SN 2024abup is a nearby broad-lined Type Ic supernova (SN Ic-bl) in NGC 0681 at a distance of 23.3 ±1.6 Mpc. As energetic explosions of massive stars, SNe Ic-bl are considered a plausible site for rapid-neutron capture nucleosynthesis (r-process) and chemical enrichment from short-lived progenitors. They may also contribute to dust production in the early Universe. We present JWST near- to mid-infrared (NIR+MIR) observations (1-14 micron) of SN Ic-bl 2024abup at +41 days after the V band maximum (+54 days after explosion), the first-ever JWST+MIR observation of a SN Ic-bl along with radio and optical data. Using the spectral synthesis code SUMO, we identify the observed broad IR line features in SN 2024abup and find significant contributions from C, O, Mg, and carbon monoxide (CO) – the earliest detection of molecules in a core-collapse SN so far. The spectrum shows continuum emission at wavelengths greater than 1.5 micron, which could be explained by dust – preexisting, newly formed, or a combination-heated by the SN. We do not find compelling evidence for infrared signatures of r-process elements, though our search is hampered by the presence of many broad and blended features from the non-r-process elements. These new observations indicate that SNe Ic-bl could be a contributor to early-universe dust production, and suggest that if r-process elements are produced, revealing their presence from spectra requires very high-quality data and models to disentangle blends.

SN 2003fg-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 2003fg-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, 2003fg-like SNe Ia occur in galaxies with lower stellar mass, lower oxygen abundance, and marginally higher specific star-formation rate (sSFR) than those of normal SNe Ia, although their hosts are not as extreme in such properties as superluminous SNe, nor representative of metal-poor dwarf-galaxy samples. Locally, the SN positions show lower star-formation-rate, stellar-mass surface densities, and lower 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 2003fg-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 2003fg-like events. Our results imply that 2003fg-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 2003fg-like SNe Ia for low-metallicity environments suggests that they may have been more common in the early Universe. (abridged)

Little Red Dots (LRDs) are compact, high-redshift sources whose physical nature remains uncertain. Their optical spectra bear many similarities to Type IIn supernovae (SNe IIn), motivating a scenario in which their emission is powered by shocks interacting with dense surrounding material. We investigate whether such interactions can power LRDs and contribute to the diffuse high-energy neutrino intensity measured by IceCube. In our simplified model, a fast central-engine outflow drives a shock through dense surrounding material before stalling near the LRD photosphere, while some material continues to flow through the shock. We explore parameter ranges motivated by SNe IIn and the observed and inferred properties of LRDs, finding solutions with shock luminosities from 2.2e43 to 3.5e44 erg/s. Using an analytical framework for cosmic-ray acceleration and hadronic interactions in dense shock environments, we calculate the resulting high-energy neutrino emission and integrate it over the cosmological LRD population. For our fiducial SNe IIn-based cosmic-ray parameters, the average predicted contribution below 2e5 GeV increases from about 0.2

Producing stable 58Ni in Type Ia supernovae (SNe Ia) requires sufficiently high-density conditions that are not predicted for all origin scenarios, so examining the distribution of 58Ni using the near-infrared (NIR) [Ni II] 1.939 mu m line may observationally distinguish between possible progenitors and explosion mechanisms. We present 79 telluric-corrected NIR spectra of 22 low-redshift SNe Ia from the Carnegie Supernova Project-II, ranging from +50 to +505 days, including 31 previously unpublished spectra. We introduce the Gaussian Peak Ratio, a detection parameter that confirms the presence of the NIR [Ni II] 1.939 mu m line in eight SNe in our sample. Nondetections occur at earlier phases (<=+100 days) when the NIR Ni line has not emerged yet or in low signal-to-noise spectra, yielding inconclusive results. Subluminous 86G-like SNe Ia show the earliest NIR Ni features around similar to+50 days, whereas normal-bright SNe Ia do not exhibit NIR Ni until similar to+150 days. NIR Ni features detected in our sample have low peak velocities (v similar to 1200 km s-1) and narrow line widths (<= 3500 km s-1), indicating stable 58Ni is centrally located. This implies high-density burning conditions in the innermost regions of SNe Ia and could be due to higher mass progenitors (i.e., near-Mch). NIR spectra of the nearly two dozen SNe Ia in our sample are compared to various model predictions and paired with early-time properties to identify ideal observation windows for future SNe Ia discovered by upcoming surveys with Rubin-LSST or the Roman Space Telescope.

Spectropolarimetry provides a unique probe of ejecta asphericities, offering direct insights into the underlying explosion physics of Type Ia supernovae (SNe Ia). We analyze the statistical properties of premaximum spectropolarimetric data for 24 SNe Ia observed with the FOcal Reducer and low dispersion Spectrograph on the Very Large Telescope, focusing on the Si ii λ 6355 Å line. Previous studies have revealed a correlation between the peak Si ii polarization degree and the expansion velocity. Here, we combine these observations with multidimensional nonthermodynamical equilibrium radiative transfer simulations. We consider two asphericity classes: (i) lopsided abundance distributions produced by off-center delayed-detonation transitions in near- M _Ch white dwarfs (WDs) or, for example, WD collisions (class I), and (ii) global axisymmetric density asphericities such as those arising from explosions of rapidly rotating WDs or mergers (class II). Our model grid spans normal to subluminous SNe Ia and successfully reproduces the observed Si ii velocity–polarization trend, with higher velocities associated with stronger asphericities. Consistent with observations, transitional SNe Ia and the faint end of the normal SN Ia population show the highest Si ii polarization and are best explained by class I scenarios. In contrast, subluminous SNe Ia are dominated by class II asphericities, characterized by lower Si ii polarization but significant continuum polarization. The observed distribution of Si ii polarization depends on both the observer’s viewing angle θ and the intrinsic asphericity. Statistical analysis of these spectropolarimetric snapshots enables the separation of class I and class II contributions and highlights the intrinsic diversity among SNe Ia. Our results imply viewing-angle-dependent luminosities in our local sample, which may have implications when using high-redshift SNe Ia as evidence for the need for nonstandard cosmology.

Context. Core-collapse supernovae (CCSNe) may have contributed a significant amount of dust in the early Universe. Freshly formed coolant molecules (e.g., CO) and warm dust can be found in CCSNe as early as similar to 100 d after the SN explosion, allowing the study of their evolution with time series observations. Aims. Through study of the Type II SN 2023ixf, we aim to investigate the temporal evolution of the temperature, velocity, and mass of CO and compare them with other CCSNe, exploring their implications for the dust formation in CCSNe. From observations of velocity profiles of lines of other species (e.g., H and He), we also aim to characterize and understand the interaction of the SN ejecta with preexisting circumstellar material (CSM). Methods. We present a time series of 16 near-infrared spectra of SN 2023ixf from 9 to 307 d, taken with multiple instruments: Gemini/GNIRS, Keck/NIRES, IRTF/SpeX, and MMT/MMIRS. Results. The early (t less than or similar to 70 d) spectra indicate interaction between the expanding ejecta and nearby CSM. At t less than or similar to 20 d, intermediate-width line profiles corresponding to the ejecta-wind interaction are superposed on evolving broad P Cygni profiles. We find intermediate-width and narrow lines in the spectra until t less than or similar to 70 d, which suggest continued CSM interaction. We also observe and discuss high-velocity absorption features in H alpha and H beta line profiles formed by CSM interaction. The spectra contain CO first overtone emission between 199 and 307 d after the explosion. We modeled the CO emission and found the CO to have a higher velocity (3000-3500 km s(-1)) than that in Type II-pec SN 1987A (1800-2000 km s(-1)) during similar phases (t = 199 - 307 d) and a comparable CO temperature to SN 1987A. A flattened continuum at wavelengths greater than 1.5 mu m accompanies the CO emission, suggesting that the warm dust is likely formed in the ejecta. The warm dust masses are estimated to be on the order of similar to 10(-5) M-circle dot.

Carbon–oxygen (CO) detonation with reactions terminating either after burning of ^12 C in the leading ^12 C + ^12 C reaction or after burning of ^12 C and ^16 O to Si-group elements may occur in the low-density outer layers of exploding white dwarfs and be responsible for the production of intermediate-mass elements observed in the outer layers of Type Ia supernovae. Basic one-dimensional properties of CO-detonations have been summarized in our previous work. This paper presents the results of two- and three-dimensional numerical simulations of low-density CO-detonations and discusses their multidimensional stability, cellular structure, and propagation through a constant low-density background. We find three-dimensional CO detonations to be strikingly different from their one-dimensional and two-dimensional counterparts. Three-dimensional detonations are significantly more robust and capable of propagating without decay compared to highly unstable and marginal one- and two-dimensional detonations. The detonation cell size and whether burning of ^12 C in a three-dimensional detonation wave is followed by the subsequent ^16 O burning are sensitive to both the background density and the initial ^12 C to ^16 O mass ratio. We also discuss the possible implications for understanding the observed early-time bumps in light curves.

The death of massive stars is triggered by an infall-induced bounce shock that disrupts the star. How such a shock is launched and propagates through the star is a decade-long puzzle. Some models assume that the shock can be reenergized by absorbing neutrinos, leading to highly aspherical explosions. Other models involve jet-powered shocks that lead to bipolar explosions reflected in the geometry of the shock-breakout emission. We report measurement of the geometry of the shock breakout through unprecedentedly early spectropolarimetry of the nearby type II supernova 2024ggi starting ~1.2 days after the explosion. The measurement indicates a well-defined symmetry axis of the shock breakout, which is also shared by the hydrogen-rich envelope that emerged after the circumstellar matter was engulfed by the ejecta, revealing a persisting and prominent symmetry axis throughout the explosion. These findings suggest that the physical mechanism driving the explosion of massive stars manifests a well-defined axial symmetry and acts on large scales.

We present an analysis of three near-infrared (NIR; 1.0–2.4 μ m) spectra of the SN 2003fg–like/“super-Chandrasekhar” Type Ia supernovae (SNe Ia) SN 2009dc, SN 2020hvf, and SN 2022pul at respective phases of +372, +296, and +294 days relative to the epoch of B -band maximum. We find that all objects in our sample have asymmetric, or “tilted,” [Fe ii ] 1.257 and 1.644 μ m profiles. We quantify the asymmetry of these features using five methods: velocity at peak flux, profile tilts, residual testing, velocity fitting, and comparison to deflagration–detonation transition models. Our results demonstrate that, while the profiles of the [Fe ii ] 1.257 and 1.644 μ m features are widely varied between 2003fg-likes, these features are correlated in shape within the same SNe. This implies that line blending is most likely not the dominant cause of the asymmetries inferred from these profiles. Instead, it is more plausible that 2003fg-like SNe have aspherical chemical distributions in their inner regions. These distributions may come from aspherical progenitor systems, such as double white dwarf mergers, or off-center delayed-detonation explosions of near-Chandrasekhar mass carbon–oxygen white dwarfs. Additional late-phase NIR observation of 2003fg-like SNe and detailed 3D non-LTE modeling of these two explosion scenarios are encouraged.

Thermodynamical explosions of White Dwarfs (WD)are one of the keys to high precision cosmology. Nebular spectra, namely mid-infrared (MIR) with JWST are an effective tool to probe for the multi-dimensional imprints of the explosion physics of WDs and their progenitor systems but also pose a challenge for simulations. What we observe as SNe Ia are low-energy photons, namely light curves, and spectra detected some days to years after the explosion. The light is emitted from a rapidly expanding envelope consisting of a low-density and low-temperature plasma with atomic population numbers far from thermodynamical equilibrium. SNe Ia are powered radioactive decays which produce hard X- and gamma-rays and MeV leptons which are converted within the ejecta to low-energy photons. We find that the optical and IR nebular spectra depend sensitively on the proper treatment of the physical conversion of high to low energies. The low-energy photons produced by forbidden line transitions originate from a mostly optically thin envelope. However, the UV is optically thick because of a quasi-continuum formed by allowed lines and bound-free transitions even several years after the explosion. We find that stimulated recombination limits the over-ionization of high ions with populations governed by the far UV. The requirements to simulate nebular spectra are well beyond both classical stellar atmospheres and nebulae. Using our full non-LTE HYDrodynamical RAdiation code (HYDRA) as a test-bed, the sensitivity on the physics on synthetic spectra are demonstrated using observations as a benchmark. At some examples, we establish the power of high-precision nebular spectroscopy as quantitative tool. Centrally ignited, off-center delayed-detonation near Chandrasekhar-mass models can reproduce line-ratios and line profiles of Branch-normal and underluminous SNe Ia observed with JWST.

We present optical photometric and spectroscopic observations of the peculiar Type Ia supernovae (SNe Ia) ASASSN-20jq/SN 2020qxp. It is a low-luminosity object, with a peak absolute magnitude of M-B = -17.1 +/- 0.5 mag, while its post-peak light-curve decline rate of Delta m(15)(B) = 1.35 +/- 0.09 mag and color-stretch parameter of s(BV) & gap; 0.82 is similar to that of normal luminosity SNe Ia. That makes it a prevalent outlier in both the SN Ia luminosity-width and the luminosity-color-stretch relations. The analysis of the early light curves indicates a possible "bump" during the first approximate to 1.4 days of explosion. ASASSN-20jq synthesized a low radioactive Ni-56 mass of 0.09 +/- 0.01 M-circle dot. The near-maximum light spectra of the supernova show strong Si II absorption lines, indicating a cooler photosphere than normal SNe Ia; however, it lacks Ti II absorption lines. Additionally, it shows unusually strong absorption features of O I lambda 7773 and the Ca II near-infrared triplet. The nebular spectra of ASASSN-20jq show a remarkably strong but narrow forbidden [Ca II] lambda lambda 7291, 7324 doublet emission that has not been seen in SNe Ia except for a handful of Type Iax events. There is also a marginal detection of the [O I] lambda lambda 6300, 6364 doublet emission in nebular spectra, which is extremely rare. Both the [Ca II] and [O I] lines are redshifted by roughly 2000 km s(-1). ASASSN-20jq also exhibits a strong [Fe II] lambda 7155 emission line with a tilted-top line profile, which is identical to the [Fe II] lambda 16433 line profile. The asymmetric [Fe II] line profiles, along with the redshifted [Ca II] and emission lines, suggest a high central density white dwarf progenitor that underwent an off-center delayed-detonation explosion mechanism, synthesizing roughly equal amounts of Ni-56 during the deflagration and detonation burning phases. The equal production of Ni-56 in both burning phases distinguishes ASASSN-20jq from normal bright and subluminous SNe Ia. Assuming this scenario, we simultaneously modeled the optical and near-infrared nebular spectra, achieving a good agreement with the observations. The light curve and spectroscopic features of ASASSN-20jq do not align with any single sub-class of SNe Ia. However, the significant deviation from the luminosity versus light-curve shape relations (along with several light-curve and spectroscopic features) exhibits similarities to some 2002es-like objects. Therefore, we have identified ASASSN-20jq as an extreme candidate within the broad and heterogeneous parameter space of 2002es-like SNe Ia.

We present 3D hydrodynamical modelling of supernova (SN)-induced binary-interaction-powered SNe; a scenario proposed for the peculiar type Ic SN SN2022jli. In this scenario, SN ejecta of a stripped-envelope star impact a close-by stellar companion, temporarily inflating the envelope. The expanded envelope engulfs the neutron star (NS), causing strong mass accretion at super-Eddington rates. Feedback from the accretion powers the SN light curve with periodic undulations. Our simulations capture key features of SN2022jli, both the overall decline and the superimposed undulations of the light curve. Based on our parameter study, we find that (i) the accretion feedback should be sufficiently geometrically confined and (ii) the eccentricity of the post-SN binary orbit should be 0.8 ≲ e ≲ 0.9 to sustain a high accretion rate and match the low undulation amplitude (Δ L / L ∼ 0.1) of SN2022jli. Different combinations of parameters could account for other SNe like SN2022mop, SN2009ip and SN2015ap, which have varying undulation periods and amplitudes. We also discuss possible explanations for other key features of SN2022jli such as the γ -ray detection at ∼200 days and the rapid optical drop at ∼250 days. Finally, we speculate on the future evolution of the system and its relation to existing NS binaries.

We present the second and final release of optical spectroscopy of Type Ia Supernovae (SNe Ia) obtained during the first and second phases of the Carnegie Supernova Project (CSP-I and CSP-II). The newly released data consist of 148 spectra of 30 SNe Ia observed in the course of the CSP-I, and 234 spectra of 127 SNe Ia obtained during the CSP-II. We also present 216 optical spectra of 46 historical SNe Ia, including 53 spectra of 30 SNe Ia observed by the Calán/Tololo Supernova Survey. We combine these observations with previously published CSP data and publicly-available spectra to compile a large sample of measurements of spectroscopic parameters at maximum light, consisting of pseudo-equivalent widths and expansion velocities of selected features, for 232 CSP and historical SNe Ia (including more than 1000 spectra). Finally, we review some of the strongest correlations between spectroscopic and photometric properties of SNe Ia. Specifically, we define two samples: one consisting of SNe Ia discovered by targeted searches (most of them CSP-I objects) and the other composed of SNe Ia discovered by untargeted searches, which includes most of the CSP-II objects. The analysed correlations are similar for both samples. We find a larger incidence of SNe Ia belonging to the Cool (CL)and Broad Line (BL) Branch subtypes among the events discovered by targeted searches, Shallow Silicon (SS) SNe Ia are present with similar frequencies in both samples, while Core Normal (CN) SNe Ia are more frequent in untargeted searches.

We present JWST spectral and photometric observations of the Type IIP supernova (SN) 2022acko at  50 days past explosion. These data are the first JWST spectral observations of a core-collapse SN. We identify  30 different H I features, other features associated with products produced from the CNO cycle, and s-process elements such as Sc II and Ba II. By combining the JWST spectra with ground-based optical and NIR spectra, we construct a full Spectral Energy Distribution from 0.4 to 25 microns and find that the JWST spectra are fully consistent with the simultaneous JWST photometry. The data lack signatures of CO formation and we estimate a limit on the CO mass of < 10^-8 solar mass. We demonstrate how the CO fundamental band limits can be used to probe underlying physics during stellar evolution, explosion, and the environment. The observations indicate little mixing between the H envelope and C/O core in the ejecta and show no evidence of dust. The data presented here set a critical baseline for future JWST observations, where possible molecular and dust formation may be seen.

Dust associated with various stellar sources in galaxies at all cosmic epochs remains a controversial topic, particularly whether supernovae (SNe) play an important role in dust production. We report evidence of dust formation in the cold, dense shell behind the ejecta-circumstellar medium (CSM) interaction in the Type Ia-CSM SN 2018evt three years after the explosion, characterized by a rise in the mid-infrared (MIR) emission accompanied by an accelerated decline in the optical radiation of the SN. Such a dust-formation picture is also corroborated by the concurrent evolution of the profiles of the Ha emission line. Our model suggests enhanced CSM dust concentration at increasing distances from the SN as compared to what can be expected from the density profile of the mass loss from a steady stellar wind. By the time of the last MIR observations at day +1041, a total amount of 1.2+-0.2x10^{-2} Msun of new dust has been formed by SN 2018evt, making SN 2018evt one of the most prolific dust factories among SNe with evidence of dust formation. The unprecedented witness of the intense production procedure of dust may shed light on the perceptions of dust formation in cosmic history.

We present a method of extrapolating the spectroscopic behavior of Type Ia supernovae (SNe Ia) in the near-infrared (NIR) wavelength regime up to 2.30 $\mu$m using optical spectroscopy. Such a process is useful for accurately estimating K-corrections and other photometric quantities of SNe Ia in the NIR. Principal component analysis is performed on data consisting of Carnegie Supernova Project I & II optical and near-infrared FIRE spectra to produce models capable of making these extrapolations. This method differs from previous spectral template methods by not parameterizing models strictly by photometric light-curve properties of SNe Ia, allowing for more flexibility of the resulting extrapolated NIR flux. A difference of around -3.1% to -2.7% in the total integrated NIR flux between these extrapolations and the observations is seen here for most test cases including Branch core-normal and shallow-silicon subtypes. However, larger deviations from the observation are found for other tests, likely due to the limited high-velocity and broad-line SNe Ia in the training sample. Maximum-light principal components are shown to allow for spectroscopic predictions of the color-stretch light-curve parameter, $s_{BV}$, within approximately $\pm$0.1 units of the value measured with photometry. We also show these results compare well with NIR templates, although in most cases the templates are marginally more fitting to observations, illustrating a need for more concurrent optical+NIR spectroscopic observations to truly understand the diversity of SNe Ia in the NIR.

We present a JWST mid-infrared spectrum of the under-luminous Type Ia Supernova (SN Ia) 2022xkq. The spectrum was obtained with the medium-resolution spectrometer on the Mid-Infrared Instrument (MIRI) roughly 130 days after explosion. We identify the first MIR lines beyond 14 $\mu$m in SN Ia observations. We also find distinct features unique to under-luminous SNe Ia, including: isolated emission of stable Ni, strong blends of [Ti II], and large ratios of singly ionized to doubly ionized species in both [Ar] and [Co]. Comparisons to normal-luminosity SNe Ia spectra at similar phases show a tentative trend between the width of the [Co III] 11.888 $\mu$m feature and the SN light curve shape. Using non-LTE-multi-dimensional radiation hydro simulations and the observed electron capture elements we constrain the mass of the exploding white dwarf. The best-fitting model shows that SN 2022xkq is consistent with an off-center delayed-detonation explosion of a near-Chandrasekhar mass WD of high-central density ($\rho_{c} \geq 2.0 \times 10^{9}$ g cm$^{-3}$) seen equator on, and produced M($^{58}$Ni) $\geq 0.06$ M$_{\odot}$. The observed line width of various species are consistent with the overall abundance distribution; and the narrow stable Ni lines indicate little to no mixing in the central regions, favoring central ignition of sub-sonic carbon burning followed by an off-center DDT which begins at a single point. Observations at later epochs may further constrain the physics revealing the presence of additional species including Cr and Mn. Our work demonstrates the power of using the full coverage of MIRI in combination with detailed modeling to elucidate the physics of SNe Ia at a level not previously possible.

We present an analysis of Type Ia supernovae (SNe Ia) from the Carnegie Supernova Project I and II and extend the Hubble diagram from optical to near-infrared wavelengths ( uBgVriYJH ). We calculate the Hubble constant, H 0 , using various distance calibrators: Cepheids, the tip of the red giant branch (TRGB), and surface brightness fluctuations (SBFs). Combining all methods of calibration, we derive H 0 = 71.76 ± 0.58 (stat) ± 1.19 (sys) km s −1 Mpc −1 from the B band and H 0 = 73.22 ± 0.68 (stat) ± 1.28 (sys) km s −1 Mpc −1 from the H band. By assigning equal weight to the Cepheid, TRGB, and SBF calibrators, we derive the systematic errors required for consistency in the first rung of the distance ladder, resulting in a systematic error of 1.2 ∼ 1.3 km s −1 Mpc −1 in H 0 . As a result, relative to the statistics-only uncertainty, the tension between the late-time H 0 we derive by combining the various distance calibrators and the early-time H 0 from the cosmic microwave background is reduced. The highest precision in SN Ia luminosity is found in the Y band (0.12 ± 0.01 mag), as defined by the intrinsic scatter ( σ int ). We revisit SN Ia Hubble residual-host mass correlations and recover previous results that these correlations do not change significantly between the optical and near-infrared wavelengths. Finally, SNe Ia that explode beyond 10 kpc from their host centers exhibit smaller dispersion in their luminosity, confirming our earlier findings. A reduced effect of dust in the outskirts of hosts may be responsible for this effect.

Understanding the nature of the luminous 1991T-like supernovae (SNe) is of great importance to SN cosmology as they are likely to have been more common in the early Universe. In this paper, we explore the observational properties of 1991T-like SNe to study their relationship to other luminous, slow-declining Type Ia supernovae (SNe Ia). From the spectroscopic and photometric criteria defined in Phillips et al., we identify 17 1991T-like SNe from the literature. Combining these objects with 10 1991T-like SNe from the Carnegie Supernova Project-II, the spectra, light curves, and colors of these events, along with their host galaxy properties, are examined in detail. We conclude that 1991T-like SNe are closely related in essentially all of their UV, optical, and near-infrared properties—as well as their host galaxy parameters—to the slow-declining subset of Branch core-normal SNe and to the intermediate 1999aa-like events, forming a continuum of luminous SNe Ia. The overriding difference between these three subgroups appears to be the extent to which 56 Ni mixes into the ejecta, producing the premaximum spectra dominated by Fe iii absorption, the broader UV light curves, and the higher luminosities that characterize the 1991T-like events. Nevertheless, the association of 1991T-like SNe with the rare Type Ia circumstellar material SNe would seem to run counter to this hypothesis, in which case 1991T-like events may form a separate subclass of SNe Ia, possibly arising from single-degenerate progenitor systems.

Type II supernovae (SNeII) mark the endpoint in the lives of hydrogen-rich massive stars. Their large explosion energies and luminosities allow us to measure distances, metallicities, and star formation rates into the distant Universe. To fully exploit their use in answering different astrophysical problems, high-quality low-redshift data sets are required. Such samples are vital to understand the physics of SNeII, but also to serve as calibrators for distinct - and often lower-quality - samples. We present uBgVri optical and YJH near-infrared (NIR) photometry for 94 low-redshift SNeII observed by the Carnegie Supernova Project (CSP). A total of 9817 optical and 1872 NIR photometric data points are released, leading to a sample of high-quality SNII light curves during the first 150 days post explosion on a well-calibrated photometric system. The sample is presented and its properties are analysed and discussed through comparison to literature events. We also focus on individual SNeII as examples of classically defined subtypes and outlier objects. Making a cut in the plateau decline rate of our sample (s2), a new subsample of fast-declining SNeII is presented. The sample has a median redshift of 0.015, with the nearest event at 0.001 and the most distant at 0.07. At optical wavelengths (V), the sample has a median cadence of 4.7 days over the course of a median coverage of 80 days. In the NIR (J), the median cadence is 7.2 days over the course of 59 days. The fast-declining subsample is more luminous than the full sample and shows shorter plateau phases. Of the non-standard SNeII highlighted, SN2009A particularly stands out with a steeply declining then rising light curve, together with what appears to be two superimposed P-Cygni profiles of H-alpha in its spectra. We outline the significant utility of these data, and finally provide an outlook of future SNII science.