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1. Peculiar Spectral Evolution of the Type I Supernova 2019eix: A Possible Double Detonation from a Helium Shell on a Sub-Chandrasekhar-mass White Dwarf

   https://doi.org/10.3847/1538-4357/acdd6a  

   Padilla Gonzalez, E.; Howell, D. Andrew; Burke, J.; Dong, Yize; Hiramatsu, D.; McCully, C.; Pellegrino, C.; Kerzendorf, W.; Modjaz, M.; Terreran, G.; et al (July 2023, The Astrophysical Journal)

   Abstract We present photometric and spectroscopic data for the nearby Type I supernova (SN Ia) 2019eix (originally classified as an SN Ic), from the day of its discovery up to 100 days after maximum brightness. Before maximum light, SN 2019eix resembles a typical SN Ic, albeit lacking the usual O i feature. Its light curve is similar to the typical SN Ic with decline rates (Δ M 15, V = 0.84) and absolute magnitude M V = −18.35. However, after maximum light, this SN has unusual spectroscopic features, a large degree of line blending, significant line blanketing in the blue ( λ < 5000 Å), and strong Ca ii absorption features during and after peak brightness. These unusual spectral features are similar to models of subluminous thermonuclear explosions, specifically double-detonation models of SNe Ia. Photometrically, SN 2019eix appears to be somewhat brighter with slower decline rates than other double-detonation candidates. We modeled the spectra using the radiative-transfer code TARDIS using SN 1994I (an SN Ic) as a base model to see whether we could reproduce the unusual features of SN 2019eix and found them to be consistent with the exception of the O i feature. We also compared SN 2019eix with double-detonation models and found them to best match the observations of SN 2019eix, but failed to reproduce its full photometric and spectroscopic evolution. 

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2. Characterizing the Rapid Hydrogen Disappearance in SN 2022crv: Evidence of a Continuum between Type Ib and IIb Supernova Properties

   https://doi.org/10.3847/1538-4357/ad710e  

   Dong_董, Yize 一泽; Valenti, Stefano; Ashall, Chris; Williamson, Marc; Sand, David J; Van_Dyk, Schuyler D; Filippenko, Alexei V; Jha, Saurabh W; Lundquist, Michael; Modjaz, Maryam; et al (October 2024, The Astrophysical Journal)

   Abstract We present optical and near-infrared (NIR) observations of SN 2022crv, a stripped-envelope supernova in NGC 3054, discovered within 12 hr of explosion by the Distance Less Than 40 Mpc Survey. We suggest that SN 2022crv is a transitional object on the continuum between Type Ib supernovae (SNe Ib) and Type IIb supernovae (SNe IIb). A high-velocity hydrogen feature (∼ −20,000 to −16,000 km s⁻¹) was conspicuous in SN 2022crv at early phases, and then quickly disappeared. We find that a hydrogen envelope of ∼10⁻³M_⊙can reproduce the observed behavior of the hydrogen feature. The lack of early envelope cooling emission implies that SN 2022crv had a compact progenitor with an extremely low amount of hydrogen. A nebular spectral analysis shows that SN 2022crv is consistent with the explosion of a He star with a final mass of ∼4.5–5.6M_⊙that evolved from a ∼16 to 22M_⊙zero-age main-sequence star in a binary system with ∼1.0–1.7M_⊙of oxygen finally synthesized in the core. In order to retain such a small amount of hydrogen, the initial orbital separation of the binary system is likely larger than ∼1000R_⊙. The NIR spectra of SN 2022crv show a unique absorption feature on the blue side of the Heiline at ∼1.005μm. This is the first time such a feature has been observed in SNe Ib/IIb, and it could be due to Sr II. Further detailed modeling of SN 2022crv can shed light on the progenitor and the origin of the mysterious absorption feature in the NIR. 

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

   https://doi.org/10.3847/1538-4357/ac4030  

   Shahbandeh, M.; Hsiao, E. Y.; Ashall, C.; Teffs, J.; Hoeflich, P.; Morrell, N.; Phillips, M. M.; Anderson, J. P.; Baron, E.; Burns, C. R.; et al (February 2022, The Astrophysical Journal)

   Abstract We present 75 near-infrared (NIR; 0.8−2.5 μ m) 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 μ m region, near the He i λ 2.0581 μ m 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 λ 2.0581 μ m 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. 

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4. Massive stars exploding in a He-rich circumstellar medium: X. Flash spectral features in the Type Ibn SN 2019cj and observations of SN 2018jmt

   https://doi.org/10.1051/0004-6361/202451131  

   Wang, Z-Y; Pastorello, A; Maeda, K; Reguitti, A; Cai, Y-Z; Andrew_Howell, D; Benetti, S; Buckley, D_A H; Cappellaro, E; Carini, R; et al (November 2024, Astronomy & Astrophysics)

   We present optical and near-infrared observations of two Type Ibn supernovae (SNe), SN 2018jmt and SN 2019cj. Their light curves have rise times of about ten days, reaching an absolute peak magnitude ofM_g(SN 2018jmt) = −19.07 ± 0.37 andM_V(SN 2019cj) = −18.94 ± 0.19 mag, respectively. The early-time spectra of SN 2018jmt are dominated by a blue continuum, accompanied by narrow (600−1000 km s⁻¹) He Ilines with the P-Cygni profile. At later epochs, the spectra become more similar to those of the prototypical SN Ibn 2006jc. At early phases, the spectra of SN 2019cj show flash ionisation emission lines of C III, N III, and He IIsuperposed on a blue continuum. These features disappear after a few days, and then the spectra of SN 2019cj evolve similarly to those of SN 2018jmt. The spectra indicate that the two SNe exploded within a He-rich circumstellar medium (CSM) lost by the progenitors a short time before the explosion. We modelled the light curves of the two SNe Ibn to constrain the progenitor and the explosion parameters. The ejecta masses are consistent with either what is expected for a canonical SN Ib (∼2 M_⊙) or for a massive Wolf Rayet star (> ∼4 M_⊙), with the kinetic energy on the order of 10⁵¹erg. The lower limit on the ejecta mass (> ∼2 M_⊙) argues against a scenario involving a relatively low-mass progenitor (e.g.M_ZAMS ∼ 10 M_⊙). We set a conservative upper limit of ∼0.1 M_⊙for the⁵⁶Ni masses in both SNe. From the light curve modelling, we determined a two-zone CSM distribution, with an inner, flat CSM component and an outer CSM with a steeper density profile. The physical properties of SN 2018jmt and SN 2019cj are consistent with those expected from the core collapse of relatively massive envelope-stripped stars. 

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5. SN 2015an: a normal luminosity type II supernova with low expansion velocity at early phases

   https://doi.org/10.1093/mnras/stz2703  

   Dastidar, Raya; Misra, Kuntal; Valenti, Stefano; Burke, Jamison; Hosseinzadeh, Griffin; Gangopadhyay, Anjasha; Howell, D Andrew; Singh, Mridweeka; Arcavi, Iair; Kumar, Brijesh; et al (December 2019, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present the photometry and spectroscopy of SN 2015an, a type II Supernova (SN) in IC 2367. The recombination phase of the SN lasts up to 120 d, with a decline rate of 1.24 mag/100d, higher than the typical SNe IIP. The SN exhibits bluer colours than most SNe II, indicating higher ejecta temperatures. The absolute V-band magnitude of SN 2015an at 50 d is −16.83 ± 0.04 mag, pretty typical for SNe II. However, the 56Ni mass yield, estimated from the tail V-band light curve to be 0.021 ± 0.010 M⊙, is comparatively low. The spectral properties of SN 2015an are atypical, with low H α expansion velocity and presence of high-velocity component of H α at early phases. Moreover, the continuum exhibits excess blue flux up to 50 d, which is interpreted as a progenitor metallicity effect. The high-velocity feature indicates ejecta-circumstellar material interaction at early phases. The semi-analytical modelling of the bolometric light curve yields a total ejected mass of 12 M⊙, a pre-SN radius of 388 R⊙ and explosion energy of 1.8 foe. 

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