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1. Distribution of Si ii λ6355 velocities of Type Ia supernovae and implications for asymmetric explosions

   https://doi.org/10.1093/mnras/staa3191  

   Zhang, Keto D ; Zheng, WeiKang ; de Jaeger, Thomas ; Stahl, Benjamin E ; Brink, Thomas G ; Han, Xuhui ; Kasen, Daniel ; Shen, Ken J ; Tang, Kevin ; Filippenko, Alexei V ( November 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The ejecta velocity is a very important parameter in studying the structure and properties of Type Ia supernovae (SNe Ia) and is a candidate key parameter in improving the utility of SNe Ia for cosmological distance determinations. Here, we study the velocity distribution of a sample of 311 SNe Ia from the kaepora data base. The velocities are derived from the Si ii λ6355 absorption line in optical spectra measured at (or extrapolated to) the time of peak brightness. We statistically show that the observed velocity has a bimodal Gaussian distribution (population ratio 201:110 or 65 per cent:35 per cent) consisting of two groups of SNe Ia: Group I with a lower but narrower scatter ($11\, 000 \pm 700\, \mathrm{km\, s}^{-1}$), and Group II with a higher but broader scatter ($12\, 300 \pm 1800\, \mathrm{km\, s}^{-1}$). The true origin of the two components is unknown. Naturally, there could exist two intrinsic velocity distributions observed. However, we try to use asymmetric geometric models through statistical simulations to reproduce the observed distribution assuming that all SNe Ia share the same intrinsic distribution. In the two cases we consider, 35 per cent of SNe Ia are considered to be asymmetric in Case 1, and all SNe Ia are asymmetric in Case 2. Simulations for both cases canmore »reproduce the observed velocity distribution but require a significantly large portion ($\gt 35{{\ \rm per\ cent}}$) of SNe Ia to be asymmetric. In addition, the Case 1 result is consistent with recent SNe Ia polarization observations that higher Si ii λ6355 velocities tend to be more polarized.« less
2. Carnegie Supernova Project. II. Near-infrared Spectral Diversity and Template of Type Ia Supernovae

   https://doi.org/10.3847/1538-4357/acc100  

   Lu, Jing ; Hsiao, Eric Y. ; Phillips, Mark M. ; Burns, Christopher R. ; Ashall, Chris ; Morrell, Nidia ; Ng, Lawrence ; Kumar, Sahana ; Shahbandeh, Melissa ; Hoeflich, Peter ; et al ( May 2023 , The Astrophysical Journal)

   We present the largest and most homogeneous collection of near-infrared (NIR) spectra of Type Ia supernovae (SNe Ia): 339 spectra of 98 individual SNe obtained as part of the Carnegie Supernova Project-II. These spectra, obtained with the FIRE spectrograph on the 6.5 m Magellan Baade telescope, have a spectral range of 0.8–2.5μm. Using this sample, we explore the NIR spectral diversity of SNe Ia and construct a template of spectral time series as a function of the light-curve-shape parameter, color stretchs_BV. Principal component analysis is applied to characterize the diversity of the spectral features and reduce data dimensionality to a smaller subspace. Gaussian process regression is then used to model the subspace dependence on phase and light-curve shape and the associated uncertainty. Our template is able to predict spectral variations that are correlated withs_BV, such as the hallmark NIR features: Mgiiat early times and theH-band break after peak. Using this template reduces the systematic uncertainties inK-corrections by ∼90% compared to those from the Hsiao template. These uncertainties, defined as the meanK-correction differences computed with the color-matched template and observed spectra, are on the level of 4 × 10⁻⁴mag on average. This template can serve as the baseline spectral energymore »distribution for light-curve fitters and can identify peculiar spectral features that might point to compelling physics. The results presented here will substantially improve future SN Ia cosmological experiments, for both nearby and distant samples.

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3. The Berkeley sample of Type II supernovae: BVRI light curves and spectroscopy of 55 SNe II

   https://doi.org/10.1093/mnras/stz2714  

   de Jaeger, T. ; Zheng, W. ; Stahl, B. E. ; Filippenko, A. V. ; Brink, T. G. ; Bigley, A. ; Blanchard, K. ; Blanchard, P. K. ; Bradley, J. ; Cargill, S. K. ; et al ( September 2019 , Monthly Notices of the Royal Astronomical Society)

   In this work, BVRI light curves of 55 Type II supernovae (SNe II) from the Lick Observatory Supernova Search programme obtained with the Katzman Automatic Imaging Telescope and the 1 m Nickel telescope from 2006 to 2018 are presented. Additionally, more than 150 spectra gathered with the 3 m Shane telescope are published. We conduct an analyse of the peak absolute magnitudes, decline rates, and time durations of different phases of the light and colour curves. Typically, our light curves are sampled with a median cadence of 5.5 d for a total of 5093 photometric points. In average, V-band plateau declines with a rate of 1.29 mag (100 d)−1, which is consistent with previously published samples. For each band, the plateau slope correlates with the plateau length and the absolute peak magnitude: SNe II with steeper decline have shorter plateau duration and are brighter. A time-evolution analysis of spectral lines in term of velocities and pseudo-equivalent widths is also presented in this paper. Our spectroscopic sample ranges between 1 and 200 d post-explosion and has a median ejecta expansion velocity at 50 d post-explosion of 6500 km s−1 (H α line) and a standard dispersion of 2000 km s−1. Nebular spectra are in good agreement with theoretical models using amore »progenitor star having a mass <16M⊙. All the data are available to the community and will help to understand SN II diversity better, and therefore to improve their utility as cosmological distance indicators.

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4. Carnegie Supernova Project: The First Homogeneous Sample of Super-Chandrasekhar-mass/2003fg-like Type Ia Supernovae

   https://doi.org/10.3847/1538-4357/ac19ac  

   Ashall, C. ; Lu, J. ; Hsiao, E. Y. ; Hoeflich, P. ; Phillips, M. M. ; Galbany, L. ; Burns, C. R. ; Contreras, C. ; Krisciunas, K. ; Morrell, N. ; et al ( November 2021 , The Astrophysical Journal)

   Abstract We present a multiwavelength photometric and spectroscopic analysis of 13 super-Chandrasekhar-mass/2003fg-like Type Ia supernovae (SNe Ia). Nine of these objects were observed by the Carnegie Supernova Project. The 2003fg-like SNe have slowly declining light curves (Δ m 15 ( B ) < 1.3 mag), and peak absolute B -band magnitudes of −19 < M B < −21 mag. Many of the 2003fg-like SNe are located in the same part of the luminosity–width relation as normal SNe Ia. In the optical B and V bands, the 2003fg-like SNe look like normal SNe Ia, but at redder wavelengths they diverge. Unlike other luminous SNe Ia, the 2003fg-like SNe generally have only one i -band maximum, which peaks after the epoch of the B -band maximum, while their near-IR (NIR) light-curve rise times can be ≳40 days longer than those of normal SNe Ia. They are also at least 1 mag brighter in the NIR bands than normal SNe Ia, peaking above M H = −19 mag, and generally have negative Hubble residuals, which may be the cause of some systematics in dark-energy experiments. Spectroscopically, the 2003fg-like SNe exhibit peculiarities such as unburnt carbon well past maximum light, a large spread (8000–12,000more »km s −1 ) in Si ii λ 6355 velocities at maximum light with no rapid early velocity decline, and no clear H -band break at +10 days. We find that SNe with a larger pseudo-equivalent width of C ii at maximum light have lower Si ii λ 6355 velocities and more slowly declining light curves. There are also multiple factors that contribute to the peak luminosity of 2003fg-like SNe. The explosion of a C–O degenerate core inside a carbon-rich envelope is consistent with these observations. Such a configuration may come from the core-degenerate scenario.« less
5. Carnegie Supernova Project: kinky i -band light curves of Type Ia supernovae

   https://doi.org/10.1093/mnras/stab3593  

   Pessi, P. J. ; Hsiao, E. Y. ; Folatelli, G. ; Anderson, J. P. ; Burns, C. R. ; Uddin, S. ; Galbany, L. ; Phillips, M. M. ; Morrell, N. ; Ashall, C. ; et al ( December 2021 , Monthly Notices of the Royal Astronomical Society)

   We present detailed investigation of a specific i-band light-curve feature in Type Ia supernovae (SNe Ia) using the rapid cadence and high signal-to-noise ratio light curves obtained by the Carnegie Supernova Project. The feature is present in most SNe Ia and emerges a few days after the i-band maximum. It is an abrupt change in curvature in the light curve over a few days and appears as a flattening in mild cases and a strong downward concave shape, or a ‘kink’, in the most extreme cases. We computed the second derivatives of Gaussian Process interpolations to study 54 rapid-cadence light curves. From the second derivatives we measure: (1) the timing of the feature in days relative to i-band maximum; tdm2(i) and (2) the strength and direction of the concavity in mag d−2; dm2(i). 76 per cent of the SNe Ia show a negative dm2(i), representing a downward concavity – either a mild flattening or a strong ‘kink’. The tdm2(i) parameter is shown to correlate with the colour-stretch parameter sBV, a SN Ia primary parameter. The dm2(i) parameter shows no correlation with sBV and therefore provides independent information. It is also largely independent of the spectroscopic and environmental properties. Dividing the sample based on the strength of themore »light-curve feature as measured by dm2(i), SNe Ia with strong features have a Hubble diagram dispersion of 0.107 mag, 0.075 mag smaller than the group with weak features. Although larger samples should be obtained to test this result, it potentially offers a new method for improving SN Ia distance determinations without shifting to more costly near-infrared or spectroscopic observations.

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