Near-infrared and Optical Nebular-phase Spectra of Type Ia Supernovae SN 2013aa and SN 2017cbv in NGC 5643

Abstract

We present multiwavelength time-series spectroscopy of SN 2013aa and SN 2017cbv, two Type Ia supernovae (SNe Ia) on the outskirts of the same host galaxy, NGC 5643. This work utilizes new nebular-phase near-infrared (NIR) spectra obtained by the Carnegie Supernova Project-II, in addition to previously published optical and NIR spectra. Using nebular-phase [Fe ii] lines in the optical and NIR, we examine the explosion kinematics and test the efficacy of several common emission-line-fitting techniques. The NIR [Fe ii] 1.644 μm line provides the most robust velocity measurements against variations due to the choice of the fit method and line blending. The resulting effects on velocity measurements due to choosing different fit methods, initial fit parameters, continuum and line profile functions, and fit region boundaries were also investigated. The NIR [Fe ii] velocities yield the same radial shift direction as velocities measured using the optical [Fe ii] λ7155 line, but the sizes of the shifts are consistently and substantially lower, pointing to a potential issue in optical studies. The NIR [Fe ii] 1.644 μm emission profile shows a lack of significant asymmetry in both SNe, and the observed low velocities elevate the importance for correcting for any velocity contribution from the host galaxy’s rotation. The low [Fe ii] velocities measured in the NIR at nebular phases disfavor progenitor scenarios in close double-degenerate systems for both SN 2013aa and SN 2017cbv. The time evolution of the NIR [Fe ii] 1.644 μm line also indicates moderately high progenitor white dwarf central density and potentially high magnetic fields.

Type Ia Supernova Progenitor Properties and their Host Galaxies

“…The resulting eigenfunction may be expected to resemble the offset produced by ρ c (Figure 4). Though the 58 Ni observed in the MIR (Gerardy et al 2007;Telesco et 2015;DerKacy et al 2023;Kumar et al 2023;Kwok et al 2023) favors M Ch mass explosions, HeDs may well be in our sample.…”

“…The observed continuum polarization is 0.5−0.7%, indicating an overall asymmetry of 10%-20% (Howell et al 2001;Patat et al 2012;Patat 2017). Spectra indicate a thick layer of a significant amount of unburned C/O Patat et al 2012;Kumar et al 2023). These observations may be understood in terms of the DD of a very fast-rotating WD or dynamical mergers of two WDs (Patat et al 2012).…”

“…91T-like and 99aa-like SNe Ia have been identified as separate classes with at least two subgroups among the 99aa-like. 91T late-time NIR and MIR show a peaked [Fe II] at 1.644 μm (Mueller et al 1991) in contrast to typical SNe Ia showing round profiles for [Fe II] and the [Co III] at 11.8 μm (Höflich et al 2004;Gerardy et al 2007;Diamond et al 2015;Telesco et al 2015;Diamond et al 2018;Hoeflich et al 2021;DerKacy et al 2023;Kumar et al 2023; see Figure 2 ).…”

1991T-like Supernovae*

“…For the two 91T-like SNe, it appears that emission due to [Ca II] λ7291,7324 is also contributing to the blend. In contrast, for normal bright SNe such as SN 2011fe, [Ca II] emission does not appear strongly in the nebular spectrum until ∼+400 days (e.g., see Tucker et al 2022a;Kumar et al 2023).…”

“…Although the spectrum of SN 1991T is noisy, it is seen that the wings of the line extend to ∼15,000 km s −1 at this epoch. Overplotted for comparison in the same figure is the [Fe II] 1.644 μm line observed for the slow-declining Branch CN SN 2013aa (Kumar et al 2023). The profiles are generally similar to within the signal-to-noise ratio of the 1991T spectrum.…”