Maximilian D. Stritzinger

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.

Context. Originating from massive stars that have lost most of their hydrogen envelopes, type IIb supernovae are stellar explosions of partially stripped stars. Due to this extensive stripping, their spectra exhibit transitional signatures that bridge the gap between hydrogen-rich (type II) and helium-rich (type Ib) events, making them crucial astrophysical probes for investigating the final evolutionary stages of massive stars. Aims. We aimed to investigate the physical properties and the underlying explosion mechanisms of the type IIb SN 2025aico. Through a comprehensive analysis of early-phase optical light curves and spectroscopic data, we aimed to constrain the fundamental explosion parameters and evaluate the physical state of the event. Methods. We present early multi-band optical imaging and low-resolution optical spectroscopic follow-up observations of the type IIb SN 2025aico, spanning ∼70 days from the explosion. We constrained the properties of SN 2025aico using a hybrid model that combines shock-cooling emission and radioactively powered diffusion, as well as through an analysis of its spectroscopic evolution. We used various approaches to constrain ⁵⁶ Ni mixing from early data and compared our spectra with models to constrain the properties of the progenitor. Results. The explosion epoch of SN 2025aico is estimated to be Modified Julian Date (MJD) 61032.69, while the rise time in the r _M band is 22.30 ± 0.70 days. The peak pseudo-bolometric luminosity in the optical bands is L _opt = (4.07 ± 0.10)×10 ⁴¹ erg s ⁻¹ . The fitting yields a moderate to relatively low ⁵⁶ Ni mass of M _Ni = 0.033 _−0.004 ^+0.006 M _⊙ and an ejecta mass of M _ej = 2.79 _−0.18 ^+0.21 M _⊙ . The photospheric velocity near the bolometric peak, measured from the Fe II λ 5169 line, is 6450 ₋₁₆₀ ^+0.180 km s ⁻¹ . The derived envelope properties suggest a compact He-star progenitor possessing an H-rich envelope of M _env ≈ 0.02 M _⊙ and a radius of R _env ≈ 6–10 R _⊙ . Conclusions. The derived physical properties of SN 2025aico indicate an origin from a moderate-mass, stripped He-star in a compact binary system, characterized by a minimal residual hydrogen envelope. The explosion itself demonstrates weak to moderate ⁵⁶ Ni mixing throughout the ejecta.

Aims. The core-collapse supernovae (CCSNe) 2022prr, 2023ucy, and 2024ljc exploded within the central regions of the nearby luminous infrared galaxy (LIRG) NGC 6745. We present our follow-up data with the aim to characterise the spectrophotometric evolution of these CCSNe, to determine their host-galaxy extinction, and to define their physical nature. Methods. The presented spectrophotometric dataset covers the optical and near-infrared region for our three supernovae (SNe). The data were analysed and compared to other hydrogen-rich CCSNe, and a host-galaxy extinction value A _V ^host was derived based on broadband photometry for each of the three targets. A spectral energy distribution (SED) for the host galaxy was constructed, and a combination of different galactic components were fitted to the SED to estimate the intrinsic CCSN rate and the age of the starburst of NGC 6745. Results. SN 2022prr is a SN 2009ip-like event, whose light curve evolution consists of two consecutive luminous events A and B with peak magnitudes of M _r = −14.8 and M _r = −18.3 mag, respectively. The high degree of V - and R -band polarisation ( P ∼ 2.8%) near maximum light suggests an interaction with highly aspherical circumstellar material. The spectral evolution of the SN is dominated by narrow Balmer emission lines, which have a complex evolution over time. SN 2022prr joins the growing sample of SN 2009ip-like events that show early ‘flash ionisation’ features of C III , N III , and He II . SN 2023ucy is found to be a normal type IIP SN. However, SN 2024ljc is a rapidly evolving type IIb SN with a post-maximum r -band decline of roughly 3.2 mag by +41 d, whereas normal type IIb SNe typically decline by 1.5 ± 0.3 mag during the same timescale. While the events exploded in a LIRG, the SNe are obscured by only moderate V -band host extinctions ranging from 0 to 1 mag. Based on the SED modelling, we estimate an age of the host starburst to be ∼40 Myr, and the intrinsic CCSN rate to be 0.4 ± 0.1 SN per year.

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 report JWST spectra and photometry of the underluminous Type Iax supernova (SN Iax) 2024vjm obtained 202.8 rest-frame days postexplosion. The spectrum exhibits a rich set of forbidden lines from low-ionization, intermediate-mass, and iron-group elements, notably the [Ni ii ] 6.64 μ m resonance line, which is a direct indicator of stable nickel. Strong CO and SiO emission is detected alongside a warm dust continuum; the spectral properties are consistent with preexisting rather than newly formed dust. Synthetic spectra were computed with the generalized stellar atmospheres code PHOENIX using simplified ejecta models. The models reproduce the overall spectral energy distribution and the molecular emission features reasonably well, but substantially underestimate the strength of the mid-infrared atomic forbidden lines, leaving the synthetic spectrum dominated by molecular emission. Experiments in which the molecular opacity is suppressed do not recover the forbidden lines; instead, the emission peak migrates to Co and Fe transitions near 2 μ m. We attribute this discrepancy to poorly constrained collisional rates and possibly to an excess of iron group material in the current ejecta models. A prominent feature at 12.8 μ m is not well accounted for by the [Ne ii ] 12.81 μ m line, indicating that the 12.8 μ m feature may be largely due to [Fe iii ]. The presence of CO, SiO, and stable nickel together with the nondetection of neon places tight constraints on the total ejecta mass and the nucleosynthetic yields of SN Iax progenitor systems.

We present a photometric and spectroscopic study of the superluminous Type Ia supernova SN 2022erq. Its early spectra, dominated by iron-group elements with weak intermediate-mass features, might indicate highly efficient nuclear burning, broadly similar to that inferred for some overluminous Type Ia supernovae. The rapid emergence and persistence of narrow Balmer emission lines superposed on this iron-rich spectrum provide clear evidence of long-lived interaction with a hydrogen-rich circumstellar medium (CSM), establishing SN 2022erq as a member of the rare Ia–CSM class. SN 2022erq reached a peak bolometric luminosity of ∼8 × 10 ⁴³ erg s ⁻¹ and exhibited an exceptionally slow postpeak decline, indicating that its light curve is dominated by long duration ejecta–CSM interaction. By combining H α diagnostics with bolometric light-curve modeling, we reconstruct the preexplosion mass-loss history of the progenitor. The mass-loss rate escalated by 1 order of magnitude over the final decades, rising from ∼0.04 to ∼0.6 M _⊙ yr ⁻¹ . This surge produced a massive, extended CSM shell of ∼3 M _⊙ out to ∼3.5 × 10 ¹⁶ cm. The young stellar environment (∼100 Myr) together with this substantial, extensive CSM points to a progenitor system consisting of a white dwarf and an intermediate-mass companion that underwent increasing mass loss prior to explosion.

Massive stripped stars are expected to become unstable before core collapse, but direct evidence linking such activity to the material shaping the ensuing explosion remains rare. Here we report optical observations of SN 2026gzf, a broad-lined Type Ic supernova associated with the Einstein Probe X-ray transient EP260321a, beginning just 1.25 hours after the X-ray trigger. Lulin Observatory discovered the optical counterpart and captured a luminous blue first-day excess that cannot be reproduced by a radioactive-powered model alone. Hydrodynamic modelling indicates that interaction with approximately 0.02 solar masses of compact circumstellar material accounts for this early emission. Archival Pan-STARRS imaging reveals variable emission at the explosion site over the preceding ~12 years, with enhanced luminosity during the final ~3 years before explosion, broadly consistent with increased mass loss during late oxygen burning. To our knowledge, this is the first stripped-envelope supernova progenitor with repeatedly detected pre-explosion variability spanning more than a decade. Together, these observations provide the first direct connection in a stripped-envelope supernova between long-term pre-explosion activity, compact circumstellar material, an X-ray shock-breakout signal and the first hours of optical emission. SN 2026gzf therefore shows how late-stage instability in a stripped progenitor system can shape the immediate environment into which the star explodes.

The discovery of stripped-envelope core-collapse supernovae (SNe) interacting with dense circumstellar medium (CSM) challenges our current understanding of massive star evolution. We present late-time observations of the interacting Type Ic SN 2017dio and investigate its mass-loss mechanism and progenitor channel. We analysed late-time spectra and light curves that are dominated by a strong ejecta–CSM interaction and reveal a slow photometric evolution primarily powered by this interaction. We examined the CSM and the source of the infrared (IR) excess by modelling the radiation produced by the ejecta–CSM interaction and the IR echo from circumstellar dust. In addition, we studied the temporal evolution of spectral features, with a particular emphasis on the Hα emission line. From the combined analysis of the light curves and spectral properties, we infer that the peak mass-loss rate for the CSM reaches sim0.2 mathrm M_⊙ yr^ -1 and that the typical value over most epochs is sim0.06 mathrm M_⊙ yr^ -1 . The nearby CSM was formed over a period of 4 to 65 years before the explosion. The CSM radius begins at ∼ 1.3 cm. The IR excess identified in the light curves is consistent with the radiation from dust with a mass increasing from sim0.001 to sim0.005 in the case of carbon dust or sim0.005 to sim0.02 in the case of silicate dust. 15 M_⊙ M_⊙ From IR echo modelling, we estimate an upper limit on the dust mass of 4 -5 M_⊙ , which implies an SN progenitor mass-loss rate of 2.4 -5 M_⊙ yr^ -1 at the dust evaporation radius determined by the SN peak luminosity (0.017 pc for carbon dust, corresponding to mass loss sim170 years before the explosion). This implies a very rapid increase in the mass-loss rate ahead of the explosion. Although the progenitor of SN 2017dio has lost its helium envelope, it interacted with a hydrogen-rich CSM formed shortly before the explosion, strongly suggesting that this material originated from a companion star rather than the progenitor itself.