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1. Optical Spectroscopy of Type Ia Supernovae by the Carnegie Supernova Projects I and II

   https://doi.org/10.3847/1538-4357/ad38af  

   Morrell, N; Phillips, M M; Folatelli, G; Stritzinger, M D; Hamuy, M; Suntzeff, N B; Hsiao, E Y; Taddia, F; Burns, C R; Hoeflich, P; et al (May 2024, The Astrophysical Journal)

   Abstract 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 CSP-I and 234 spectra of 127 SNe Ia obtained during 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 analyzed correlations are similar for both samples. We find a larger incidence of SNe Ia belonging to the cool and broad-line Branch subtypes among the events discovered by targeted searches, shallow-silicon SNe Ia are present with similar frequencies in both samples, while core normal SNe Ia are more frequent in untargeted searches. 

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2. Carnegie Supernova Project: Fast-declining Type Ia Supernovae as Cosmological Distance Indicators*

   https://doi.org/10.3847/1538-4357/ae2fef  

   Phillips, M M; Uddin, Syed A; Burns, Christopher R; Suntzeff, Nicholas B; Ashall, C; Baron, E; Galbany, L; Hoeflich, P; Hsiao, E Y; Morrell, Nidia; et al (February 2026, The Astrophysical Journal)

   Abstract In this paper, the suitability of fast-declining Type Ia supernovae (SNe Ia) as cosmological standard candles is examined utilizing a “Hubble Flow” sample of 43 of these objects observed by the Carnegie Supernova Project (CSP). We confirm previous suggestions that fast-declining SNe Ia offer a viable method for estimating distances to early-type galaxies when the color-stretch parameter,s_BV, is used as a measure of the light-curve shape. As a test, we employ the Tripp method, which models the absolute magnitude at maximum as a function of light-curve shape and color. We calibrate the sample using 12 distance moduli based on published infrared surface-brightness fluctuations to derive a value of the Hubble constant that is in close agreement with the value obtained for the full sample of CSP SNe Ia using the same methodology. We also develop a new and simple method of estimating the distances of fast decliners based only on their colors at maximum (and not light-curve shape) and find that it leads to similar results as with using the Tripp method. This “color” technique is a powerful tool that is unique to fast-declining SNe Ia. We show that the colors of the fast decliners at maximum light are strongly affected by photospheric temperature differences and not solely due to dust extinction, and provide a physical rationale for this effect. 

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3. Cosmological Results from the RAISIN Survey: Using Type Ia Supernovae in the Near Infrared as a Novel Path to Measure the Dark Energy Equation of State

   https://doi.org/10.3847/1538-4357/ac755b  

   Jones, D. O.; Mandel, K. S.; Kirshner, R. P.; Thorp, S.; Challis, P. M.; Avelino, A.; Brout, D.; Burns, C.; Foley, R. J.; Pan, Y.-C.; et al (July 2022, The Astrophysical Journal)

   Abstract Type Ia supernovae (SNe Ia) are more precise standardizable candles when measured in the near-infrared (NIR) than in the optical. With this motivation, from 2012 to 2017 we embarked on the RAISIN program with the Hubble Space Telescope (HST) to obtain rest-frame NIR light curves for a cosmologically distant sample of 37 SNe Ia (0.2 ≲ z ≲ 0.6) discovered by Pan-STARRS and the Dark Energy Survey. By comparing higher- z HST data with 42 SNe Ia at z < 0.1 observed in the NIR by the Carnegie Supernova Project, we construct a Hubble diagram from NIR observations (with only time of maximum light and some selection cuts from optical photometry) to pursue a unique avenue to constrain the dark energy equation-of-state parameter, w . We analyze the dependence of the full set of Hubble residuals on the SN Ia host galaxy mass and find Hubble residual steps of size ∼0.06-0.1 mag with 1.5 σ −2.5 σ significance depending on the method and step location used. Combining our NIR sample with cosmic microwave background constraints, we find 1 + w = −0.17 ± 0.12 (statistical + systematic errors). The largest systematic errors are the redshift-dependent SN selection biases and the properties of the NIR mass step. We also use these data to measure H 0 = 75.9 ± 2.2 km s −1 Mpc −1 from stars with geometric distance calibration in the hosts of eight SNe Ia observed in the NIR versus H 0 = 71.2 ± 3.8 km s −1 Mpc −1 using an inverse distance ladder approach tied to Planck. Using optical data, we find 1 + w = −0.10 ± 0.09, and with optical and NIR data combined, we find 1 + w = −0.06 ± 0.07; these shifts of up to ∼0.11 in w could point to inconsistency in the optical versus NIR SN models. There will be many opportunities to improve this NIR measurement and better understand systematic uncertainties through larger low- z samples, new light-curve models, calibration improvements, and eventually by building high- z samples from the Roman Space Telescope. 

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4. Characterizing the standardization properties of type ia supernovae in the z band with hierarchical Bayesian modelling

   https://doi.org/10.1093/mnras/staf1056  

   Hayes, Erin E; Dhawan, Suhail; Mandel, Kaisey S; Jones, David O; Foley, Ryan J; Thorp, Stephen; Grayling, Matthew; Ward, Sam M; Do, Aaron; Langeroodi, Danial; et al (July 2025, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Type Ia supernovae (SNe Ia) are standardizable candles: their peak magnitudes can be corrected for correlations between light-curve properties and their luminosities to precisely estimate distances. Understanding SN Ia standardization across wavelength improves methods for correcting SN Ia magnitudes. Using 150 SNe Ia from the Foundation Supernova Survey and Young Supernova Experiment, we present the first study focusing on SN Ia standardization properties in the z band. Straddling the optical and near-infrared, SN Ia light in the z band is less sensitive to dust extinction and can be collected alongside the optical on CCDs. Pre-standardization, SNe Ia exhibit less residual scatter in z-band peak magnitudes than in the g and r bands. SNe Ia peak z-band magnitudes still exhibit a significant dependence on light-curve shape. Post-standardization, the z-band Hubble diagram has a total scatter of root mean square $=0.195$ mag. We infer a z-band mass step of $$\gamma _{z} = -0.105 \pm 0.031$$ mag, which is consistent within $$1\sigma$$ of that estimated from $gri$ data, assuming $$R_{V} = 2.61$$. When assuming different $$R_{V}$$ values for high and low mass host galaxies, the z band and optical mass steps remain consistent within $$1\sigma$$. Based on current statistical precision, these results suggest dust reddening cannot fully explain the mass step. SNe Ia in the z band exhibit complementary standardizability properties to the optical that can improve distance estimates. Understanding these properties is important for the upcoming Vera Rubin Observatory and Nancy G. Roman Space Telescope, which will probe the rest-frame z band to redshifts 0.1 and 1.8. 

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5. Hawai‘i Supernova Flows: a peculiar velocity survey using over a Thousand Supernovae in the near-infrared

   https://doi.org/10.1093/mnras/stae2501  

   Do, Aaron; Shappee, Benjamin J; Tonry, John L; Tully, R Brent; de Jaeger, Thomas; Rubin, David; Ashall, Chris; Burns, Christopher R; Desai, Dhvanil D; Hinkle, Jason T; et al (December 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We introduce the Hawai‘i Supernova Flows project and present summary statistics of the first 1217 astronomical transients observed, 668 of which are spectroscopically classified Type Ia Supernovae (SNe Ia). Our project is designed to obtain systematics-limited distances to SNe Ia while consuming minimal dedicated observational resources. To date, we have performed almost 5000 near-infrared (NIR) observations of astronomical transients and have obtained spectra for over 200 host galaxies lacking published spectroscopic redshifts. In this survey paper, we describe the methodology used to select targets, collect/reduce data, calculate distances, and perform quality cuts. We compare our methods to those used in similar studies, finding general agreement or mild improvement. Our summary statistics include various parametrizations of dispersion in the Hubble diagrams produced using fits to several commonly used SN Ia models. We find the lowest dispersions using the SNooPy package’s EBV_model2, with a root mean square deviation of 0.165 mag and a normalized median absolute deviation of 0.123 mag. The full utility of the Hawai‘i Supernova Flows data set far exceeds the analyses presented in this paper. Our photometry will provide a valuable test bed for models of SN Ia incorporating NIR data. Differential cosmological studies comparing optical samples and combined optical and NIR samples will have increased leverage for constraining chromatic effects like dust extinction. We invite the community to explore our data by making the light curves, fits, and host galaxy redshifts publicly accessible. 

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