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1. Signatures of progenitors of Type Ia supernovae

   P. Hoeflich, S. Chakraborty ( October 2017 , Memorie della Società astronomica italiana)

   Thermonuclear Supernovae (SNe Ia) are one of the building blocks of modern cosmology and laboratories for the explosion physics of White Dwarf star/s (WD) in close binary systems. The second star may be aWD(double degenerate systems, DD), or a non-degenerated star (SD) with a main sequence star, red giant or a helium star as companion (Branch et al. 1995; Nomoto et al. 2003; Wang & Han 2012). Light curves and spectra of the explosion look similar because a ’stellar amnesia’ (H¨oflich et al. 2006). Basic nuclear physics determines the progenitor structure and the explosion physics, breaking the link between progenitor evolution, and the explosion, resulting in three main classes of explosion scenarios: a) dynamical merging of two WD and a heating on time scales of seconds (Webbink 1984; Isern et al. 2011), b) surface helium detonations on top of a WD which ignite the central C/O by a detonation wave traveling inwards (Nomoto 1982; Hoeflich & Khokhlov 1996; Kromer et al. 2010); c) compressional heating in an accreting WD approaching the Chandrasekar mass on time of up to 108 years which may originated from SD and DD systems (Whelan & Iben 1973; Piersanti et al. 2003). Simulations of the explosions depend on the inital conditions at the onset of the explosions, namely the mass and angular momentum of the WD(s). For all scenarios, diversity in SNe Ia must be expected because the WD originates from a range of Main Sequence masses (MMS < 8M) and metallicities Z. Moreover, there is growing evidence that magnetic fields B may have to be added to the ’mix’. Only with recent advances in observations ranging from X-ray to radio, high precision spectroscopy, polarimetry and photometry and in the time-domain astronomy we obtain constraints for progenitor, on the explosion scenarios and links emerge between the progenitors and their environment with LCs and spectral signatures needed for high precision cosmology. It is too early to give final answers but we present our personal view. We will give some examples from the theory point of view and discuss future prospects with upcoming ground based, ELT, GMT and space based such as JWST, Euclide and WFIRST instruments. 

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2. Origin of High-velocity Ejecta, Excess Emission, and Redward Color Evolution in the Infant Type Ia Supernova 2021aefx

   https://doi.org/10.3847/1538-4357/ad0640  

   Ni, Yuan Qi ; Moon, Dae-Sik ; Drout, Maria R. ; Matzner, Christopher D. ; Leong, Kelvin C. C. ; Kim, Sang Chul ; Park, Hong Soo ; Lee, Youngdae ( December 2023 , The Astrophysical Journal)

   SN 2021aefxis a normal Type Ia supernova (SN) showing excess emission and redward color evolution over the first ∼ 2 days. We present analyses of this SN using our high-cadence KMTNet multiband photometry, spectroscopy, and publicly available data, including first measurements of its explosion epoch (MJD 59529.32 ± 0.16) and onset of power-law rise (t_PL= MJD 59529.85 ± 0.55; often calledfirst light) associated with the main ejecta⁵⁶Ni distribution. The first KMTNet detection of SN 2021aefx precedest_PLby ∼ 0.5 hr, indicating presence of additional power sources. Our peak-spectrum confirms its intermediate Type Ia subclassification between core-normal and broad-Line, and we estimate an ejecta mass of ∼ 1.34M_⊙. The spectral evolution identifies material reaching >40,000 km s⁻¹(fastest ever observed in Type Ia SNe) and at least two split-velocity ejecta components expanding homologously: (1) a normal-velocity (∼ 12,400 km s⁻¹) component consistent with typical photospheric evolution of near-Chandrasekhar-mass ejecta; and (2) a high-velocity (∼ 23,500 km s⁻¹) secondary component visible during the first ∼ 3.6 days post-explosion, which locates the component within the outer <16% of the ejecta mass. Asymmetric subsonic explosion processes producing a nonspherical secondary photosphere provide an explanation for the simultaneous appearance of the two components, and may also explain the excess emission via a slight⁵⁶Ni enrichment in the outer ∼ 0.5% of the ejecta mass. Our 300 days post-peak nebular-phase spectrum advances constraints against nondegenerate companions and further supports a near-Chandrasekhar-mass explosion origin. Off-center ignited delayed-detonations are likely responsible for the observed features of SN 2021aefx in some normal Type Ia SNe.

    

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3. Constraining the Progenitor System of the Type Ia Supernova 2021aefx

   https://doi.org/10.3847/2041-8213/ac7cef  

   Hosseinzadeh, Griffin ; Sand, David J. ; Lundqvist, Peter ; Andrews, Jennifer E. ; Bostroem, K. Azalee ; Dong 董, Yize 一泽 ; Janzen, Daryl ; Jencson, Jacob E. ; Lundquist, Michael ; Meza Retamal, Nicolas E. ; et al ( July 2022 , The Astrophysical Journal Letters)

   We present high-cadence optical and ultraviolet light curves of the normal Type Ia supernova (SN) 2021aefx, which shows an early bump during the first two days of observation. This bump may be a signature of interaction between the exploding white dwarf and a nondegenerate binary companion, or it may be intrinsic to the white dwarf explosion mechanism. In the case of the former, the short duration of the bump implies a relatively compact main-sequence companion star, although this conclusion is viewing-angle dependent. Our best-fit companion-shocking and double-detonation models both overpredict the UV luminosity during the bump, and existing nickel-shell models do not match the strength and timescale of the bump. We also present nebular spectra of SN 2021aefx, which do not show the hydrogen or helium emission expected from a nondegenerate companion, as well as a radio nondetection that rules out all symbiotic progenitor systems and most accretion disk winds. Our analysis places strong but conflicting constraints on the progenitor of SN 2021aefx; no current model can explain all of our observations.

    

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4. Flight of the Bumblebee: the Early Excess Flux of Type Ia Supernova 2023bee Revealed by TESS, Swift, and Young Supernova Experiment Observations

   https://doi.org/10.3847/1538-4357/ad0edb  

   Wang, Qinan ; Rest, Armin ; Dimitriadis, Georgios ; Ridden-Harper, Ryan ; Siebert, Matthew R. ; Magee, Mark ; Angus, Charlotte R. ; Auchettl, Katie ; Davis, Kyle W. ; Foley, Ryan J. ; et al ( February 2024 , The Astrophysical Journal)

   We present high-cadence ultraviolet through near-infrared observations of the Type Ia supernova (SN Ia) 2023bee atD= 32 ± 3 Mpc, finding excess flux in the first days after explosion, particularly in our 10 minutes cadence TESS light curve and Swift UV data. Compared to a few other normal SNe Ia with early excess flux, the excess flux in SN 2023bee is redder in the UV and less luminous. We present optical spectra of SN 2023bee, including two spectra during the period where the flux excess is dominant. At this time, the spectra are similar to those of other SNe Ia but with weaker Siii, Cii,and Caiiabsorption lines, perhaps because the excess flux creates a stronger continuum. We compare the data to several theoretical models on the origin of early excess flux in SNe Ia. Interaction with either the companion star or close-in circumstellar material is expected to produce a faster evolution than observed. Radioactive material in the outer layers of the ejecta, either from double detonation explosion or from a⁵⁶Ni clump near the surface, cannot fully reproduce the evolution either, likely due to the sensitivity of early UV observable to the treatment of the outer part of ejecta in simulation. We conclude that no current model can adequately explain the full set of observations. We find that a relatively large fraction of nearby, bright SNe Ia with high-cadence observations have some amount of excess flux within a few days of explosion. Considering potential asymmetric emission, the physical cause of this excess flux may be ubiquitous in normal SNe Ia.

    

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5. The Origin and Evolution of the Normal Type Ia SN 2018aoz with Infant-phase Reddening and Excess Emission

   https://doi.org/10.3847/1538-4357/aca9be  

   Ni, Yuan Qi ; Moon, Dae-Sik ; Drout, Maria R. ; Polin, Abigail ; Sand, David J. ; González-Gaitán, Santiago ; Kim, Sang Chul ; Lee, Youngdae ; Park, Hong Soo ; Howell, D. Andrew ; et al ( March 2023 , The Astrophysical Journal)

   SN 2018aoz is a Type Ia SN with aB-band plateau and excess emission in infant-phase light curves ≲1 day after the first light, evidencing an over-density of surface iron-peak elements as shown in our previous study. Here, we advance the constraints on the nature and origin of SN 2018aoz based on its evolution until the nebular phase. Near-peak spectroscopic features show that the SN is intermediate between two subtypes of normal Type Ia: core normal and broad line. The excess emission may be attributable to the radioactive decay of surface iron-peak elements as well as the interaction of ejecta with either the binary companion or a small torus of circumstellar material. Nebular-phase limits on Hαand Heifavor a white dwarf companion, consistent with the small companion size constrained by the low early SN luminosity, while the absence of [Oi] and Heidisfavors a violent merger of the progenitor. Of the two main explosion mechanisms proposed to explain the distribution of surface iron-peak elements in SN 2018aoz, the asymmetric Chandrasekhar-mass explosion is less consistent with the progenitor constraints and the observed blueshifts of nebular-phase [Feii] and [Niii]. The helium-shell double-detonation explosion is compatible with the observed lack of C spectral features, but current 1D models are incompatible with the infant-phase excess emission,$B_{\max }–V_{\max }$color, and weak strength of nebular-phase [Caii]. Although the explosion processes of SN 2018aoz still need to be more precisely understood, the same processes could produce a significant fraction of Type Ia SNe that appear to be normal after ∼1 day.

    

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