arXiv subjects

Anders Jerkstrand

Publications and source records attributed to Anders Jerkstrand.

At least 19 recordsLinked to original sources

arXiv2026

ExTraSS: a Domain Decomposed 3D NLTE Radiative Transfer spectral synthesis code for nebular phase transients

In the nebular phase, supernovae are powered by radioactive decay and continuously fade, while their densities have decreased enough such that the expanding nebula becomes (largely) optically thin and the entire structure contributes to the emission. Models for the nebular phase need to take Non-Local Thermodynamic Equilibrium (NLTE) effects into account, while at the same time radiative transfer effects often cannot be ignored. To account for the asymmetric morphologies of SNe, 3D input ejecta models must be used. In this work, we present the $\texttt{ExTraSS}$ (EXplosive TRAnsient Spectral Simulator) code, which has been upgraded to be fully capable of 3D NLTE radiative transfer calculations in order to generate synthetic spectra for explosive transients in the nebular phase, with a focus on supernovae. We solve a long-standing difficulty of 3D NLTE radiative transfer -- to manage generation and storage of millions of photoexcitation rates over $\gtrsim10^{5}$ of cells -- by developing a new Domain Decomposition algorithm. We describe this new methodology and general code operations in detail, and analyse convergence and accuracy for $\texttt{ExTraSS}$.

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arXiv2026

Recombination Rates for $5f^N$ Uranium Ions Using the Analogous $4f^4$ Nd III as a Guideline and Their Effect on Kilonova Nebular SpectraAn Exploration of Recombination of Uranium with application to Kilonovae Spectra

Dielectronic recombination (DR) is expected to be the dominant recombination process during the non-local thermodynamic equilibrium (non-LTE) phase of kilonovae, yet reliable DR data remain unavailable for most heavy ions. Current spectral models therefore rely on simplified recombination prescriptions, introducing significant uncertainties into predicted spectra. We present an optimization strategy for open f-shell ions using \texttt{AUTOSTRUCTURE}, targeting uranium ions U II--U IV relevant to actinide-producing kilonovae. As a benchmark case, calculations are performed for Nd III to validate the treatment of the f-shell structure and its impact on DR. The resulting DR rate coefficients are of order $10^{-10}$--$10^{-12}$ cm$^{3}$\,s$^{-1}$ over temperatures relevant to kilonova plasmas. The optimized rates are implemented into the radiative-transfer code \texttt{SUMO}, and their impact on kilonova spectra studied. The Nd III benchmark demonstrates that refinements to the atomic structure can produce measurable changes in spectral features supporting the atomic structure optimisation performed for U II--IV.

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arXiv2026

Spectral synthesis techniques for supernovae and kilonovae

Supernovae (SNe) and kilonovae (KNe) are the most violent explosions in cosmos, signalling the destruction of a massive star (core-collapse SN), a white dwarf (thermonuclear SN) and a neutron star (KN), respectively. The ejected debris in these explosions is believed to be the main cosmic source of most elements in the periodic table. However, decoding the spectra of these transients is a challenging task requiring sophisticated spectral synthesis modelling. Here, the techniques for such modelling is reviewed, with particular focus on the computational aspects. We build from a historical review of how methodologies evolved from modelling of stellar winds, to supernovae, to kilonovae, studying various approximations in use for the central physical processes. Similarities and differences in the numeric schemes employed by current codes are discussed, and the path towards improved models is laid out.

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arXiv2026

NLTE Spectral Modelling of the Nearby Stripped-Envelope Supernova 2024ehs

We present a detailed study of the Type IIb supernova 2024ehs, discovered in March 2024 in the nearby galaxy NGC 3443 at a distance of $23.8\pm 0.9$ Mpc. Using photometric and spectroscopic observations spanning 10 months, we analyse its light curve, spectral evolution, and physical properties with the \texttt{SUMO} radiative transfer code. SN 2024ehs exhibits a narrow light-curve peak, rapid decline, and weak helium lines, distinguishing it from typical Type IIb supernovae. Comparisons with other objects, including SNe 1993J and 2020acat, and modelling of nebular spectra suggest a low ejecta mass, high velocities ($\sim$20,000 \kms), and a $^{56}$Ni mass just below $\sim0.1 \, \mathrm{M}_{\odot}$. Furthermore, the nebular spectral models indicate a progenitor with a helium core mass of $\sim 6 \, \mathrm{M}_{\odot}$, consistent with a zero-age main-sequence mass of $\sim 23\, \mathrm{M}_{\odot}$. The composition of spectra is explored through photospheric modelling, finding a link between expansion velocity and the relative strength of different element lines. This work discusses further the diversity of stripped-envelope supernovae and the role of binary interactions for their progenitors, and demonstrates the need for further modelling to refine $^{56}$Ni mass estimates and to understand the physical mechanisms driving their evolution.

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arXiv2026

SN 2023rve: A Type II Supernova with No Nebular Oxygen

We report on multiband photometric and spectroscopic observations of SN 2023rve, a nearby Type II supernova (SN II) discovered in galaxy NGC 1097 (D=$15.4 \pm 3.2$ Mpc). Nearby SNe II provide constraints on late-stage evolution and progenitor mass loss, particularly the role of circumstellar material (CSM) in shaping SN II observables. SN 2023rve peaks with an absolute V-band magnitude of -17.1 and declines at a rate of $0.90 \pm 0.02$ mag/50 days during the plateau. The bolometric light curve implies a $^{56}$Ni mass of 0.0064 $M_\odot$. Using hydrodynamic light-curve modeling, we infer an intermediate-mass progenitor (~14-18 $M_\odot$), a low explosion energy of 0.27 $\times 10^{51}$ ergs, and a dense CSM component with radial extent of 2900 $R_\odot$ and density of $10^{18}$g cm$^{-1}$. This supports growing evidence that enhanced pre-SN mass loss influences the diversity of SNe II. The nebular spectra of SN 2023rve show narrow He I lines and an absence of [O I] lines unprecedented among Type II SNe. Comparison with other SNe II shows that only two other known objects, both with higher velocities, lack oxygen signatures at similar epochs, <10% of the sample. The lack of oxygen emission combined with low explosion energy, a long plateau, and a small synthesized nickel mass may be consistent with partial fallback of material onto the compact remnant. We also discuss alternative explanations for the suppressed oxygen emission, including dust formation, oxygen-calcium mixing, and ongoing CSM interaction.

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arXiv2026

JWST observations of SN 2024abup: First Detection of CO in a broad-lined Type Ic Supernova and Constraints on r-process Nucleosynthesis

SN 2024abup is a nearby broad-lined Type Ic supernova (SN Ic-bl) in NGC 0681 at a distance of 23.3 \pm 1.6 Mpc. As energetic explosions of massive stars, SNe Ic-bl are considered a plausible site for rapid-neutron capture nucleosynthesis (r-process) and chemical enrichment from short-lived progenitors. They may also contribute to dust production in the early Universe. We present JWST near- to mid-infrared (NIR+MIR) observations (1-14 micron) of SN Ic-bl 2024abup at +41 days after the V band maximum (+54 days after explosion), the first-ever JWST+MIR observation of a SN Ic-bl along with radio and optical data. Using the spectral synthesis code SUMO, we identify the observed broad IR line features in SN 2024abup and find significant contributions from C, O, Mg, and carbon monoxide (CO) -- the earliest detection of molecules in a core-collapse SN so far. The spectrum shows continuum emission at wavelengths greater than 1.5 micron, which could be explained by dust -- preexisting, newly formed, or a combination-heated by the SN. We do not find compelling evidence for infrared signatures of r-process elements, though our search is hampered by the presence of many broad and blended features from the non-r-process elements. These new observations indicate that SNe Ic-bl could be a contributor to early-universe dust production, and suggest that if r-process elements are produced, revealing their presence from spectra requires very high-quality data and models to disentangle blends.

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arXiv2026

Low-Luminosity Type IIP Supernovae from the Zwicky Transient Facility Census of the Local Universe. III: Hunting for electron-capture supernovae using nebular spectroscopy

Electron-capture supernovae (ECSNe) may arise from ONeMg-core collapse in super-asymptotic giant branch (sAGB) stars near the low-mass core-collapse limit ($\approx\!8$--$10$\,\Msun). At early times, models predict that ECSNe resemble low-mass red supergiant iron-core-collapse SNe (FeCCSNe), making the two channels difficult to distinguish. Nebular spectroscopy, however, can reveal differences in ejecta composition. We present a systematic sample of nebular spectra of 19 low-luminosity Type IIP (LLIIP) SNe from the ZTF CLU survey, obtained 115$-$450\,d after explosion. Their low velocities expose narrow lines blended in brighter SNe, which we identify and model to constrain progenitor properties. We find a strong correlation between the FWHM of H\,\textsc{i}\,$λ$6563 and peak luminosity, showing that LLIIP SNe occupy the low-energy end of the core-collapse population, but no correlation with plateau duration, suggesting that envelope and core properties are not tightly linked. Only one SN reaches the extremely low H\,\textsc{i}\,$λ$6563 widths predicted for the weakest $\sim$9\,M$_\odot$ explosion models, implying that such low-energy events are intrinsically rare. Combining our sample with 118 literature nebular spectra of Type II SNe, we infer an IMF slope of $2.1\pm1.2$. We also introduce an `ECSN score'' based on the absence of He- and O-shell emission lines, and identify two plausible ECSN candidates, SN~2023bvj and SN~2024btj. However, neither shows the extremely narrow nebular lines predicted by current ECSN models. If ECSNe arise predominantly through the LLIIP channel, we infer an upper limit on the ECSN rate of $\lesssim (5$--$8)\times10^{2}\,\mathrm{Gpc^{-3}\,yr^{-1}}$, corresponding to a narrow sAGB progenitor mass window of $ΔM_{\rm sAGB} \lesssim 0.02$--$0.06\,\mathrm{M_\odot}$.

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arXiv2026

Emission line models for the lowest mass core-collapse supernovae -- II. 3D NLTE radiative transfer modelling of a $9.0\,M_\odot$ neutrino-driven explosion

The nebular phase of a supernova (SN) occurs several months to years after the explosion, with asymmetries created by the explosion encoded into the line profiles of the emission lines. To make accurate predictions for these line profiles, Non-Local Thermodynamic Equilibrium (NLTE) radiative transfer calculations need to be carried out. In this work, we use $\texttt{ExTraSS}$ (EXplosive TRAnsient Spectral Simulator) -- which was recently upgraded into a full 3D NLTE radiative transfer code (including photoionization and line-by-line transfer effects) -- to perform such calculations. $\texttt{ExTraSS}$ is applied to a 3D explosion model of a $9.0\,M_\odot$ H-rich progenitor, evolved into the homologous phase. Synthetic spectra are computed and lines from different elements are studied for varying viewing angles. Line profile properties strongly correlate with a primary Ni plume in the ejecta. The model spectra are compared against observations of SN 1997D and SN 2016bkv. The model can create good line profile matches for both SNe, and reasonable luminosity matches for He, C, O, and Mg lines for SN 1997D -- however H$α$ and Fe I lines are too strong. Key diagnostic lines of low-mass core-collapse SNe (CCSNe), e.g. differentiating Fe CCSNe from electron capture SNe, are upheld from 1D to 3D. However, both line profiles and line luminosities differ in 3D across viewing angles, enabling the possibility of detailed comparisons to observed spectra to infer asymmetries imprinted by the explosion. We show that even the fastest $^{56}$Ni is traceable in nebular phase line profiles.

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arXiv2026

JWST Nebular Spectroscopy of SN 2023qov: Circumstellar Dust Emission in a Normal Type Ia Supernova

We present panchromatic observations of the Type Ia supernova (SN Ia) 2023qov, ranging from $\sim$2 weeks before to $\sim$1 year after maximum light. \textit{JWST} near- and mid-infrared spectra at $+$276 and $+$363~days show $\sim$400 K dust emission that cools by $\sim$75 K between epochs, the first unambiguous spectroscopic detection of dust emission in a normal SN Ia. We find that the emission is well described by models of carbonaceous dust placed within $\sim$1 light year of the SN, with a dust mass of $\sim$$10^{-4}$ M$_{\odot}$. We do not see evidence of active dust creation, suggesting an infrared light echo by pre-existing circumstellar dust as the likely source of the emission. The \textit{JWST} nebular line profiles suggest asymmetric, stratified ejecta, similar to other normal SNe Ia, though a slight double-horn structure in the argon lines indicate a toroidal enhancement. SN 2023qov exhibits a slightly red, fast-declining early light curve ($Δm_{15}(B) = 1.47 \pm 0.05$ mag), from which we determine a $^{56}$Ni mass of $M_{56} = 0.21 \pm 0.04$ M$_{\odot}$, and a distance of $d = 36.0 \pm 1.8$ Mpc to the SN and its host, NGC 7029.

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arXiv2026

The Double-Peaked Calcium-Strong SN 2025coe: Progenitor Constraints from Early Interaction and Ejecta Asymmetries

Supernova (SN) 2025coe at a distance of $\sim$25 Mpc is the second-closest calcium-strong (CaST) transient. It was discovered at a large projected offset of $\sim$34 kpc from its potential host galaxy NGC 3277. Multiband photometry of SN 2025coe indicates the presence of two peaks at day $\sim$2 and day $\sim$11 after explosion. Modeling the bolometric light curve, we find that the first peak can be reproduced either by shock cooling of a compact envelope ($R_\mathrm{env}$ $\approx $6-40 $R_{\odot}$; $M_\mathrm{env}$ $\approx $0.1-0.2 $M_{\odot}$) or by interaction with close-in circumstellar material (CSM; $R_{\mathrm{CSM}} \lesssim 6 \times10^{14}$ cm), or a combination of both. The second peak is dominated by radioactive decay of $^{56}$Ni ($M_{\mathrm{ej}} \approx $0.4-0.5 $M_{\odot}$; $M_{^{56}\mathrm{Ni}} \approx 1.4 \times 10^{-2}$ $M_{\odot}$). SN 2025coe rapidly evolves from the photospheric phase dominated by He I P-Cygni profiles to nebular phase spectra dominated by strong [Ca II] $λλ$7291, 7323 and weak [O I] $λλ$6300, 6364 emission lines. Simultaneous line profile modeling of [Ca II] and [O I] at nebular phases shows that an asymmetric core-collapse explosion of a low-mass ($\lesssim$3.3 $M_{\odot}$) He-core progenitor can explain the observed line profiles. Alternatively, lack of local star formation at the site of the SN explosion combined with a low ejecta mass is also consistent with a thermonuclear explosion due to a low-mass hybrid He-C/O white dwarf + C/O white dwarf merger.

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arXiv2026

Lanthanide Impact on the Infra-Red Spectra of Nebular Phase Kilonovae

Nebular phase kilonovae (KNe) have significant infra-red (IR) emission thought to be mostly forbidden emission lines from rapid neutron capture (r-process) species in neutron star merger ejecta. Lanthanide elements in particular have complex, open f-shell atomic structures with many IR transitions. Using non-local thermodynamic equilibrium (NLTE) radiative transfer simulations, we explore the impact of lanthanides on the IR spectra of KNe in the nebular phase, exploring a parameter space of ejecta mass and lanthanide fraction. We find that lanthanide impact is greater at higher densities, corresponding to earlier epochs and greater ejecta masses. The wavelengths most affected are found to be $λ\lesssim 4~μ$m, with the species Ce\,\textsc{iii} and Nd \textsc{ii} being the most important contributors to spectral formation. We also find significant emission from species proposed in observations, notably Te\,\textsc{iii} at 2.1 $μ$m, and Se\,\textsc{iii} at 4.5 and 5.7 $μ$m, while W\,\textsc{iii} is subdominant at 4.5 $μ$m. The Te\,\textsc{iii} feature at 2.1 $μ$m is always blended, particularly with Zr\,\textsc{ii}, Ce\,\textsc{iii}, and Nd\,\textsc{ii}. We do not reproduce the smooth blackbody-like continua observed in AT2023vfi. Based on our results, we argue that line opacity alone is likely insufficient to produce optically thick continua in the nebular phase, even in the case of lanthanide/actinide-rich ejecta, as our models are optically thin in the IR at these epochs. Given that lanthanide contributions are dominant below 4 $μ$m, we suggest that NIR observations best probe these elements, while MIR spectroscopy with \textit{JWST} can reliably probe non-lanthanide emission even in relatively lanthanide-rich cases.

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arXiv2025

Infrared spectral signatures of light r-process elements in kilonovae

A central question regarding neutron star mergers is whether they are able to produce all the r-process elements, from first to third peak. The high abundances of first-peak elements (atomic number $Z \sim 31-40$) in the solar composition means they may dominate the ejecta mass in kilonovae. We here study theoretical infrared signatures of such light elements with spectral synthesis modelling. By combining state-of-the-art NLTE physics with new radiative and collisional data for these elements, we identify several promising diagnostic lines from Ge, As, Se, Br, Kr and Zr. The models give self-consistent line luminosities and indicate specific features that probe emission volumes at early phases ($\sim$10d), the product of ion mass and electron density in late phases ($\gtrsim$75d), and in some cases direct ionic masses at intermediate phases. Emission by [Se I] 5.03 \mum\ + [Se III] 4.55 \mum\ can produce satisfactory fits to the Spitzer photometry of AT2017gfo. However, the models show consistently that with a Kr/Te and Se/Te ratio following the solar r-process pattern, Kr + Se emission is dominant over Te for the blend at 2.1 \mum\ observed in both AT2017gfo and AT2023vfi. The somewhat better line profile fit with [Te III] may suggest that both AT2017gfo and AT2023vfi had a strongly sub-solar production of the light r-process elements. An alternative scenario could be that Kr + Se in an asymmetric morphological distribution generates the feature. Further JWST spectral data, in particular covering the so far unobserved $>5$ \mum\ region, holds promise to determine the light r-process production of kilonovae, and in particular whether the light elements are made in a slow disk wind or in a fast proto-NS outflow. We identify specific needs for further atomic data on recombination rates and collision strengths for $Z=31-40$ elements.

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arXiv2025

Formation and Diagnostic Use of Carbon Lines in Stripped-Envelope Supernovae

Carbon is one of the main end products of nucleosynthesis in massive stars. In this work, we study the emission signatures of carbon in spectra of stripped envelope supernovae (SESNe). A grid of model nebular spectra is created using the NLTE radiative transfer code SUMO, with stellar evolution- and explosion models as inputs. In the models, [C I] λ8727 and [C I] λλ9824, 9850 are identified as the only significant optical carbon lines, with contribution from both the O/C and He/C zones. To obtain estimates of L_[C I] λ8727, which is blended with the Ca II triplet, we introduce and apply the CaNARY code, a publicly available Monte Carlo scattering code. We study carbon lines in a sample of SESNe, and find that luminosities of [C I] λλ9824, 9850 relative to the optical spectrum increase with time, just as in our model grid. However, the relative luminosities of both [C I] λλ9824, 9850 and [C I] λ8727 are overproduced in our models. Multiple explanations for this discrepancy, such as too high carbon abundances in the stellar evolution models and underestimated cooling through molecule formation, are investigated. For those SNe where both lines are clearly observed, we use an analytical formalism to constrain their ejected carbon masses to the range ~0.2 - 2 Msun. However, several SNe yield upper limits of 0.05 Msun. We also show that [C I] λλ9824, 9850 is a useful line to diagnose both carbon mass and the extent of the He/C zone. We strongly encourage observers and instrumentalists to target [C I] λλ9824, 9850 in future SN observing campaigns.

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arXiv2025

SN 2021aaev: a Hydrogen-Rich Superluminous Supernova with Early Flash and Long-Lived Circumstellar Interaction in an Unusual Host Environment

We present photometric and spectroscopic observations of SN\,2021aaev, a hydrogen-rich, superluminous supernova with persistent (at least $\sim100$ days) narrow Balmer lines (SLSN-IIn) at redshift $z=0.1557$. We observed SN\,2021aaev to rise in $32.5 \pm 1.0$ days since first light and reach a peak absolute magnitude of $-21.46 \pm 0.01$ in the ATLAS $o$ band. The pre-peak spectra resemble those of typical SNe IIn with flash-ionization features arising from the interaction with a dense, confined circumstellar medium (CSM), albeit the flash timescale is longer than usual ($>20$ days). Post peak, the narrow emission lines evolve slowly, and the absence of ejecta features indicates strong deceleration by the CSM. The total radiated energy (about $1.41\times10^{51}$~ergs) is possible with a low-mass (1--$2\,M_{\odot}$) ejecta ploughing into a massive (9--$19\,M_{\odot}$), extended (outer radius $>1\times10^{16}$~cm) H-rich CSM, or alternatively, with magnetar-powered models. Interestingly, the host environment consists of a spiral galaxy with a red substructure in the south-eastern part, and the SN's exact location coincided with the quiescent red substructure (star-formation rate$=0.02^{+0.13}_{-0.02}\,M_{\odot}$~yr$^{-1}$). Given the atypical environment and the obscuring effect of the massive CSM, a thermonuclear (Type Ia-CSM) origin cannot be ruled out. Altogether, SN\,2021aaev is a compelling case to study the diversity of SLSN-IIn features and their host environment.

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arXiv2025

Nebular spectra of kilonovae with detailed recombination rates -- I. Light r-process composition

To investigate spectra of kilonovae in the NLTE phase (t>= 1 week), we perform atomic calculations for dielectronic recombination (DR) rates for the light r-process elements Se (Z = 34), Rb (Z = 37), Sr (Z = 38), Y (Z = 39), and Zr (Z = 40) using the HULLAC code. For the different elements, our results for the DR rate coefficients for recombining from the ionization states of II to I, III to II, and IV to III vary between 2x10^{-12} - 5x10^{-11} cm^3/s, 10^{-13} - 5x10^{-11} cm^3/s and 2x10^{-15} - 10^{-11} cm^3/s, respectively, at a temperature of T = 10,000 K. Using this new atomic data (DR), we study the impact on kilonova model spectra at phases of t = 10 days and t = 25 days after the merger using the spectral synthesis code SUMO. Compared to models using the previous treatment of recombination as a constant rate, the new models show significant changes in ionization and temperature, and correspondingly, in emergent spectra. With the new rates, we find that Zr (Z = 40) plays a yet more dominant role in kilonova spectra for light r-process compositions. Further, we show that previously predicted mid-infrared (e.g. [Se III] 4.55 mum) and optical (e.g. Rb I 7802, 7949 Å) lines weaken in the new model. Instead [Se I] 5.03 mum emerges as a signature. These results demonstrate the importance of considering the detailed microphysics for modelling and interpreting the late-time kilonova spectra.

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arXiv2025

Core-collapse supernovae

Core-collapse supernovae (CCSNe) are the explosive end-points of stellar evolution for $M_{ZAMS} \gtrsim 8$ $M_\odot$ stars. The cores of these stars collapse to neutron stars, a process in which high neutrino luminosity drives off the overlying stellar layers, which get ejected with thousands of kilometers per second. These supernovae enrich their host galaxies with elements made both during the star's life and in the explosion, providing the main cosmic source of elements such as oxygen, neon and silicon. Their high luminosities ($\sim$ $10^{42}$ erg s$^{-1}$ at peak) make SNe beacons to large distances, and their light curves and spectra provide rich information on single and binary stellar evolution, nucleosynthesis, and a diverse set of high-energy physical processes. As the SN ejecta sweep up circumstellar and interstellar matter, it eventually enters a supernova remnant phase, exemplified by nearby, spatially resolved remnants such as Cas A and the Crab Nebula. In this phase, shocks and pulsar winds continue to light up the interior of the exploded stars, giving detailed information about their 3D structure. We review the central concepts of CCSNe, from the late stages of evolution of massive stars, through collapse, explosion, and electromagnetic display, to the final remnant phase. We briefly discuss still open questions, and current and future research avenues.

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arXiv2025

Actinide signatures in low electron fraction kilonova ejecta

Neutron star (NS) mergers are known to produce heavy elements through rapid neutron capture (r-process) nucleosynthesis. Actinides are expected to be created solely by the r-process in the most neutron rich environments. Confirming if NS mergers provide the requisite conditions for actinide creation is therefore central to determining their origin in the Universe. Actinide signatures in kilonova (KN) spectra may yield an answer, provided adequate models are available in order to interpret observational data. In this study, we investigate actinide signatures in neutron rich merger ejecta. We use three ejecta models with different compositions and radioactive power, generated by nucleosynthesis calculations using the same initial electron fraction ($Y_e = 0.15$) but with different nuclear physics inputs and thermodynamic expansion history. These are evolved from 10 - 100 days after merger using the SUMO non-local thermodynamic equilibrium (NLTE) radiative transfer code. We highlight how uncertainties in nuclear properties, as well as choices in thermodynamic trajectory, may yield entirely different outputs for equal values of $Y_e$. We consider an actinide-free model and two actinide-rich models, and find that the emergent spectra and lightcurve evolution are significantly different depending on the amount of actinides present, and the overall decay properties of the models. We also present potential key actinide spectral signatures, of which doubly ionized $_{89}$Ac and $_{90}$Th may be particularly interesting as spectral indicators of actinide presence in KN ejecta.

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arXiv2024

Luminous Type II Short-Plateau SN 2023ufx: Asymmetric Explosion of a Partially-Stripped Massive Progenitor

We present supernova (SN) 2023ufx, a unique Type IIP SN with the shortest known plateau duration ($t_\mathrm{PT}$ $\sim$47 days), a luminous V-band peak ($M_{V}$ = $-$18.42 $\pm$ 0.08 mag), and a rapid early decline rate ($s1$ = 3.47 $\pm$ 0.09 mag (50 days)$^{-1}$). By comparing observed photometry to a hydrodynamic MESA+STELLA model grid, we constrain the progenitor to be a massive red supergiant with M$_\mathrm{ZAMS}$ $\simeq$19 - 25 M$_{\odot}$. Independent comparisons with nebular spectral models also suggest an initial He-core mass of $\sim$6 M$_{\odot}$, and thus a massive progenitor. For a Type IIP, SN 2023ufx produced an unusually high amount of nickel ($^{56}$Ni) $\sim$0.14 $\pm$ 0.02 M$_{\odot}$, during the explosion. We find that the short plateau duration in SN 2023ufx can be explained with the presence of a small hydrogen envelope (M$_\mathrm{H_\mathrm{env}}$ $\simeq$1.2 M$_{\odot}$), suggesting partial stripping of the progenitor. About $\simeq$0.09 M$_{\odot}$ of CSM through mass loss from late-time stellar evolution of the progenitor is needed to fit the early time ($\lesssim$10 days) pseudo-bolometric light curve. Nebular line diagnostics of broad and multi-peak components of [O I] $λλ$6300, 6364, H$α$, and [Ca II] $λλ$7291, 7323 suggest that the explosion of SN 2023ufx could be inherently asymmetric, preferentially ejecting material along our line-of-sight.

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