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

   Pickens, C; DeSoto, S; Bilinski, C; Hoffman, J; Williams, G; Leonard, D; Shrestha, M (February 2024, Americal 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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3. ASASSN-18am/SN 2018gk: an overluminous Type IIb supernova from a massive progenitor

   https://doi.org/10.1093/mnras/stab629  

   Bose, Subhash; Dong, Subo; Kochanek, C S; Stritzinger, M D; Ashall, Chris; Benetti, Stefano; Falco, E; Filippenko, Alexei V; Pastorello, Andrea; Prieto, Jose L; et al (March 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT ASASSN-18am/SN 2018gk is a newly discovered member of the rare group of luminous, hydrogen-rich supernovae (SNe) with a peak absolute magnitude of MV ≈ −20 mag that is in between normal core-collapse SNe and superluminous SNe. These SNe show no prominent spectroscopic signatures of ejecta interacting with circumstellar material (CSM), and their powering mechanism is debated. ASASSN-18am declines extremely rapidly for a Type II SN, with a photospheric-phase decline rate of ∼6.0 mag (100 d)−1. Owing to the weakening of H i and the appearance of He i in its later phases, ASASSN-18am is spectroscopically a Type IIb SN with a partially stripped envelope. However, its photometric and spectroscopic evolution shows significant differences from typical SNe IIb. Using a radiative diffusion model, we find that the light curve requires a high synthesized 56Ni mass $$M_{\rm Ni} \sim 0.4\, \rm {M_{\odot }}$$ and ejecta with high kinetic energy Ekin = (7–10) × 1051 erg. Introducing a magnetar central engine still requires $$M_{\rm Ni} \sim 0.3\, \rm {M_{\odot }}$$ and Ekin = 3 × 1051 erg. The high 56Ni mass is consistent with strong iron-group nebular lines in its spectra, which are also similar to several SNe Ic-BL with high 56Ni yields. The earliest spectrum shows ‘flash ionization’ features, from which we estimate a mass-loss rate of $$\dot{M}\approx 2\times 10^{-4} \, \rm \rm {M_{\odot }}\,yr^{-1}$$. This wind density is too low to power the luminous light curve by ejecta–CSM interaction. We measure expansion velocities as high as 17 000 $$\rm {\, km\, s^{-1}}$$ for Hα, which is remarkably high compared to other SNe II. We estimate an oxygen core mass of 1.8–3.4 M⊙ using the [O i] luminosity measured from a nebular-phase spectrum, implying a progenitor with a zero-age main-sequence mass of 19–26 M⊙. 

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4. Extreme Mass Loss in Low-mass Type Ib/c Supernova Progenitors

   https://doi.org/10.3847/2041-8213/ac9b3d  

   Wu, Samantha_C; Fuller, Jim (November 2022, The Astrophysical Journal Letters)

   Abstract Many core-collapse supernovae (SNe) with hydrogen-poor and low-mass ejecta, such as ultra-stripped SNe and type Ibn SNe, are observed to interact with dense circumstellar material (CSM). These events likely arise from the core collapse of helium stars that have been heavily stripped by a binary companion and have ejected significant mass during the last weeks to years of their lives. In helium star models run to days before core collapse we identify a range of helium core masses ≈2.5–3M_⊙whose envelopes expand substantially due to the helium shell burning while the core undergoes neon and oxygen burning. When modeled in binary systems, the rapid expansion of these helium stars induces extremely high rates of late-stage mass transfer ( $\dot{M}\gtrsim {10}^{-2}{M}_{\odot }{\mathrm{yr}}^{-1}$) beginning weeks to decades before core collapse. We consider two scenarios for producing CSM in these systems: either mass transfer remains stable and mass loss is driven from the system in the vicinity of the accreting companion, or mass transfer becomes unstable and causes a common envelope event (CEE) through which the helium envelope is unbound. The ensuing CSM properties are consistent with the CSM masses (∼10⁻²–1M_⊙) and radii (∼10¹³–10¹⁶cm) inferred for ultra-stripped SNe and several type Ibn SNe. Furthermore, systems that undergo a CEE could produce short-period neutron star binaries that merge in less than 100 Myr. 

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5. Probing Presupernova Mass Loss in Double-peaked Type Ibc Supernovae from the Zwicky Transient Facility

   https://doi.org/10.3847/1538-4357/ad595f  

   Das, Kaustav_K; Kasliwal, Mansi_M; Sollerman, Jesper; Fremling, Christoffer; Irani, I.; Leung, Shing-Chi; Yang, Sheng; Wu, Samantha; Fuller, Jim; Anand, Shreya; et al (August 2024, The Astrophysical Journal)

   Abstract Eruptive mass loss of massive stars prior to supernova (SN) explosion is key to understanding their evolution and end fate. An observational signature of pre-SN mass loss is the detection of an early, short-lived peak prior to the radioactive-powered peak in the lightcurve of the SN. This is usually attributed to the SN shock passing through an extended envelope or circumstellar medium. Such an early peak is common for double-peaked Type IIb SNe with an extended hydrogen envelope but uncommon for normal Type Ibc SNe with very compact progenitors. In this paper, we systematically study a sample of 14 double-peaked Type Ibc SNe out of 475 Type Ibc SNe detected by the Zwicky Transient Facility. The rate of these events is ∼3%–9% of Type Ibc SNe. A strong correlation is seen between the peak brightness of the first and the second peak. We perform a holistic analysis of this sample’s photometric and spectroscopic properties. We find that six SNe have ejecta mass less than 1.5M_⊙. Based on the nebular spectra and lightcurve properties, we estimate that the progenitor masses for these are less than ∼12M_⊙. The rest have an ejecta mass >2.4M_⊙and a higher progenitor mass. This sample suggests that the SNe with low progenitor masses undergo late-time binary mass transfer. Meanwhile, the SNe with higher progenitor masses are consistent with wave-driven mass loss or pulsation-pair instability-driven mass-loss simulations. 

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