SN 2023zaw: An Ultrastripped, Nickel-poor Supernova from a Low-mass Progenitor

We present SN 2023zaw—a subluminous (M r = −16.7 mag) and rapidly evolving supernova (t 1/2,r = 4.9 days), with the lowest nickel mass (≈0.002 M ⊙) measured among all stripped-envelope supernovae discovered to date. The photospheric spectra are dominated by broad He i and Ca near-infrared emission lines with velocities of ∼10,000−12,000 km s−1. The late-time spectra show prominent narrow He i emission lines at ∼1000 km s−1, indicative of interaction with He-rich circumstellar material. SN 2023zaw is located in the spiral arm of a star-forming galaxy. We perform radiation-hydrodynamical and analytical modeling of the lightcurve by fitting with a combination of shock-cooling emission and nickel decay. The progenitor has a best-fit envelope mass of ≈0.2 M ☉ and an envelope radius of ≈50 R ⊙. The extremely low nickel mass and low ejecta mass (≈0.5 M ⊙) suggest an ultrastripped SN, which originates from a mass-losing low-mass He-star (zero-age main-sequence mass < 10 M ⊙) in a close binary system. This is a channel to form double neutron star systems, whose merger is detectable with LIGO. SN 2023zaw underscores the existence of a previously undiscovered population of extremely low nickel mass (<0.005 M ☉) stripped-envelope supernovae, which can be explored with deep and high-cadence transient surveys.


Introduction
Modern wide-field and high-cadence transient surveys such as the Zwicky Transient Facility (ZTF; Bellm et al. 2019;Graham et al. 2019) and the Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018;Smith et al. 2020) have expanded the discovery space of unusual supernovae (SNe).The class of rapidly evolving, faint SNe constitutes such a population of peculiar SNe that evolve on the shortest timescales of a few days and have a peak absolute magnitude fainter than −17 mag.There are only a handful of well-studied examples in the literature-SN 2005ek (Drout et al. 2013), SN 2010X (Kasliwal et al. 2010), iPTF14gqr (De et al. 2018a), SN 2018kzr (McBrien et al. 2019), SN 2019dge (Yao et al. 2020), SN 2019wxt (Agudo et al. 2023;Shivkumar et al. 2023), SN 2019bkc (Chen et al. 2020;Prentice et al. 2020), and SN 2022agco (Yan et al. 2023).
In this Letter, we present SN 2023zaw-a Type Ib SN with the lowest nickel mass measured among all stripped-envelope SNe discovered to date.The late-time spectra are dominated by narrow He I emission lines, suggestive of interaction with Herich circumstellar material (CSM).SN 2023zaw exhibits the most rapid rise and fade time among all stripped-envelope SNe in the literature.Additionally, it is the nearest SN among all fast and faint SNe in the literature.The photometric and spectroscopic properties suggest that it is an ultrastripped SN, originating from a progenitor with an initial mass <10 M e , which likely formed an NS.

Discovery and Follow-up Observations
2.1.Discovery SN 2023zaw (ZTF23absbqun) was discovered with the ZTF camera (Dekany et al. 2020), which is mounted on the 48 inch Samuel Oschin Telescope at the Palomar Observatory.It was first detected at α = 04 h 29 m 20 235, δ = +70°25′37 52 (J2000) on 2023 December 7, at 05:34:06 UTC and reported to the Transient Name Server19 (Sollerman 2023).At the time of discovery, the AB apparent magnitude in the g band was 19.34 ± 0.15 mag.The transient stood out due to its faintness and extremely fast evolution (see Figure A1).It was also flagged as a fast transient candidate by the ZTFReST framework for kilonova and fast-transient discovery (Andreoni et al. 2021) and was saved as part of the magnitude-limited (Bright Transient Survey; Fremling et al. 2020) and volumelimited (Census of the Local Universe; De et al. 2020) surveys of ZTF.
SN 2023zaw is located in the UGC 03048 galaxy at a redshift of z = 0.0101 (Springob et al. 2005).It is located on the edge of one of its spiral arms (see Figure 1).The angular separation from the nucleus of the galaxy is 21 44, which corresponds to a physical separation of 4.36 kpc.We correct for the Virgo, Great Attractor, and Shapley supercluster infall (Mould et al. 2000) based on the NASA Extragalactic Database20 object page for UGC 03048.We adopt a Hubbleflow distance of 43.9 ± 3.1 Mpc, which corresponds to a distance modulus of 33.21 ± 0.15 mag.

Optical Photometry
We obtained multiple epochs of g-, r-, and i-band photometry with the ZTF camera.The images were reduced using the ZTF image analysis pipeline (Masci et al. 2019).We also perform forced photometry at the location of the transient for the ZTF images in the gri bands.We obtained c-and o-band photometry from the forced-photometry service of the ATLAS (Tonry et al. 2018;Smith et al. 2020;Shingles et al. 2021).We took regular cadence multiband photometry in the g, r, i, and z bands with the optical imager (IO:O) at the 2.0 m robotic Liverpool Telescope (LT; Steele et al. 2004).In addition, photometric data were obtained with the rainbow camera on the automated Palomar 60 inch telescope (P60; Cenko et al. 2006).LT and P60 images were processed using the FPipe (Fremling et al. 2016) image-subtraction pipeline with PanSTARRS (Chambers et al. 2016) reference images.The Alhambra Faint Object Spectrograph and Camera (ALFOSC) at the 2.5 m Nordic Optical Telescope was used to obtain g-, r-, and i-band photometry.The ALFOSC data was reduced using the PyNOT21 pipeline.The collage of all the multiband lightcurves is shown in Figure 2. The photometry data are listed in Table C1.

Optical Spectroscopy
We acquired four epochs of spectroscopy with the Spectral Energy Distribution Machine (SEDM; Blagorodnova et al. 2018) on the P60.The SEDM data was reduced using the pipeline described in Rigault et al. (2019) and Kim et al. (2022).We obtained three epochs of spectroscopy with the Low Resolution Imaging Spectrometer (LRIS; Oke et al. 1995) on the Keck I telescope, with data reduced using the automated LPIPE (Perley 2019) pipeline.The ALFOSC instrument at the 2.5 m Nordic Optical Telescope was used to obtain one epoch of lowresolution spectrum, which was reduced using the PyNOT22 pipeline.We took one spectrum with the Binospec spectrograph (Fabricant et al. 2019) on the Multiple Mirror Telescope, which was reduced using the PypeIt package (Prochaska et al. 2020a(Prochaska et al. , 2020b)).We used the Keck Cosmic Web Imager (KCWI; Martin et al. 2010;Morrissey et al. 2018) to obtain spectra of the transient and its host environment.This was reduced using the KCWI data reduction pipeline. 23We also show one spectrum observed with the Gemini-GMOS instrument from the Transient Name Server. 24The collage of all the spectra is shown in Figure D1, and the spectroscopy log can be found in Table E1.

Extinction Correction
We correct for Milky Way extinction using the dust maps from Schlafly & Finkbeiner (2011).Along the line of sight of SN 2023zaw, E(B − V ) = 0.25 mag.For reddening corrections, we use the extinction law in Cardelli et al. (1989) with R V = 3.1.
For host-extinction correction, we use the Na I D absorption lines of the host galaxy (Poznanski et al. 2012;Stritzinger et al. 2018).We measure an equivalent width of 1.5 ± 0.2 Å from the KCWI spectrum obtained at a phase of +2.5 days past r-band peak.To compute A V , we use A mag 0.78 0.15 et al. 2018).We note that there are caveats in using the Na I D EW measurement to estimate host extinction, especially for low-resolution spectra (Poznanski et al. 2011) and in the presence of CSM (Phillips et al. 2013).
We plot the g − r and r − i colors of SN 2023zaw before and after host-extinction correction in Figure F1 and compare with other rapidly evolving transients and the Type Ib SN color template from Stritzinger et al. (2018).

Explosion Epoch Estimation
We fit for the r-band maximum epoch with a polynomial fit to the r-band photometry and obtain a peak magnitude of −16.7 mag and peak MJD of 60287.7.All phases mentioned in the Letter will be in rest-frame days measured from this r-band peak epoch.The last nondetection before the first detection was on 60283 MJD (−4.7 days), with an upper limit of −14.8 mag in the o band.We perform a power-law fit to the r-band data prior to the maximum epoch to estimate the explosion epoch.We find that the explosion epoch is 60284.4± 0.5 MJD (−3.3 ± 0.5 days).

Lightcurve Properties
In Figures 2 and G1, we compare the g-and r-band evolution of SN 2023zaw to other fast-evolving and subluminous transients in the literature: fast-declining Type I SNe-SN 2005ek (Drout et al. 2013), SN 2010X (Kasliwal et al. 2010), SN 2018kzr (McBrien et al. 2019), SN 2019bkc (Chen et al. 2020;Prentice et al. 2020), and the kilonova AT 2017gfo (Abbott et al. 2017) and ultrastripped SN candidates-SN 2019dge (Yao et al. 2020), iPTF14gqr (De et al. 2018b), SN 2019wxt (Agudo et al. 2023;Shivkumar et al. 2023), and SN 2022agco (Yan et al. 2023).We also compare with lightcurve templates for Type Ibn (Hosseinzadeh et al. 2017) and Type Ibc SNe (Taddia et al. 2015).SN 2023zaw is much fainter and faster evolving than the typical Type Ibn or Ibc SN.However, we note that some Type Ibn/Icn SNe also show rapid evolution: LSQ13ccw (Hosseinzadeh et al. 2017) and SN 2023emq (Pursiainen et al. 2023).We fit a polynomial to the lightcurves and measure the rise time and fade time as the duration above the half maximum of the peak.SN 2023zaw has a rise time (t rise 1 2 / ) of 1.8 days and a decline time (t dec 1 2 / ) of 3.1 days.The time above half maximum of the peak is shorter than for all other fast-evolving faint transients in the literature (see Figures 2, A1, and G1).The lightcurve declines by 2.7 mag in 10 days in the r band and by 2.7 mag in 6 days in the g band.SN 2023zaw is only slower than kilonova AT 2017gfo and has a comparable decline timescale to SN 2019bkc and SN 2018kzr.In the r band, SN 2023zaw has a very sharp initial decline of ∼0.28 mag day −1 followed by a transition to a relatively slower decline rate of ∼0.07 mag day −1 after ∼10 days since peak.In the g band, SN 2023zaw shows a very sharp initial decline of ∼0.44 mag day −1 until 10 days postpeak.The peak absolute magnitude in the r band before and after host-extinction correction is −15.6 mag and −16.7 mag, respectively.In the g band, the peak absolute magnitude before and after host-extinction correction is −15.2 mag and −16.6 mag, respectively.We compare the time above half maximum and the absolute luminosity of SN 2023zaw to a sample of stripped-envelope supernovae (SESNe) detected by ZTF and other fast and faint SNe in the literature in Figure A1.SN 2023zaw shows the fastest evolution among all transients at similar luminosities.

Spectral Properties
In the early SEDM spectra taken before r-band peak, we see a blue, featureless continuum.We see multiple broad helium lines-He I λλ5876, 6678, and 7065, in the subsequent spectra.The expansion velocity measured from the absorption in the P-Cygni profile of the He I lines evolved from 12,000 to 9700 km s −1 from 0 to 8 days postpeak.These He I absorption lines resemble those seen in Type Ib SNe and we obtain a decent fit to Type Ib SNe at similar or later phases using the SuperNova Identification (Blondin & Tonry 2007) code.This is consistent with the initial classification made by Gillanders et al. (2023).There is also a redshifted component to the He I λ5876 line profile in the spectrum taken at +8 and +29 days, probably due to asymmetrical ejecta.He I line velocities drop to ∼3000 km s −1 at a phase of 29 days.Prominent narrow He I emission lines can be seen in the LRIS spectra obtained at 29 and 61 days after peak.The FWHM velocities of the He I emission lines in the latest LRIS spectrum are ∼1050 ± 100 km s −1 .
In the photospheric phase, we do not see any broad O I λ7774 emission line, which is common in stripped-envelope SNe.We also see broad, high-velocity lines of the Ca II NIR triplet at ∼12,000 km s −1 as early as 2 days after peak.Such high-velocity early Ca NIR absorption lines are not seen in other fast-evolving SNe (see Figure 3).Also, unlike other rapid-declining SNe, SN 2023zaw does not reach nebular phase even at 29 days after peak.Broad nebular lines such as [O I] λλ 6300, 6364, and Ca II NIR triplet are also not seen in the latest LRIS spectrum taken at +61 days.

Bolometric Lightcurve
We bin the data in intervals of 1 day and fit with the Planck blackbody function where there are detections in at least three filters.We perform the fit using the Python EMCEE package (Foreman-Mackey et al. 2013).The blackbody temperature, radius, and luminosity thus obtained are shown in Table H1.The errors in the model parameters are determined by extracting the 16th and 84th percentiles of the posterior probability distribution.For the later epochs, where we have photometry in only one band, we use the extrapolated blackbody temperatures obtained by fitting an exponential decay function to the temperature data from previous epochs.The temperature stays roughly constant at ≈4000 K.For these later epochs, we also estimate the bolometric luminosity as νL ν .These estimates agree with each other within 3σ, with the latter method slightly underestimating the bolometric luminosity as expected.
We plot the best-fit blackbody parameters and compare these with other ultrastripped SN candidates in Figure 4. We can see that the bolometric lightcurve of SN 2023zaw is similar in peak brightness to the other SNe but has a faster decline rate within the first 10 days after r-band peak.The decline of the bolometric luminosity slows down after around 6 days postpeak.The photospheric radius increases up to 4 days post-peak, reaching a maximum of ∼10,000 R e .The temperature decreases over time, from a maximum of ∼17,000 K on day 0 to ∼4000 K on day 8.

Radiation-hydrodynamics Modeling
We use the pre-SN model grid presented in Wu & Fuller (2022).This grid was produced through binary evolution of He stars with an initial mass of 2.5-3 M e evolved up to silicon burning using the Modules for Experiments in Stellar Astrophysics code (MESA; Paxton et al. 2010).The binary systems evolved in Wu & Fuller (2022) include orbital periods of 1, 10, and 100 days.Predictions of the properties of the unbound CSM in the vicinity of each pre-SN model were also produced.In this work, we compare our data to both models with and without unbound CSM.
We use the SuperNova Explosion Code (SNEC; Morozova et al. 2015) to explode the grid of pre-SN stellar models described above.First, we exploded all models in the grid using a thermal bomb with E kin = 0.4 × 10 51 erg and M Ni = 0.0035 M e , with 56 Ni mixed all the way through the remaining star after mass excision of 1.4 M e in the center.
The resulting bolometric lightcurves and expansion velocities were then compared to our data on SN 2023zaw.We find that the model Wu & Fuller (2022) produced from a He star with an initial mass of 2.651 M e and an orbit period of 100 days reproduces the overall behavior seen in our data well, including the expansion velocities.Other models fail to reproduce both the early cooling phase of the bolometric lightcurve (governed by the radius and mass in the bound envelope) or the late-time decline rate (governed by the total ejecta mass).To produce our best-fitting MESA+SNEC model shown in Figure 4, we modified the bound envelope radius of the raw MESA model so that it extends to ≈50 R e by cutting the model grid appropriately (compared to ≈110 R e in the raw model).This would correspond to an orbital period between 10 and 100 days.We also modified the mass excision in the center to be 1.3 M e .These modifications result in the explosion parameters E kin = 0.4 × 10 51 erg, M Ni = 0.0035 M e , with complete mixing of 56 Ni through the ejecta, which has a total mass of M ej = 0.56 M e , including the bound envelope.Models including the unbound CSM predicted by Wu & Fuller (2022) are largely inconsistent with our data.The luminosity during the first week can be similar to what we observe if the explosion energy is adjusted to be low (<0.1 × 10 51 erg).
However, such models are inconsistent (too bright) at later times and inconsistent with the overall decay rate.

Analytical Modeling
The bolometric luminosity declines at a rapid rate for the first ∼6 days after peak (see Figure 4).Radioactivity alone is unable to power this early-time luminosity under the constraint that the ejecta mass must be greater than the nickel mass.The peak luminosity of 2.5 × 10 42 erg s −1 and timescale of evolution (≈4.9 days) falls in the forbidden region of nickel-powered lightcurves (see Figure 1 in Kasen 2017).Instead, we model this early lightcurve as being powered by shock-cooling emission from the shock-heated bound or unbound extended stellar material.We use the shock-cooling-emission model presented in Piro et al. (2021) to fit the early-time multiband photometry.We obtain a good fit for all the filters for photometry up to 6 days past peak (Figure 4).The explosion time is constrained to be −2.7 ± 0.2 days before the peak.Based on the fit, we find that the best-fit envelope mass is 0.20 ± 0.01 M e and envelope radius is 55 ± 11 R e .This is comparable to the envelope properties measured from shockcooling-emission modeling in other Type Ibc SNe (see Figure I1).The envelope properties of SN 2023zaw are also consistent with theoretical models for bound and unbound stellar material (Figure I2).
For estimating the Ni mass, we first subtract the contribution of the shock-cooling emission estimated above from the bolometric luminosity, which we then assume to be entirely powered by radioactive decay.We use the analytical expression for a Ni-powered bolometric lightcurve provided in Arnett et al. (1989), Valenti et al. (2008), and Wheeler et al. (2015).Further details on the model fitting can be found in Yao et al. (2020, their Appendix B).Based on the fit, we find that the required Ni mass to power the SN is 0.24 ± 0.14 × 10 −2 M e .We measure a photon diffusion timescale (τ m ) of6.21 0.11 0.11 -+ days and characteristic γ-ray diffusion timescale (t o ) of 36.04 2.74 3.41 -+ days.We assume that the photospheric velocity is ∼12,000 km s −1 , as assumed from the He I absorption lines in the ALFOSC spectrum taken at r-band peak.We use an opacity (κ) value of 0.07 cm 2 g −1 , appropriate for H-poor SNe (Taddia et al. 2018).We estimate the ejecta mass using the following relation: -.These explosion parameters are an order of magnitude lower than for regular Type Ibc SNe.The estimated ejecta mass is comparable to other ultrastripped SNe in the literature, while the Ni mass is smaller by a factor of ∼5.The rapid lightcurve decline is explained by the low ejecta mass, which allows γ-rays to escape already a few days after the peak.Overall, SN 2023zaw occupies a unique location in the phase-space plot of nickel mass versus ejecta mass (see Figure 5).For comparison, we also plot the nickel and ejecta masses estimated for a sample of stripped-envelope SNe from Taddia et al. (2018) and doublepeaked Type Ibc SNe from Das et al. (2023b).We note that the double-peaked SN 2021inl (Jacobson-Galán et al. 2022;Das et al. 2023b) is the other SN with Ni mass <0.01 M e .
We also use the analytical models given in Khatami & Kasen (2019) to obtain a nickel mass estimate of ≈2.5 × 10 −2 M e and an ejecta mass estimate of ≈0.45 M e .

Host Galaxy Environment
We observed SN 2023zaw and its immediate environment with the integral-field unit KCWI (Section 2.3).These data  B1.
We infer the attenuation by ionized gas from the flux ratio between Hβ and Hγ (Momcheva et al. 2013).Using the Calzetti et al. (2000) attenuation model, we measure E(B − V ) gas = 0.58 ± 0.20 mag, which translates to a stellar continuum color excess of 0.26 ± 0.09 mag.This value is comparable to the reddening inferred from the Na I D absorption lines (Section 3.1) and corroborates that the line of sight is suffering from significant host attenuation.
We note that the stellar populations near SN 2023zaw are evolved and their spectra show conspicuous Balmer absorption lines.This could introduce a bias into the observed Balmer decrement.To quantify the impact, we center a box annulus on the SN position and extract a spectrum from the blue KCWI data between 2 and 4 px from the SN position.We fit the stellar continuum with the stellar population fitting code FIREFLY version 1.0.-utilizing the relationship between the Hα luminosity and the SFR from Kennicutt (1998) and the scaling factor from Madau & Dickinson (2014) to convert from the Salpeter to the Chabrier IMF in the Kennicutt (1998) relation.The error estimate includes the uncertainty of the flux measurements, the gas-phase attenuation, and the luminosity distance of SN 2023zaw's host galaxy.We stress that the large SFR error reflects the uncertainty in the attenuation correction, not a low precision of the Hα flux measurement (Table B1).
Detecting emission lines from hydrogen, nitrogen, and oxygen also allows us to measure the metallicity of the starforming region.Using the O3N2 metallicity indicator and the calibration from Curti et al. (2017), we infer a metallicity of 1.18 ± 0.02 solar (statistical error), adopting a solar oxygen abundance of 8.67 (Asplund et al. 2009).
Next, we draw our attention to SN 2023zaw's neighborhood.Figure 1 shows a map of the star formation activity out to a distance of 6.7 kpc from SN 2023zaw's location.This image was generated from the KCWI data.First, we extracted a 20 Å wide image centered on the wavelength of Hα.Then, we extracted two additional 20 Å wide images from emission-linefree regions adjacent to Hα and interpolated between them to remove the galaxy/SN flux at the wavelength of Hα.Finally, the intensity scale was corrected for MW extinction, scaled to the SN photometry, and converted to SFR as we did for the explosion site.The SFRs in Figure 1 are strict lower limits.They account for neither host attenuation nor stellar Balmer absorption lines.
The explosion site, marked by the blue circle in Figure 1, is located near the outskirts of a larger region with ongoing star formation activity.Although the level of star formation activity at the explosion site is in the lower half of the intensity distribution, this star-forming region extends much farther.Inspecting the neighboring spaxels reveals that the star formation activity extends to regions of even less intensity.Thus, the location of SN 2023zaw favors a massive star progenitor, but it does not rule out a thermonuclear origin.

Discussion: Progenitor and Evolutionary Pathways
In this section, we discuss the possible progenitors and evolutionary pathways for SN 2023zaw.

Thermonuclear ".Ia" SN
In the ".Ia" SN scenario, the progenitor is a WD in a close binary (P orb < 1 hr) system, accreting helium from its companion.They have a tenth of the ejecta mass and explosion energy of a normal Type Ia SN (Bildsten et al. 2007).We compare the r-band lightcurve and bolometric luminosity with ".Ia" SN models from Shen et al. (2010) in Figure 6.The peak luminosities range from 0.5 to 5 × 10 42 erg s −1 or peakbolometric magnitudes from −15.5 to −18 mag, consistent with SN 2023zaw.We highlight the best-fit ".Ia" SN bolometric lightcurve, which is an excellent fit to the earlytime data before host-extinction correction.Prominent He I features from unburnt He are predicted, which is consistent with the spectra of SN 2023zaw.We note that the absence of broad O I λ7774 and the presence of Ca II NIR lines at early phases resemble the spectra of SN 2010X (Kasliwal et al. 2010), which is a ".Ia" SN candidate (see Figure 3).However, the fact that the decline of the r-band lightcurve slows down after ∼10 days postpeak is not consistent with the ".Ia" SN models (Shen et al. 2010).

AIC of a WD
O-Ne WDs collapse to form an NS when it approaches Chandrasekhar mass.Numerical simulations predict very low explosion energy (∼0.5 × 10 49 erg).We compare the bolometric lightcurve prediction from Darbha et al. (2010) with SN 2023zaw in Figure 6.The predicted peak luminosity is ∼10 times fainter than that observed for SN 2023zaw.The models do not predict the late-time tail seen in SN 2023zaw.The nickel mass predicted in AIC models (0.001M ☉ ) is slightly smaller than that estimated for SN 2023zaw.The ejecta velocities predicted for AIC explosions are very high (∼0.1c),which is not consistent with SN 2023zaw.

WD-NS/Black Hole Merger
Spectroscopic models of WD-NS/black hole (BH) mergers (Gillanders et al. 2020) predict that the ejecta should be rich in intermediate elements such as O, Mg, and S.This is not consistent with the observed spectra for SN 2023zaw.

Nickel-free Core-collapse SN
A potential powering mechanism for a fast-evolving, low-Ni mass SN is the "Ni-free" SN scenario (Kleiser & Kasen 2014;Kleiser et al. 2018).In this case, the SN is not powered by decay of radionuclides like 56 Ni but is instead powered by shock interactions with an extended hydrogen-poor CSM.We compare the r-band lightcurve and bolometric luminosity of SN 2023zaw with Ni-free SN models from Kleiser et al. (2018) in Figure 6.If there is a significant amount of Ni present (>0.05M e ), it shows up as a second peak in the model lightcurves, which is not seen for SN 2023zaw.The models with 0 or 0.01 M e of nickel produce bright and short-lived peaks.However, none of the lightcurves can explain the rapid rise and the slower-evolving late-time lightcurve tail of SN 2023zaw.

ECSN
The low nickel mass is consistent with super-asymptotic giant branch (sAGB) progenitor stars with an O-Ne-Mg core that explodes as an ECSN.However, an sAGB star has a hydrogen-rich envelope.Since, the spectra do not show hydrogen, a binary system is necessary, where the hydrogen has been stripped off through Roche-lobe overflow or common envelope ejection.Stripped-envelope ECSN models in Moriya & Eldridge (2016) predict ejecta mass of ∼0.3−0.6 M ☉ , nickel mass of ∼0.003 M e , and explosion energy of 10 50 erg, which is consistent with SN 2023zaw.Enhanced production of calcium is predicted for these SN, which is also consistent with the broad Ca emission lines seen in SN 2023zaw, as early as 2 days after peak.We compared the theoretical lightcurves from Moriya & Eldridge (2016) with SN 2023zaw in Figure 6.The predicted peak-bolometric luminosities (∼10 41 erg s −1 ) are ∼10 times fainter than the peak luminosity measured for SN 2023zaw.

Ultrastripped Core-collapse SN from a Low-mass
Progenitor Embedded in a He-rich CSM An ultrastripped SN (Tauris et al. 2015) is a possible channel for the formation of double-NS systems.They arise from a binary star system where a He star undergoes a high degree of stripping by its smaller companion, such that the remaining ejecta mass is <1 M e .The nickel mass is also expected to be quite low (∼0.01 M e ).Due to the low nickel and ejecta mass, the lightcurves are expected to be faint and show fast evolution.The multiband lightcurve and bolometric properties of SN 2023zaw are consistent with an ultrastripped SN scenario (Figures 2, 4, and G1).We compare the measured ejecta and nickel mass for SN 2023zaw with other ultrastripped SN candidates in the literature in Table 1 and Figure 5.The estimated ejecta mass is comparable with other ultrastripped SN candidates, while the nickel mass is a factor of ∼5 lower.We could also reproduce the observed lightcurve by using ultrastripped SN progenitor models evolved with MESA and exploded using SNEC (see Section 3.6.1).

Implication of Narrow He I Lines in the Late-time Spectra
The presence of narrow He I emission lines in the late-time spectra at 30 and 60 days post-peak implies the presence of a Herich dense CSM shell, which is consistent with the predictions of ultrastripped SN models.We note that we do not see prominent narrow He I emission lines in the spectra taken prior to 9 days after peak.It is likely the broad He I lines from the ejecta dominate over the narrow CSM emission lines in the initial days.But if there is actually no CSM interaction until +8 days, we can constrain the CSM shell to be at ∼10 15 cm, which is consistent with the short-period binary models from Wu & Fuller (2022).While the mass-transfer model in Wu & Fuller (2022) assumes that the stellar material lost through the L2 point is distributed spherically, a torus-like distribution of the CSM is also likely (Pejcha et al. 2016;Lu et al. 2023).The viewing angle will strongly influence the observed spectral line widths.In this scenario, the early spectra do not show narrow lines as the CSM is engulfed in the SN ejecta, similar to the model described in Andrews & Smith (2018) for iPTF14hls.Once the photosphere recedes, the CSM is accelerated by the collision of the ejecta to intermediate velocities of ≈1000 km s −1 .In this scenario, the absence of narrow lines in the ALFOSC spectrum taken at +0 day can be explained if the CSM is confined within 10 14 cm, which is consistent with the majority of binary mass-transfer models in Wu & Fuller (2022).We note that the CSM interaction does not show up as an additional bump in the optical lightcurve.This could be explained by a very low-mass, He-rich CSM, which is consistent with the estimated low progenitor and ejecta masses.If CSM interaction does contribute significantly to the luminosity, the nickel mass we estimate should be considered as an upper limit.

Implication of Very Low Nickel Mass
Observations and parameterized 1D models of massive star explosions suggest that typically ∼0.05 M e Ni should be ejected from a core-collapse SN.Anderson (2019) and Meza & Anderson (2020) measured the Ni mass for a sample of Type II and Type Ibc SNe in the literature and found that the Ni mass synthesized in SESNe was significantly higher than for Type II SNe.The lowest Ni mass in their Type Ibc sample was 0.015 M e , and 25% of all Type II SNe has a Ni mass below this limit.However, this might be due to an observational bias as low-Ni-mass SESNe are harder to detect as they are likely to be faint as well as fast evolving.The extremely low amount of Ni mass in SN 2023zaw (∼0.002M e ) could imply a fallback event, where most of the Ni produced in the core-collapse immediately fall into the newly formed BH (Turatto et al. 1998;Balberg et al. 2000;Sollerman 2002).However, the observed high velocities do not favor this scenario.
The other explanation is that the progenitor's initial mass lies in the low-mass end of core-collapse SN progenitors, as stars with low-mass iron cores produce a low amount of Ni.Stockinger et al. (2020) and Sandoval et al. (2021) obtained ∼0.002-0.005M e Ni mass for a 9.6 M e progenitor.More recent 3D simulations (Burrows et al. 2024) estimate 0.002-0.006M e Ni mass for a 9 M e zero-age main-sequence mass progenitor and >0.01 M e for progenitors with initial mass greater than 9.25 M e .We note that these models are not for stripped-envelope SNe.However, if they undergo mass loss after He-core burning, the core evolution of a star is decoupled from the envelope evolution and the nickel mass is expected to be the same.The amount of Ni synthesized in ultrastripped models for low-mass CO stars is 0.01 M e (Moriya et al. 2017;Sawada et al. 2022), which is consistent with our estimate for SN 2023zaw.(Agudo et al. 2023;Shivkumar et al. 2023), and SN 2022agco (Yan et al. 2023).

Conclusion
In this Letter, we have presented the observations and analysis of an unusual stellar explosion-SN 2023zaw.It has the lowest nickel mass among all stripped-envelope SNe discovered so far.SN 2023zaw also shows the fastest evolution among all stripped-envelope SNe, with a time above half maximum of 4.9 days in the r band.The photospheric spectra show broad He I emission lines.It also shows broad Ca NIR emission lines as early as 2 days after peak.
We performed radiation-hydrodynamical and analytical modeling of the lightcurve by fitting with a combination of shock-cooling emission and nickel decay.The early-time lightcurve fits well with shock-cooling emission, with an envelope mass of ≈0.2 M e and an envelope radius of ≈50 R e .The estimated ejecta and nickel masses are ≈0.5 M e and ≈0.002 M e , respectively.
We interpret SN 2023zaw as an ultrastripped SN from a lowmass He star-a compact object binary.The late-time evolution of the He star results in an extended He-rich envelope.There is also spectral evidence for thick He-rich CSM, likely from pre-SN mass-loss episodes from late-time Roche-lobe overflow.The very low nickel mass, ejecta mass, and explosion energy can only be explained by a progenitor with an initial mass less than around 10 M e .
The late-time spectra show prominent narrow He I at a FWHM velocity of ∼1000 km s −1 , indicating the presence of He-rich CSM.It is unclear if CSM interaction also provides a powering mechanism for SN 2023zaw.A number of other models for faint and rapidly evolving transients such as the nickel-free SN, AIC, and ECSN were ruled out based on the peak luminosity and the presence of a late-time tail in the lightcurve of SN 2023zaw.The excess luminosity might be explained if the luminosity is boosted by CSM interaction.In this case, the estimated nickel mass should be treated as an upper limit.Including CSM interaction modeling in the future will help conclusively favor or rule out these scenarios.Also, late-time multiwavelength follow-up will provide useful diagnostics in determining the CSM properties, the pre-SN mass-loss mechanism, and the evolutionary channel (Matsuoka & Maeda 2020;Kashiyama et al. 2022).
Spectroscopic modeling is currently missing for most of the theoretical scenarios for producing faint and rapid transients.Detailed modeling is required to explain some unusual spectroscopic features of SN 2023zaw, such as the presence of strong Ca NIR lines close to peak, redshifted component of He I λ5876 emission line, not reaching nebular phase until 30 days in spite of the low ejecta mass, the emergence of narrow He I emission lines, and the absence of [Ca II], [O I], and Ca NIR emission lines in the nebular phase.
The extremely low nickel mass of SN 2023zaw gives rise to the question-What is the lowest nickel mass that is synthesized in a stripped-envelope SN? Current literature shows that the nickel mass measured in stripped-envelope SNe is significantly higher than in Type II SNe (Anderson 2019).It is likely that this is due to an observational bias, as SESNe with low nickel masses are likely to be faint and fast evolving and hence harder to detect, classify, and follow up.SN 2023zaw underscores the existence of this undiscovered population of low nickel mass SESNe (<0.005 M ☉ ) with progenitor masses in the low-mass end of core-collapse SNe (<10 M ☉ ).A systematic study of the lowest-nickel-mass strippedenvelope SNe, including their rates and the nickel mass distribution of low-luminosity SESNe, will be explored in future work.Candidates for such low-nickel-mass stripped-envelope SNe include double-peaked Type Ibc SNe such as SN 2021inl (Jacobson-Galán et al. 2022;Das et al. 2023b), rapidly evolving (t 1/2,r < 10 days) Type IIb SNe (e.g., Das et al. 2023a;Ho et al. 2023;C. Fremling et al. 2024, in preparation), and rapidly declining H-poor SNe such as SN 2020ghq (Q.Wang et al. 2024 in preparation).For SNe with extremely low Ni mass, the shockcooling emission is likely to dominate over nickel decay.The ultrastripped SNe candidates in the literature show an early luminosity excess that is powered by shock cooling.Future widefield UV surveys such as ULTRASAT (Sagiv et al. 2014;Shvartzvald et al. 2024), UVEX (Kulkarni et al. 2021), and deep ground-based surveys such as LSST (Ivezić et al. 2019) in synergy with high-cadenced surveys such as ZTF will provide an exciting opportunity to explore this phase space.(This table is available in its entirety in machine-readable form in the online article.)

Figure 1 .
Figure 1.Left: Pan-STARRS image of the host galaxy of SN 2023zaw, UGC 03048 in the grizy filters.The location of the transient is indicated by the white cross at the center.Right: SFR map of the environment around SN 2023zaw (see Section 3.7).SN 2023zaw (position marked by the red circle) exploded in a star-forming region of the host galaxy close to regions of more vigorously star-forming regions.The image has a size of 4.1 × 6.7 kpc.The SFR scale is not corrected for attenuation.

Figure 4 .
Figure 4. Top: comparison of the bolometric luminosity, best-fit radius, and temperature evolution of SN 2023zaw from the blackbody fits described in Section 3.5 with ultrastripped SN candidates-SN 2019dge (Yao et al. 2020), iPTF14gqr (De et al. 2018b), SN 2019wxt (Agudo et al. 2023; Shivkumar et al. 2023), and SN 2022agco (Yan et al. 2023).Bottom left: shock-cooling-emission model fit to the early-phase multiband lightcurve as described in Section 3.6.2.Bottom right: best-fits for bolometric luminosity with radiation-hydrodynamics and analytical models as described in Section 3.6.
3 (Wilkinson et al. 2017; Neumann et al. 2022), utilizing the stellar population models from Maraston et al. (2020) and assuming the Kroupa initial mass function (IMF), a spectral resolution of 1000-1250 appropriate for our spectroscopic observation (Morrissey et al. 2018), and a velocity dispersion of 130 ± 20 km s −1 .The best-fit yields an E(B − V ) star of 0.36 mag, comparable to the value at the explosion site.Henceforth, we use E(B − V ) gas = 0.58 ±0.20 mag to characterize the properties of the star-forming region further.The attenuation-corrected Hα flux of 1.2 × 10 −14 erg cm −2 s −1 translates to a star formation rate (SFR) of1.4

Table B1
Emission-line Flux Measurements of the Star-forming Region at the SN Site Note.No measurement is corrected for reddening.
Note.The full machine-readable version will be made available on GitHub (https://github.com/kaustavkdas/SN2023zaw)after publication.The photometry data have been corrected for Milky Way extinction.