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1. Probing the Circumstellar Environment of the Highly Luminous Type IIn Supernova ASASSN-14il

   https://doi.org/10.3847/1538-4357/ad7e11  

   Dukiya, Naveen; Gangopadhyay, Anjasha; Misra, Kuntal; Hosseinzadeh, Griffin; Bostroem, K Azalee; Ailawadhi, Bhavya; Howell, D Andrew; Valenti, Stefano; Arcavi, Iair; McCully, Curtis; et al (November 2024, The Astrophysical Journal)

   Abstract We present long-term photometric and spectroscopic studies of circumstellar material (CSM)–ejecta interacting supernova (SN) ASASSN-14il in the galaxy PGC 3093694. The SN reaches a peakr-band magnitude of ∼−20.3 ± 0.2 mag, rivaling SN 2006tf and SN 2010jl. The multiband and the pseudo-bolometric lightcurves show a plateau lasting ∼50 days. Semi-analytical CSM interaction models can match the high luminosity and decline rates of the lightcurves but fail to faithfully represent the plateau region and the bumps in the lightcurves. The spectral evolution resembles a typical Type IIn SN dominated by CSM interaction, showing blue continuum and narrow Balmer lines. The lines are dominated by electron scattering at early epochs. The signatures of the underlying ejecta are visible as the broad component in the Hαprofile from as early as day 50, hinting at asymmetry in the CSM. A narrow component is persistent throughout the evolution. The SN shows remarkable photometric and spectroscopic similarity with SN 2015da. However, the different polarization in ASASSN-14il compared to SN 2015da suggests an alternative viewing angle. The late-time blueshift in the Hαprofile supports dust formation in the post-shock CSM or ejecta. The mass-loss rate of 2–7M_⊙yr⁻¹suggests a luminous blue variable progenitor in an eruptive phase for ASASSN-14il. 

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2. From Discovery to the First Month of the Type II Supernova 2023ixf: High and Variable Mass Loss in the Final Year before Explosion

   https://doi.org/10.3847/2041-8213/acf299  

   Hiramatsu, Daichi; Tsuna, Daichi; Berger, Edo; Itagaki, Koichi; Goldberg, Jared A.; Gomez, Sebastian; De, Kishalay; Hosseinzadeh, Griffin; Bostroem, K. Azalee; Brown, Peter J.; et al (September 2023, The Astrophysical Journal Letters)

   Abstract We present the discovery of the Type II supernova SN 2023ixf in M101 and follow-up photometric and spectroscopic observations, respectively, in the first month and week of its evolution. Our discovery was made within a day of estimated first light, and the following light curve is characterized by a rapid rise (≈5 days) to a luminous peak (M_V≈ − 18.2 mag) and plateau (M_V≈ − 17.6 mag) extending to 30 days with a fast decline rate of ≈0.03 mag day⁻¹. During the rising phase,U−Vcolor shows blueward evolution, followed by redward evolution in the plateau phase. Prominent flash features of hydrogen, helium, carbon, and nitrogen dominate the spectra up to ≈5 days after first light, with a transition to a higher ionization state in the first ≈2 days. Both theU−Vcolor and flash ionization states suggest a rise in the temperature, indicative of a delayed shock breakout inside dense circumstellar material (CSM). From the timescales of CSM interaction, we estimate its compact radial extent of ∼(3–7) × 10¹⁴cm. We then construct numerical light-curve models based on both continuous and eruptive mass-loss scenarios shortly before explosion. For the continuous mass-loss scenario, we infer a range of mass-loss history with 0.1–1.0M_⊙yr⁻¹in the final 2−1 yr before explosion, with a potentially decreasing mass loss of 0.01–0.1M_⊙yr⁻¹in ∼0.7–0.4 yr toward the explosion. For the eruptive mass-loss scenario, we favor eruptions releasing 0.3–1M_⊙of the envelope at about a year before explosion, which result in CSM with mass and extent similar to the continuous scenario. We discuss the implications of the available multiwavelength constraints obtained thus far on the progenitor candidate and SN 2023ixf to our variable CSM models. 

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3. Extended Shock Breakout and Early Circumstellar Interaction in SN 2024ggi

   https://doi.org/10.3847/2041-8213/ad6907  

   Shrestha, Manisha; Bostroem, K Azalee; Sand, David J; Hosseinzadeh, Griffin; Andrews, Jennifer E; Dong_董, Yize 一泽; Hoang, Emily; Janzen, Daryl; Pearson, Jeniveve; Jencson, Jacob E; et al (August 2024, The Astrophysical Journal Letters)

   Abstract We present high-cadence photometric and spectroscopic observations of supernova (SN) 2024ggi, a Type II SN with flash spectroscopy features, which exploded in the nearby galaxy NGC 3621 at ∼7 Mpc. The light-curve evolution over the first 30 hr can be fit by two power-law indices with a break after 22 hr, rising fromM_V≈ −12.95 mag at +0.66 day toM_V≈ −17.91 mag after 7 days. In addition, the densely sampled color curve shows a strong blueward evolution over the first few days and then behaves as a normal SN II with a redward evolution as the ejecta cool. Such deviations could be due to interaction with circumstellar material (CSM). Early high- and low-resolution spectra clearly show high-ionization flash features from the first spectrum to +3.42 days after the explosion. From the high-resolution spectra, we calculate the CSM velocity to be 37 ± 4 km s⁻¹. We also see the line strength evolve rapidly from 1.22 to 1.49 days in the earliest high-resolution spectra. Comparison of the low-resolution spectra with CMFGEN models suggests that the pre-explosion mass-loss rate of SN 2024ggi falls in the range of 10⁻³–10⁻²M_☉yr⁻¹, which is similar to that derived for SN 2023ixf. However, the rapid temporal evolution of the narrow lines in the spectra of SN 2024ggi (R_CSM∼ 2.7 × 10¹⁴cm) could indicate a smaller spatial extent of the CSM than in SN 2023ixf (R_CSM∼ 5.4 × 10¹⁴cm), which in turn implies a lower total CSM mass for SN 2024ggi. 

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4. SN 2021foa: The “Flip-flop” Type IIn/Ibn Supernova

   https://doi.org/10.3847/1538-4357/ad8cd3  

   Farias, D; Gall, C; Narayan, G; Rest, S; Villar, V A; Angus, C R; Auchettl, K; Davis, K W; Foley, R J; Gagliano, A; et al (December 2024, The Astrophysical Journal)

   Abstract We present a comprehensive analysis of the photometric and spectroscopic evolution of SN 2021foa, unique among the class of transitional supernovae for repeatedly changing its spectroscopic appearance from hydrogen-to-helium-to-hydrogen dominated (IIn-to-Ibn-to-IIn) within 50 days past peak brightness. The spectra exhibit multiple narrow (≈300–600 km s⁻¹) absorption lines of hydrogen, helium, calcium, and iron together with broad helium emission lines with a full width at half-maximum (FWHM) of ∼6000 km s⁻¹. For a steady, wind mass-loss regime, light-curve modeling results in an ejecta mass of ∼8M_⊙and circumstellar material (CSM) mass below 1M_⊙, and an ejecta velocity consistent with the FWHM of the broad helium lines. We obtain a mass-loss rate of ≈2M_⊙yr⁻¹. This mass-loss rate is 3 orders of magnitude larger than derived for normal Type II supernovae. We estimate that the bulk of the CSM of SN 2021foa must have been expelled within half a year, about 12 yr ago. Our analysis suggests that SN 2021foa had a helium-rich ejecta that swept up a dense shell of hydrogen-rich CSM shortly after explosion. At about 60 days past peak brightness, the photosphere recedes through the dense ejecta-CSM region, occulting much of the redshifted emission of the hydrogen and helium lines, which results in an observed blueshift (∼−3000 km s⁻¹). Strong mass-loss activity prior to explosion, such as those seen in SN 2009ip-like objects and SN 2021foa as precursor emission, are the likely origin of a complex, multiple-shell CSM close to the progenitor star. 

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5. SN 2020zbf: A fast-rising hydrogen-poor superluminous supernova with strong carbon lines

   https://doi.org/10.1051/0004-6361/202348166  

   Gkini, A; Lunnan, R; Schulze, S; Dessart, L; Brennan, S J; Sollerman, J; Pessi, P J; Nicholl, M; Yan, L; Omand, C_M B; et al (May 2024, Astronomy & Astrophysics)

   SN 2020zbf is a hydrogen-poor superluminous supernova (SLSN) atz = 0.1947 that shows conspicuous C IIfeatures at early times, in contrast to the majority of H-poor SLSNe. Its peak magnitude isM_g = −21.2 mag and its rise time (≲26.4 days from first light) places SN 2020zbf among the fastest rising type I SLSNe. We used spectra taken from ultraviolet (UV) to near-infrared wavelengths to identify spectral features. We paid particular attention to the C IIlines as they present distinctive characteristics when compared to other events. We also analyzed UV and optical photometric data and modeled the light curves considering three different powering mechanisms: radioactive decay of⁵⁶Ni, magnetar spin-down, and circumstellar medium (CSM) interaction. The spectra of SN 2020zbf match the model spectra of a C-rich low-mass magnetar-powered supernova model well. This is consistent with our light curve modeling, which supports a magnetar-powered event with an ejecta massM_ej = 1.5 M_⊙. However, we cannot discard the CSM-interaction model as it may also reproduce the observed features. The interaction with H-poor, carbon-oxygen CSM near peak light could explain the presence of C IIemission lines. A short plateau in the light curve around 35–45 days after peak, in combination with the presence of an emission line at 6580 Å, can also be interpreted as being due to a late interaction with an extended H-rich CSM. Both the magnetar and CSM-interaction models of SN 2020zbf indicate that the progenitor mass at the time of explosion is between 2 and 5M_⊙. Modeling the spectral energy distribution of the host galaxy reveals a host mass of 10^8.7M_⊙, a star formation rate of 0.24_−0.12^+0.41M_⊙yr⁻¹, and a metallicity of ∼0.4Z_⊙. 

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