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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 Producing stable⁵⁸Ni in Type Ia supernovae (SNe Ia) requires sufficiently high-density conditions that are not predicted for all origin scenarios, so examining the distribution of⁵⁸Ni using the near-infrared (NIR) [NiII] 1.939μm line may observationally distinguish between possible progenitors and explosion mechanisms. We present 79 telluric-corrected NIR spectra of 22 low-redshift SNe Ia from the Carnegie Supernova Project-II, ranging from +50 to +505 days, including 31 previously unpublished spectra. We introduce the Gaussian Peak Ratio, a detection parameter that confirms the presence of the NIR [NiII] 1.939μm line in eight SNe in our sample. Nondetections occur at earlier phases (≤+100 days) when the NIR Ni line has not emerged yet or in low signal-to-noise spectra, yielding inconclusive results. Subluminous 86G-like SNe Ia show the earliest NIR Ni features around ∼+50 days, whereas normal-bright SNe Ia do not exhibit NIR Ni until ∼+150 days. NIR Ni features detected in our sample have low peak velocities (v∼ 1200 km s⁻¹) and narrow line widths (≤3500 km s⁻¹), indicating stable⁵⁸Ni is centrally located. This implies high-density burning conditions in the innermost regions of SNe Ia and could be due to higher mass progenitors (i.e., near-M_ch). NIR spectra of the nearly two dozen SNe Ia in our sample are compared to various model predictions and paired with early-time properties to identify ideal observation windows for future SNe Ia discovered by upcoming surveys with Rubin-LSST or the Roman Space Telescope. 

Abstract We present panchromatic 0.4–21μm observations of the nearby (∼7.2 Mpc) Type II supernova (SN) 2024ggi, obtained during the plateau phase at ∼55 days past explosion. Our data set includes JWST spectra spanning 1.7–14μm, mid-infrared (MIR) imaging at 7.7 and 21μm, and near-simultaneous ground-based optical and near-infrared (NIR) spectra covering 0.32−1.8μm. The NIR and MIR spectral features of SN 2024ggi are dominated by Hiemission. We present line IDs and a toy PHOENIX/1D model that reproduces the observations well, especially the continuum redward of 0.9μm. We compare SN 2024ggi to SN 2022acko and SN 2023ixf, two other Type II SNe that were also observed by JWST, and highlight key similarities and differences in their spectral features. No evidence for a MIR excess or dust is found at these epochs, with the model matching the observed flux out to 21μm. We discuss the model’s shortcomings, focusing on the density profile, which suppresses line blanketing and produces features in the optical that are too narrow. Our results show the power of panchromatic studies in both exploring the nature of the SN ejecta and constraining detailed models of SNe. 

Abstract We present and analyze panchromatic (0.35–14μm) spectroscopy of the Type II supernova 2023ixf, including near- and mid-infrared spectra obtained 33.6 days after explosion during the plateau phase, with the James Webb Space Telescope (JWST). This is the first in a series of papers examining the evolution of SN 2023ixf with JWST during the initial 1000 days after explosion and monitoring the formation and growth of molecules and dust in ejecta and the surrounding environment. The JWST infrared spectra are overwhelmingly dominated by H lines, whose profiles reveal ejecta structures, including flat tops, blue notches, and red shoulders, unseen in the optical spectra. We characterize the nature of these structures, concluding that they likely result from a combination of ejecta geometry, viewing angle, and opacity effects. We find no evidence for the formation of dust precursor molecules such as carbon monoxide (CO), nor do we observe an infrared excess attributable to dust. These observations imply that the detections of molecules and dust in SN 2023ixf at later epochs arise either from freshly synthesized material within the ejecta or circumstellar material at radii not yet heated by the supernova at this epoch. 

Abstract We present a JWST MIRI medium-resolution spectrometer spectrum (5–27μm) of the Type Ia supernova (SN Ia) SN 2021aefx at +415 days pastB-band maximum. The spectrum, which was obtained during the iron-dominated nebular phase, has been analyzed in combination with previous JWST observations of SN 2021aefx to provide the first JWST time series analysis of an SN Ia. We find that the temporal evolution of the [Coiii] 11.888μm feature directly traces the decay of⁵⁶Co. The spectra, line profiles, and their evolution are analyzed with off-center delayed-detonation models. Best fits were obtained with white dwarf (WD) central densities ofρ_c= 0.9−1.1 × 10⁹g cm⁻³, a WD mass ofM_WD= 1.33–1.35M_⊙, a WD magnetic field of ≈10⁶G, and an off-center deflagration-to-detonation transition at ≈0.5M_⊙seen opposite to the line of sight of the observer (−30°). The inner electron capture core is dominated by energy deposition fromγ-rays, whereas a broader region is dominated by positron deposition, placing SN 2021aefx at +415 days in the transitional phase of the evolution to the positron-dominated regime. The formerly “flat-tilted” profile at 9μm now has a significant contribution from [Niiv], [Feii], and [Feiii] and less from [Ariii], which alters the shape of the feature as positrons mostly excite the low-velocity Ar. Overall, the strength of the stable Ni features in the spectrum is dominated by positron transport rather than the Ni mass. Based on multidimensional models, our analysis is consistent with a single-spot, close-to-central ignition with an indication of a preexisting turbulent velocity field and excludes a multiple-spot, off-center ignition. 

Abstract We present the nebular phase spectroscopic and photometric observations of the nearby hydrogen-rich core-collapse supernova (CCSN) SN 2023ixf, obtained through our JWST programs. These observations, combined with ground-based optical and near-infrared spectra, cover +252.67–719.96 days, creating a comprehensive, panchromatic time-series data set spanning 0.32–30μm. In this second paper of the series, we focus on identifying key spectral emission features and tracking their evolution through the nebular phase. The JWST data reveal hydrogen emission from the Balmer to Humphreys series, as well as prominent forbidden lines from Ne, Ar, Fe, Co, and Ni. NIRSpec observations display strong emission from the first-overtone and fundamental bands of carbon monoxide, which weaken with time as the ejecta cools and dust emission dominates. The spectral energy distribution shows a clear infrared excess emerging by +252.67 days peaking around 10.0μm, with a secondary bump at 18.0μm developing by +719.96 days. We suggest that this evolution could arise from multiple warm dust components. In upcoming papers in this series, we will present detailed modeling of the molecular and dust properties. Overall, this work provides the community with a unique data set that can be used to advance our understanding of the mid-infrared properties of CCSNe, offering an unprecedented resource for studying their late-time line, molecular, and dust emission. 

Context.Type II supernovae (SNe II) mark the endpoint in the lives of hydrogen-rich massive stars. Their large explosion energies and luminosities allow us to measure distances, metallicities, and star formation rates into the distant Universe. To fully exploit their use in answering different astrophysical problems, high-quality low-redshift data sets are required. Such samples are vital to understand the physics of SNe II, but also to serve as calibrators for distinct – and often lower-quality – samples. Aims.We presentuBgVrioptical andYJHnear-infrared (NIR) photometry for 94 low-redshift SNe II observed by the Carnegie Supernova Project (CSP). A total of 9817 optical and 1872 NIR photometric data points are released, leading to a sample of high-quality SN II light curves during the first ∼150 days post explosion on a well-calibrated photometric system. Methods.The sample is presented and its properties are analysed and discussed through comparison to literature events. We also focus on individual SNe II as examples of classically defined subtypes and outlier objects. Making a cut in the plateau decline rate of our sample (s₂), a new subsample of fast-declining SNe II is presented. Results.The sample has a median redshift of 0.015, with the nearest event at 0.001 and the most distant at 0.07. At optical wavelengths (V), the sample has a median cadence of 4.7 days over the course of a median coverage of 80 days. In the NIR (J), the median cadence is 7.2 days over the course of 59 days. The fast-declining subsample is more luminous than the full sample and shows shorter plateau phases. Of the non-standard SNe II highlighted, SN 2009A particularly stands out with a steeply declining then rising light curve, together with what appears to be two superimposed P-Cygni profiles of Hαin its spectra. We outline the significant utility of these data, and finally provide an outlook of future SN II science. 

ABSTRACT We present ultraviolet (UV) to near-infrared (NIR) observations and analysis of the nearby Type Ia supernova SN 2021fxy. Our observations include UV photometry from Swift/UVOT, UV spectroscopy from HST/STIS, and high-cadence optical photometry with the Swope 1-m telescope capturing intranight rises during the early light curve. Early B − V colours show SN 2021fxy is the first ‘shallow-silicon’ (SS) SN Ia to follow a red-to-blue evolution, compared to other SS objects which show blue colours from the earliest observations. Comparisons to other spectroscopically normal SNe Ia with HST UV spectra reveal SN 2021fxy is one of several SNe Ia with flux suppression in the mid-UV. These SNe also show blueshifted mid-UV spectral features and strong high-velocity Ca ii features. One possible origin of this mid-UV suppression is the increased effective opacity in the UV due to increased line blanketing from high velocity material, but differences in the explosion mechanism cannot be ruled out. Among SNe Ia with mid-UV suppression, SNe 2021fxy and 2017erp show substantial similarities in their optical properties despite belonging to different Branch subgroups, and UV flux differences of the same order as those found between SNe 2011fe and 2011by. Differential comparisons to multiple sets of synthetic SN Ia UV spectra reveal this UV flux difference likely originates from a luminosity difference between SNe 2021fxy and 2017erp, and not differing progenitor metallicities as suggested for SNe 2011by and 2011fe. These comparisons illustrate the complicated nature of UV spectral formation, and the need for more UV spectra to determine the physical source of SNe Ia UV diversity. 

Abstract We present early-time photometric and spectroscopic observations of the Type Ia supernova (SN Ia) 2021aefx. The early-time u -band light curve shows an excess flux when compared to normal SNe Ia. We suggest that the early excess blue flux may be due to a rapid change in spectral velocity in the first few days post explosion, produced by the emission of the Ca ii H&K feature passing from the u to the B bands on the timescale of a few days. This effect could be dominant for all SNe Ia that have broad absorption features and early-time velocities over 25,000 km s −1 . It is likely to be one of the main causes of early excess u -band flux in SNe Ia that have early-time high velocities. This effect may also be dominant in the UV filters, as well as in places where the SN spectral energy distribution is quickly rising to longer wavelengths. The rapid change in velocity can only produce a monotonic change (in flux-space) in the u band. For objects that explode at lower velocities, and have a more structured shape in the early excess emission, there must also be an additional parameter producing the early-time diversity. More early-time observations, in particular early spectra, are required to determine how prominent this effect is within SNe Ia. 

Type II supernovae (SNe II) show great photometric and spectroscopic diversity which is attributed to the varied physical characteristics of their progenitor and explosion properties. In this study, the third of a series of papers where we analyse a large sample of SNe II observed by the Carnegie Supernova Project-I, we present correlations between their observed and physical properties. Our analysis shows that explosion energy is the physical property that correlates with the highest number of parameters. We recover previously suggested relationships between the hydrogen-rich envelope mass and the plateau duration, and find that more luminous SNe II with higher expansion velocities, faster declining light curves, and higher 56 Ni masses are consistent with higher energy explosions. In addition, faster declining SNe II (usually called SNe IIL) are also compatible with more concentrated 56 Ni in the inner regions of the ejecta. Positive trends are found between the initial mass, explosion energy, and 56 Ni mass. While the explosion energy spans the full range explored with our models, the initial mass generally arises from a relatively narrow range. Observable properties were measured from our grid of bolometric LC and photospheric velocity models to determine the effect of each physical parameter on the observed SN II diversity. We argue that explosion energy is the physical parameter causing the greatest impact on SN II diversity, that is, assuming the non-rotating solar-metallicity single-star evolution as in the models used in this study. The inclusion of pre-SN models assuming higher mass loss produces a significant increase in the strength of some correlations, particularly those between the progenitor hydrogen-rich envelope mass and the plateau and optically thick phase durations. These differences clearly show the impact of having different treatments of stellar evolution, implying that changes in the assumption of standard single-star evolution are necessary for a complete understanding of SN II diversity.