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Abstract We investigate thermonuclear explosions within the delayed detonation framework. While spherical delayed detonation models generally reproduce key observational features, a fundamental inconsistency emerges in three dimensions: 3D hydrodynamic simulations exhibit insufficient white dwarf expansion during the deflagration phase. We identify the early deflagration stage, when the burning is dominated by the laminar speed, as a critical phase and explore potential solutions using 3D magnetohydrodynamic simulations performed with theFLASHcode. In absence of preexisting small-scale velocity fields, hydrodynamical simulations of the early deflagration phase produce large pockets of unburned C/O, leading to inefficient burning. Much of the released energy is deposited into buoyantly rising plumes rather than into the global preexpansion of the white dwarf, which is required to produce the partially burned layers characteristic of SNe Ia. In contrast, when preexisting turbulent velocity fields on scales expected from the smoldering phase are included, the entrainment of burned material into unburned pockets enables the conductive ignition of the surrounding unburned fuel. The effective burning approaches that in spherical models, addressing a long-standing problem in multidimensional deflagration models. For magnetic fields considered here, ≲1% of the saturation strength, we find that the effective burning rate is dominated by the turbulence. Magnetic fields only marginally suppress the rising of burned plumes and the formation of small structures, leading to a slightly more confined burning region and a reduced burning rate. 

Abstract We present a new public-domain Molecular Fitting Analysis Tool (MOFAT) designed to probe molecule-forming regions in supernovae through analysis of molecular features in the near- and mid-infrared. MOFAT employs a novel data-driven approach to explore the physical properties of these regions using time-independent radiative transfer simulations that include multidimensional, clump-like structures, constrained by high-precision observations. Such structures are required to reproduce the flux ratio between fundamental and overtone bands, overcoming limitations of traditional one-zone forward modeling, such as optical depth effects and initial configurations. Our approach enables spectral fits that can reconstruct overall abundances and temperatures and determine parameterized small-scale structures associated with physical instabilities. We systematically study the relationship between physical parameters and the profiles of CO and SiO, showing that free parameters are constrained, while detection of small-scale structure requires optically thick bands. As a demonstration, MOFAT is applied to SN 2024ggi at +285 and +385 days postexplosion. We find that CO formation triggers SiO formation in the inner layers of the CO-rich region previously studied. The inner edge of the SiO-emitting region recedes with velocities ofv₁ ≈ 1500–1000 km s⁻¹, indicating continued SiO formation. The SiO mass decreases from ∼(2–6) × 10⁻³M_⊙by roughly an order of magnitude, suggesting ongoing evaporation. SiO features indicate clumping, but most of the flux originates from optically thin regions. SiO contributes negligibly to cooling, and we find no evidence for dust formation. Finally, we discuss observational strategies to trace the evolution of molecule formation and its connection to dust formation. 

Abstract We present the first data release of the Hawaii Infrared Supernova Study (HISS), consisting of a large sample of near-infrared (NIR) spectra, 0.7–2.5μm, obtained with the Keck-II/NIRES and IRTF/SpeX spectrographs. This sample is comprised of 90 NIR spectra of 48 transients, spanning from hours after explosion to ≥+350 days. Acquired over 3 yr (2021–2024), this data release includes 17 Type Ia supernovae (SNe), 15 Type II SNe, eight stripped envelope SNe, six interacting SNe, one tidal disruption event, and one SLSN-I. These spectra were all systematically reduced using either the Python-based reduction codePypeitor theIDL-basedSpextooland constitute one of the largest NIR samples of transients available to the astrophysical community. We show the utility of NIR spectra and identify the key spectral features across multiple types of SNe. We demonstrate how both early-time and nebular-phase NIR spectra of SNe can be used to investigate the physics of the explosion, and to reveal the properties of the progenitor. With the addition of this data set, the number of publicly available NIR spectra spanning multiple transient types has been substantially increased. In its next phase, HISS will leverage target-of-opportunity spectral observations and NIR imaging from telescopes on Maunakea. Expanding the NIR data set of SNe is vital to the transient community, particularly in light of the increasing emphasis on the infrared regime following the recent launch of the James Webb Space Telescope and the forthcoming launch of the Nancy Grace Roman Space Telescope. 

We present infant-phase observations of the broad-line Type Ic supernova (SN Ic-BL) 2020lao, including optical spectroscopy beginning within about 48 hours of the inferred explosion epoch and extending to nearly 100 days. The explosion time was constrained by power-law fits to the rising TESS and ZTF light curves, with the first ZTF detection occurring only ∼27 hours after explosion. The optical light curves show a rapid rise that lasted for ≈8.8 days and a peak luminosity typical of SNe Ic-BL (i.e.,M_r ≃ −18.5 mag). Unlike some engine-driven SN Ic-BL events, the early light curve of SN 2020lao shows no evidence of an optical afterglow or excess emission, and the absence of any detectable shock–cooling component in the TESS and ZTF data constrains the progenitor to a compact Wolf-Rayet-like star whoseR_★is less than or equal to a few times theR_⊙, ruling out any extended envelope. The spectra resemble those of the X-ray-flash-associated SN 2006aj but with systematically higher expansion velocities. From Arnett-type fits to the bolometric light curve and measured Fe IIλ5169 line velocities, we infer a⁵⁶Ni mass of 0.23 ± 0.03 M_⊙, an ejecta mass (M_ej) of 3.2 ± 0.8 M_⊙, and a kinetic energy (E_K) of ∼(23.1 ± 12.4)×10⁵¹erg, corresponding to a specific kinetic energy (E_K/M_ej) of ≈(7.2 ± 3.5)×10⁵¹ergM_⊙⁻¹. Spectral synthesis modeling broadly reproduces the photospheric-phase spectra of SN 2020lao and suggestsE_K/M_ej ≈ 4.9 × 10⁵¹ergM_⊙⁻¹. SN 2020lao and SN 2006aj synthesized comparable amounts of⁵⁶Ni, yet SN 2020lao exhibitsE_K/M_ejvalues on the order of 5–10 times larger. Published VLA andSwift/XRT non-detections reveal no afterglow emission, allowing us to place stringent limits on relativistic ejecta and dense circumstellar material. Given that SN 2020lao reaches a specific kinetic energy typical of engine-driven SNe Ic-BL, the lack of an early optical excess together with the non-detections in the radio and X-ray bands suggests that if a relativistic jet was launched, the explosion must have been viewed far off axis or the jet was choked before breakout. If there was no relativistic jet, SN 2020lao would therefore be an extreme nonrelativistic SN Ic-BL. This underscores the importance of continued infant-phase, multiwavelength monitoring of these explosions. 

Abstract We present James Webb Space Telescope (JWST) Near-Infrared Spectrograph observations of SN 2024ggi, spanning wavelengths of 1.7–5.5μm at +285.51 and +385.27 days postexplosion. These nebular spectra are dominated by asymmetric emission lines from atomic species including H, Ca, Ar, C, Mg, Ni, Co, and Fe, indicative of an aspherical explosion. The other strong features are molecular CO vibrational bands from the fundamental and first overtone. We introduce a novel, data-driven approach using non–local thermodynamic equilibrium three-dimensional (3D) radiative transfer simulations to model the CO emission with high fidelity. This method enables us to constrain the 3D CO distribution and its radial temperature structure. CO formation is found to occur prior to day +285, with subsequent evolution characterized by progressive evaporation. The CO mass decreases from approximately 8.7 to 1.3 ×10⁻³M_⊙, while the average temperature drops from ≈2900 to ≈2500 K. Concurrently, the CO distribution transitions from nearly homogeneous to highly clumped (density contrast increasing fromfc≈ 1.2 to 2). The minimum velocity of the CO-emitting region remains nearly constant (v₁≈ 1200 to 1100 km s⁻¹), significantly above the receding photosphere velocity (v_ph≈ 500 km s⁻¹), suggesting the photosphere resides within Si-rich layers. However, the temperature profile indicates that only a narrow zone reaches the conditions necessary for SiO formation. Due to a lack of observational constraints, SiO clumping is not modeled, and thus, synthetic SiO profiles for mass estimates are not highlighted. We discuss the implications of these findings for dust formation processes in SN 2024ggi. 

Context. Type Ibn supernovae (SNe) are characterized by narrow helium (He I) lines from photons produced by the unshocked circumstellar material (CSM). About 80 SNe Ibn have been discovered to date and only a handful of them have extensive observational records. Thus, many open questions remain regarding the progenitor system and the origin of CSM. Aims. Here, we investigate potential correlations between the spectral features of the prominent He Iλ5876 Å line and the optical and X-ray light curve properties of Type Ibn SNe (SNe Ibn). Methods. We compiled the largest sample of 61 SNe Ibn to date, of which 24 SNe have photometric and spectroscopic data available from the Young Supernova Experiment and 37 SNe benefit from archival datasets. We fit 24 SNe Ibn with sufficient photometric coverage (Btozbands) using semi-analytical models fromMOSFiT. Results. We demonstrate that the light curves of SNe Ibn are more diverse than previous analyses suggest, with absoluter-band peak magnitudes (r_max) of −19.4 ± 0.6 mag, along with rise (from −10 days to peak,γ₋₁₀) and decay rates (from peak to +10 days;γ₊₁₀) of −0.08 ± 0.06 and 0.08 ± 0.03 mag/day, respectively. We find that the majority of SNe Ibn in the subsample are consistent with a low-energy explosion (< 10⁵¹erg) of a star with a compact envelope surrounded by ~0.1 M_⊙of helium-rich CSM. The inferred ejecta masses are small (M_ej~ 1 M_⊙) and expand with a velocity of ~5000 km/s. Our spectroscopic analysis shows that the mean velocity of the narrow component of the He Ilines, associated with the CSM, peaks at ~1100 km/s. Conclusions. The mean CSM and ejecta masses inferred for a subsample of SNe Ibn indicate that their progenitors are not massive (~10 M_⊙) single stars at the moment of explosion; rather, they are likely to be binary systems. This finding is in agreement with detections of potential companion stars of SNe Ibn progenitors and inferred CSM properties from stellar evolution models. 

We report observations of a Type Ia supernova (SN Ia) 2021hem that was discovered within 48 hours of last nondetection and is located in an apparently hostless environment. With a peak absoluteB-band magnitude ofM_B, max = −19.96 ± 0.29 mag, SN 2021hem lies at the luminous end of the SNe Ia distribution. Its near-infrared andi-band light curves lack the secondary maximum, which is otherwise ubiquitous to normal and 1991T-like SNe Ia. Instead, these properties cause SN 2021hem to closely resemble 2003fg-like events. The slowly evolving light curves (characterized by Δm₁₅(B) = 1.02 ± 0.02 mag;s_BV = 0.94 ± 0.05) and the earliest spectrum showing C IIλ6580 andλ7235 absorption lines further support this classification. Other spectroscopic features, including Si IIline diagnostics, resemble those of normal SNe Ia. A fit of a fireball model to the early-time light curves yields a time of first light oft_first= −16.43^+0.45_−0.38days relative toB-band maximum. The first photometric detection occurs 1.51^+0.45_−0.38days before the onset of fireball-like flux rise. This early emission, together with the intrinsic (g − r)₀color, is inconsistent with circumstellar or companion interaction. Instead, shallow⁵⁶Ni mixing or an asymmetric⁵⁶Ni distribution offers a plausible explanation for the delayed onset of the fireball flux rise, while a double-detonation scenario with a thin helium shell remains a less likely alternative. Notably, SN 2021hem represents the fifth known 2003fg-like SN that has early-time activity or excess flux emission. The estimated mass of radioactive⁵⁶Ni synthesized in SN 2021hem is 1.00 ± 0.09 M_⊙. Deep GTC imaging obtained 2.5 years after the explosion, with an estimated limiting magnitude ofm_lim, r = 24.4 mag and a surface-brightness limit ofμ_lim, r = 26.3 mag arcsec⁻², revealed no coincident host. Most faint dwarf and ultradiffuse galaxies (UDGs) are therefore ruled out. Alternatively, if the nearest plausible AGN host galaxy located at a projected distance of 104 kpc is assumed, the progenitor would need to be a hypervelocity star ejected at ≈2200 km s⁻¹from the host by AGN interaction. A faint diffuse feature ≈6 kpc from the SN site has also been detected in the GTC image, and its surface brightness is within the limits of UDGs. It is unclear whether it is a galaxy and is associated with SN 2021hem, however. Based on its large normalized directional light distance (d_DLR ≈ 3 − 4) from the SN and its unusual elongation, the probability that this is the candidate host galaxy of SN 2021hem is low. These results identify SN 2021hem as one of the strongest candidates for a hostless SN Ia and underscore the diversity of luminous slowly evolving 2003fg-like explosions and the wide range of environments in which they may occur. 

We present optical photometric and spectroscopic observations of the peculiar Type Ia supernovae (SNe Ia) ASASSN-20jq/SN 2020qxp. It is a low-luminosity object, with a peak absolute magnitude ofM_B = −17.1 ± 0.5 mag, while its post-peak light-curve decline rate of Δm₁₅(B) = 1.35 ± 0.09 mag and color-stretch parameter ofs_BV ⪆ 0.82 is similar to that of normal luminosity SNe Ia. That makes it a prevalent outlier in both the SN Ia luminosity-width and the luminosity-color-stretch relations. The analysis of the early light curves indicates a possible “bump” during the first ≈1.4 days of explosion. ASASSN-20jq synthesized a low radioactive⁵⁶Ni mass of 0.09 ± 0.01 M_⊙. The near-maximum light spectra of the supernova show strong Si IIabsorption lines, indicating a cooler photosphere than normal SNe Ia; however, it lacks Ti IIabsorption lines. Additionally, it shows unusually strong absorption features of O Iλ7773 and the Ca IInear-infrared triplet. The nebular spectra of ASASSN-20jq show a remarkably strong but narrow forbidden [Ca II]λλ7291, 7324 doublet emission that has not been seen in SNe Ia except for a handful of Type Iax events. There is also a marginal detection of the [O I]λλ6300, 6364 doublet emission in nebular spectra, which is extremely rare. Both the [Ca II] and [O I] lines are redshifted by roughly 2000 km s⁻¹. ASASSN-20jq also exhibits a strong [Fe II]λ7155 emission line with a tilted-top line profile, which is identical to the [Fe II]λ16433 line profile. The asymmetric [Fe II] line profiles, along with the redshifted [Ca II] and emission lines, suggest a high central density white dwarf progenitor that underwent an off-center delayed-detonation explosion mechanism, synthesizing roughly equal amounts of⁵⁶Ni during the deflagration and detonation burning phases. The equal production of⁵⁶Ni in both burning phases distinguishes ASASSN-20jq from normal bright and subluminous SNe Ia. Assuming this scenario, we simultaneously modeled the optical and near-infrared nebular spectra, achieving a good agreement with the observations. The light curve and spectroscopic features of ASASSN-20jq do not align with any single sub-class of SNe Ia. However, the significant deviation from the luminosity versus light-curve shape relations (along with several light-curve and spectroscopic features) exhibits similarities to some 2002es-like objects. Therefore, we have identified ASASSN-20jq as an extreme candidate within the broad and heterogeneous parameter space of 2002es-like SNe Ia. 

Abstract In recent years, multiple Type Ia supernovae (SNe Ia) have been observed with “bumps” in their rising light curves shortly after the explosion. Here, we present SN 2021qvo: an SN Ia that exhibits a clear early bump in photometry obtained by the Young Supernova Experiment. Photometric and spectroscopic observations of SN 2021qvo show that it has a broader light curve, higher peak luminosity, shallower Siiiλ5972 pseudoequivalent width, and lower ejecta velocities than normal SNe Ia, which are all consistent with the characteristics of the 2003fg-like (often called “super-Chandrasekhar”) SN subtype. Including SN 2021qvo, just four known 2003fg-like SNe Ia have sufficient prepeak data to reveal a rising light-curve bump, and all four have bump detections. A host-galaxy analysis reveals that SN 2021qvo exploded in a low-mass galaxy $\log (M_{*}/M_{\odot })=7.83_{-0.24}^{+0.17}$ , also consistent with other members of this class. The current leading early bump 2003fg-like SN Ia progenitor model involves an interaction between the circumstellar material (CSM) and the SN ejecta. We test the validity of this theory by modeling the early bump and subsequent light-curve evolution of SN 2021qvo with the Modular Open Source Fitter for Transients. We find that the bump can be modeled with a best-fit CSM mass in the rangeM_CSM= 3.31−8.51 × 10⁻³M_⊙. SN 2021qvo adds to the small but growing number of 2003fg-like SNe Ia with rising light-curve bumps; as the number of these SNe Ia with CSM estimates continues to grow, population-level inferences about the CSM distribution will be able to constrain the progenitor scenario for these SNe Ia. 

We discuss ASASSN-24fw, a 13th-magnitude star that optically faded by $\Delta g=4.12\pm 0.02$ mag starting in September 2024 after over a decade of quiescence in ASAS-SN. The dimmimg lasted $$8 months before returning to quiescence in late May 2025. The spectral energy distribution (SED) before the event is that of a pre-main sequence or a modestly evolved F star with some warm dust emission. The shape of the optical SED during the dim phase is unchanged and the optical and near-infrared spectra are those of an F star. The SED and the dilution of some of the F star infrared absorption features near minimum suggest the presence of a $$ $0.25$ M_$$ M dwarf binary companion. The 43.8 year period proposed by Nair & Denisenko (2024) appears correct and is probably half the precession period of a circumbinary disk. The optical eclipse is nearly achromatic, although slightly deeper in bluer filters, $\Delta (g-z)=0.31\pm 0.15$ mag, and the $V$ band emission is polarized by up to 4%. The materials most able to produce such small optical color changes and a high polarization are big ($$20 $\mu$ m) carbonaceous or water ice grains. Particle distributions dominated by big grains are seen in protoplanetary disks, Saturn-like ring systems and evolved debris disks. We also carry out a survey of occultation events, finding 46 additional systems, of which only 7 (4) closely match $\epsilon$ Aurigae (KH 15D), the two archetypes of stars with long and deep eclipses. The full sample is widely distributed in an optical color-magnitude diagram, but roughly half show a mid-IR excess. It is likely many of the others have cooler dust since it seems essential to produce the events.