M. M. Phillips

The local environment of a Type Ia supernova (SN Ia) traces the local stellar-population age (a statistical proxy for the progenitor age), star formation, dust, and chemical enrichment that can influence both its light-curve properties and its standardized luminosity. We assembled a homogeneous explosion-site dataset to test how near-ultraviolet (UV) imaging combined with optical integral-field spectroscopy improves the characterization of SN Ia environments and their connection to SN Ia observables. We analyzed 340 low-redshift SN Ia explosion sites observed with HST/WFC3 in F275W together with their matched VLT/MUSE spectroscopy. For each site, we measured local photometry and extracted a 1 kpc-aperture spectrum. We then modeled the stellar continuum with STARLIGHT in a joint UV+optical fit, derived local stellar and gas-phase properties, and fit the SN light curves with sncosmo /SALT3-NIR. We find that UV emission is detected at 235 of the 337 sites with valid HST imaging. The UV- and H α -based star-formation-rate surface densities are strongly correlated, with the UV estimates systematically higher by about 0.5 dex. Including the UV constraint in the STARLIGHT fit shifts local luminosity-weighted ages toward older values by a median of +0.20 dex for UV detections and +0.45 dex for UV nondetections, showing that UV data help break age-dust-metallicity degeneracies and substantially alter and tighten the inferred local stellar-population ages within the adopted modeling framework. Consistently, F275W− r is found to be very strongly correlated with the joint-fit stellar age. Within the 169-SN Ia calibration sample, the joint stellar age, the H α specific star-formation rate (sSFR), and the UV sSFR all correlate strongly with the SALT3 stretch parameter, x ₁ , with significances above 6.8 σ . After standardization, the residual Hubble-residual trends are weaker: the strongest trend is found for the local mass-weighted age, while the UV and H α sSFRs show residual offsets of comparable sign and amplitudes of order ∼0.05 mag. None of the environmental dependencies of the Hubble residual exceeds 3 σ . The results point to a dominant empirical association between light-curve stretch and a covariant young-stellar-population axis, largely absorbed by the standard light-curve corrections, while any residual luminosity dependence on the local environment is weaker and more subtle. If confirmed with larger samples, these residual trends may still become relevant for the younger SN Ia populations expected at higher redshift. Therefore, they should be tested in cosmology analyses with forthcoming Roman and JWST SN samples.

SN 2003 fg-like events are a peculiar type Ia supernova (SN Ia) subtype characterized by broader light curves, higher near-infrared luminosities, and stronger carbon absorptions at early times. Here we present observations of the largest compilation of 2003 fg-like SN Ia host galaxies to date, obtained with integral field spectroscopy (IFS). For 20 objects, we study both the global host-galaxy properties and, for the first time for a sizeable sample, the local environment at the SN position. Globally, 2003 fg-like SNe Ia occur in galaxies with a lower stellar mass, lower oxygen abundance, and marginally higher specific star formation rate (sSFR) than those of normal SNe Ia, although their hosts are neither as extreme in such properties as superluminous SNe nor representative of metal-poor dwarf-galaxy samples. Locally, the SN positions show a lower star formation rate, stellar-mass surface densities, and sSFR than normal SN Ia environments, consistent with a significant preference for the outskirts of their hosts, while their stellar age indicators are typical. The most distinctive local property is metallicity, with 2003 fg-like SNe Ia occupying the most metal-poor environments among SNe Ia. We also find a tentative positive correlation between the light-curve width and the oxygen abundance for 2003 fg-like events. Our results imply that 2003 fg-like SNe Ia arise from the merger of two white dwarfs (WDs) or the core-degenerate scenario, but disfavor the single, rapidly rotating super-M _ch C-O WD progenitor, as this channel requires a young stellar population that we do not observe at the SN positions. The preference of 2003 fg-like SNe Ia for low-metallicity environments suggests that they may have been more common in the early Universe. Since they are overluminous after light-curve standardization (negative Hubble residuals), they will inevitably enter distant SN Ia samples used for cosmology, introducing potential systematic uncertainties in future dark energy experiments. A better understanding of these peculiar SNe Ia is therefore crucial, not only to decipher their progenitors and explosion mechanism, but also to reduce systematic errors in cosmological parameters.

We present observations of the 1991 bg-like type Ia supernova (SN Ia) 2023 vjh associated with the elliptical galaxy MCG+04-10-013. Its light-curve shape parameters (∆ m ₁₅ ( B ) = 1.89 ± 0.01 mag and s _BV = 0.45 ± 0.03) together with its spectroscopic evolution place it firmly within the class of fast-declining subluminous SNe Ia. The near-peak spectra show prominent features of Si II , Ca II , and Ti II consistent with a cool photosphere and place SN 2023 vjh within the ‘cool’ and extremely cool regions on the classification diagrams. In addition, the three late-phase near-infrared (NIR) spectra display the Ca II NIR triplet, Fe II , and Co II absorptions but no obvious H -band break. Although SN 2023 vjh falls in the same regions of the classification diagrams as other well-studied 91 bg-like events, it shows some deviation within this class. In particular, it is systematically fainter than predicted by explosion models and fainter than other well-studied 91 bg-like SNe. Light-curve fitting and colour-based analyses indicate a relatively large reddening ( E ( B − V ) _host ∼ 0.2–0.35 mag), which is unusual for 91 bg-like SNe. Yet the lack of detectable Na I D absorption in its spectra, along with its large projected distance from the centre of its passive host galaxy (6.8 kpc), suggests that interstellar extinction along the line of sight is minimal. SN 2023 vjh appears to be fainter than typical 91 bg-like SNe, and it could be affected by circumstellar material (CSM). Comparisons with explosion models and alternative extinction prescriptions show that including CSM-like extinction improves agreement in the blue bands, but residual discrepancies in the i -band may reflect limitations in the model.

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 of v ₁ ≈ 1500–1000 km s ⁻¹ , indicating continued SiO formation. The SiO mass decreases from ∼(2–6) × 10 ⁻³ M _⊙ 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.

Producing stable ⁵⁸ Ni in Type Ia supernovae (SNe Ia) requires sufficiently high-density conditions that are not predicted for all origin scenarios, so examining the distribution of ⁵⁸ Ni using the near-infrared (NIR) [Ni II ] 1.939 μ m 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 [Ni II ] 1.939 μ m 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 stable ⁵⁸ Ni 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.

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.

We present James Webb Space Telescope (JWST) Near-Infrared Spectrograph observations of SN 2024ggi, spanning wavelengths of 1.7–5.5 μ m 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 ⁻³ M _⊙ , while the average temperature drops from ≈2900 to ≈2500 K. Concurrently, the CO distribution transitions from nearly homogeneous to highly clumped (density contrast increasing from fc ≈ 1.2 to 2). The minimum velocity of the CO-emitting region remains nearly constant ( v ₁ ≈ 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.

We present and analyze panchromatic (0.35–14 μ m) 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.