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1. Multi-Epoch Spectropolarimetry Reveals Asphericity in Type IIb Supernova Explosions

   Pickens, C; DeSoto, S; Bilinski, C; Hoffman, JL; Williams, GG; Leonard, DC; Shrestha, M (February 2024, Bulletin of the American Astronomical Society)

   Type IIb supernovae (SNe IIb) are core-collapse events whose optical spectra show strong hydrogen features that disappear over time, implying that their progenitors were nearly, but not completely, stripped of their hydrogen envelopes prior to core collapse. Thus, compared to hydrogen-rich SNe II, SNe IIb can provide a closer examination of the underlying structure of the progenitor system, particularly during early photospheric phases (less than +70 days relative to max. light). I will present early-time multi-epoch optical spectropolarimetry of several SNe IIb, obtained using the SPOL instrument at the University of Arizona. Using polarization diagnostics provides a way to track structural changes in the depleted hydrogen envelopes of these SNe as deeper layers of helium and other elements emerge and evolve. I find significant temporal polarization increases in the absorption wings of their H and He lines. Some of these line features make "loops" in Stokes Q-U diagrams, suggesting non-axisymmetic structure in the ejecta, perhaps arising from a transient absorbing clump. Furthermore, the majority of these SNe show polarimetric evidence for aspherical explosions along a preferred, or dominant, axis. I discuss the implications these findings have on the 3D geometry of the explosions by comparing the observed polarization to published synthetic spectropolarimetry that models axial symmetry and clump structures in stripped-envelope, core-collapse SNe. This comparative study naturally facilitates a broader discussion around the unresolved question as to what extent this SNe subclass shows common polarization characteristics. 

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2. Spectropolarimetric Snapshots of Stripped Envelope Supernovae

   Pickens, C. (January 2021, American Astronomical Society meeting)

   null (Ed.)

   Supernovae of Type IIb (SNe IIb) are relatively rare events, constituting roughly 10% of all core-collapse supernovae. However, comparative optical spectroscopy of core-collapse events suggest that SNe IIb represent an important transition from the SNe II to SNe Ib sub-type. SNe IIb progenitors are thought to have been stripped of most, but not all, of their hydrogen envelopes by stellar winds or mass transfer in binary systems. Thus, they provide an opportunity to study the effects of mass-loss on stellar evolution. Spectropolarization signatures of these SNe can provide unique information on the spatial distribution of their ejecta and circumstellar environment/interaction. We present multi-epoch spectropolarimetry of SNe IIb selected from the database of the SNSPOL. The observations were obtained using the SPOL instrument at the University of Arizona telescopes. We analyze the time-dependent spectropolarimetric signatures of the objects in this sample and discuss what these imply about the evolution of the overall geometrical structure of the ejecta and circumstellar material in SNe IIb. These time-dependent "snapshots" provide an important window into stripped-envelope SNe and provide further clues regarding the relationship between SNe IIb and other SNe sub-types. 

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3. Carnegie Supernova Project-II: Near-infrared Spectroscopy of Stripped-envelope Core-collapse Supernovae*

   https://doi.org/10.3847/1538-4357/ac4030  

   Shahbandeh, M.; Hsiao, E. Y.; Ashall, C.; Teffs, J.; Hoeflich, P.; Morrell, N.; Phillips, M. M.; Anderson, J. P.; Baron, E.; Burns, C. R.; et al (February 2022, The Astrophysical Journal)

   Abstract We present 75 near-infrared (NIR; 0.8−2.5 μ m) spectra of 34 stripped-envelope core-collapse supernovae (SESNe) obtained by the Carnegie Supernova Project-II (CSP-II), encompassing optical spectroscopic Types IIb, Ib, Ic, and Ic-BL. The spectra range in phase from pre-maximum to 80 days past maximum. This unique data set constitutes the largest NIR spectroscopic sample of SESNe to date. NIR spectroscopy provides observables with additional information that is not available in the optical. Specifically, the NIR contains the strong lines of He i and allows a more detailed look at whether Type Ic supernovae are completely stripped of their outer He layer. The NIR spectra of SESNe have broad similarities, but closer examination through statistical means reveals a strong dichotomy between NIR “He-rich” and “He-poor” SNe. These NIR subgroups correspond almost perfectly to the optical IIb/Ib and Ic/Ic-BL types, respectively. The largest difference between the two groups is observed in the 2 μ m region, near the He i λ 2.0581 μ m line. The division between the two groups is not an arbitrary one along a continuous sequence. Early spectra of He-rich SESNe show much stronger He i λ 2.0581 μ m absorption compared to the He-poor group, but with a wide range of profile shapes. The same line also provides evidence for trace amounts of He in half of our SNe in the He-poor group. 

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4. The aspherical explosion of the Type IIP SN 2017gmr†

   https://doi.org/10.1093/mnrasl/slz119  

   Nagao, T; Cikota, A; Patat, F; Taubenberger, S; Bulla, M; Faran, T; Sand, D J; Valenti, S; Andrews, J E; Reichart, D E (October 2019, Monthly Notices of the Royal Astronomical Society: Letters)

   ABSTRACT Type IIP supernovae (SNe IIP), which represent the most common class of core-collapse (CC) SNe, show a rapid increase in continuum polarization just after entering the tail phase. This feature can be explained by a highly asymmetric helium core, which is exposed when the hydrogen envelope becomes transparent. Here we report the case of an SN IIP (SN 2017gmr) that shows an unusually early rise of the polarization, ≳30 d before the start of the tail phase. This implies that SN 2017gmr is an SN IIP that has very extended asphericity. The asymmetries are not confined to the helium core, but reach out to a significant part of the outer hydrogen envelope, hence clearly indicating a marked intrinsic diversity in the aspherical structure of CC explosions. These observations provide new constraints on the explosion mechanism, where viable models must be able to produce such extended deviations from spherical symmetry, and account for the observed geometrical diversity. 

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5. SN 2020bio: A Double-peaked, H-poor Type IIb Supernova with Evidence of Circumstellar Interaction

   https://doi.org/10.3847/1538-4357/ace595  

   Pellegrino, C.; Hiramatsu, D.; Arcavi, I.; Howell, D. A.; Bostroem, K. A.; Brown, P. J.; Burke, J.; Elias-Rosa, N.; Itagaki, K.; Kaneda, H.; et al (August 2023, The Astrophysical Journal)

   Abstract We present photometric and spectroscopic observations of SN 2020bio, a double-peaked Type IIb supernova (SN) discovered within a day of explosion, primarily obtained by Las Cumbres Observatory and Swift. SN 2020bio displays a rapid and long-lasting initial decline throughout the first week of its light curve, similarly to other well-studied Type IIb SNe. This early-time emission is thought to originate from the cooling of the extended outer hydrogen-rich (H-rich) envelope of the progenitor star that is shock heated by the SN explosion. We compare SN 2020bio to a sample of other double-peaked Type IIb SNe in order to investigate its progenitor properties. Analytical model fits to the early-time emission give progenitor radius (≈100–1500 R ⊙ ) and H-rich envelope mass (≈0.01–0.5 M ⊙ ) estimates that are consistent with other Type IIb SNe. However, SN 2020bio displays several peculiarities, including (1) weak H spectral features indicating a greater amount of mass loss than other Type IIb progenitors; (2) an underluminous secondary light-curve peak that implies a small amount of synthesized 56 Ni ( M Ni ≈0.02 M ⊙ ); and (3) low-luminosity nebular [O i ] and interaction-powered nebular features. These observations are more consistent with a lower-mass progenitor ( M ZAMS ≈ 12 M ⊙ ) that was stripped of most of its H-rich envelope before exploding. This study adds to the growing diversity in the observed properties of Type IIb SNe and their progenitors. 

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