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1. Carnegie Supernova Project-II: The Near-infrared Spectroscopy Program

   Hsiao, E. Y. ; Phillips, M. M. ; Marion, G. H. ; Kirshner, R. P. ; Morrell, N. ; Sand, D. J. ; Burns, C. R. ; Contreras, C. ; Hoeflich, P. ; Stritzinger, M. D. ; et al ( January 2019 , Publications of the Astronomical Society of the Pacific)

   We present the H-band wavelength region of 37 postmaximum light near-infrared spectra of three normal, nine transitional, and four subluminous type Ia supernovae (SNe Ia), extending from +5 days to +20 days relative to the epoch of B-band maximum. We introduce a new observable, the blue-edge velocity, v _edge, of the prominent Fe/Co/Ni-peak H-band emission feature, which is quantitatively measured. The v _edge parameter is found to decrease over subtype ranging from around ‑14,000 km s^‑1 for normal SNe Ia, to ‑10,000 km s^‑1 for transitional SNe Ia, down to ‑5000 km s^‑1 for the subluminous SNe Ia. Furthermore, inspection of the +10 ± 3 days spectra indicates that v _edge is correlated with the color-stretch parameter, s _BV , and hence with peak luminosity. These results follow the previous findings that brighter SNe Ia tend to have ⁵⁶Ni located at higher velocities as compared to subluminous objects. As v _edge is a model-independent parameter, we propose it can be used in combination with traditional observational diagnostics to provide a new avenue to robustly distinguish between leading SNe Ia explosion models. This Letter includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.
2. 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
3. A Speed Bump: SN 2021aefx Shows that Doppler Shift Alone Can Explain Early Excess Blue Flux in Some Type Ia Supernovae

   https://doi.org/10.3847/2041-8213/ac7235  

   Ashall, C. ; Lu, J. ; Shappee, B. J. ; Burns, C. R. ; Hsiao, E. Y. ; Kumar, S. ; Morrell, N. ; Phillips, M. M. ; Shahbandeh, M. ; Baron, E. ; et al ( June 2022 , The Astrophysical Journal Letters)

   Abstract We present early-time photometric and spectroscopic observations of the Type Ia supernova (SN Ia) 2021aefx. The early-time u -band light curve shows an excess flux when compared to normal SNe Ia. We suggest that the early excess blue flux may be due to a rapid change in spectral velocity in the first few days post explosion, produced by the emission of the Ca ii H&K feature passing from the u to the B bands on the timescale of a few days. This effect could be dominant for all SNe Ia that have broad absorption features and early-time velocities over 25,000 km s −1 . It is likely to be one of the main causes of early excess u -band flux in SNe Ia that have early-time high velocities. This effect may also be dominant in the UV filters, as well as in places where the SN spectral energy distribution is quickly rising to longer wavelengths. The rapid change in velocity can only produce a monotonic change (in flux-space) in the u band. For objects that explode at lower velocities, and have a more structured shape in the early excess emission, there must also be an additional parameter producing themore »early-time diversity. More early-time observations, in particular early spectra, are required to determine how prominent this effect is within SNe Ia.« less
4. Linear spectropolarimetry of 35 Type Ia supernovae with VLT/FORS: an analysis of the Si ii line polarization

   https://doi.org/10.1093/mnras/stz2322  

   Cikota, Aleksandar ; Patat, Ferdinando ; Wang, Lifan ; Wheeler, J. Craig ; Bulla, Mattia ; Baade, Dietrich ; Höflich, Peter ; Cikota, Stefan ; Clocchiatti, Alejandro ; Maund, Justyn R. ; et al ( August 2019 , Monthly Notices of the Royal Astronomical Society)

   Spectropolarimetry enables us to measure the geometry and chemical structure of the ejecta in supernova explosions, which is fundamental for the understanding of their explosion mechanism(s) and progenitor systems. We collected archival data of 35 Type Ia supernovae (SNe Ia), observed with Focal Reducer and Low-Dispersion Spectrograph (FORS) on the Very Large Telescope at 127 epochs in total. We examined the polarization of the Si ii λ6355 Å line ($p_{\rm Si\, \small {II}}$) as a function of time, which is seen to peak at a range of various polarization degrees and epochs relative to maximum brightness. We reproduced the $\Delta m_{15}\!-\!p_{\rm Si\, \small {II}}$ relationship identified in a previous study, and show that subluminous and transitional objects display polarization values below the $\Delta m_{15}\!-\!p_{\rm Si\, \small {II}}$ relationship for normal SNe Ia. We found a statistically significant linear relationship between the polarization of the Si ii λ6355 Å line before maximum brightness and the Si ii line velocity and suggest that this, along with the $\Delta m_{15}\!-\!p_{\rm Si\, \small {II}}$ relationship, may be explained in the context of a delayed-detonation model. In contrast, we compared our observations to numerical predictions in the $\Delta m_{15}\!-\!v_{\rm Si\, \small {II}}$ plane and found a dichotomy in the polarization propertiesmore »between Chandrasekhar and sub-Chandrasekhar mass explosions, which supports the possibility of two distinct explosion mechanisms. A subsample of SNe displays evolution of loops in the q–u plane that suggests a more complex Si structure with depth. This insight, which could not be gleaned from total flux spectra, presents a new constraint on explosion models. Finally, we compared our statistical sample of the Si ii polarization to quantitative predictions of the polarization levels for the double-detonation, delayed-detonation, and violent-merger models.

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5. A Physical Basis for the H-band Blue-edge Velocity and Light-curve Shape Correlation in Context of Type Ia Supernova Explosion Physics

   https://doi.org/https://ui.adsabs.harvard.edu/link_gateway/2019ApJ...878...86A/doi:10.3847/1538-4357/ab204b  

   C. Ashall, 1 P. ( July 2019 , The Astrophysical journal)

   Our recent work demonstrates a correlation between the high-velocity blue edge, vedge, of the ironpeak Fe/Co/Ni H-band emission feature and the optical light curve shape of normal, transitional and sub-luminous type Ia Supernovae (SNe Ia). We explain this correlation in terms of SN Ia physics. vedge corresponds to the sharp transition between the complete and incomplete silicon burning regions in the ejecta. It measures the point in velocity space where the outer 56Ni mass fraction, XNi, falls to the order of 0.03-0.10. For a given 56Ni mass, M(56Ni), vedge is sensitive to the speci c kinetic energy Ekin(M(56Ni)=MWD) of the corresponding region. Combining vedge with light curve parameters (i.e., sBV ,
   m15;s in B and V ) allows us to distinguish between explosion scenarios. The correlation between vedge and light-curve shape is consistent with explosion models near the Chandrasekhar limit. However, the available sub-MCh WD explosion model based on SN 1999by exhibits velocities which are too large to explain the observations. Finally, the sub-luminous SN 2015bo exhibits signatures of a dynamical merger of two WDs demonstrating diversity among explosion scenarios at the faint end of the SNe Ia population.