SN 2022vqz: a peculiar subluminous Type Ia supernova with prominent early excess emission Open Access

ABSTRACT

We present extensive photometric and spectroscopic observations of the peculiar Type Ia supernova (SN Ia) 2022vqz. It shares many similarities with the SN 2002es-like SNe Ia, such as low luminosity (⁠|$M_{B,\rm max}=-18.11\pm 0.16$| mag) and moderate post-peak decline rate (Δm_15,B = 1.33 ± 0.11 mag). The nickel mass synthesized in the explosion is estimated as 0.20 ± 0.04 M_⊙ from the bolometric light curve, which is obviously lower than that of normal SNe Ia. SN 2022vqz is also characterized by slowly expanding ejecta, with Si ii velocities persisting around 7000 km s⁻¹ since 16 d before peak brightness, unique among all known SNe Ia. While all of these properties imply a lower energy thermonuclear explosion that should leave a considerable amount of unburnt materials, the absent signature of unburnt carbon in spectra of SN 2022vqz is puzzling. A prominent early peak is clearly detected in the Asteroid Terrestrial-impact Last Alert System c- and o-band light curves and in the Zwicky Transient Facility gr-band data within days after the explosion. Possible mechanisms for the early peak are discussed, including the sub-Chandrasekhar-mass double-detonation model and interaction of SN ejecta with circumstellar material. We find that both models face some difficulties in replicating all aspects of the observed data. As an alternative, we propose a hybrid C–O–Ne white dwarf as the progenitor of SN 2022vqz; it can simultaneously reconcile the tension between low ejecta velocity and the absence of carbon. We further discuss the diversity of SN 2002es-like objects and their origin in the context of different scenarios.

1 INTRODUCTION

Although it is commonly accepted that Type Ia supernovae (SNe Ia) are thermonuclear explosions of carbon–oxygen (C–O) white dwarfs (WDs) in close binary systems (Hoyle & Fowler 1960; Bloom et al. 2012), the detailed properties of their progenitors, as well as the exact explosion mechanism, are still inconclusive (Livio & Mazzali 2018; Liu, Röpke & Han 2023). Observationally, about 70 per cent of SNe Ia display relatively homogeneous properties, especially after some empirical corrections based on their light-curve decline rates (Phillips 1993) and colours (Wang et al. 2005; Guy et al. 2007; Jha, Riess & Kirshner 2007). These SNe Ia are known as ‘Branch-normal’ ones (Branch, Fisher & Nugent 1993) and are used as excellent cosmological standardizable candles (Riess et al. 1998, 2022; Schmidt et al. 1998; Perlmutter et al. 1999 ). Note that even Branch-normal SNe Ia can be further classified into several subclasses that may also have different physical origins (Benetti et al. 2005; Wang et al. 2009, 2013, 2019).

Beside the normal ones, the remaining SNe Ia show a substantial diversity, and are classified into different subclasses depending on their photometric and spectroscopic properties (Filippenko 1997; Parrent, Friesen & Parthasarathy 2014; Taubenberger 2017). In particular, the subluminous group represented by SN 1991bg (‘91bg-like’) is characterized by its rapidly evolving light curves, low ejecta velocity, more prominent intermediate-mass elements (IMEs), and strong Ti ii absorption in spectra at maximum light (Filippenko et al. 1992; Leibundgut et al. 1993). These peculiarities are generally interpreted as signatures of a lower burning efficiency and a cooler ejecta temperature (Mazzali et al. 1997; Hachinger et al. 2009).

Another peculiar subluminous subclass of SNe Ia, which is represented by SN 2002es (dubbed ‘02es-like’), shares most of the characteristics with the 91bg-like objects, except for displaying broader light curves that are comparable to those of normal SNe Ia (Ganeshalingam et al. 2012; White et al. 2015). The normal light-curve decline rate combined with the low peak luminosity put 02es-like objects at a position that was long believed to be devoid of events on the Phillips-relation diagram (Phillips 1993). In comparison with other subtypes, the subclass of 02es-like SNe Ia is relatively rare, contributing to ∼2.5 per cent of all SNe Ia (Ganeshalingam et al. 2012). By now, only about a dozen events have been identified as 02es-like (Burke et al. 2021). The prototype SN 2002es itself shows a subluminous peak luminosity (M_B ≈ −17.78 mag) and a slow ejecta velocity (v_ej ≈ 6000 km s⁻¹ around maximum light). Its optical spectra are also characterized by strong Si ii λ5972, O i, and Ti ii lines, similar to those of the 91bg-like SNe Ia. It is expected that the 02es-like SNe Ia should have subluminous peaks and slowly expanding ejecta. However, some events with properties close to those of normal ones were also classified as 02es-like. For example, the peak magnitudes of SN 2006bt (Taubenberger 2017), SN 2016jhr (Jiang et al. 2017), and SN 2022ywc (Srivastav et al. 2023) are as bright as M_B ≈ −18.8 mag. The Si ii velocities of SN 2006bt (Foley et al. 2010) and PTF10ops (Maguire et al. 2011) measured near maximum light are ∼10 000 km s⁻¹. Another proposed candidate is SN 2019yvq (Miller et al. 2020; Burke et al. 2021; Tucker et al. 2021), which showed an unprecedented high velocity of ∼15 000 km s⁻¹. This greatly extends the range of 02es-like SNe Ia if they have similar origins. All of the above-mentioned events show normal light curves but have the spectral characteristics of low-luminosity SNe Ia, especially the distinct Ti ii absorption. These commonalities make them classified as 02es-like SNe Ia and demonstrate the diversity in this subclass.

Observations of 02es-like SNe Ia revealed many other properties of this subclass. The nebular spectra of 02es-like SN 2010lp (Taubenberger et al. 2013) and iPTF14atg (Kromer et al. 2016) are characterized by broad and strong [Fe ii] and [Ca ii] emission, as in low-luminosity SNe Ia. However, in contrast to the 91bg-like events, they do not display narrow [Fe iii] or [Co iii] emission but rather narrow [O i] in their nebular spectra. The light curves of SN 2002es show a rapid decline from ∼1 month after the peak (Ganeshalingam et al. 2012), which is not observed in other 02es-like SNe Ia. White et al. (2015) noticed that the 02es-like SNe Ia tend to reside in luminous, early-type galaxies with minimal star formation, and located far from the centre of their hosts, suggesting longevity of their progenitor systems (Taubenberger 2017). There are also outliers, such as SN 2016ije, located in a bluer, low-mass, star-forming galaxy (Li et al. 2023).

More recent studies indicate that all the 02es-like SNe Ia with good early photometric coverage, including iPTF14atg (Cao et al. 2015), SN 2016jhr, 2019yvq, and 2022ywc, show early blue/ultraviolet (UV) excesses/peaks within days after the explosion. An early-time excess has been considered to be evidence for a non-degenerate companion in the single-degenerate (SD) scenario (Whelan & Iben 1973), interacting with the expanding ejecta and creating additional UV/optical flux. However, owing to the effect of the viewing-angle distribution, the excess is predicted to be observable in only ∼10 per cent of the events (Kasen 2010). This prediction is more consistent with the observed rate of early excesses in normal SNe Ia (Burke et al. 2022), which is ∼20 per cent (Deckers et al. 2022; Magee et al. 2022).

A promising scenario for 02es-like SNe Ia is that they arise from a double-degenerate (DD) system, in which the companion of the exploding WD is another He–C–O WD (Iben & Tutukov 1984; Webbink 1984). Maguire et al. (2011) and Ganeshalingam et al. (2012) suggested that PTF10ops and SN 2002es can be explained by a violent-merger model of two 0.9 M_⊙ WDs (Pakmor et al. 2010). Kromer et al. (2013, 2016) found that a revised merger model with 0.9 + 0.76 M_⊙ WDs and different metallicities provide both light curves and spectra that match the observations of SN 2010lp and iPTF14atg. This model also leaves unburnt O near the centre of the ejecta, potentially reproducing the [O i] emission in the nebular spectra of 02es-like events. However, the merger model does not naturally explain the universal appearance of early-time excesses. A possible solution refers to the interaction between ejecta and extended circumstellar material (CSM; Piro & Morozova 2016; Piro, Haynie & Yao 2021). The CSM may be ejected before the merging state as tidal tails, disc winds, or jets (Raskin & Kasen 2013; Levanon, Soker & García-Berro 2015).

The DD scenario has a quite large parameter space. If the two merging WDs have larger masses, they can potentially result in a more luminous super-M_Ch event, falling into the category of SN 2003fg-like SNe Ia (Howell et al. 2006; Scalzo et al. 2010; van Kerkwijk, Chang & Justham 2010). There are also some similarities between 02es-like and 03fg-like subtypes that are distinct from other SNe Ia, such as non-monotonic early-time bumps and unique UV colour evolution, suggesting that they may originate from a single DD scenario with different WD masses (Hoogendam et al. 2023).

An alternative explanation proposed for the early peak of 02es-like objects is the double-detonation model, in which a sub-Chandrasekhar-mass (sub-M_Ch) C–O WD is detonated by ignition of a surface He layer (Polin, Nugent & Kasen 2019). A surface He layer of |$\sim\!\! 0.05~\rm {M}_\odot$| was adopted to explain the early-time excesses of SN 2016jhr (Jiang et al. 2017) and SN 2019yvq (Miller et al. 2020; Siebert et al. 2020; Burke et al. 2021). However, in the case of SN 2019yvq, the double-detonation model faces some issues that are difficult to reconcile (Tucker et al. 2021). The inferred WD masses from early-time photometry (0.96 M_⊙; Miller et al. 2020) and nebular-phase spectroscopy (1.15 M_⊙; Siebert et al. 2020) show a large discrepancy. Observing along the detonation pole of a thick He layer may reconcile the overluminous-model prediction of a more massive WD (Siebert et al. 2020), but this will also introduce additional peculiar signals that were not observed in SN 2019yvq, such as severe line blanketing from He ashes (Tucker et al. 2021).

In this paper, we present our observations and analysis of SN 2022vqz, a subluminous 02es-like SN Ia with a remarkable early peak and extremely low ejecta velocity. Section 2 describes our observations and data reduction. Detailed analysis of the photometry and spectroscopy is presented in Sections 3 and 4, respectively. In Section 5, we discuss possible explosion models for SN 2022vqz. We summarize the main results in Section 6.

2 OBSERVATIONS AND DATA REDUCTION

SN 2022vqz was discovered at 18.38 mag on MJD 59846.369 (2022 September 24.369; utc dates are used throughout this paper) by the Zwicky Transient Facility (ZTF; Fremling 2022) in the g band. This transient was also detected by the Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018; Smith et al. 2020) survey on MJD 59845.471, ∼1 d earlier than the ZTF discovery, at ∼19 mag in their o band. The last non-detection from the ATLAS survey was on MJD 59844.484, posting a strict constraint on its explosion time. Later, two spectra taken by Tucker (2022) and Maguire et al. (2022) classified this transient as a peculiar SN Ia. It is located at J2000 coordinates α = 00^h51^m05|${_{.}^{\rm s}}$|78, δ = +29°32′12|${_{.}^{\prime\prime}}$|2, which is 4.9 arcsec west and 3.2 arcsec south of the centre of its host galaxy MCG+05−03−011, a lenticular (S0) galaxy with a heliocentric redshift of z = 0.016 995. Fig. 1(a) shows the BVr-band composite image of SN 2022vqz.

2.1 Photometric data

We performed follow-up optical photometric observations of SN 2022vqz with multiple facilities, including the Tsinghua-National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) 80 cm telescope (TNT) at Xinglong Observatory of NAOC (Wang et al. 2008; Huang et al. 2012), the AZT-22 1.5 m telescope at Maidanak Observatory (AZT; Ehgamberdiev 2018), and the Schmidt 67/92 cm and the 1.82 m Copernico Telescope at Cima Ekar Observing Station (Padova).Our multiband photometric monitoring began on 2022 September 27.80 (MJD 59849.80) and has good coverage over a period of ∼2.5 months. Standard Image Reduction and Analysis Facility (IRAF¹) routines were adopted to reduce the CCD images, including bias and flat-field corrections. The magnitudes are calibrated using a set of nearby reference stars from the Sloan Digital Sky Survey (SDSS) catalogue (York et al. 2000; Gunn et al. 2006). The number of reference stars ranges from 30 to 40 for the different facilities. For flux calibration of the UBVRI-band photometry from AZT and BV-band photometry from TNT, the SDSS ugriz magnitudes of the reference stars are converted to standard UBVRI magnitudes.²

Although SN 2022vqz is offset from the centre of its host galaxy, background-light contamination is still prominent, especially for its late-time light curves. Usually, a host-galaxy template can be taken when the SN fades away, but this will take a long time for SN 2022vqz. As an alternative, we construct the host-galaxy templates of SN 2022vqz using two different methods. The first is smoothly interpolating the host-galaxy flux at the location of the SN by solving a diffusive equation bounded by surrounding starlight (Zhang et al. 2004). The second is using the flux information from the opposite side as a template since the host galaxy looks symmetric. The subtraction process follows Zrutyphot (Mo et al., in preparation). With the templates constructed by the above two methods, the final results of the photometry are consistent. Fig. 1 shows an example of an original image, constructed template, and their difference image.

In our analysis, we also included the publicly available gr-band forced photometry from ZTF (Masci et al. 2019)³ and co-band (cyan and orange) forced photometry from ATLAS. These photometric results were obtained after subtraction of the corresponding template images taken before the SN explosion, and hence are more reliable. The first observation of ATLAS was on 2022 September 23.471 (MJD 59845.471), less than 1 d after the latest non-detection. Combined with the early ZTF observations, which started ∼1 d later, the early-time photometric evolution of SN 2022vqz can be well constrained. The photometric data of SN 2022vqz collected with TNT, AZT, and Padova telescopes are listed in Table 3, Table 4, and Table 5, respectively. The photometric results from ZTF and ATLAS are also given in Table 6 and Table 7, respectively. Fig. 2 shows the multiband light curves of SN 2022vqz.

2.2 Spectroscopic data

Our spectroscopy of SN 2022vqz covered phases from −7.1 to +97.4 d relative to its peak brightness. We obtained nine spectra with the Beijing Faint Object Spectrograph and Camera (BFOSC) mounted on the 2.16 m telescope of the Xinglong Observatory (XLT), five spectra with the Yunnan Faint Object Spectrograph and Camera (YFOSC) mounted on the Lijiang 2.4 m telescope (LJT; Fan et al. 2015) of Yunnan Observatories, three spectra with the Kast spectrograph (Miller & Stone 1994) on the 3 m Shane reflector at Lick Observatory, and one spectrum with the Asiago Faint Object Spectrograph and Camera (AFOSC) mounted on the 1.82 m Copernico Telescope of Padova.In addition, one spectrum was taken of the host galaxy MCG+05−03−011 with XLT + BFOSC. The Lick/Kast spectra were taken with the slit oriented along the parallactic angle to minimize the effects of atmospheric dispersion (Filippenko 1982). The journal of spectroscopic observations is shown in Table 1.

Standard IRAF routines were adopted for reduction of all the optical spectra taken by LJT, XLT, Lick 3 m Shane, and Padova 1.82 m Copernico telescopes, including bias, flat-field corrections, and removal of cosmic rays. The Copernico spectrum is reduced using a dedicated pipeline foscgui.⁴ The wavelength scale was calibrated with comparison-lamp spectra, and the fluxes were calibrated with standard stars observed on the same night at similar airmasses. All spectra were further corrected for atmospheric extinction using the extinction curves obtained at each observatory, and telluric lines were also removed from the spectra.

We also included two classification spectra from the Spectroscopic Classification of Astronomical Transients (Tucker 2022; Tucker et al. 2022) and ZTF (Maguire et al. 2022), and one spectrum taken by Las Cumbres Observatory Global Telescope (LCOGT; Brown et al. 2013) in our analysis.

3 PHOTOMETRIC RESULTS

3.1 Light curves and photometric properties

Fig. 2 shows the multiband light curves of SN 2022vqz. The data reveal an early-time peak that is clearly separated from the main peak. The SN brightness increased by ∼0.6 mag in the o band in the first day after discovery, reaching an early peak and then dropping to a minimum in another few days, followed by a slow rise to maximum light. No secondary peak or significant shoulder can be seen in the i and R bands, consistent with the light curves of subluminous SNe Ia such as the 02es-like or 91bg-like objects.

Applying a polynomial fit to the B-band light curve around maximum light indicates that SN 2022vqz reaches the peak on MJD 59863.43 ± 0.48, with |$m_{B,\rm max}=16.205\pm 0.053$| mag. We use the time of B maximum as the reference epoch throughout this paper. The decline in the first 15 d after peak is measured as Δm_15,B = 1.33 ± 0.11 mag, slightly fast among normal SNe Ia.

The distance modulus of SN 2022vqz can be estimated based on the redshift of its host galaxy, which is in the Hubble flow: z_CMB = 0.015 94 after correcting for the solar peculiar velocity. Adopting a cosmological model with Ω_m = 0.27, |$\Omega _\Lambda =0.73$|⁠, and H₀ = 73.04 km s⁻¹Mpc⁻¹ (Riess et al. 2022), the corresponding distance modulus is m − M = 34.11 ± 0.15 mag. With this value, the absolute peak magnitude of SN 2022vqz is |$M_{B,\rm max}=-18.11\pm 0.16$| mag, after accounting for a Galactic extinction of A_B = 0.21 mag (Schlafly & Finkbeiner 2011). The extinction of the host galaxy is assumed to be negligible, considering that the SN is offset from the centre of the host and no Na i D absorption can be recognized in the SN spectra.

3.2 Comparison of light and colour curves

Fig. 3 shows the location of SN 2022vqz in the Δm_15,B–|$M_{B,\rm max}$| diagram, together with those of some comparison objects including normal SNe Ia. Its low peak luminosity and moderate decline rate fit into the category of 02es-like objects. One may note that with the discovery of some events in recent years, especially SN 2019yvq, the gap between 02es-like objects and normal SNe Ia now seems to be filled by a continuous distribution. However, the absolute magnitude of SN 2019yvq is highly uncertain owing to large discrepancies (over 2 mag) of its host-galaxy extinction estimates from different methods (Burke et al. 2021). In Fig. 3, we adopt the value reported by Burke et al. (2021).

We further compare in Fig. 4 the absolute UBgV and rRiI light curves of SN 2022vqz with those of some well-observed SNe Ia, including 02es-like SN 2002es (Ganeshalingam et al. 2012), SN 2016ije (Li et al. 2023), SN 2019yvq (Burke et al. 2021), iPTF14atg (Cao et al. 2015), 91bg-like SN 1999by (Garnavich et al. 2004), and the normal SN 2004eo (Pastorello et al. 2007) but with a similar Δm_15,B (1.46 mag) as SN 2022vqz.

The light-curve morphology of SN 2022vqz is overall similar to that of iPTF14atg and SN 2019yvq. We notice that SN 2022vqz and other 02es-like objects do not exhibit a rapid decline after t ≈ 1 month when compared to SN 2002es. This indicates an additional peculiarity intrinsic to SN 2002es that is still unexplained (Taubenberger 2017). The absolute magnitudes of SN 2022vqz are slightly brighter than all objects in the 02es-like comparison sample, while they are all bounded by subluminous 91bg-like SN 1999by and normal SN 2004eo. Note that in Fig. 4 we assumed zero host extinction for all 02es-like SNe. SN 2019yvq appears not to be brighter than other 02es-like objects without applying a significant host extinction correction. Some evolution features seen in SN 2016ije, like slower B and V decline rates after t ≈ +20 d and strong line blanketing in the U band, are absent in SN 2022vqz. These features can be interpreted as the result of a broader line-forming region and a smaller total amount of iron-group elements (IGEs) distributed in the line of sight. A more extended line-forming region could result in stronger line blanketing at early phases, while less IGEs can reduce their absorptions in the spectra, especially at short wavelengths, thereby providing more luminous late-time fluxes and flatter light curves in BV (Li et al. 2023). In contrast, SN 2022vqz should have a less extended line-forming region for IGEs, suggesting a more stratified and less mixed IGE distribution.

Fig. 5 shows the comparison of the colour evolution, with all data corrected for Galactic extinction. Overall, the colour curves of SN 2022vqz show a close resemblance to those of other 02es-like SNe, while it seems to have a more extended and rapid evolution in g − i relative to iPTF14atg. In B − V, SN 2022vqz shows slower evolution than SN 1999by, and it is ∼0.2–0.3 mag redder than SN 2004eo before t ≈ 15 d. Immediately after the explosion, SN 2022vqz appears exceptionally blue, g − r ≈ −0.15 ± 0.09 mag at t ≈ −15 d from maximum brightness, and it then evolves rapidly to a red peak 2 d later. This trend is coincident with the declining part of the early peak of SN 2022vqz, suggesting a blue colour of the early peak luminosity.

3.3 Time of explosion

An accurate explosion time estimation is crucial for further analysis of the explosion physics of SN 2022vqz. A non-detection was reported ∼1 d before the discovery, indicating that this SN exploded not long before the first observations. The time of explosion can be estimated from the early phase of the multiband light curves by adopting a simple ‘fireball’ model (Riess et al. 1999; Nugent et al. 2011). Assuming that the early-time SN can be described by a photosphere expanding at a constant velocity, its luminosity L should be proportional to (t − t_exp)², where t_exp is the time of explosion and should be same for all bands.

Note that SN 2022vqz shows an early peak, which lasts for ∼3 d since time of discovery and cannot be fitted by the fireball model. To avoid its influence, photometric data before MJD 59848 are excluded from the fit. The fitting method follows that described by Xi et al. (2022), and data earlier than 5 d before B maximum are used. The fitting procedure gives t_exp = MJD 59844.48 ± 0.20, which coincides with the last non-detection time, MJD 59844.484. Given the uncertainty of t_exp, the SN may have either not yet exploded or been too dim for detection at the latest time of non-detection. The rise time is thereby given as t_rise = 18.63 d. Fig. 6 shows the fitting result.

3.4 Bolometric light curve and ⁵⁶Ni mass

Based on the UBgVrRiI-band photometry, we adopted snoopy2 (Burns et al. 2011, 2014) to construct the bolometric light curve of SN 2022vqz. Since SN 2022vqz is a subluminous object, we use spectral energy distribution (SED) templates of subluminous 91bg-like SNe Ia (Nugent, Kim & Perlmutter 2002) for flux integration.

The bolometric light curve is found to reach its peak at |$L\approx 5.1 \times 10^{42}~\rm {erg~s}^{-1}$| on MJD 59863.3. According to Arnett’s rule (Arnett 1982; Stritzinger & Leibundgut 2005), the mass of synthesized radioactive ⁵⁶Ni can be inferred as M_Ni ≈ 0.25 M_⊙. A more robust way for Ni mass estimation is to fit the full bolometric light curve using the radiation diffusion model of Arnett (1982) (see also Chatzopoulos, Wheeler & Vinko 2012; Chatzopoulos et al. 2013). The best fit yields the explosion time t₀, the initial mass of radioactive nickel M_Ni, the light-curve time-scale t_lc, and the gamma-ray leaking time-scale t_γ as (respectively) MJD 59848.6 ± 2.2, 0.20 ± 0.04 M_⊙, 12.20 ± 3.87 d, and 36.26 ± 4.73 d. The explosion time inferred from the bolometric light-curve fit is about 4 d later than that from the early-time light curve, and well positioned at the minimum light between the early and main peaks, indicating a delay between the SN explosion and the emergence of the radioactive nickel-powered light curve. Such a delay was predicted as a ‘dark phase’ (Piro & Nakar 2013; Piro & Morozova 2016) caused by the location of the radioactive ⁵⁶Ni deep within the ejecta. This delay will also result in a shorter rise time, hence a lower ⁵⁶Ni mass estimation.

The early peak bolometric luminosity was estimated as |$4.5 \times 10^{41}~\rm {erg~s}^{-1}$|⁠. However, this value may be inaccurate since it only uses the ZTF gr-band photometry, and the rest of the flux was extrapolated from the template SED.

4 SPECTROSCOPIC RESULTS

4.1 Spectral evolution

Fig. 7 shows the spectral evolution of SN 2022vqz, spanning from t ≈ −15.7 to +97.4 d relative to B maximum, along with a spectrum of the host galaxy. The spectral evolution generally exhibits some common properties of subluminous SNe Ia. The Si ii λ5972 and O i λ7774 absorptions are relatively prominent around maximum light, implying a low ejecta temperature and a significant amount of unburnt materials. We measured the equivalent width (W) of the Si ii λλ6355 and 5972 features in the −0.7 d spectrum: W(5972) = 59 ± 12 Å and W(6355) = 137 ± 12 Å. These values make SN 2022vqz an extreme object in the ‘cool’ group proposed by Branch et al. (2006). As explained by Branch et al. (2006), there is a temperature threshold of ∼7000–8000 K (Hoeflich, Mueller & Khokhlov 1993; Höflich et al. 2002), below which the line optical depth will abruptly change owing to variations in key ionization ratios, so the ‘cool’ objects can be well separated from normal ones in the W(6355)–W(5972) plane. The equivalent width of O i λ7774 absorption in the same spectrum is measured to be W(7774) = 116 ± 14 Å, significantly higher than those in normal SNe Ia (≲70 Å; Zhao et al. 2016). In Fig. 8, the spectroscopic properties of SN 2022vqz are compared with those of other subluminous SNe Ia, including 02es-like objects iPTF14atg (Cao et al. 2015), PTF10ops (Maguire et al. 2011), SN 2002es (Ganeshalingam et al. 2012), and SN 2016ije (Li et al. 2023). A 91bg-like object SN 1999by (Garnavich et al. 2004) and the normal SN Ia 2004eo (Pastorello et al. 2007) are also included for comparison.

At t ≈ −7 d (Fig. 8a), the spectrum of SN 2022vqz is characterized by broad P Cygni lines of IMEs typical among SNe Ia: the Si ii λλ6355 and 5972 absorptions, the ‘W’-shaped S ii feature around 5400 Å, and deep absorptions of O i λ7774 and the Ca ii near-infrared triplet.The overall morphology is quite similar to that of iPTF14atg. However, no C ii λ6580 is detectable in SN 2022vqz, while it is strong in iPTF14atg. The weaker features of Si ii and Fe ii around 5000 Å are narrower and hence less blended compared to PTF10ops or SN 2004eo.

Near maximum light (Fig. 8b), all 02es-like objects show an absorption feature of C ii λ6580, except SN 2022vqz. The IGE lines around 5000 Å are deeper and narrower than in iPTF14atg or SN 2016ije, indicating a more concentrated density distribution of the ejecta. The Ti ii feature near 4200 Å, which is characteristic of subluminous SNe Ia, is less developed than those of SN 2002es and 91bg-like SN 1999by at this epoch, and is more similar to iPTF14atg.

By t ≈ 14 d (Fig. 8c), the spectra of SN 2022vqz resemble those of iPTF14atg and SN 2002es. The Ti ii lines have developed in the spectrum of SN 2022vqz, notably the deep trough around 4200 Å and the W-shaped feature around 6800 Å. These absorptions are less prominent or absent in SN 2016ije or SN 2004eo. The O i and Ca ii lines remain strong, but narrower than those seen near maximum light.

At t ≈ 40 d after maximum light (Fig. 8d), the spectra are dominated by IGE lines. Si ii λ6355 is still pronounced in SN 2022vqz, while it is contaminated by the Fe ii lines and hardly noticeable in other objects of the comparison sample. The Si ii λ5972 absorption is also replaced by Na i absorption, while the O i and Ca ii lines are still persistent in the spectra.

4.2 Ejecta velocity

The expansion velocity of SN ejecta can be measured from the blueshifted absorption of P Cygni profiles. Differences in line velocities can reveal the element distribution within the ejecta. The velocity evolution measured from absorptions of some IMEs, including S ii λλ5460, 5640, O i λ7774, Ca ii λ8542, and Si ii λ6355, appear to be exceptionally flat as shown in Fig. 9(a). Despite having relatively large uncertainties, the Si ii and S ii velocities even show a global rise at early phases, conflicting with the trend that the line velocities should decrease rapidly after SN explosion as a result of the receding photosphere. This complex velocity evolution might be related to the double-peaked light curves of SN 2022vqz. Considering energy components, each results in a declining line velocity. When the first peak fades away and the second takes control, the apparent line velocity, as a weighted average of the two components, may show a temporary increase. The Si ii velocity measured around the time of maximum light is v_Si,max = 6900 ± 500 km s⁻¹, which is significantly lower than the typical value of ∼10 500 km s⁻¹ for normal SNe Ia, and it is among the lowest values even for subluminous 91bg-like and 02es-like objects. The post-peak velocity decline of SN 2022vqz is |$\dot{v}=-55\pm 10~\mathrm{km~s^{-1}~d^{-1}}$|⁠, making it a low-velocity-gradient object according to the classification scheme proposed by Benetti et al. (2005). We do not find signatures of high-velocity features (HVFs) in SN 2022vqz, while the absence of HVFs seems to be a common property of subluminous SNe.

The velocity evolution of Si ii λ6355 is further compared to that of some other SNe Ia, including 02es-like iPTF14atg, SN 2019yvq, SN 2016ije, subluminous SN 1991bg (Turatto et al. 1996), normal SN 2004eo (Pastorello et al. 2007), and SN 2011fe (Pereira et al. 2013; Zhang et al. 2016). The last two are overplotted to visualize the typical velocity evolution of normal SNe Ia. The flat evolution and low velocity make SN 2022vqz stand out in Fig. 9(b). We also notice that, although SN 2019yvq was classified as a member of the 02es-like subclass according to its underluminous signatures (Burke et al. 2021), its velocity is extremely high for a 02es-like object, especially when compared with SN 2022vqz. This large diversity in ejecta velocity will be very challenging if a single mechanism is adopted to explain all 02es-like objects.

4.3 Host-galaxy parameters

SN 2022vqz is hosted by the face-on lenticular galaxy MCG+05−03−011 with a distance modulus of 34.11 ± 0.15 mag. The resulting g-band absolute magnitude is −19.73 ± 0.15 mag and the g − i colour is 1.1 mag (SDSS; Almeida et al. 2023), which are typical for hosts of 02es-like SNe (White et al. 2015). Note that most 02es-like SNe Ia tend to explode in massive, early-type galaxies with little or no star-forming activity, such as SNe 2002es (Ganeshalingam et al. 2012), PTF10bvr, 10acdh, 10ujn (White et al. 2015), and iPTF14atg (Cao et al. 2015). For SN 2022vqz, however, the host spectrum exhibits prominent H α emission, indicating relatively active star formation at least near the centre of the host galaxy.

To determine the metallicity of the host galaxy, we measured the intensity ratio of [N ii] λ6583 and H α as log([N ii]/H α) = −0.4, and that of [O iii] λ5007 and H β as log([O iii]/H β) = −0.1. These values can be converted to a metallicity estimate of 12 + log(O/H) = 8.64 using an empirical relationship (Kewley & Ellison 2008, equation A9). We also used Firefly (Wilkinson et al. 2017) to fit the host spectrum with combinations of single-burst stellar population models, and obtained a stellar mass of log(M/M_⊙) = 9.79, an age of 4.05 Gyr, and a metallicity [Z/H] = 0.14 dex. The metallicities inferred from the two methods are roughly consistent and comparable to the solar metallicity. Thus, we conclude that the host of SN 2022vqz is a medium-mass, solar-metallicity, star-forming galaxy.

5 DISCUSSION

5.1 Double-detonation model

Early-time photometry provides a distinct perspective and is crucial for discriminating different explosion mechanisms of SNe Ia (Maeda et al. 2018). Recent statistical studies based on ZTF samples found that ∼20–30 per cent of normal SNe Ia show some early-time flux excesses (Deckers et al. 2022; Magee et al. 2022). There is also large diversity in brightness, colours, time-scales, and light-curve morphologies of these excesses. Different physical models have been proposed to explain these features, such as interaction with a non-degenerate companion star (Kasen 2010), outward mixing of radioactive ⁵⁶Ni (Piro & Nakar 2013; Piro & Morozova 2016), detonation of a surface He layer (Polin et al. 2019), and interaction with CSM (Piro & Morozova 2016; Piro et al. 2021). The rate of early excess in normal SNe Ia is related to the actual population of different physical channels. Even if all the normal SNe Ia are from the SD scenario, an early excess from companion interaction could only be detected in ∼10 per cent of the events owing to the viewing-angle effect (Kasen 2010; Burke et al. 2022).

Obtaining high-quality early-time photometry necessitates early discovery, rapid classification, and high-cadence follow-up photometry of young SNe Ia, which require both extensive efforts and good luck. Early excess emission has been observed in detail only for a handful of normal SNe Ia, such as SN 2017cbv (Hosseinzadeh et al. 2017), SN 2018oh (Dimitriadis et al. 2019; Li et al. 2019; Shappee et al. 2019), SN 2019np (Sai et al. 2022), and SN 2023bee (Hosseinzadeh et al. 2023; Wang et al. 2023). In the future, with specially designed robotic facilities dedicated to high-cadence, multiband observations of young, nearby SNe Ia, we expect more such events to be observed and analysed in detail.

Although early-time flux excesses are relatively rare among normal SNe Ia, a series of recent studies reveals that the 02es-like SNe Ia tend to show them: see iPTF14atg (Cao et al. 2015), iPTF14dpk (Cao et al. 2016; Jiang et al. 2018), SN 2016jhr (Jiang et al. 2017), SN 2019yvq (Miller et al. 2020; Burke et al. 2021), SN 2022ywc (Srivastav et al. 2023), and SN 2022vqz (this work). This generic feature supports the idea that the 02es-like subclass likely originates from a single channel. However, the diversity in other aspects, including ejecta velocity, peak luminosity, and existence of early line blanketing, challenges the potential progenitor scenario and explosion mechanism. One possible scenario is the double-detonation model, in which the ignition of a surface He layer detonates the underlying C–O WD (Polin et al. 2019). The burning of the He layer will leave radioactive IGEs in the outermost ejecta and produce an early-time excess. This model was adopted to explain the early excesses of SNe 2016jhr and 2019yvq.

We compared the observations of SN 2022vqz with a grid of double-detonation models (Polin et al. 2019). The grid contains models with WD masses ranging from 0.6 to 1.3 M_⊙ and He-shell masses varying between 0.01 and 0.1 M_⊙. The explosion epoch is taken as the value inferred in Section 3.3. The best-fitting model has a WD mass of 1.0 M_⊙ and an He shell of 0.04 M_⊙. Comparisons between this model and the observed gr-band light curves, the g − r colour, and the spectra are shown in Fig. 10.

The early gr peaks of SN 2022vqz appear to be well reproduced, and the relative strength of the early peak to the main peak is also satisfactory. In particular, the ‘red bump’ in g − r colour ∼4 d after the explosion is overpredicted by the model. The general trend of the subsequent light-curve evolution is also reproduced, with minor differences. The model light curves are faster, bluer at early epochs and redder at later times, and overluminous by a factor of ∼2 (0.8 mag). All of this makes the model light curves more consistent with those of fast decliners among normal SNe Ia rather than 02es-like events.

Most spectral features can be reproduced by the double-detonation model, but the predicted velocities are higher than the observed ones by 3000–5000 km s⁻¹. This is not unexpected given that SN 2022vqz is a peculiar object with an extremely low velocity, as described in Section 4.2. No carbon can be detected in the model and the observed spectra. O i λ7774 is underpredicted, especially in the +14.6 d spectrum. Moreover, the model predicts a strong high-velocity Ca ii feature, but this is not observed in the +14.6 d spectrum.

Although the double-detonation model is capable of reproducing the early-time peak, all of the other discrepancies, especially those in spectral velocities, are difficult to reconcile. Reducing the WD mass to 0.9 M_⊙ yields lower luminosity and ejecta velocities, providing a better match with the observations. On the other hand, changing the above parameters would give a worse match with the overall light-curve and colour-curve evolution, and the velocities of Si ii and Ca ii features would still be unreasonably high.

5.2 O–Ne WD

It is commonly accepted that SNe Ia are explosions of C–O WDs. Assuming that the WD is composed of equal parts of C and O, and is fully burnt into iron-peak elements, the released specific energy is 1.55 × 10⁵¹ erg |${\rm M}_\odot ^{-1}$| (Branch 1992). This energy will be turned into kinetic energy of the ejecta, after compensating the binding energy of the WD. Since heavier WDs are more compact and have a larger specific binding energy, the thermonuclear explosion of a sub-M_Ch WD is expected to result in an SN with higher ejecta velocity than that of an M_Ch WD, assuming the same progenitor abundance and burning efficiency. This trend is beneficial in explaining high-velocity SNe like SN 2019ein (Xi et al. 2022) and SN 2019yvq (Burke et al. 2021), but causes problems for the low-velocity SN 2022vqz.

Reducing the burning efficiency will result in lower energy yields and slower ejecta velocities, leaving signatures of unburnt elements in the spectra. Most of the subluminous 02es-like SNe show a noticeable C ii λ6580 feature around maximum light, indicating low-efficiency burning of C–O WDs. However, the spectra of SN 2022vqz do not reveal any trace of carbon. Thus, we tentatively consider a possibility that the progenitor WD is made of oxygen and neon (O–Ne). By replacing ¹²C with ²⁰Ne, the specific fusion energy reduces to 1.21 × 10⁵¹ erg |${\rm M}_\odot ^{-1}$| and hence causes a decrease in the ejecta velocity by ∼2000 km s⁻¹, which could potentially explain the low velocity observed in SN 2022vqz.

We further use SYNAPPS (Thomas, Nugent & Meza 2011) to identify the potential signatures of unburnt Ne in the spectra of SN 2022vqz. Fig. 11 shows the decomposition of the spectrum synthesized by SYNAPPS compared to the t = −7.1 d spectrum of SN 2022vqz. We noticed that the shallow absorption features around 6800 and 7100 Å could be well fit by Ne i. These features are relatively common in subluminous SNe (see Fig. 8). Since burning of a C–O WD may also produce Ne, this cannot be treated as an exclusive signature of an O–Ne WD progenitor. The absorption around 6800 Å might be also attributed to Ti ii. However, the 6800 Å Ti ii feature usually emerges at later epochs and is accompanied by another Ti ii feature at ∼6600 Å, which is invisible in this spectrum. Thus, we still identify the 6800 Å absorption feature in the t = −7.1 d spectrum as Ne i rather than Ti ii.

The electron-degenerate cores made of O–Ne are believed to result from carbon burning in ‘heavy-weight’ intermediate-mass stars (i.e. 8 M_⊙ ≲ M ≲ 11 M_⊙), and they are expected to collapse into neutron stars through electron capture (Miyaji et al. 1980). However, O–Ne WDs can be ignited at lower central densities if a tiny amount of carbon (∼1.5 per cent) exists in their cores (Gutiérrez, Canal & García-Berro 2005), leading to thermonuclear SNe Ia. This tiny amount of carbon can be explained as the remnant of an off-centre carbon-burning process, which does not propagate all the way to the centre. Bravo et al. (2016) and Willcox et al. (2016) examined the delayed detonation of such hybrid C–O–Ne cores. They found that the produced ejecta are characterized by slightly lower masses of ⁵⁶Ni and substantially less kinetic energy than those of normal C–O WDs, making this kind of event an interesting candidate for the subluminous class of SNe Ia.

While the C–O–Ne model can simultaneously explain the deficiency of C and extremely low velocity of SN 2022vqz, it does not naturally reproduce the early excess emission, unless combined with the double-detonation model (see Section 5.1). Most of the observables can be well reproduced or explained by detonation of a C–O–Ne core triggered by a thick He shell. The companion star could be either a helium star or another degenerate He WD. Marquardt et al. (2015) studied a set of double-detonation models of ∼1.2 M_⊙ O–Ne WDs. The ejecta structure and spectral morphology of O–Ne models are similar to those of the C–O models of similar WD masses, but the resultant explosions are less energetic with peak luminosity being fainter by ∼0.2 mag in B and ejecta velocity slower by ∼3000 km s⁻¹, respectively.

However, it should be noted that the minimal mass of a C–O–Ne core is ∼1.08 M_⊙, which is the required mass for a C–O core to start off-centre carbon burning to Ne (Hurley, Pols & Tout 2000; Marquardt et al. 2015). This value is slightly larger than the 1.0 M_⊙ inferred from model fitting in Section 5.1 and will predict a brighter peak magnitude. It is not clear whether this mass excess can be compensated by the subluminous nature of O–Ne core explosions compared with C–O cores. Viewing angle may also affect the observed luminosity since double detonation is intrinsically asymmetric. Further analysis requires a more thorough numerical examination of double detonation of C–O–Ne WDs, exploring the full mass range and viewing-angle effects of such events.

5.3 CSM interaction

The early excesses of some 02es-like events are attributed to interaction between ejecta and CSM (Hu et al. 2023). Kromer et al. (2016) suggest that interaction with a non-spherical CSM may be able to account for the early UV emission in iPTF14atg. Srivastav et al. (2023) also attribute the strong early-time peak observed in SN 2022ywc to CSM interaction, and rule out other scenarios like surface helium detonation, surface Ni distribution, or companion interaction.

Following the methodology of Srivastav et al. (2023), we use the CSMNI model of Modular Open Source Fitter for Transients (mosfit; Nicholl, Guillochon & Berger 2017; Guillochon et al. 2018) to fit the gr-band photometry of SN 2022vqz. The best-fitting parameters are |$E_{\rm k}=0.51^{+0.15}_{-0.07}\times 10^{51}$| erg, |$M_{\rm CSM}=0.0056^{+0.0042}_{-0.0015}~{\rm M}_\odot$|⁠, |$M_{\rm ej}=0.87^{+0.11}_{-0.06}~{\rm M}_\odot$|⁠, |$R_0=2.6^{+2.1}_{-1.6}\times 10^{14}$| cm, and |$t_{\rm exp}={\rm MJD}~59844.75^{+0.08}_{-0.11}$|⁠, where E_k is the ejecta kinetic energy, M_CSM is the CSM mass, M_ej is the ejecta mass, R₀ is the inner radius of the CSM shell, and t_exp is the explosion time. Fig. 12 shows the gr-band light curves of SN 2022vqz compared with those from the CSMNI model.

The CSMNI model reproduces the time-scale and shape of the early peak very well, and it also provides a good fit to the main bulk of the light curves, with a slightly fainter predicted g-band peak. The estimated explosion time is ∼0.3 d later than that inferred from the fireball model in Section 3.3, but within the quoted uncertainty. The ejecta mass is significantly less than M_Ch, making near-M_Ch models like the delayed detonation unfavourable. This model requires a CSM distribution at ∼3 × 10¹⁴ cm, similar to that of SN 2022ywc, but with a mass of about one order of magnitude lower (i.e. ∼0.006 M_⊙).

The CSM could originate from interactions between two merging WDs, specifically the violent merger scenario. This scenario was favoured for multiple 02es-like events, including SNe 2002es, 2010lp, PTF10ops, and iPTF14atg. The secondary WD can be disrupted by the primary, forming a centrifugally supported disc (Yoon, Podsiadlowski & Rosswog 2007; Shen et al. 2012), or eject disc-originated material (DOM; e.g. disc winds or jets) along the polar directions (Levanon et al. 2015). Simulations of WD mergers indicate that the mass of CSM could reach ∼10⁻⁴ to 10⁻² M_⊙ for discs formed by tidal tails (Raskin & Kasen 2013; Dan et al. 2014), or ∼10⁻² to 10⁻¹ M_⊙ for disc wind-driven DOM (Levanon & Soker 2019). The non-spherical configuration of CSM may lead to a few 02es-like events that have very bright early excesses, as SN 2022ywc, if the viewing angle aligns with the polar axis (Srivastav et al. 2023). The large parameter space and asymmetry of the violent merger scenario may account for the observed diversity in luminosity and ejecta velocity of 02es-like objects.

5.4 Other models for the early excess

Some other mechanisms could also produce early excess emission in SNe Ia. One is the collision between SN ejecta and a non-degenerate companion star (Kasen 2010; Dimitriadis et al. 2019); a bright UV/optical excess will be produced when the SN ejecta reach the surface of the companion. The separation between the SN and its companion should be comparable to the Roche lobe radius of the companion star, a/R_companion = 2–3, where a is the separation. For typical velocities of SN and radius of main-sequence stars, the interaction flux should appear a few minutes after the SN explosion, or a couple of hours in the case of a red-giant companion (Kasen 2010).

Assuming that the density profile of the outer SN ejecta follows a power law, ρ ∝ r⁻¹⁰, the isotropic equivalent collision-powered luminosity (L_c) and effective temperature (T_eff) can be analytically estimated as

where a is the distance between the WD and its companion, M_ej is the ejecta mass, v_ej is the ejecta velocity, κ_e is the effective opacity in optical, and t is the time after explosion, respectively. Assuming that the early-time SED can be treated as a blackbody radiation, multiband model photometry and colour evolution can be constructed from L_c and T_eff.

We fit the observed bolometric light curve and g − r colour evolution to the analytical model. As the early-time bolometric light curve is not sufficiently sampled, to reduce the free parameters in the fitting, the opacity κ_e is set to 0.2 cm² g⁻¹ according to the assumption made by Kasen (2010), whereas M_ej and v_ej are fixed to M_ch and 7000 km s⁻¹, respectively. Fig. 13 shows the best-fitting companion-collision model with separation a = 4.3 R_⊙. While the decreasing part of the early bolometric peak is well fitted, the model curve does not show an initial increase before the early peak as observed in ATLAS o and ZTF r bands. Moreover, the model g − r colour curve is too blue, and lacks a red bump as seen in the observed data of SN 2022vqz.

Another possible explanation for the early flux excesses is ⁵⁶Ni mixing to the outermost layers of the SN ejecta (Piro & Nakar 2013; Sai et al. 2022). The emission from the outer ⁵⁶Ni can escape earlier than that from the deeper one, and will cause an early excess in first few days after the SN explosion. Piro & Morozova (2016) examined a series of theoretical models with different levels of Ni mixing, measured by the width of a boxcar smoothing routine on the Ni distribution curve (0.05 ≤ boxcar ≤ 0.25). Their model bolometric and colour curves are also overplotted in Fig. 13 for comparison. When the level of mixing increases, the model bolometric light curve shows a more rapid increase at early times, and the colour curve becomes flatter and bluer. However, neither the early bolometric peak nor the red-bump feature of SN 2022vqz can be reproduced by the Ni mixing models. The absence of a separate early peak is not surprising, since the outer Ni in these models is from mixing of a concentrated distribution, and the mass fraction of Ni still decreases monotonically from centre to the surface. In contrast, the outermost Ni in the double-detonation model is produced by surface He, thus ⁵⁶Ni can be more congregated in the outermost region of ejecta and produce a clear early peak.

The main strengths and weaknesses of the considered models in this paper are summarized in Table 2.

6 CONCLUSION

In this paper, we presented extensive photometric and spectroscopic observations of SN 2022vqz, a peculiar SN Ia located in a solar-metallicity, medium-mass, star-forming galaxy. SN 2022vqz has an absolute B-band peak magnitude of |$M_{B,\rm max}=-18.11\pm 0.16$| mag and a post-peak decline rate of Δm_15,B = 1.33 ± 0.11 mag, which fits well into the subclass of subluminous 02es-like SNe Ia. The bolometric light curve of SN 2022vqz is constructed from multiband photometry and fitted to the radiation diffusion model of Arnett (1982), implying a ⁵⁶Ni mass of M_Ni = 0.20 ± 0.04 M_⊙.

The light curves and spectra of SN 2022vqz resemble those of the classical (i.e. subluminous, low ejecta velocity) 02es-like object iPTF14atg very well. However, while lower ejecta velocity indicates a less energetic explosion and lower burning efficiency, no C is detected in the spectra of SN 2022vqz, in contrast to the prominent C ii absorption seen in pre-peak and maximum-light spectra of iPTF14atg.

SN 2022vqz also displays a clear blue early-time peak within ∼4 d after the explosion, confirming the recently established trend that most, if not all, 02es-like events have early blue/UV excesses. We discussed the possible origin of this early excess. A sub-M_Ch double-detonation model of ∼0.9–1.0 M_⊙ WD  + 0.04 M_⊙ He shell can well reproduce the strength and epoch of the early peak, but it also predicts a ‘red bump’ after the early peak that is significantly stronger than the observed data. Double-detonation models generally leave little or no C in spectra, which is consistent with SN 2022vqz. However, the model spectral features are significantly blueshifted compared to the observed ones, overpredicting the ejecta velocity by 3000–5000 km s⁻¹. This discrepancy may be reconciled by considering a hybrid C–O–Ne WD model, which releases lower specific fusion energy when ignited and produces SN Ia explosions with slightly lower ⁵⁶Ni masses and substantially slower ejecta velocities.

We also use the code mosfit to explore the CSM interaction scenario as an alternative explanation for the early peak. A CSM mass of ∼0.006 M_⊙ at ∼3 × 10¹⁴ cm is required to reproduce the early peak. This amount of CSM could arise from interactions of WDs in a violent merger scenario, which has been proposed for several 02es-like events. The large amount of asymmetry and extent of parameter space may account for the observed diversity in luminosity, ejecta velocity, and strength of the early excess in the 02es-like subclass. However, since the CSM is formed by disruption of a secondary C–O WD, it should be C–O rich and produce C signatures in spectra of the SN. The lack of a C signature in SN 2022vqz is still a challenge for this model.

Additional models like SN ejecta interaction with a non-degenerate companion and outwards Ni mixing are also examined. None can simultaneously reproduce the morphology of the bolometric light curve and colour evolution of the early peak. Moreover, they do not provide additional clues for the cool, subluminous, and slow-expansion nature of the 02es-like SNe Ia; thus, we disfavour these models for SN 2022vqz.

In conclusion, we suggest that double detonation of a low-mass (≲1.1 M_⊙) C–O–Ne WD is the most viable model for SN 2022vqz. However, there are still some tensions in peak luminosity and synthesized ⁵⁶Ni mass that require further modelling to resolve. SN 2022vqz extends the diversity of the 02es-like subclass with its extremely low ejecta velocity and absence of carbon features. Additional observations of this SN at nebular phases would be important for discriminating between different possible progenitor scenarios. More efforts in exploring the observational properties of progenitors that previously drew less attention, like WDs made of C–O–Ne, will likely help in determining the possible origin of this rare subclass of SNe Ia. Three-dimensional numerical simulations that explore the whole parameter space and viewing angles of double detonation and violent merger models, although challenging, will also help in understanding the diversity in 02es-like events. For now, the sample size of 02es-like SNe Ia is still limited. Future surveys that concentrate on quick response for young transients and early-time multiwavelength photometric observations may greatly enrich the sample, providing better constraints on the origin of the seemingly universal early-time excesses of 02es-like SNe Ia.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (NSFC; grants 12288102, 12033003, and 11633002), the Scholar Program of Beijing Academy of Science and Technology (DZ:BS202002), and the Tencent Xplorer Prize. Y-zC and J-jZ were supported by the International Centre of Supernovae, Yunnan Key Laboratory (No. 202302AN360001). Y-zC was supported by NSFC grant 12303054. AVF’s group at U.C. Berkeley received financial support from the Christopher R. Redlich Fund, Briggs and Kathleen Wood (TGB is a Wood Specialist in Astronomy), Alan Eustace (WZ is a Eustace Specialist in Astronomy), and many individual donors. AP, AR, and LT acknowledge support from the PRIN-INAF 2022, ‘Shedding light on the nature of gap transients: from the observations to the models’.

A major upgrade of the Kast spectrograph on the Shane 3 m telescope at Lick Observatory was made possible through generous gifts from William and Marina Kast as well as the Heising-Simons Foundation. Research at Lick Observatory is partially supported by a generous gift from Google. Observations at AZT-22 of the Maidanak Observatory were supported by Uzbekistan’s Ministry of Innovative Development (grant A-FA-2021-36). This work is partially based on observations collected with the Copernico 1.82 m telescope and the Schmidt 67/92 telescope (Asiago Mount Ekar, Italy) INAF – Osservatorio Astronomico di Padova. The ZTF forced-photometry service was funded under Heising-Simons Foundation grant 12540303 (PI: M. Graham). This work makes use of the NASA/IPAC Extragalactic Database (NED), which is funded by NASA and operated by the California Institute of Technology.

This work has made use of data from the ATLAS project. The ATLAS project is primarily funded to search for near-Earth objects (NEOs) through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogues from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and by STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and the Millennium Institute of Astrophysics (MAS), Chile.

DATA AVAILABILITY

The photometric data underlying this article are available in the article, and the spectroscopic data will be available in the Weizmann Interactive Supernova Data Repository (WISeREP) at https://www.wiserep.org/object/21648.

Footnotes

References