Discovery of the Fastest Early Optical Emission from Overluminous SN Ia 2020hvf: A Thermonuclear Explosion within a Dense Circumstellar Environment

The early-excess SN Ia was originally proposed as a powerful indicator of the single-degenerate progenitor system because the interaction between a nondegenerate companion star (e.g., red giant, main-sequence star) and SN ejecta may cause prominent brightness excess in the first few days of the explosion. Recently, thanks to the progress of numerical simulations for early-phase light curves and remarkable discoveries of the early-excess SNe Ia, another three scenarios, i.e., the He-det, the CSM–ejecta interaction, and the surface-⁵⁶Ni-decay scenarios, have also been proposed to explain previous early-excess SNe Ia. In order to understand the prompt pulse-like early excess of SN 2020hvf, we systematically investigate the four early-excess scenarios. A few representative models for each scenario are shown in a panel of Figure 4.

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We first introduce a "confined" CSM and compute the interaction between the SN ejecta and the CSM. The prescriptions here are similar to Piro & Morozova (2016). The CSM composition is assumed to be dominated by C + O with solar metals. Adopting a power-law density distribution as a function of radius with an index of −3, the properties of the CSM are specified by the CSM mass and outer radius. The ejecta properties are fixed by modeling the post-flash behaviors, and here we adopt 2.1 M_⊙ for the ejecta mass (i.e., "super-M_Ch" WD) and 1.4 × 10⁵¹ erg for the kinetic energy (see Section 5.2). Now that the ejecta and CSM properties are specified, the system is evolved with the radiation-hydrodynamic mode of SNEC (the SuperNova Explosion Code, Morozova et al. 2015), which provides multiband light curves powered by the CSM interaction. As shown in panel (a) of Figure 4, a model assuming a CSM mass of 0.01 M_⊙ with a characteristic (outer edge) radius of 1.0 × 10¹³ cm can nicely fit the prominent early flash of SN 2020hvf. The CSM outer radius mostly affects the luminosity and the color, while the CSM mass mainly affects the duration (Piro & Morozova 2016; Maeda et al. 2018). As such, observational data can be used to strongly constrain the nature of the CSM in this scenario.

In terms of other early-excess scenarios, we find that the other three scenarios would not reproduce the observed combination of the high luminosity and the short duration of the early excess found for SN 2020hvf. In the following, we briefly introduce how the model parameters in each scenario are set to produce the early excess as bright as observed. These models are shown in Figure 4, demonstrating that either the timescale or characteristic evolution is not compatible with the observation of SN 2020hvf. Difficulties of interpreting the early excess of SN 2020hvf by the other three scenarios are further discussed by taking into account the discrepancies of the overall features/conditions expected by early-excess-related explosion mechanisms (e.g., He-det) and progenitor systems (e.g., companion–ejecta interaction) in Section 6.1.

For the He-detonation scenario, we examine a grid of 1D models computed previously (Maeda et al. 2018) that covers the detonated He-shell mass of 0.003–0.13M_⊙. In order to produce the high luminosity of the first-night light curve, a helium shell of about 0.1 M_⊙ is required (panel (b) in Figure 4; refer to models 8A and 9A of Maeda et al. (2018) for detailed model descriptions). The predicted timescale is, however, much longer than observed.

In the grid of the companion–ejecta interaction models with a range of binary configuration, a separation of ∼2.0 × 10¹² cm between a companion star and the SN ejecta can roughly explain the prominent early excess while the early-excess timescale is significantly longer than that of SN 2020hvf (panel (c) in Figure 4; refer to model D2e12 of Maeda et al. 2018 for details).

Synthesized light curves with a series of ⁵⁶Ni distributions are shown in panel (d) of Figure 4. In these models, we add the surface ⁵⁶Ni to our fiducial ejecta model. Specifically, the mass fraction of ⁵⁶Ni is set as 50% from the surface down to a specified velocity, taken to be either 10,000, 15,000, or 20,000 km s⁻¹; the corresponding ⁵⁶Ni mass in the outermost region is 0.16, 0.05, or 0.01 M_⊙, respectively. For the latter two, the ⁵⁶Ni distributions are indeed not monotonic and the major ⁵⁶Ni core and the surface-⁵⁶Ni region are separated. For these three models, the early-phase light curves are computed with SNEC as in the same manner as with the CSM–ejecta interaction model. Despite the model sequence including the non-monotonic ⁵⁶Ni distribution, we find that the light curve always shows a monotonically rising behavior unlike the observed one, and thus we conclude that the excess in the ⁵⁶Ni mass in the outermost region would never explain the observed early-phase behavior.

To interpret the overall light curve and spectral features, we have constructed a series of phenomenological ejecta models. The ejecta model construction follows the set up as described by Maeda et al. (2018). The density structure is assumed to follow an exponential function in velocity, specified by the ejecta mass and the kinetic energy. The abundance stratification is then assumed to be divided into the electron-capture core, ⁵⁶Ni-rich core, the Si-rich zone, then the O-rich layer from the inner to the outer region. No He-detonation layer is introduced for the present work. In summary, we have the kinetic energy and the masses of the four abundance zones as the input model parameters. Indirectly, the model parameters provide a required binding energy of the progenitor WD by subtracting the amount of the nuclear energy generation (specified by the composition structure) by the final kinetic energy adopted in the model.

The ejecta models are used as an input to model detailed radiation transfer simulations using HEIMDALL (Handling Emission In Multi-Dimension for spectrAL and light curve calculations; Maeda et al. 2014). Note that the ejecta models have been input to the SNEC radiation-hydrodynamic simulations to study the CSM–ejecta interaction in Section 5.1. However, to study the overall light curve and spectral features, we directly use the ejecta model without CSM interaction. For the CSM properties we finally adopt (0.01 M_⊙ and 10¹³ cm), the energy created by the CSM interaction is quickly lost, and the density structure is changed only in the outermost region. Therefore, in the post-early-flash phase as a main interest of this section, the effect of the CSM interaction is largely negligible (more details will be given in a forthcoming paper; K. Maeda et al. 2022, in preparation).

We first try to obtain a reasonable model sequence that can explain the post-early-flash phase. As the first trial we fix the ejecta mass as 1.4 M_⊙ (hereafter M_Ch mo del), but never find a model that explains the data; the model light curves always evolve too quickly for a reasonable range of the near-Chandrasekhar-mass WD binding energy given that nearly entire ejecta must be converted to ⁵⁶Ni to reach the peak luminosity as high as observed, leading to a high ratio of the kinetic energy to the mass and thus short diffusion timescale. In Figure 5, a dashed–dotted line corresponding to the bolometric light curve of a M_Ch model for which all the ejecta are converted to the ⁵⁶Ni-rich abundance (90% ⁵⁶Ni, 5% stable Ni, 5% stable Fe; i.e., a total ⁵⁶Ni of 1.26 M_⊙) evolves too quickly to be compatible with the slow-evolving light curve of SN 2020hvf.

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The discrepancy demonstrates the difficulty in explaining the light curve of SN 2020hvf by variants of a M_Ch model—reducing the ⁵⁶Ni mass will slow down the evolution speed but decrease the luminosity and never bring this specific model light curve to the characteristic slow and bright combination for the SN 2020hvf. We then increase the ejecta mass and find a reasonable solution for the ejecta mass of ∼2.1 M_⊙ and the kinetic energy of 1.4 × 10⁵¹ erg. In our fiducial model we fix the mass of the ⁵⁶Ni-rich zone to be 1.6 M_⊙ (1.44 M_⊙ of ⁵⁶Ni assuming a typical fraction of ⁵⁶Ni as 90%). By also varying the Si- and O-rich zones, we adopt 0.3 and 0.2 M_⊙ for these zones, respectively.

The light curve of this ejecta model is shown as the fiducial model in Figure 6 (dashed lines). The spectral evolution is shown in Figure 3 (red). Given that our main focus here is to constrain the general ejecta properties and test the super-M_Ch WD hypothesis for SN 2020hvf, we have not tuned the model to obtain a better fit (see below for further discussion). Rather, the model spectra are shown for demonstration purposes, as a sanity check to show that the model constructed based on the light-curve properties can also provide spectra largely consistent with the observed ones. As shown in Figure 6, the overall light curve is explained reasonably well, and the key features in the spectra (Figure 3) are also explained without fine-tuning. Specifically, the evolution of the Si ii velocity is roughly reproduced. The relatively high velocity of the Si ii line at maximum brightness is found to be consistent with the super-M_Ch WD explosion model. It is interesting to note that the model naturally produces the very high velocity in the initial phase and it quickly decreases to normal velocity without introducing additional modification to the standard exponential density structure. This is indeed a challenge when explaining similar high-velocity features in normal SNe Ia (Gerardy et al. 2004; Tanaka et al. 2008). For the present model, the combination of the high luminosity and the small photospheric radius during the early-phase results in sufficient amounts of single-ionized Si in the outermost layer, leading to the formation of the high-velocity feature in the initial phase.

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Another key spectral feature of SN 2020hvf as a carbon-rich overluminous SN Ia is the existence of the C ii absorption. We, however, did not attempt to explain this feature; the ejecta model indeed does not contain carbon (i.e., there is no carbon introduced in the outermost O layer). The formation of C ii is not well understood in general, not only in the overluminous SNe Ia but also in normal SNe Ia. In addition, the distance uncertainty could limit the detailed modeling of relatively weak features like C ii, as the line formation can be very sensitive to the model temperature and thus the intrinsic luminosity. More details will be discussed in a separate paper (K. Maeda et al. 2022, in preparation).

Among the four early-excess scenarios, i.e., (a) CSM–ejecta interaction, (b) He-det, (c) companion–ejecta interaction, and (d) surface-⁵⁶Ni-decay scenarios, the CSM–ejecta interaction model can nicely reproduce the earliest phase light curve, while the other three have difficulty in explaining the combination of the luminosity and the duration (or the characteristic evolution). In this section, we discuss details of this issue.

The strong early emission and high brightness in UV/NUV wavelengths soon after the early emission of SN 2020hvf conflict with the prediction of the He-det scenario. This is because, in the He-det scenario, a considerable amount of titanium and iron-group element absorptions below ∼4500 Å will be generated in order to explain the high luminosity of the early flash of SN 2020hvf (Jiang et al. 2017; Maeda et al. 2018), which results in a much redder B − V color in the next few days after the early excess (panel (b) in Figure 4). In addition, theoretically, the He-det-induced explosion mechanisms (e.g., the double-detonation scenario; Livne & Glasner 1990; Woosley & Weaver 1994) predict that the onset of the SN explosion occurs before the mass of the primary WD approaching the Chandrasekhar limit, leading to a subluminous or normal-brightness SN Ia (Fink et al. 2010; Kromer et al. 2010; Pakmor et al. 2013; Tanikawa et al. 2019).

The surface-⁵⁶Ni-decay-induced early excess requires an extended ⁵⁶Ni distribution and a considerable amount of ⁵⁶Ni has to be formed at the outermost region of the SN ejecta (Piro & Morozova 2016; Magee et al. 2020). To explain such a bright pulse-like early excess isolated from the major light curve with the surface-⁵⁶Ni-decay scenario, most of the ⁵⁶Ni should be confined to the inner layers while a considerable amount of ⁵⁶Ni has to be formed at the surface of the ejecta. However, such a configuration is hard to realize for SNe Ia; the "bulk" ⁵⁶Ni distribution is already extended to a high velocity and there is no room for a sufficient amount of materials between the ⁵⁶Ni core and the surface (which is required to produce the "dip" after the earliest phase). Our simulations (panel (d) in Figure 4) do confirm that in order to reach the brightness of the early flash via the surface-⁵⁶Ni decay, we should see a bright but bump-like early excess (Jiang et al. 2018, 2020), which contradicts the bright "pulse-like" early excess discovered in SN 2020hvf. Furthermore, unlike 91T/99aa-like overluminous SNe Ia, prominent IME adsorptions were found in the early-phase spectra of SN 2020hvf (Figure 3), suggesting a less efficient detonation approaching the outermost region of the SN ejecta.

A series of analytical and numerical simulations were performed to test whether or not the two interaction-induced early-excess scenarios can generate the bright pulse-like early excess as shown in Section 5.1. Although the luminosity of the companion-interaction-induced early excess can be comparable to that of SN 2020hvf as shown in panel (c) in Figure 4, its timescale is much longer than the observed early excess. For example, our model with 5.0 × 10¹¹ cm separation between the SN center and the companion provides an optical luminosity already below the observation while it still has a longer timescale. Given that a smaller separation (and thus companion radius) leads to a fainter and faster early-excess emission, to have an even shorter timescale, we will need to further reduce the separation/radius, which will result in even fainter luminosity. In addition, within the single-degenerate scenario, we would not expect a configuration where the companion star is much smaller in radius than the models here. In contrast, the early excess caused by the CSM–ejecta interaction evolves generally faster due to a shorter characteristic timescale of the evolution in the bolometric light curve of the CSM-induced early excess (Maeda et al. 2018), which nicely fits the prominent pulse-like early excess (panel (a), Figure 4) for a CSM mass of 0.01 M_⊙ and a characteristic (outer edge) radius of 1.0 × 10¹³ cm. The nondetection of H ii and He ii features in spectra of SN 2020hvf suggests that the CSM is likely carbon-rich, originating from either debris of a disrupted CO WD or surface materials of a spin-down super-M_Ch CO WD.

Our understanding on the origin(s) of carbon-rich overluminous SNe Ia has been limited mainly due to the uncertainty of the energy source that contributes to the ultra-high luminosity (Taubenberger 2017). Although a significant amount of ⁵⁶Ni from a super-M_Ch WD explosion can intuitively explain the high luminosity of carbon-rich overluminous SNe Ia, other scenarios have been proposed to avoid the abnormal super-M_Ch WD progenitor.

One prevailing scenario is the interaction between a very extended, massive CSM envelope and the ejecta from a normal WD explosion (Hicken et al. 2007; Scalzo et al. 2010; Taubenberger et al. 2011, 2013). Thanks to the stringent constraint given by early-excess modeling, the confined CSM distribution derived from the early emission rules out the possibility of a CSM-interaction origin of the high luminosity of SN 2020hvf; if the peak luminosity is powered by the CSM interaction, that must dilute any short-timescale variability in the rising phase, irrespective of the origin of such a flash.

Another scenario that might explain overluminous SNe Ia with a smaller amount of ⁵⁶Ni is an asymmetric ⁵⁶Ni distribution after the SN explosion (Hillebrandt et al. 2007; Leung et al. 2021). However, the asymmetric ⁵⁶Ni distribution is unlikely to explain SN 2020hvf. If the main power source is the asymmetrically distributed ⁵⁶Ni, this cannot create the pulse-like early excess of SN 2020hvf within the same context and, instead, it is possible that a broader rising-phase light curve with a bump-like early excess commonly shown in 91T/99aa-like overluminous SNe Ia can be expected (i.e., similar to the surface-⁵⁶Ni-decay scenario discussed above). Thus, a different mechanism is required to explain the early-excess feature of SN 2020hvf. Also, there is no viewing-angle effect shown among the carbon-rich overluminous SNe Ia. For example, we do not see correlations among the SN luminosity, the strengths of intermediate-mass element absorptions, and the light-curve evolving speed (Ashall et al. 2021).

The main drawback of the super-M_Ch WD progenitor model is the poor fitting of the V-band light curve around the peak and overfitting in blue wavelengths in the declining phase (Figure 6). We further investigate model light curves by reducing the ⁵⁶Ni amount in the inner region of the ejecta, and the result shows that the overall light curve cannot be promisingly improved by reducing ∼17% of ⁵⁶Ni (the "fiducial-hole" model; dotted lines in Figure 6). Given the unpromising output from the less massive ⁵⁶Ni model and a reasonable fitting of the overall bolometric light curves, especially around the peak, via our fiducial super-M_Ch WD model (Figure 5), we suggest that the super-M_Ch WD model is a promising scenario and the models shown here are thus regarded as defining a range of the super-M_Ch SN Ia light curves. However, additional hydrodynamic and radiation transfer simulations are required for further understanding of the multiband light-curve behavior of the SN 2020hvf.

For the super-M_Ch WD progenitor, both the mechanism and progenitor system leading to such an abnormal condition are under debate. In the early 2000s, theoretical studies have predicted that a WD can reach 1.2–2.7 M_⊙ by increasing its spin speed through the single-degenerate channel (Langer et al. 2000; Yoon & Langer 2005; Hachisu et al. 2012; Benvenuto et al. 2015). Recently, a series of smoothed particle hydrodynamic simulations have been performed to investigate if a merger of two WDs via the double-degenerate channel can finally lead to SN Ia events. The results show a large uncertainty in producing SNe Ia without a foregoing He-shell detonation that triggers a central carbon ignition of the primary WD (Dan et al. 2011, 2012, 2014; Schwab et al. 2012; Shen et al. 2012; Tanikawa et al. 2015). Therefore, it is still an open question as to whether typical SNe Ia can be produced through the merger of two CO WDs with a total mass above the Chandrasekhar limit. However, the simulations suggest that ultraluminous SNe Ia can be expected once the total mass of the binary CO WD system is significantly beyond the Chandrasekhar limit (i.e., a merger of two massive CO WDs; Dan et al. 2012, 2014; Zhu et al. 2013). As such massive systems will be rare in the universe, this might explain the extremely low event rate of the carbon-rich overluminous SNe Ia.

A further question is how the (carbon-abundant) CSM envelope can be formed. We expect that the existence and the nature of the CSM can distinguish between the single- and double-degenerate channels (among other possible scenarios) leading to the formation of a super-massive WD and a subsequent explosion. If the massive WD, beyond the Chandrasekhar limit for a nonrotating configuration, was created by accretion from a nondegenerate companion star, the WD likely evolves through the critical rotation where the centrifugal force is marginally balanced with the gravity near the surface (Uenishi et al. 2003; Yoon & Langer 2005). This requires the angular momentum redistribution, and a fraction of the accretion materials may finally form a disk or torus around the WD. Alternatively, but through a similar mechanism, a binary WD merger likely creates a massive torus as an immediate outcome of the merger, which then evolves into a hot envelope due to further angular momentum redistribution, probably through the magnetic field (Schwab et al. 2012; Shen et al. 2012). In the former case, the composition could be either H/He-rich or carbon-rich, while in the latter, it is expected to be carbon-rich. Future studies, both on placing a stronger constraint on the CSM composition and on the detailed evolution calculations leading to the super-M_Ch WD, will therefore be necessary.

A remaining question is whether all carbon-rich overluminous SNe Ia discovered so far have the same origin. The required binding energy of 1.6 × 10⁵¹ erg of the WD progenitor of SN 2020hvf (2.1 M_⊙) is within the theoretical model sequence with a range of the central density (Yoon & Langer 2005). The WD mass is very close to the upper limit of the theoretical prediction (2.1 M_⊙ in the model sequence by Yoon & Langer 2005). If the ⁵⁶Ni mass is larger for a more massive WD, it may require an even more massive WD in order to generate more luminous carbon-rich overluminous SNe Ia such as SN 2009dc. Furthermore, the low velocity seen in some carbon-rich overluminous SNe Ia would require a higher binding energy, which will raise another challenge; indeed, most of the previous examples of carbon-rich overluminous SNe Ia show observed ejecta velocities significantly lower than the prediction (Maeda & Iwamoto 2009; Hachinger et al. 2012).

In contrast, the high ejecta velocity of SN 2020hvf perfectly matches the prediction of exploding a super-M_Ch WD. Interestingly, the other high-velocity carbon-rich overluminous SN Ia, ASASSN-15pz, likely shows a pulse-like early emission, though a large photometric uncertainty makes it difficult to give a robust judgement (Chen et al. 2019). The detection of (probable) pulse-like early excess in both high-velocity carbon-rich overluminous SNe Ia may indicate a specific pathway of forming the super-M_Ch WD. It also suggests a different origin of the 20hvf-like overluminous SNe Ia compared to the typical carbon-rich overluminous (i.e., 03fg/09dc-like) SNe Ia. To figure out the possible multiple origins of carbon-rich overluminous SNe Ia, the early-phase intranight photometry will play an irreplaceable role in enlightening us about the CSM distribution, explosion mechanism, and progenitor system of the carbon-rich overluminous SNe Ia. With ongoing high-cadence transient surveys such as the Zwicky Transient Facility (ZTF; Bellm et al. 2019a, 2019b; Graham et al. 2019), the Tomo-e Gozen high-cadence survey, and the MUlti-band Subaru Survey for Early-phase Supernovae (MUSSES; Jiang et al. 2017), systematical studies of the early-phase carbon-rich overluminous SNe Ia can be achieved in the coming years.

In this Letter we report the observation of a carbon-rich, overluminous SN Ia, SN 2020hvf, discovered by the Tomo-e Gozen camera within about 0.2 day of the explosion. The prominent pulse-like early flash is explained almost exclusively by an interaction between the supernova ejecta and ∼0.01 M_⊙ CSM extending to a distance of ∼10¹³ cm. On the basis of the CSM-induced prompt early excess, the overluminous light curve, and the high ejecta velocity of the SN 2020hvf, we suggest that the SN 2020hvf may originate from a thermonuclear explosion of a super-M_Ch WD. A detailed comparison with models is limited, however, by several factors; there is no detailed explosion model developed so far for the super-M_Ch WD explosion. The distance uncertainty affects the derived ejecta density and temperature, which substantially affect the light curve and spectral formation. Given the large diversity of carbon-rich overluminous SNe Ia discovered so far, systematical investigations from the very early phase via high-cadence time-domain surveys are required to figure out the origin(s) of this extreme SN Ia subclass.