass loss rate and pre-SN mass, we find that He-stars of 6–8 M⊙ initially (ZAMS mass of 23–28 M⊙) match the properties of standard SNe Ibc adequately. This finding agrees with the offset in progenitor masses inferred from the environments of SNe Ibc relative to SNe II. Our results for less massive He stars are more perplexing since the predicted spectra are not seen in nature. They may be missed by current surveys or associated with Type Ibn SNe in which interaction power dominates over decay power.

Helium stars exploding in circumstellar material and the origin of Type Ibn supernovae

“…This model exhibits a very peculiar spectrum by SN standards, but quite typical of Type Ibn SNe several weeks after bolometric maximum. With its strong Fe ii emission between 3000 and 5500 Å it is in fact reminiscent of the predicted nebular-phase models obtained for heliumstar explosions in the mass range 2.6−4.0 M (see Dessart et al 2021). The bottom panel of Fig.…”

“…We enforce a chemical mixing by running four times a boxcar width of 0.06M ej through the ejecta. In our calculations, we consider solar-metallicity progenitor models with an initial heliumstar mass between 2.9 and 12 M (for details on these models, see Dessart et al 2021). Following our assumption for the late-time conditions, we put the total mass of each model into the dense shell.…”

“…We choose a Gaussian power profile whose volume integral yields a prescribed total powerwe explore with values of 2-10 × 10 42 erg s −1 , which are typical of the high-luminosity phase of Type Ibn SNe like 2006jc. For the decay power, we adopt the same initial 56 Ni mass as given in each helium-star explosion model of Dessart et al (2021), and we assume that at the time of the computation, the 56 Ni has entirely decayed into a mixture composed of 50% 56 Co and 50% 56 Fe, as expected for a post-explosion epoch of about three months, roughly representative of the nebular epochs that we aim to study here 5 . When comparing to specific observations, using the inferred post-explosion time (and the adequate mixture of 56 Co and 56 Fe -only traces of 56 Ni would remain) would have been superior but it is not essential for this first exploration on the basic properties of Type Ibn SNe.…”

Luminous Fast Blue Optical Transients and Type Ibn/Icn SNe from Wolf-Rayet/Black Hole Mergers

“…The initial spectra at t 15 days were mostly featureless and indicated large photosphere expansion velocities v 0.1c and temperatures ∼ 3 × 10 5 K. Later spectra revealed a persistent optically-thick photosphere and the emergence of H and He emission features, which abruptly decrease to velocities v ∼ 3000 − 4000 km s −1 with no evidence for ejecta cooling (e.g., Perley et al 2019;Margutti et al 2019;Xiang et al 2021), quite unlike standard shock break-out emission. However, the presence of narrow He emission lines 300 km s −1 exhibit similarities with SNe Ibn or transitional SNe Ibn/IIn objects and point to shock interaction between the ejecta and H-depleted circumstellar material (CSM; e.g., Fox & Smith 2019;Dessart et al 2021).…”

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

“…From previous work on nebular spectra of stripped-envelope SNe values of χ ∼ 30 − 200 have been indicated (Jerkstrand et al 2015), although the number is not very well constrained, see e.g. discussion in Dessart et al (2021). For the standard model we use χ=100, making the density of the O/Si/S, O/Ne/Mg and O/C zones 100 times larger than that of the Fe/He zone.…”