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1. 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)

   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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2. A Rapid Ionization Change in the Nebular-phase Spectra of the Type Ia SN 2011fe

   https://doi.org/10.3847/2041-8213/ac4fbd  

   Tucker, M. A. ; Ashall, C. ; Shappee, B. J. ; Kochanek, C. S. ; Stanek, K. Z. ; Garnavich, P. ( February 2022 , The Astrophysical Journal Letters)

   We present three new spectra of the nearby Type Ia supernova (SN Ia) 2011fe covering ≈480–850 days after maximum light and show that the ejecta undergoes a rapid ionization shift at ∼500 days after explosion. The prominent Feiiiemission lines at ≈4600 Å are replaced with Fei+Feiiblends at ∼4400 Å and ∼5400 Å. The ≈7300 Å feature, which is produced by [Feii]+[Niii] at ≲400 days after explosion, is replaced by broad (≈±15,000 km s⁻¹) symmetric [Caii] emission. Models predict this ionization transition occurring ∼100 days later than what is observed, which we attribute to clumping in the ejecta. Finally, we use the nebular-phase spectra to test several proposed progenitor scenarios for SN 2011fe. Nondetections of H and He exclude nearby nondegenerate companions, [Oi] nondetections disfavor the violent merger of two white dwarfs, and the symmetric emission-line profiles favor a symmetric explosion.

    

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3. High-resolution Spectroscopy of SN 2023ixf’s First Week: Engulfing the Asymmetric Circumstellar Material

   https://doi.org/10.3847/1538-4357/acf366  

   Smith, Nathan ; Pearson, Jeniveve ; Sand, David J. ; Ilyin, Ilya ; Bostroem, K. Azalee ; Hosseinzadeh, Griffin ; Shrestha, Manisha ( October 2023 , The Astrophysical Journal)

   We present a series of high-resolution echelle spectra of SN 2023ixf in M101, obtained nightly during the first week or so after discovery using PEPSI on the Large Binocular Telescope. NaiD absorption in these spectra indicates a host reddening ofE(B−V) = 0.031 mag and a systemic velocity of +7 km s⁻¹relative to the average redshift of M101. Dramatic changes are seen in the strength and shape of strong emission lines emitted by circumstellar material (CSM), including Heiiλ4686, Civλλ5801,5811, Hα, and Nivλλ7109,7123. In general, these narrow lines broaden to become intermediate-width lines before disappearing from the spectrum within a few days, indicating a limited extent to the dense CSM of around 20–30 au (or ≲10^14.7cm). Hαpersists in the spectrum for about a week as an intermediate-width emission line with P Cyg absorption at 700–1300 km s⁻¹arising in the post-shock shell of swept-up CSM. Early narrow emission lines are blueshifted and indicate an expansion speed in the pre-shock CSM of about 115 km s⁻¹, but with even broader emission in higher-ionization lines. This is faster than the normal winds of red supergiants, suggesting some mode of eruptive mass loss from the progenitor or radiative acceleration of the CSM. A lack of narrow blueshifted absorption suggests that most of the CSM is not along our line of sight. This and several other clues indicate that the CSM of SN 2023ixf is significantly aspherical. We find that CSM lines disappear after a few days because the asymmetric CSM is engulfed by the supernova photosphere.

    

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4. Circumstellar Medium Constraints on the Environment of Two Nearby Type Ia Supernovae: SN 2017cbv and SN 2020nlb

   https://doi.org/10.3847/1538-4357/ac20da  

   Sand, D. J. ; Sarbadhicary, S. K. ; Pellegrino, C. ; Misra, K. ; Dastidar, R. ; Brown, P. J. ; Itagaki, K. ; Valenti, S. ; Swift, Jonathan J. ; Andrews, J. E. ; et al ( November 2021 , The Astrophysical Journal)

   Abstract We present deep Chandra X-ray observations of two nearby Type Ia supernovae, SN 2017cbv and SN 2020nlb, which reveal no X-ray emission down to a luminosity L X ≲ 5.3 × 10 37 and ≲ 5.4 × 10 37 erg s −1 (0.3–10 keV), respectively, at ∼16–18 days after the explosion. With these limits, we constrain the pre-explosion mass-loss rate of the progenitor system to be M ̇ < 7.2 × 10 −9 and < 9.7 × 10 −9 M ⊙ yr −1 for each (at a wind velocity v w = 100 km s −1 and a radius of R ≈ 10 16 cm), assuming any X-ray emission would originate from inverse Compton emission from optical photons upscattered by the supernova shock. If the supernova environment was a constant-density medium, we would find a number density limit of n CSM < 36 and < 65 cm −3 , respectively. These X-ray limits rule out all plausible symbiotic progenitor systems, as well as large swathes of parameter space associated with the single degenerate scenario, such as mass loss at the outer Lagrange point and accretion winds. We also present late-time optical spectroscopy of SN 2020nlb, and set strong limits on any swept up hydrogen ( L H α < 2.7 × 10 37 erg s −1 ) and helium ( L He, λ 6678 < 2.7 × 10 37 erg s −1 ) from a nondegenerate companion, corresponding to M H ≲ 0.7–2 × 10 −3 M ⊙ and M He ≲ 4 × 10 −3 M ⊙ . Radio observations of SN 2020nlb at 14.6 days after explosion also yield a non-detection, ruling out most plausible symbiotic progenitor systems. While we have doubled the sample of normal Type Ia supernovae with deep X-ray limits, more observations are needed to sample the full range of luminosities and subtypes of these explosions, and set statistical constraints on their circumbinary environments. 

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5. Hubble Space Telescope Imaging Reveals That SN 2015bh Is Much Fainter than Its Progenitor

   https://doi.org/10.3847/2041-8213/ac867c  

   Jencson, Jacob E. ; Sand, David J. ; Andrews, Jennifer E. ; Smith, Nathan ; Strader, Jay ; Aghakhanloo, Mojgan ; Pearson, Jeniveve ; Valenti, Stefano ( August 2022 , The Astrophysical Journal Letters)

   We present Hubble Space Telescope (HST) imaging of the site of SN 2015bh in the nearby spiral galaxy NGC 2770 taken between 2017 and 2019, nearly four years after the peak of the explosion. In 2017–2018, the transient fades steadily in optical filters before declining more slowly toF814W= −7.1 mag in 2019, ≈4 mag below the level of its eruptive luminous blue variable (LBV) progenitor observed with HST in 2008–2009. The source fades at a constant color ofF555W−F814W= 0.4 mag until 2018, similar to SN 2009ip and consistent with a spectrum dominated by interaction of the ejecta with circumstellar material (CSM). A deep optical spectrum obtained in 2021 lacks signatures of ongoing interaction (L_Hα≲ 10³⁸erg s⁻¹for broadened emission ≲2000 km s⁻¹), but indicates the presence of a nearby Hiiregion (≲300 pc). The color evolution of the fading source makes it unlikely that emission from a scattered-light echo or binary OB companion of the progenitor contributes significantly to the flattening of the late-time light curve. The remaining emission in 2019 may plausibly be attributed an evolved/inflated companion or an unresolved (≲3 pc), young stellar cluster. Importantly, the color evolution of SN 2015bh rules out scenarios in which the surviving progenitor is obscured by nascent dust and does not clearly indicate a transition to a hotter, optically faint state. The simplest explanation is that the massive progenitor did not survive. SN 2015bh likely represents a remarkable example of the terminal explosion of a massive star preceded by decades of end-stage eruptive variability.

    

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