Carnegie Supernova Project-II: Near-infrared Spectroscopy of Stripped-envelope Core-collapse Supernovae*

Abstract

We present 75 near-infrared (NIR; 0.8−2.5 μm) spectra of 34 stripped-envelope core-collapse supernovae (SESNe) obtained by the Carnegie Supernova Project-II (CSP-II), encompassing optical spectroscopic Types IIb, Ib, Ic, and Ic-BL. The spectra range in phase from pre-maximum to 80 days past maximum. This unique data set constitutes the largest NIR spectroscopic sample of SESNe to date. NIR spectroscopy provides observables with additional information that is not available in the optical. Specifically, the NIR contains the strong lines of He i and allows a more detailed look at whether Type Ic supernovae are completely stripped of their outer He layer. The NIR spectra of SESNe have broad similarities, but closer examination through statistical means reveals a strong dichotomy between NIR “He-rich” and “He-poor” SNe. These NIR subgroups correspond almost perfectly to the optical IIb/Ib and Ic/Ic-BL types, respectively. The largest difference between the two groups is observed in the 2 μm region, near the He i λ2.0581 μm line. The division between the two groups is not an arbitrary one along a continuous sequence. Early spectra of He-rich SESNe show much stronger He i λ2.0581 μm absorption compared to the He-poor group, but with a wide range of profile shapes. The same line also provides evidence for trace amounts of He in half of our SNe in the He-poor group.

Ultraviolet Spectroscopy and TARDIS Models of the Broad-lined Type-Ic Supernova 2014ad

“…In Figure 3, we also show two epochs of near-infrared (NIR) spectra for SN 2014ad from Shahbandeh et al (2022 and 2014 March 25 (+7d) that closely match the phase of our two epochs of HST UV+Optical spectra on 2014 March 19 (+1d) and 2014 March 25 (+7d). Combined, these data give the composite UV + Optical + NIR spectra of SN 2014ad for two epochs near maximum light.…”

“…However, the metallicity of SN 2014ad (∼ 0.5 Z ) is not as low as required for a very high CO core mass, and our models likely cannot be reconciled with the theoretical CO core mass limit in this way. Thus, an ejecta mass of ∼ 20.5 M would require several solar masses of leftover hydrogen and/or helium on top of the CO core; there is no sign of such material in the observations, specifically from the NIR observations which exhibit no trace of helium at early epochs (Shahbandeh et al 2022).…”

“…NIR spectra are particularly interesting for stripped-envelope SNe because two strong He absorption features in the NIR can probe the amount of helium leftover in the explosion. The He I λ1 µm absorption can be contaminated by a mix of C I λ1.0693 µm or Mg II λ1.0927 µm lines, so Shahbandeh et al (2022) find that the He I λ2.0581 µm absorption line is a better indicator of He in the supernova ejecta. As noted in Figure C2 & C3 of Shahbandeh et al (2022), at early times for SN 2014ad, the He I absorption features in both the 1 µm and 2 µm regions are not strong enough to be fit.…”

“…The He I λ1 µm absorption can be contaminated by a mix of C I λ1.0693 µm or Mg II λ1.0927 µm lines, so Shahbandeh et al (2022) find that the He I λ2.0581 µm absorption line is a better indicator of He in the supernova ejecta. As noted in Figure C2 & C3 of Shahbandeh et al (2022), at early times for SN 2014ad, the He I absorption features in both the 1 µm and 2 µm regions are not strong enough to be fit. These results indicate that there is no trace of He in SN 2014ad at early times and SN 2014ad has been thoroughly stripped of its helium envelope.…”

“…We can combine the analysis above with the results from Shahbandeh et al (2022) to understand the UV + Optical + NIR composite spectrum. NIR spectra are particularly interesting for stripped-envelope SNe because two strong He absorption features in the NIR can probe the amount of helium leftover in the explosion.…”