Professor Stephen Smartt CBE FRS MRIA

Abstract:

Calcium-strong supernovae (Ca-strong SNe) are a class of thermonuclear transients characterised by rapid photometric evolution (rise times of ≤16 d) and nebular-phase spectra dominated by strong [Ca II] λλ 7291,7324 emission, compared to the strength of [O I] λλ 6300,6364 emission. Despite their unique spectroscopic signatures and association with remote environments at large physical offsets from their host galaxy, their origins remain uncertain. Using new discoveries from surveys such as the Zwicky Transient Facility, supplemented by archival data, we compiled a sample of 46 Ca-strong SNe, including 8 newly classified objects – the largest sample presented to date. We present a comprehensive optical photometric and spectroscopic study of Ca-strong SNe, aimed at better constraining their diversity, progenitor systems, and explosion mechanisms. This includes Gaussian decomposition of the [Ca II] and [O I] nebular emission lines to measure bulk velocities and line profile evolution from peak brightness to deep nebular-phase epochs. Of the 35 Ca-strong SNe with spectra near peak light, we find that 32 objects exhibit significant [Ca II] emission within 10 days of peak. Forbidden [Ca II] emission is typically expected to emerge after the ejecta have expanded, cooled, and reached a sufficiently low density for forbidden transitions to occur. The prevalence of these early features in Ca-strong SNe, however, remains unexplained by current explosion models. In several objects, we identify complex, multi-component line profiles that suggest a transition between two distinct velocity regimes. These transitional features, combined with the common presence of early forbidden emission, lead us to favour a scenario in which extended material surrounds the progenitor and pre-dates the SN itself. Our results suggest that the diverse spectroscopic behaviours of Ca-strong SNe are consistent with white dwarfs exploding in extended environments polluted by prior activity of the progenitor system.

Abstract:

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.

Abstract:

Abstract Tidal disruption event (TDE) light curves are increasingly used to infer the masses of quiescent supermassive black holes (MBH), offering a powerful probe of low-mass black hole demographics independent of host-galaxy scaling relations. However, most semi-analytic TDE models invoke assumptions aligned with a full stellar disruption, despite theoretical expectations that partial disruptions dominate the TDE population. In this work we test the robustness of current TDE models using three repeating partial TDEs (rpTDEs), in which the multiple flares produced by the same surviving stellar core must yield consistent black hole masses. We present spectroscopic observations establishing AT 2023adr as a rpTDE, making it the third such spectroscopically confirmed event. We independently model the flares of the three rpTDEs; 2020vdq, 2022dbl, and 2023adr, applying fallback-accretion fits, stream–stream collision scaling relations, luminosity-based empirical relations, accretion disc models, and cooling-envelope fits. After accounting for statistical and model-specific systematics, we find that all TDE models generally return self-consistent MBH values between flares, and are broadly consistent with host-galaxy MBH proxies, recovering MBH to within 0.3–0.5 dex. However, the convergence of fallback models towards unphysical stellar masses and impact parameters reveals limitations in existing fallback model grids. We also show that light curve coverage, particularly in the near-ultraviolet, is critical for constraining model parameters. This has direct implications for interpreting the thousands of TDE light curves expected from upcoming surveys such as the Rubin Observatory’s Legacy Survey of Space and Time, where from MOSFiT simulations, we find that MBH may be underestimated on average by 0.1 – 0.5 dex depending on peak light curve coverage and the availability of follow-up data.

Abstract:

We report the results of a photometric and spectroscopic follow-up campaign of the unusual interacting supernova (SN) 2022pda. Precursor variability lasting ∼100 days is observed before the explosion. The SN light curve has a double-peak shape. It reached a first maximum of Mr = −19.6 ± 0.2 mag, followed by an initial 2 month decline and a second, broad peak lasting about 6 months. The early spectra show a blue continuum with dominant H and He emission lines. A high-resolution pre-maximum spectrum shows that the profile of the He i λ 5876 line consists of a moderately narrow (∼1900 km s−1) P Cygni absorption superposed on a broader (∼3300 km s−1) component. In the blue region, several spectral features are identified, including C iii/N iii/O ii blends. Two broad bumps at 4600–5200 Å and 6400–6800 Å regions reveal a complex profile, which are likely due to blends of H, He, and other emission lines. Late-time spectra are still dominated by prominent and broad H and He lines in emission. Shock-driven model fits to the bolometric light curve suggest that the SN is powered by interaction with a massive CSM with enhanced mass-loss rates ∼5 M⊙yr−1, expelled during two events that occurred ∼1 and ∼0.2 yr before the explosion. The overall SN evolution indicates that SN 2022pda is a transitional event between an H-rich SN IIn (SN 2009ip-like) and an He-rich SN Ibn. Our findings suggest that the progenitor was likely a luminous blue variable transitioning towards a Wolf–Rayet stage.

Abstract:

ATLAS (Asteroid Terrestrial Last Alert System) is a time-domain survey using four telescopes, covering the entire sky. It has observed 8378 spectroscopically confirmed Type Ia supernovae (SNe Ia), with thousands of cosmology-grade light curves (to be released as TITAN DR1). To prepare this massive, low-redshift dataset for cosmology, we evaluate and cross-calibrate ATLAS forced photometry using tertiary stars from the DES (Dark Energy Survey) Y6 release. The 5000 deg2 DES footprint overlaps regions both in and out of the PS1 (Pan-STARRS DR1) footprint, allowing tests of the primary calibrator for the ATLAS Refcat2 catalog. Initial offsets are at the ∼40 mmag scale. To improve this, we determine Δ zero-point offsets for two cases: (1) pixel-to-pixel offsets within individual CCDs (reduced from ∼8 to ∼4 mmag rms) and (2) chip-to-chip offsets across the nine CCDs and filters (reduced from ∼17 to ∼3 mmag rms). We also identify the largest systematic uncertainty as a transmission-function color dependence, requiring shifts in the assumed ATLAS filters at the ∼30 mmag level if uncorrected. We validate our calibration using (a) CALSPEC standards, (b) an independent tertiary catalog, and (c) distance moduli of cross-matched SNe Ia, all showing improved consistency. Overall, we estimate combined calibration-related systematics at the ∼5–10 mmag level, supporting competitive cosmological constraints with the TITAN SN Ia dataset.