Joseph P. Anderson

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.

Type Ibn supernovae (SNe) are a class of interacting SNe characterised by narrow helium lines in their spectra. We present an extensive observational dataset of the Type Ibn SN 2025kzr at 51 Mpc, including the discovery of a precursor outburst with a peak brightness of M _r ≈ − 13.6 mag beginning ≈55 days before explosion. Our photometry indicates the SN was discovered within the first day of explosion, and shows fast-rising, ultraviolet-bright emission peaking at M _r = −19.26 ± 0.09 mag and a peak blackbody temperature of T ≈29 000 K, consistent with shock breakout within a region of dense and confined circumstellar material (CSM). Our high-cadence spectroscopic dataset spanning 1.9–58.5 days post-explosion shows flash-ionised emission features during the first ten days. In our SALT spectrum at 3.8 days, we observe a pronounced blueshift of the He II lines by 460 km s ⁻¹ compared to the He I lines at zero velocity, while a Pickering-decrement analysis reveals a CSM that is fully hydrogen free. The timing of the disappearance of the flash features combined with the CSM velocity of 1500 km s ⁻¹ imply a mass-loss event ≈66 days before explosion, in close agreement with the timing of the precursor observed 55 days before explosion and strongly suggestive of a physical link. We derive a CSM mass of 0.03–1.7 M _⊙ and a corresponding high mass-loss rate of ≳10 ⁻¹ M _⊙ yr ⁻¹ . The precursor timescale and energetics suggest an extreme mass-loss event that might be explained by wave-driven mass loss during the late stages of nuclear burning, in particular the oxygen-burning phase. Overall, we favour a single massive Wolf-Rayet progenitor with M _ZAMS ~30–40 M _⊙ to explain SN 2025kzr, although a binary origin cannot be excluded.

Type Ia supernovae (SNe Ia) are thermonuclear runaways of some white dwarfs in binary systems. They have been extensively studied, yet their progenitor and explosion mechanisms remain poorly understood. We studied a large sample of SNe Ia, comparing the narrow interstellar absorption features in their spectra with various photometric and spectroscopic supernova properties as well as environmental characteristics. Sodium absorption is significantly stronger in younger more star-forming and more centrally located SNe Ia, as expected. However, we also show that there is a relation with intrinsic properties that is independent of the environment. In fact, there is substantial evidence of two environmental SN Ia populations, an old and a young one, with the young population showing significantly different distributions of sodium strength when divided according to the Si II ejecta velocity, nebular velocity, extinction, E ( B - V ), and reddening curve ( R _V ). Performing a clustering of the SNe Ia, we recovered an old population of SNe with a low extinction and normal ejecta velocity, while the young population can indeed be subdivided into a group of highly extincted high-velocity SNe Ia with much stronger blueshifted sodium absorption and another group of low-extincted normal-velocity objects with little sodium absorption. We interpret this relation of intervening material with intrinsic properties as evidence of the young SN Ia population (occurring in young and star-forming environments) having asymmetric radiation that interacts with nearby material and observables that depend on the viewing angle. Finally, we show that the cosmological mass step is consistent with these populations.

We investigate the thermal emission and the extinction from dust associated with the nearby ( z = 0.0267) superluminous supernova (SLSN) 2018bsz. Our dataset combines daily cadence, simultaneous optical and near-infrared (NIR) coverage up to ∼100 days with late-time (+ 1.7 yr) mid-infrared (MIR) observations. At 230 days after the light-curve peak, the supernova (SN) is not detected in the optical, but shows a surprisingly strong NIR excess, with r − J > 3 mag and r − K _s > 5 mag. The temporal evolution of the infrared (IR) light curve allows us to assess whether the MIR emission originates from newly formed dust, either in the SN ejecta or in a postshock circumstellar shell, or instead from an IR echo produced by preexisting circumstellar or interstellar dust heated by the SN radiation. Our radiative-transfer modeling shows that the circumstellar and interstellar echo scenarios cannot simultaneously reproduce the observed IR luminosity and its temporal evolution, and we therefore disfavor these interpretations. In contrast, a scenario in which new dust forms in the SN ejecta at epochs ≳200 days provides a self-consistent explanation for the evolution of the SN flux. We can fit the spectral energy distribution well at +230 days with 5 × 10 ⁻⁴ M _⊙ of carbon dust, increasing over the following several hundred days to 10 ⁻² M _⊙ by +535 days. SN 2018bsz is the first SLSN showing evidence for dust formation within the SN ejecta, and appears to form 10 times more dust than normal core-collapse supernovae (SNe) at similar epochs. Together with their preference for low-mass, low-metallicity host galaxies, we suggest that superluminous SNe may be a significant contributor to dust formation in the early Universe.

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.

Context. Type II supernovae (SNe) are the most common terminal stellar explosions in the Universe. With SNe now being detected within days after first light, there is growing evidence that most Type II SNe (SNe II) show signs of interaction with a dense, confined circumstellar material (CSM) in the first few days after first light. Aims. We aim to bridge the gap between single SN studies showing early-time interaction in their spectra and statistical studies of early-time SN light curves, which imply the existence of CSM. Methods. We present a sample of 68 Type II SNe with both early photometric data obtained by the ATLAS survey and spectroscopic data obtained by the ePESSTO+ collaboration. A subset of the sample is classified based on the presence or absence of narrow spectral features with electron-scattered broadened wings in the early spectra, indicative of interaction with the CSM. We characterised the photometric and spectroscopic properties of the sample by measuring rise times to maximum light, peak magnitudes, decline rates, and line velocities. Additionally, we measured the ratio of absorption to emission (a/e) of the H α P-Cygni profile. Results. Our analysis reveals that SNe II showing early spectroscopic signs of interaction with the CSM decline faster and are brighter than those without. However no difference was found in rise times between the two groups. A clear separation can be observed in the a/e ratio, namely, SNe with signs of interaction exhibit lower a/e ratios at all epochs compared to those without. Our results highlight that understanding SN II ejecta-CSM interaction requires large uniform samples of photometric and spectroscopic data, such as the one presented in this work.

Hydrogen-rich superluminous supernovae without narrow emission lines (SLSNe-II) are rare transients whose powering mechanisms remain debated, particularly the role of circumstellar medium (CSM) interaction. We present photometric and spectroscopic observations of SN 2019cqc to investigate its power source and progenitor mass loss. We model the lightcurve using MOSFiT with the csm and csmni models to constrain the explosion and CSM parameters. SN 2019cqc peaked at M _g = −20.21 ± 0.07 and emitted ∼1.7 × 10 ⁵⁰ erg of energy in the form of radiation. Photometric and spectroscopic modeling indicates that CSM interaction dominates in both scenarios, with best-fit CSM and ejecta masses of ∼4.5 M _⊙ and ∼32 M _⊙ , respectively. The inferred CSM properties imply an extreme eruptive mass loss at a rate of ∼0.2–0.3 M _⊙ , yr ⁻¹ in the years preceding the core collapse, consistent with a variable progenitor of luminous blue variable progenitor. We observe a persistent blueshifted asymmetry in the H α emission line. At early times (≲+110 days), this is attributed to electron scattering with the bulk velocity of ejecta, while the profile at late epochs (≳ +402 days onward) suggests the subsequent formation of dust in the ejecta. Additionally, we identify a distinct, short-lived feature at ∼4600 Å, likely a blend of ionized C iii /N iii lines powered by the interaction of the SN-shock with the extended atmosphere.

Understanding dust attenuation toward extragalactic transients is critical for recovering their intrinsic properties and probing the local environments of distant galaxies. A popular diagnostic is the Na i D absorption equivalent width, widely applied to extragalactic transients. In this paper, we present early-time optical and near-infrared observations of the Type Ib supernova (SN Ib) SN 2024vjc, followed for ∼130 days postexplosion. SN 2024vjc exhibits only weak Na I D absorption, indicating only a modest host attenuation with E ( B − V ) _host ∼ 0.18 mag; if this argument were applied, SN 2024vjc would be a peculiar, faint, and red SN Ib while showing the light-curve shape and spectral evolution broadly consistent with those of canonical Type Ib supernovae. We show that this is not the case; SN-based diagnostics (intrinsic color templates, color-curve evolution, and spectral dereddening) together with the Balmer decrement indicate substantial attenuation, E ( B − V ) ∼ 0.45−0.8 mag. The weak Na i D absorption may result from photoionization of neutral sodium by the intense radiation field of a young H ii region at the explosion site; this scenario is directly supported by the detection of narrow H α , [N ii ], and H β emission lines and a blue continuum excess in late-time spectroscopy. SN 2024vjc represents a clear counterexample to the common assumption that weak or absent Na I D absorption implies negligible host-galaxy attenuation, and highlights the importance of employing multiple, independent attenuation diagnostics.

Mapping how the explosion properties of Type II supernovae (SNe II) relate to the properties of their progenitors can provide strong constraints for understanding the final evolutionary stages of massive stars. Type IIP SNe, linked to the explosions of single red supergiant (RSG) stars, have recently been found to require some form of interaction with circumstellar material (CSM) to reproduce the rapid rise to the plateau often seen in their light curves. In this work, we present observations and analysis of the Type IIP SN 2020bij, characterized by a slow rise to its plateau as well as high expansion velocities. We identify four other SNe IIP from the literature (ASASSN-14 kg, SN 2018fif, SN 2021yja, and SN 2023axu) with similarly slowly rising light curves and find that they also show high expansion velocities. Using both analytical and numerical models, all five events can be explained with weak to no CSM interaction. We therefore propose that these events constitute a new subclass of Type IIP SNe that could be associated with relatively confined CSM. Early and dense photometric coverage of future SNe IIP together with early spectroscopic observations will further map this subclass and its physical properties. Understanding such rare events could be key to constraining the diversity of late-stage mass loss in RSGs.