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1. Multiple Peaks and a Long Precursor in the Type IIn Supernova 2021qqp: An Energetic Explosion in a Complex Circumstellar Environment

   https://doi.org/10.3847/1538-4357/ad2854  

   Hiramatsu, Daichi; Matsumoto, Tatsuya; Berger, Edo; Ransome, Conor; Villar, V Ashley; Gomez, Sebastian; Cendes, Yvette; De, Kishalay; Bostroem, K Azalee; Farah, Joseph; et al (March 2024, The Astrophysical Journal)

   Abstract We present optical photometry and spectroscopy of the Type IIn supernova (SN) 2021qqp. Its unusual light curve is marked by a long precursor for ≈300 days, a rapid increase in brightness for ≈60 days, and then a sharp increase of ≈1.6 mag in only a few days to a first peak ofM_r≈ −19.5 mag. The light curve then declines rapidly until it rebrightens to a second distinct peak ofM_r≈ −17.3 mag centered at ≈335 days after the first peak. The spectra are dominated by Balmer lines with a complex morphology, including a narrow component with a width of ≈1300 km s⁻¹(first peak) and ≈2500 km s⁻¹(second peak) that we associate with the circumstellar medium (CSM) and a P Cygni component with an absorption velocity of ≈8500 km s⁻¹(first peak) and ≈5600 km s⁻¹(second peak) that we associate with the SN–CSM interaction shell. Using the luminosity and velocity evolution, we construct a flexible analytical model, finding two significant mass-loss episodes with peak mass loss rates of ≈10 and ≈5M_⊙yr⁻¹about 0.8 and 2 yr before explosion, respectively, with a total CSM mass of ≈2–4M_⊙. We show that the most recent mass-loss episode could explain the precursor for the year preceding the explosion. The SN ejecta mass is constrained to be ≈5–30M_⊙for an explosion energy of ≈(3–10) × 10⁵¹erg. We discuss eruptive massive stars (luminous blue variable, pulsational pair instability) and an extreme stellar merger with a compact object as possible progenitor channels. 

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2. 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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3. Circumstellar Interaction Signatures in the Low-luminosity Type II SN 2021gmj

   https://doi.org/10.3847/1538-4357/ad4d55  

   Meza-Retamal, Nicolás; Dong_董, Yize_一泽; Bostroem, K_Azalee; Valenti, Stefano; Galbany, Lluís; Pearson, Jeniveve; Hosseinzadeh, Griffin; Andrews, Jennifer_E; Sand, David_J; Jencson, Jacob_E; et al (August 2024, The Astrophysical Journal)

   Abstract We present comprehensive optical observations of SN 2021gmj, a Type II supernova (SN II) discovered within a day of explosion by the Distance Less Than 40 Mpc survey. Follow-up observations show that SN 2021gmj is a low-luminosity SN II (LL SN II), with a peak magnitudeM_V= −15.45 and an Feiivelocity of ∼1800 km s⁻¹at 50 days past explosion. Using the expanding photosphere method, we derive a distance of ${17.8}_{-0.4}^{+0.6}$Mpc. From the tail of the light curve we obtain a radioactive nickel mass of $M_{{}^{56}\mathrm{Ni}}$= 0.014 ± 0.001M_⊙. The presence of circumstellar material (CSM) is suggested by the early-time light curve, early spectra, and high-velocity Hαin absorption. Analytical shock-cooling models of the light curve cannot reproduce the fast rise, supporting the idea that the early-time emission is partially powered by the interaction of the SN ejecta and CSM. The inferred low CSM mass of 0.025M_⊙in our hydrodynamic-modeling light-curve analysis is also consistent with our spectroscopy. We observe a broad feature near 4600 Å, which may be high-ionization lines of C, N, or/and Heii. This feature is reproduced by radiation-hydrodynamic simulations of red supergiants with extended atmospheres. Several LL SNe II show similar spectral features, implying that high-density material around the progenitor may be common among them. 

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4. SN 2024ggi in NGC 3621: Rising Ionization in a Nearby, Circumstellar-material-interacting Type II Supernova

   https://doi.org/10.3847/1538-4357/ad5c64  

   Jacobson-Galán, W V; Davis, K W; Kilpatrick, C D; Dessart, L; Margutti, R; Chornock, R; Foley, R J; Arunachalam, P; Auchettl, K; Bom, C R; et al (September 2024, The Astrophysical Journal)

   Abstract We present UV–optical–near-infrared observations and modeling of supernova (SN) 2024ggi, a type II supernova (SN II) located in NGC 3621 at 7.2 Mpc. Early-time (“flash”) spectroscopy of SN 2024ggi within +0.8 days of discovery shows emission lines of Hi, Hei, Ciii, and Niiiwith a narrow core and broad, symmetric wings (i.e., “IIn-like”) arising from the photoionized, optically thick, unshocked circumstellar material (CSM) that surrounded the progenitor star at shock breakout (SBO). By the next spectral epoch at +1.5 days, SN 2024ggi showed a rise in ionization as emission lines of Heii, Civ, Niv/v, and Ovbecame visible. This phenomenon is temporally consistent with a blueward shift in the UV–optical colors, both likely the result of SBO in an extended, dense CSM. The IIn-like features in SN 2024ggi persist on a timescale oft_IIn= 3.8 ± 1.6 days, at which time a reduction in CSM density allows the detection of Doppler-broadened features from the fastest SN material. SN 2024ggi has peak UV–optical absolute magnitudes ofM_w2= −18.7 mag andM_g= −18.1 mag, respectively, that are consistent with the known population of CSM-interacting SNe II. Comparison of SN 2024ggi with a grid of radiation hydrodynamics and non–local thermodynamic equilibrium radiative-transfer simulations suggests a progenitor mass-loss rate of $\dot{M}={10}^{-2}{M}_{\odot }$yr⁻¹(v_w= 50 km s⁻¹), confined to a distance ofr< 5 × 10¹⁴cm. Assuming a wind velocity ofv_w= 50 km s⁻¹, the progenitor star underwent an enhanced mass-loss episode in the last ∼3 yr before explosion. 

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5. Final Moments. II. Observational Properties and Physical Modeling of Circumstellar-material-interacting Type II Supernovae

   https://doi.org/10.3847/1538-4357/ad4a2a  

   Jacobson-Galán, W V; Dessart, L; Davis, K W; Kilpatrick, C D; Margutti, R; Foley, R J; Chornock, R; Terreran, G; Hiramatsu, D; Newsome, M; et al (July 2024, The Astrophysical Journal)

   Abstract We present ultraviolet/optical/near-infrared observations and modeling of Type II supernovae (SNe II) whose early time (δt< 2 days) spectra show transient, narrow emission lines from shock ionization of confined (r< 10¹⁵cm) circumstellar material (CSM). The observed electron-scattering broadened line profiles (i.e., IIn-like) of Hi, Hei/ii, Civ, and Niii/iv/vfrom the CSM persist on a characteristic timescale (t_IIn) that marks a transition to a lower-density CSM and the emergence of Doppler-broadened features from the fast-moving SN ejecta. Our sample, the largest to date, consists of 39 SNe with early time IIn-like features in addition to 35 “comparison” SNe with no evidence of early time IIn-like features, all with ultraviolet observations. The total sample includes 50 unpublished objects with a total of 474 previously unpublished spectra and 50 multiband light curves, collected primarily through the Young Supernova Experiment and Global Supernova Project collaborations. For all sample objects, we find a significant correlation between peak ultraviolet brightness and botht_IInand the rise time, as well as evidence for enhanced peak luminosities in SNe II with IIn-like features. We quantify mass-loss rates and CSM density for the sample through the matching of peak multiband absolute magnitudes, rise times,t_IIn, and optical SN spectra with a grid of radiation hydrodynamics and non-local thermodynamic equilibrium radiative-transfer simulations. For our grid of models, all with the same underlying explosion, there is a trend between the duration of the electron-scattering broadened line profiles and inferred mass-loss rate: $t_{\mathrm{IIn}}\approx 3.8[\dot{M}/$(0.01M_⊙yr⁻¹)] days. 

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