‘High-velocity features’ (HVFs) are spectral features in Type Ia supernovae (SNe Ia) that have minima indicating significantly higher (by greater than about 6000 km s⁻¹) velocities than typical ‘photospheric-velocity features’ (PVFs). The PVFs are absorption features with minima indicating typical photospheric (i.e. bulk ejecta) velocities (usually ∼9000–15 000 km s⁻¹ near B-band maximum brightness). In this work, we undertake the most in-depth study of HVFs ever performed. The data set used herein consists of 445 low-resolution optical and near-infrared (NIR) spectra (at epochs up to 5 d past maximum brightness) of 210 low-redshift SNe Ia that follow the ‘Phillips relation’. A series of Gaussian functions is fit to the data in order to characterize possible HVFs of Ca ii H&K, Si ii λ6355, and the Ca ii NIR triplet. The temporal evolution of the velocities and strengths of the PVFs and HVFs of these three spectral features is investigated, as are possible correlations with other SN Ia observables. We find that while HVFs of Ca ii are regularly observed (except in underluminous SNe Ia, where they are never found), HVFs of Si ii λ6355 are significantly rarer, and they tend to exist at the earliest epochs and mostly in objects with large photospheric velocities. It is also shown that stronger HVFs of Si ii λ6355 are found in objects that lack C ii absorption at early times and that have red ultraviolet/optical colours near maximum brightness. These results lead to a self-consistent connection between the presence and strength of HVFs of Si ii λ6355 and many other mutually correlated SN Ia observables, including photospheric velocity.

Observational evidence points to a red supergiant (RSG) progenitor for SN 2023ixf. The progenitor candidate has been detected in archival images at wavelengths (≥0.6 μm) where RSGs typically emit profusely. This object is distinctly variable in the infrared (IR). We characterize the variability using pre-explosion mid-IR (3.6 and 4.5 μm) Spitzer and ground-based near-IR (JHK _s ) archival data jointly covering 19 yr. The IR light curves exhibit significant variability with rms amplitudes in the range 0.2–0.4 mag, increasing with decreasing wavelength. From a robust period analysis of the more densely sampled Spitzer data, we measure a period of 1091 ± 71 days. We demonstrate using Gaussian process modeling that this periodicity is also present in the near-IR light curves, thus indicating a common physical origin, which is likely pulsational instability. We use a period–luminosity relation for RSGs to derive a value of M _K = −11.58 ± 0.31 mag. Assuming a late M spectral type, this corresponds to log ( L / L ⊙ ) = 5.27 ± 0.12 at T _eff = 3200 K and to log ( L / L ⊙ ) = 5.37 ± 0.12 at T _eff = 3500 K. This gives an independent estimate of the progenitor’s luminosity, unaffected by uncertainties in extinction and distance. Assuming the progenitor candidate underwent enhanced dust-driven mass loss during the time of these archival observations, and using an empirical period–luminosity–based mass-loss prescription, we obtain a mass-loss rate of around (2–4) × 10⁻⁴ M _⊙ yr⁻¹. Comparing the above luminosity with stellar evolution models, we infer an initial mass for the progenitor candidate of 20 ± 4 M _⊙, making this one of the most massive progenitors for a Type II SN detected to date.

Supernova (SN) explosions have been sought for decades as a possible source of dust in the Universe, providing the seeds of galaxies, stars, and planetary systems. SN 1987A offers one of the most promising examples of significant SN dust formation, but until the James Webb Space Telescope (JWST), instruments have traditionally lacked the sensitivity at both late times (>1 yr post-explosion) and longer wavelengths (i.e. >10 μm) to detect analogous dust reservoirs. Here we present JWST/MIRI observations of two historic Type IIP SNe, 2004et and SN 2017eaw, at nearly 18 and 5 yr post-explosion, respectively. We fit the spectral energy distributions as functions of dust mass and temperature, from which we are able to constrain the dust geometry, origin, and heating mechanism. We place a 90 per cent confidence lower limit on the dust masses for SNe 2004et and 2017eaw of >0.014 and >4 × 10⁻⁴ M_⊙, respectively. More dust may exist at even colder temperatures or may be obscured by high optical depths. We conclude dust formation in the ejecta to be the most plausible and consistent scenario. The observed dust is radiatively heated to ∼100–150 K by ongoing shock interaction with the circumstellar medium. Regardless of the best fit or heating mechanism adopted, the inferred dust mass for SN 2004et is the second highest (next to SN 1987A) mid-infrared inferred dust mass in extragalactic SNe thus far, promoting the prospect of SNe as potential significant sources of dust in the Universe.

We present extensive optical (UBV RI, , and open CCD) and near-infrared (ZY JH) photometry for the very nearby Type IIP SN 2013ej extending from +1 to +461 days after shock breakout, estimated to be MJD 56496.9 ± 0.3. Substantial time series ultraviolet and optical spectroscopy obtained from +8 to +135 days are also presented. Considering well-observed SNe IIP from the literature, we derive UBV RIJHK bolometric calibrations from UBV RI and unfiltered measurements that potentially reach 2% precision with a B − V color-dependent correction. We observe moderately strong Si ii as early as +8 days. The photospheric velocity () is determined by modeling the spectra in the vicinity of Fe ii whenever observed, and interpolating at photometric epochs based on a semianalytic method. This gives km s⁻¹ at +50 days. We also observe spectral homogeneity of ultraviolet spectra at +10–12 days for SNe IIP, while variations are evident a week after explosion. Using the expanding photosphere method, from combined analysis of SN 2013ej and SN 2002ap, we estimate the distance to the host galaxy to be Mpc, consistent with distance estimates from other methods. Photometric and spectroscopic analysis during the plateau phase, which we estimated to be 94 ± 7 days long, yields an explosion energy of erg, a final pre-explosion progenitor mass of 15.2 ± 4.2 and a radius of 250 ± 70 . We observe a broken exponential profile beyond +120 days, with a break point at +183 ± 16 days. Measurements beyond this break time yield a ⁵⁶Ni mass of 0.013 ± 0.001 M .