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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. 

Thermonuclear explosions of C/O white dwarf stars in binary systems known as Type Ia supernovae (SNe Ia) remain poorly understood. The complexity of their progenitor systems, explosion physics, and intrinsic diversity poses challenges in understanding these phenomena as astrophysical objects, as well as their standardization and use as cosmological probes. Near-infrared (NIR) observations offer a promising avenue for studying the physics of SNe Ia and for reducing systematic uncertainties in distance estimations, as they exhibit lower dust extinction and smaller dispersion in peak luminosity than optical bands. In this work, we applied a principal component analysis (PCA) to a sample of SNe Ia with well-sampled NIR (YJH-band) light curves to identify the dominant components of their variability and constrain physical underlying properties. The theoretical models are used for the physical interpretation of the PCA components, where we found that the⁵⁶Ni mass best describes the dominant variability. Other factors, such as mixing and metallicity, were found to contribute significantly as well. However, some differences are seen among the components of the NIR bands, which could be attributed to differences in the explosion aspects they each trace. Additionally, we compared the PCA components to various light curve parameters, identifying strong correlations between the first component inJandHbands (second component inY) and peak brightness in both the NIR and optical bands, particularly in theYband. When applying a PCA to NIR color curves, we found interesting correlations with the host-galaxy mass, where SNe Ia with redder NIR colors are predominantly found in less massive (potentially more metal-poor) galaxies. We also investigated the potential for improved standardization in theYband by incorporating PCA coefficients as correction parameters, leading to a reduction in the scatter of the intrinsic luminosity of SNe Ia. As new NIR observations become available, our findings can be further tested, ultimately refining our understanding of SNe Ia physics and enhancing their reliability as cosmological distance indicators. 

Abstract Thermodynamical Supernovae (SNe Ia) are one of the keys to high precision cosmology and decipher the nature of the dark energy and matter. They provide a playground for numerical astrophysical processes for a diverse group of explosions of White Dwarf (WD) stars. At late times during the nebular phase, mid-infrared (MIR) observations are an effective tool to probe for the multi-dimensional imprints of the explosion physics of WDs and their progenitor systems. What we observe as SNe Ia are low-energy photons, namely light curves, and spectra detected some days to years after the explosion. The light is emitted from a rapidly expanding envelope consisting of a low-density and low-temperature plasma with atomic population numbers far from thermodynamical equilibrium. SNe Ia are powered radioactive decays which produce hard X- andγrays and MeV leptons which are converted within the ejecta to low-energy photons. We find that the optical and IR nebular spectra depend sensitively on the proper treatment of the physical conversion of high to low energies. The low-energy photons produced by forbidden line transitions originate from a mostly optically thin envelope. However, the UV is optically thick because of a quasi-continuum formed by allowed lines and bound-free transitions even several years after the explosion. The requirements to simulate nebular spectra are well beyond both ‘classical’ stellar atmospheres and nebulae. Using our full non-LTE HYDrodynamical RAdiation code (HYDRA) as a testbed, the sensitivity on the physics on synthetic spectra are demonstrated using observations as a benchmark. At some examples, we establish the power of high-precision nebular spectroscopy as quantitative tool. Centrally ignited, off-center delayed-detonation nearM_Chmodels can reproduce line-ratios and line profiles. 

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 Carbon–oxygen (CO) detonation with reactions terminating either after burning of¹²C in the leading¹²C + ¹²C reaction or after burning of¹²C and¹⁶O to Si-group elements may occur in the low-density outer layers of exploding white dwarfs and be responsible for the production of intermediate-mass elements observed in the outer layers of Type Ia supernovae. Basic one-dimensional properties of CO-detonations have been summarized in our previous work. This paper presents the results of two- and three-dimensional numerical simulations of low-density CO-detonations and discusses their multidimensional stability, cellular structure, and propagation through a constant low-density background. We find three-dimensional CO detonations to be strikingly different from their one-dimensional and two-dimensional counterparts. Three-dimensional detonations are significantly more robust and capable of propagating without decay compared to highly unstable and marginal one- and two-dimensional detonations. The detonation cell size and whether burning of¹²C in a three-dimensional detonation wave is followed by the subsequent¹⁶O burning are sensitive to both the background density and the initial¹²C to¹⁶O mass ratio. We also discuss the possible implications for understanding the observed early-time bumps in light curves. 

Abstract In this paper, the suitability of fast-declining Type Ia supernovae (SNe Ia) as cosmological standard candles is examined utilizing a “Hubble Flow” sample of 43 of these objects observed by the Carnegie Supernova Project (CSP). We confirm previous suggestions that fast-declining SNe Ia offer a viable method for estimating distances to early-type galaxies when the color-stretch parameter,s_BV, is used as a measure of the light-curve shape. As a test, we employ the Tripp method, which models the absolute magnitude at maximum as a function of light-curve shape and color. We calibrate the sample using 12 distance moduli based on published infrared surface-brightness fluctuations to derive a value of the Hubble constant that is in close agreement with the value obtained for the full sample of CSP SNe Ia using the same methodology. We also develop a new and simple method of estimating the distances of fast decliners based only on their colors at maximum (and not light-curve shape) and find that it leads to similar results as with using the Tripp method. This “color” technique is a powerful tool that is unique to fast-declining SNe Ia. We show that the colors of the fast decliners at maximum light are strongly affected by photospheric temperature differences and not solely due to dust extinction, and provide a physical rationale for this effect. 

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.