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1. Evolution of a Peculiar Type Ibn Supernova SN 2019wep

   https://doi.org/10.3847/1538-4357/ac6187  

   Gangopadhyay, Anjasha; Misra, Kuntal; Hosseinzadeh, Griffin; Arcavi, Iair; Pellegrino, Craig; Wang, Xiaofeng; Andrew Howell, D.; Burke, Jamison; Zhang, Jujia; Kawabata, Koji; et al (May 2022, The Astrophysical Journal)

   Abstract We present a high-cadence short term photometric and spectroscopic monitoring campaign of a type Ibn SN 2019wep, which is one of the rare SN Ibn after SNe 2010al and 2019uo to display signatures of flash ionization (He ii , C iii , N iii ). We compare the decline rates and rise time of SN 2019wep with other SNe Ibn and fast transients. The post-peak decline in all bands (0.1 mag day −1 ) are consistent with SNe Ibn but less than the fast transients. On the other hand, the Δ m 15 values are slightly lower than the average values for SNe Ibn but consistent with the fast transients. The rise time is typically shorter than SNe Ibn but longer than fast transients. SN 2019wep lies at the fainter end of SNe Ibn but possesses an average luminosity among the fast transients sample. The peculiar color evolution places it between SNe Ib and the most extreme SNe Ibn. The bolometric light-curve modeling shows resemblance with SN 2019uo with ejecta masses consistent with SNe Ib. SN 2019wep belongs to the P cygni subclass of SNe Ibn and shows faster evolution in line velocities as compared to the emission subclass. The post-maximum spectra show close resemblance with ASASSN-15ed hinting it to be of SN Ib nature. The low He i CSM velocities and residual H α further justifies it and provide evidence of an intermittent progenitor between Wolf-Rayet and LBV stars. 

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2. The Type Ibn Supernova 2019kbj: Indications for Diversity in Type Ibn Supernova Progenitors

   https://doi.org/10.3847/1538-4357/acb432  

   Ben-Ami, Tom; Arcavi, Iair; Newsome, Megan; Farah, Joseph; Pellegrino, Craig; Terreran, Giacomo; Burke, Jamison; Hosseinzadeh, Griffin; McCully, Curtis; Hiramatsu, Daichi; et al (March 2023, The Astrophysical Journal)

   Abstract Type Ibn supernovae (SNe) are a rare class of stellar explosions whose progenitor systems are not yet well determined. We present and analyze observations of the Type Ibn SN 2019kbj, and model its light curve in order to constrain its progenitor and explosion parameters. SN 2019kbj shows roughly constant temperature during the first month after peak, indicating a power source (likely circumstellar material interaction) that keeps the continuum emission hot at ∼15,000 K. Indeed, we find that the radioactive decay of⁵⁶Ni is disfavored as the sole power source of the bolometric light curve. A radioactive decay + circumstellar material (CSM) interaction model, on the other hand, does reproduce the bolometric emission well. The fits prefer a uniform-density CSM shell rather than CSM due to a steady mass-loss wind, similar to what is seen in other Type Ibn SNe. The uniform-density CSM shell model requires ∼0.1M_⊙of⁵⁶Ni and ∼1M_⊙total ejecta mass to reproduce the light curve. SN 2019kbj differs in this manner from another Type Ibn SN with derived physical parameters, SN 2019uo, for which an order of magnitude lower⁵⁶Ni mass and larger ejecta mass were derived. This points toward a possible diversity in SN Ibn progenitor systems and explosions. 

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3. The X-Ray Luminous Type Ibn SN 2022ablq: Estimates of Preexplosion Mass Loss and Constraints on Precursor Emission

   https://doi.org/10.3847/1538-4357/ad8bc5  

   Pellegrino, C.; Modjaz, M.; Takei, Y.; Tsuna, D.; Newsome, M.; Pritchard, T.; Baer-Way, R.; Bostroem, K_A; Chandra, P.; Charalampopoulos, P.; et al (November 2024, The Astrophysical Journal)

   Abstract Type Ibn supernovae (SNe Ibn) are rare stellar explosions powered primarily by interaction between the SN ejecta and H-poor, He-rich material lost by their progenitor stars. Multiwavelength observations, particularly in the X-rays, of SNe Ibn constrain their poorly understood progenitor channels and mass-loss mechanisms. Here we present Swift X-ray, ultraviolet, and ground-based optical observations of the Type Ibn SN 2022ablq, only the second SN Ibn with X-ray detections to date. While similar to the prototypical Type Ibn SN 2006jc in the optical, SN 2022ablq is roughly an order of magnitude more luminous in the X-rays, reaching unabsorbed luminositiesL_X∼ 4 × 10⁴⁰erg s⁻¹between 0.2–10 keV. From these X-ray observations we infer time-varying mass-loss rates between 0.05 and 0.5M_⊙yr⁻¹peaking 0.5–2 yr before explosion. This complex mass-loss history and circumstellar environment disfavor steady-state winds as the primary progenitor mass-loss mechanism. We also search for precursor emission from alternative mass-loss mechanisms, such as eruptive outbursts, in forced photometry during the 2 yr before explosion. We find no statistically significant detections brighter thanM≈ −14—too shallow to rule out precursor events similar to those observed for other SNe Ibn. Finally, numerical models of the explosion of an ∼15M_⊙helium star that undergoes an eruptive outburst ≈1.8 yr before explosion are consistent with the observed bolometric light curve. We conclude that our observations disfavor a Wolf–Rayet star progenitor losing He-rich material via stellar winds and instead favor lower-mass progenitor models, including Roche-lobe overflow in helium stars with compact binary companions or stars that undergo eruptive outbursts during late-stage nucleosynthesis stages. 

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4. A low-mass helium star progenitor model for the Type Ibn SN 2020nxt

   https://doi.org/10.1093/mnras/stae1038  

   Wang, Qinan; Goel, Anika; Dessart, Luc; Fox, Ori D; Shahbandeh, Melissa; Rest, Sofia; Rest, Armin; Groh, Jose H; Allan, Andrew; Fransson, Claes; et al (May 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT A growing number of supernovae (SNe) are now known to exhibit evidence for significant interaction with a dense, pre-existing, circumstellar medium (CSM). SNe Ibn comprise one such class that can be characterized by both rapidly evolving light curves and persistent narrow He i lines. The origin of such a dense CSM in these systems remains a pressing question, specifically concerning the progenitor system and mass-loss mechanism. In this paper, we present multiwavelength data of the Type Ibn SN 2020nxt, including HST/STIS ultraviolet spectra. We fit the data with recently updated CMFGEN models designed to handle configurations for SNe Ibn. The UV coverage yields strong constraints on the energetics and, when combined with the CMFGEN models, offer new insight on potential progenitor systems. We find the most successful model is a ≲4 M⊙ helium star that lost its $$\sim 1\, {\rm M}_\odot$$ He-rich envelope in the years preceding core collapse. We also consider viable alternatives, such as a He white dwarf merger. Ultimately, we conclude at least some SNe Ibn do not arise from single, massive (>30 M⊙) Wolf–Rayet-like stars. 

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5. The luminous and rapidly evolving SN 2018bcc: Clues toward the origin of Type Ibn SNe from the Zwicky Transient Facility

   https://doi.org/10.1051/0004-6361/201936308  

   Karamehmetoglu, E.; Fransson, C.; Sollerman, J.; Tartaglia, L.; Taddia, F.; De, K.; Fremling, C.; Bagdasaryan, A.; Barbarino, C.; Bellm, E. C.; et al (May 2021, Astronomy & Astrophysics)

   null (Ed.)

   Context. Supernovae (SNe) Type Ibn are rapidly evolving and bright ( M R, peak  ∼ −19) transients interacting with He-rich circumstellar material (CSM). SN 2018bcc, detected by the ZTF shortly after explosion, provides the best constraints on the shape of the rising light curve (LC) of a fast Type Ibn. Aims. We used the high-quality data set of SN 2018bcc to study observational signatures of the class. Additionally, the powering mechanism of SN 2018bcc offers insights into the debated progenitor connection of Type Ibn SNe. Methods. We compared well-constrained LC properties obtained from empirical models with the literature. We fit the pseudo-bolometric LC with semi-analytical models powered by radioactive decay and CSM interaction. Finally, we modeled the line profiles and emissivity of the prominent He  I lines, in order to study the formation of P-Cygni profiles and to estimate CSM properties. Results. SN 2018bcc had a rise time to peak of the LC of 5.6 −0.1 +0.2 days in the restframe with a rising shape power-law index close to 2, and seems to be a typical rapidly evolving Type Ibn SN. The spectrum lacked signatures of SN-like ejecta and was dominated by over 15 He emission features at 20 days past peak, alongside Ca and Mg, all with V FWHM ∼ 2000 km s −1 . The luminous and rapidly evolving LC could be powered by CSM interaction but not by the decay of radioactive 56 Ni. Modeling of the He  I lines indicated a dense and optically thick CSM that can explain the P-Cygni profiles. Conclusions. Like other rapidly evolving Type Ibn SNe, SN 2018bcc is a luminous transient with a rapid rise to peak powered by shock interaction inside a dense and He-rich CSM. Its spectra do not support the existence of two Type Ibn spectral classes. We also note the remarkable observational match to pulsational pair instability SN models. 

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