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1. The Immediate, Exemplary, and Fleeting Echelle Spectroscopy of SN 2023ixf: Monitoring Acceleration of Slow Progenitor Circumstellar Material Driven by Shock Interaction

   https://doi.org/10.3847/1538-4357/adc108  

   Dickinson, Danielle; Milisavljevic, Dan; Garretson, Braden; Dessart, Luc; Margutti, Raffaella; Chornock, Ryan; Subrayan, Bhagya; Hillier, D John; Golub, Eli; Li, Dan; et al (April 2025, The Astrophysical Journal)

   Abstract We present high-resolution WIYN/NEID echelle spectroscopy (R ≈ 70,000) of the supernova (SN) 2023ixf in M101, obtained 1.51 to 18.51 days after explosion over nine epochs. Daily monitoring for the first 4 days after explosion shows narrow emission features (≤200 km s⁻¹), exhibiting predominantly blueshifted velocities that rapidly weaken, broaden, and vanish in a manner consistent with radiative acceleration and the SN shock eventually overrunning or enveloping the full extent of the dense circumstellar medium (CSM). The most rapid evolution is in the Heiemission, which is visible on day 1.51 but disappears by day 2.62. We measure the maximum pre-SN speed of Heito be 25 ${}_{-5}^{+0}\pm 2$km s⁻¹, where the error is attributable to the uncertainty in how much the Heihad already been radiatively accelerated and to measurement of the emission-line profile. The radiative acceleration of CSM is likely driven by the shock–CSM interaction, and the CSM is accelerated to ≥200 km s⁻¹before being completely swept up by the SN shock to ∼2000 km s⁻¹. We compare the observed spectra with spherically symmetric r1w6bHERACLES/CMFGENmodel spectra and find the line evolution to generally be consistent with radiative acceleration, optical depth effects, and evolving ionization state. The progenitor of SN 2023ixf underwent an enhanced mass-loss phase ≳4 yr prior to core collapse, creating a dense, asymmetric CSM region extending out to approximatelyr_CSM = 3.7  × 10¹⁴(v_shock/9500 km s⁻¹) cm. 

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2. Asymmetries and Circumstellar Interaction in the Type II SN 2024bch

   https://doi.org/10.3847/1538-4357/ada555  

   Andrews, Jennifer E; Shrestha, Manisha; Bostroem, K Azalee; Dong_董, Yize 一泽; Pearson, Jeniveve; Fausnaugh, M M; Sand, David J; Valenti, S; Ravi, Aravind P; Hoang, Emily; et al (February 2025, The Astrophysical Journal)

   Abstract We present a comprehensive multi-epoch photometric and spectroscopic study of SN 2024bch, a nearby (19.9 Mpc) Type II supernova (SN) with prominent early high-ionization emission lines. Optical spectra from 2.8 days after the estimated explosion reveal narrow lines of H i, He ii, C iv, and N ivthat disappear by day 6. High-cadence photometry from the ground and Transiting Exoplanet Survey Satellite show that the SN brightened quickly and reached a peakM_V ~ −17.8 mag within a week of explosion, and late-time photometry suggests a⁵⁶Ni mass of 0.050M_⊙. High-resolution spectra from days 7.9 and 43 trace the unshocked circumstellar medium (CSM) and indicate a wind velocity of 30–40 km s⁻¹, a value consistent with a red supergiant (RSG) progenitor. Comparisons between models and the early spectra suggest a pre-SN mass-loss rate of $\dot{M}~10^{-3}–10^{-2}{M}_{\odot }{\mathrm{yr}}^{-1}$, which is too high to be explained by quiescent mass loss from RSGs, but is consistent with some recent measurements of similar SNe. Persistent blueshifted H iand [O i] emission lines seen in the optical and near-IR spectra could be produced by asymmetries in the SN ejecta, while the multicomponent Hαmay indicate continued interaction with an asymmetric CSM well into the nebular phase. SN 2024bch provides another clue to the complex environments and mass-loss histories around massive stars. 

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3. SN 2023ixf in Messier 101: Photo-ionization of Dense, Close-in Circumstellar Material in a Nearby Type II Supernova

   https://doi.org/10.3847/2041-8213/acf2ec  

   Jacobson-Galán, W. V.; Dessart, L.; Margutti, R.; Chornock, R.; Foley, R. J.; Kilpatrick, C. D.; Jones, D. O.; Taggart, K.; Angus, C. R.; Bhattacharjee, S.; et al (September 2023, The Astrophysical Journal Letters)

   Abstract We present UV and/or optical observations and models of SN 2023ixf, a type II supernova (SN) located in Messier 101 at 6.9 Mpc. Early time (flash) spectroscopy of SN 2023ixf, obtained primarily at Lick Observatory, reveals emission lines of Hi, Hei/ii, Civ, and Niii/iv/vwith a narrow core and broad, symmetric wings arising from the photoionization of dense, close-in circumstellar material (CSM) located around the progenitor star prior to shock breakout. These electron-scattering broadened line profiles persist for ∼8 days with respect to first light, at which time Doppler broadened the features from the fastest SN ejecta form, suggesting a reduction in CSM density atr≳ 10¹⁵cm. The early time light curve of SN 2023ixf shows peak absolute magnitudes (e.g.,M_u= −18.6 mag,M_g= −18.4 mag) that are ≳2 mag brighter than typical type II SNe, this photometric boost also being consistent with the shock power supplied from CSM interaction. Comparison of SN 2023ixf to a grid of light-curve and multiepoch spectral models from the non-LTE radiative transfer codeCMFGENand the radiation-hydrodynamics codeHERACLESsuggests dense, solar-metallicity CSM confined tor= (0.5–1) × 10¹⁵cm, and a progenitor mass-loss rate of $\dot{M}={10}^{-2}{M}_{\odot }$yr⁻¹. For the assumed progenitor wind velocity ofv_w= 50 km s⁻¹, this corresponds to enhanced mass loss (i.e.,superwindphase) during the last ∼3–6 yr before explosion. 

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4. Early Spectroscopy and Dense Circumstellar Medium Interaction in SN 2023ixf

   https://doi.org/10.3847/2041-8213/acf9a4  

   Bostroem, K. Azalee; Pearson, Jeniveve; Shrestha, Manisha; Sand, David J.; Valenti, Stefano; Jha, Saurabh W.; Andrews, Jennifer E.; Smith, Nathan; Terreran, Giacomo; Green, Elizabeth; et al (October 2023, The Astrophysical Journal Letters)

   Abstract We present the optical spectroscopic evolution of SN 2023ixf seen in subnight cadence spectra from 1.18 to 15 days after explosion. We identify high-ionization emission features, signatures of interaction with material surrounding the progenitor star, that fade over the first 7 days, with rapid evolution between spectra observed within the same night. We compare the emission lines present and their relative strength to those of other supernovae with early interaction, finding a close match to SN 2020pni and SN 2017ahn in the first spectrum and SN 2014G at later epochs. To physically interpret our observations, we compare them to CMFGEN models with confined, dense circumstellar material around a red supergiant (RSG) progenitor from the literature. We find that very few models reproduce the blended Niii(λλ4634.0,4640.6)/Ciii(λλ4647.5,4650.0) emission lines observed in the first few spectra and their rapid disappearance thereafter, making this a unique diagnostic. From the best models, we find a mass-loss rate of 10⁻³–10⁻²M_⊙yr⁻¹, which far exceeds the mass-loss rate for any steady wind, especially for an RSG in the initial mass range of the detected progenitor. These mass-loss rates are, however, similar to rates inferred for other supernovae with early circumstellar interaction. Using the phase when the narrow emission features disappear, we calculate an outer dense radius of circumstellar materialR_CSM,out≈ 5 × 10¹⁴cm, and a mean circumstellar material density ofρ= 5.6 × 10⁻¹⁴g cm⁻³. This is consistent with the lower limit on the outer radius of the circumstellar material we calculate from the peak Hαemission flux,R_CSM,out≳ 9 × 10¹³cm. 

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5. Circumstellar Interaction in the Ultraviolet Spectra of SN 2023ixf 14–66 Days After Explosion

   https://doi.org/10.3847/2041-8213/ad7855  

   Bostroem, K_Azalee; Sand, David_J; Dessart, Luc; Smith, Nathan; Jha, Saurabh_W; Valenti, Stefano; Andrews, Jennifer_E; Dong_董, Yize_一泽; Filippenko, Alexei_V; Gomez, Sebastian; et al (September 2024, The Astrophysical Journal Letters)

   Abstract SN 2023ixf was discovered in M101 within a day of the explosion and rapidly classified as a Type II supernova with flash features. Here we present ultraviolet (UV) spectra obtained with the Hubble Space Telescope 14, 19, 24, and 66 days after the explosion. Interaction between the supernova ejecta and circumstellar material (CSM) is seen in the UV throughout our observations in the flux of the first three epochs and asymmetric Mgiiemission on day 66. We compare our observations to CMFGEN supernova models that include CSM interaction ( $\dot{M}<{10}^{-3}$M_⊙yr⁻¹) and find that the power from CSM interaction is decreasing with time, fromL_sh≈ 5 × 10⁴²erg s⁻¹toL_sh≈ 1 × 10⁴⁰erg s⁻¹between days 14 and 66. We examine the contribution of individual atomic species to the spectra on days 14 and 19, showing that the majority of the features are dominated by iron, nickel, magnesium, and chromium absorption in the ejecta. The UV spectral energy distribution of SN 2023ixf sits between that of supernovae, which show no definitive signs of CSM interaction, and those with persistent signatures assuming the same progenitor radius and metallicity. Finally, we show that the evolution and asymmetric shape of the Mgiiλλ2796, 2802 emission are not unique to SN 2023ixf. These observations add to the early measurements of dense, confined CSM interaction, tracing the mass-loss history of SN 2023ixf to ∼33 yr prior to the explosion and the density profile to a radius of ∼5.7 × 10¹⁵cm. They show the relatively short evolution from a quiescent red supergiant wind to high mass loss. 

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