Cameron M. Pfeffer

Cometary activity from interstellar objects provides a unique window into the environs of other stellar systems. We report blue-sensitive integral field unit (IFU) spectroscopy of the interstellar object 3I/ATLAS from the Keck II–mounted Keck Cosmic Web Imager on 2025 August 24 UT. We confirm the previously reported CN and Ni outgassing and, for the first time, present the radial profiles of Ni and CN emission in 3I/ATLAS. We find characteristic e -folding radii of 593.7 ± 14.8 km for Ni and 841.0 ± 15.4 km for CN; this suggests that the Ni emission is more centrally concentrated in the comet’s nucleus and favors hypotheses involving easily dissociated species, such as metal carbonyls or metal–polycyclic aromatic hydrocarbon (PAH) molecules. Additional integral field spectroscopy after perihelion will provide an ongoing opportunity to determine the evolution of the radial distributions of species in the interstellar comet 3I/ATLAS.

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.

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.

Type Ia supernovae explosively destroy C/O white dwarfs (WDs) in multiple stellar systems. They produce ≈50% of the iron-group elements in the universe, synthesize electron-capture (EC) elements, drive nuclear physics experiments, and underpin high-precision cosmology. To first order, the outcome is governed by nuclear physics, a property often described as “stellar amnesia.” Recently, this stellar amnesia has begun to be broken by the nearly universal detection of EC elements with James Webb Space Telescope. These elements trace high-density burning, largely ruling out the currently popular He-triggered, sub-MCh detonation models as the dominant channel. Instead, the ubiquitous presence of EC is shifting back the focus to dynamical and secular mergers and near-MCh explosions similar to the deflagration model W7, but in which the nuclear flame undergoes a deflagration-to-detonation transition. The early deflagration phase is especially important because spherical simulations identify the central WD density, and thus the WD mass, as a key parameter governing the explosion. Here, we present detailed magneto-hydrodynamical simulations. We find that small-scale, preexisting turbulence expected from the pre-explosion smoldering phase is essential for overcoming the fundamental challenges imposed by the intrinsic 3D physics. This turbulence systematically reduces the production of EC elements by about a factor of two, implying the need for WD central densities closer to those associated with accretion-induced collapse to a neutron star. We also demonstrate the effect of magnetic fields near the saturation field strength and highlight the need for higher-precision EC rates at low Ye.

Interstellar objects are the ejected building blocks of other solar systems. As such, they enable the acquisition of otherwise inaccessible information about nascent extrasolar systems. The discovery of the third interstellar object, 3I/ATLAS, provides an opportunity to explore the properties of a small body from another solar system and to compare it to the small bodies in our own. To that end, we present spectrophotometric observations of 3I/ATLAS taken using the SuperNova Integral Field Spectrograph on the University of Hawaii 2.2-m telescope. Our data includes the earliest λ ≤ 3800 A spectrum of 3I/ATLAS, obtained ∼ 12.5 hours after the discovery announcement. Later spectra confirm previously reported cometary activity, including Ni and CN emission. The data show wavelength-varying spectral slopes ( S ≈ ( 0%-29%)/1000 A, depending on wavelength range) throughout the pre-perihelion ( r h = 4.4 – 2.5 au) approach of 3I/ATLAS. We perform synthetic photometry on our spectra and find 3I/ATLAS shows mostly stable color evolution over the period of our observations, with g − r colors ranging from ∼ 0.69-0.75 mag, r − i colors ranging from ∼ 0.26-0.30 mag, and c − o colors ranging from ∼ 0.50-0.55 mag. Ongoing post-perihelion observations of 3I/ATLAS will provide further insight into its potentially extreme composition.

The environs of other stellar systems may be directly probed by analyzing the cometary activity of interstellar objects. The recently discovered interstellar object 3I/ATLAS was the subject of an intensive worldwide follow-up campaign in its preperihelion approach. Now, 3I/ATLAS has begun its postperihelion departure from the solar system. In this paper, we report the first postperihelion blue-sensitive integral-field unit spectroscopy of 3I/ATLAS using the Keck Cosmic Web Imager on 2025 November 16. We confirm previously reported CN, Fe, and Ni outgassing along with detections of carbon chain molecules C ₂ and C ₃ . We calculate production rates for each species. We find Fe and Ni production rates of Q _Fe = (9.55 ± 3.96) × 10 ²⁵ atoms s ⁻¹ and Q _Ni = (6.61 ± 2.74) × 10 ²⁵ atoms s ⁻¹ , resulting in a ratio of log ( Q Ni / Q Fe ) = − 0.16 ± 0.03 , which matches solar system comets well and continues the preperihelion trend of declining log ( Q Ni / Q Fe ) with r _h . We investigate the radial distributions of these elemental species and find characteristic e -folding radii of 3880 ± 39 km for Ni, 6053 ± 68 km for CN, 4194 ± 45 km for C ₂ , and 3833 ± 45 km for C ₃ . Compared to preperihelion measurements, these radii have increased by a factor of ∼6.5–7. Our postperihelion observations reveal that 3I/ATLAS continues to exhibit cometary behavior broadly consistent with solar system comets.

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 the FLASH code. 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.

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.

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.