Abstract Recently, the Covarying Coupling Constants and Tired Light (CCC+TL) hybrid model was proposed to explain the unexpectedly small angular diameters of high-redshift galaxies observed by the James Webb Space Telescope (JWST) that are challenging to reconcile with the ΛCDM model. In this work, we test the CCC+TL model against model-independent Hubble parameter [H(z)] measurements obtained from cosmic chronometers. It turns out that the parameter set optimized for the type-Ia supernova (SN Ia) dataset within the CCC+TL model fails to reproduce the H(z) data, but the ΛCDM model works well. Statistical comparison using the Δχ2 strongly favors ΛCDM over CCC+TL for the H(z) data, with Δχ2 = 61.52. Crucially, the CCC+TL framework exhibits a severe internal tension, where the SN Ia-optimized speed-of-light variation index α is rejected by the H(z) dataset with a likelihood ratio of $\mathcal {R} \approx 1.7 \times 10^{-14}$. Our result suggests that the tension posed by JWST observations of compact high-z galaxies may originate from the intrinsic properties and evolution of galaxies in the early universe.

Abstract A previous article by some of the authors introduced a Brans-Dicke-like framework wherein the scalar field φ is composed of both G and c which, for this reason, co-vary according to c3/G = constant. In the present paper, we use observational data to constrain the supposed co-varying G and c, under the hypothesis of the validity of the standard Lemaitre formula 1 + z ∼ a−1. The datasets include SN Ia, BAO, and the value of θ extracted from CMB data. A proxy function is demanded for the varying c since the framework does not provide a closed set of equations for computing the functional form of either G or c uniquely. Accordingly, we choose three separate parameterizations for c(z) inspired both by desirable properties of the varying speed of light (VSL) and by successful phenomenological models from the literature—including the one by Gupta (CCC framework). When combined with DESI, Pantheon+ data strongly favor a variable speed of light with more than 3σ confidence level for all parameterizations considered in this paper, whereas Union2.1 suggests no variation of the speed of light. As we shall demonstrate, this apparent discrepancy is due to a strong correlation that emerges between H0 and VSL.

Abstract We present BayeSN-TD, an enhanced implementation of the probabilistic type Ia supernova (SN Ia) BayeSN SED model, designed for fitting multiply-imaged, gravitationally lensed type Ia supernovae (glSNe Ia). BayeSN-TD fits for magnifications and time-delays across multiple images while marginalising over an achromatic, Gaussian process-based treatment of microlensing, to allow for time-dependent deviations from a typical SN Ia SED caused by gravitational lensing by stars in the lensing system. BayeSN-TD is able to robustly infer time delays and produce well-calibrated uncertainties, even when applied to simulations based on a different SED model and incorporating chromatic microlensing, strongly validating its suitability for time-delay cosmography. We then apply BayeSN-TD to publicly available photometry of the glSN Ia SN H0pe, inferring time delays between images BA and BC of $\Delta T_{BA}=121.9^{+9.5}_{-7.5}$ days and $\Delta T_{BC}=63.2^{+3.2}_{-3.3}$ days along with absolute magnifications β for each image, $\beta _A = 2.38^{+0.72}_{-0.54}$, $\beta _B=5.27^{+1.25}_{-1.02}$ and $\beta _C=3.93^{+1.00}_{-0.75}$. Combining our constraints on time-delays and magnifications with existing lens models of this system, we infer $H_0=69.3^{+12.6}_{-7.8}$ km s−1 Mpc−1, consistent with previous analysis of this system; incorporating additional constraints based on spectroscopy yields $H_0=66.8^{+13.4}_{-5.4}$ km s−1 Mpc−1. While this is not yet precise enough to draw a meaningful conclusion with regard to the ‘Hubble tension’, upcoming analysis of SN H0pe with more accurate photometry enabled by template images, and other glSNe, will provide stronger constraints on H0; BayeSN-TD will be a valuable tool for these analyses. The BayeSN-TD code is available at https://github.com/bayesn/bayesn-td.

Abstract We present an overview of the Weighing Halos Accurately, Locally, and Efficiently with Supernovae (WHALES) survey, the first to discover and measure Type Ia supernovae (SNe Ia) in and around galaxy superclusters. By building a sample of SNe Ia around these massive environments, we aim to provide new constraints on bulk-flow models while laying the groundwork for improved estimates of supercluster masses. Here, we present data from the first two seasons targeting the Shapley Supercluster (0.02 < z < 0.06), which is responsible for a large but unknown fraction of our Local Group’s motion. Until now, no supernovae (SNe) had been analyzed in the direction of Shapley. Through the WHALES survey, we have identified 12 likely SNe Ia in this region using SkyMapper, including eight with spectroscopic confirmation. We present the first light curves of these SNe and combine our observations with data from the Asteroid Terrestrial-impact Last Alert System. We demonstrate that the low number of discovered SNe Ia per season is consistent with various rate calculations, highlighting the need for future surveys to monitor superclusters over a multiyear time span. Finally, we present simulations of SN Ia observations in the environments of massive galaxy clusters, demonstrating how the inferred peculiar velocities can constrain cluster masses, and highlighting the added complexity within superclusters. We find that a sample of 100 SNe Ia would enable a 25% precision measurement of the total mass of the Shapley Supercluster.

We report observations of a Type Ia supernova (SN Ia) 2021hem that was discovered within 48 hours of last nondetection and is located in an apparently hostless environment. With a peak absolute B -band magnitude of M B , max = −19.96 ± 0.29 mag, SN 2021hem lies at the luminous end of the SNe Ia distribution. Its near-infrared and i -band light curves lack the secondary maximum, which is otherwise ubiquitous to normal and 1991T-like SNe Ia. Instead, these properties cause SN 2021hem to closely resemble 2003fg-like events. The slowly evolving light curves (characterized by Δ m 15 ( B ) = 1.02 ± 0.02 mag; s BV = 0.94 ± 0.05) and the earliest spectrum showing C II λ 6580 and λ 7235 absorption lines further support this classification. Other spectroscopic features, including Si II line diagnostics, resemble those of normal SNe Ia. A fit of a fireball model to the early-time light curves yields a time of first light of t first = −16.43 +0.45 −0.38 days relative to B -band maximum. The first photometric detection occurs 1.51 +0.45 −0.38 days before the onset of fireball-like flux rise. This early emission, together with the intrinsic ( g − r ) 0 color, is inconsistent with circumstellar or companion interaction. Instead, shallow 56 Ni mixing or an asymmetric 56 Ni distribution offers a plausible explanation for the delayed onset of the fireball flux rise, while a double-detonation scenario with a thin helium shell remains a less likely alternative. Notably, SN 2021hem represents the fifth known 2003fg-like SN that has early-time activity or excess flux emission. The estimated mass of radioactive 56 Ni synthesized in SN 2021hem is 1.00 ± 0.09 M ⊙ . Deep GTC imaging obtained 2.5 years after the explosion, with an estimated limiting magnitude of m lim, r = 24.4 mag and a surface-brightness limit of μ lim, r = 26.3 mag arcsec −2 , revealed no coincident host. Most faint dwarf and ultradiffuse galaxies (UDGs) are therefore ruled out. Alternatively, if the nearest plausible AGN host galaxy located at a projected distance of 104 kpc is assumed, the progenitor would need to be a hypervelocity star ejected at ≈2200 km s −1 from the host by AGN interaction. A faint diffuse feature ≈6 kpc from the SN site has also been detected in the GTC image, and its surface brightness is within the limits of UDGs. It is unclear whether it is a galaxy and is associated with SN 2021hem, however. Based on its large normalized directional light distance ( d DLR ≈ 3 − 4) from the SN and its unusual elongation, the probability that this is the candidate host galaxy of SN 2021hem is low. These results identify SN 2021hem as one of the strongest candidates for a hostless SN Ia and underscore the diversity of luminous slowly evolving 2003fg-like explosions and the wide range of environments in which they may occur.

Abstract Early excess emission observed in Type Ia supernovae (SNe Ia) within ∼1 day of explosion provides a critical window into their progenitor systems. In the present study, we investigate formation of the circumstellar matter (CSM) in double white-dwarf (WD) mergers. We further study the interaction between the CSM and the SN ejecta. We first model the orbital evolution and super-Eddington mass transfer/ejection in the double WD systems. We then conduct hydrodynamical and light-curve (LC) simulations of the SN–CSM interaction, assuming a prompt SN Ia explosion in the context of the carbon-ignited violent merger (C-ignited VM). Our simulations show that at the moment of the merger, the binary system has the CSM distribution following ρ CSM ≃ D ( r / 1 0 14 cm ) − 3.5 ( D ≃ 1 0 − 14 – 1 0 − 13 g cm − 3 ) . The simulated LCs reproduce the early flux excesses across optical to UV bands, as well as their color evolution, observed in the VM candidates, i.e., 03fg/02es-like SNe Ia. This supports that 03fg/02es-like objects originate from the VM explosions. We also discuss the case of the helium-ignited VM, which might be realized in some WD–WD mergers depending on the He content in the system. Focused here is the timing when the explosion is initiated, and we find that the explosion is initiated after the companion WD is, at least partially, tidally disrupted also in this case; we thus expect the formation of the CSM through the mass transfer phase also for the helium-ignited VM scenario.

Abstract In recent years, multiple Type Ia supernovae (SNe Ia) have been observed with “bumps” in their rising light curves shortly after the explosion. Here, we present SN 2021qvo: an SN Ia that exhibits a clear early bump in photometry obtained by the Young Supernova Experiment. Photometric and spectroscopic observations of SN 2021qvo show that it has a broader light curve, higher peak luminosity, shallower Si ii λ 5972 pseudoequivalent width, and lower ejecta velocities than normal SNe Ia, which are all consistent with the characteristics of the 2003fg-like (often called “super-Chandrasekhar”) SN subtype. Including SN 2021qvo, just four known 2003fg-like SNe Ia have sufficient prepeak data to reveal a rising light-curve bump, and all four have bump detections. A host-galaxy analysis reveals that SN 2021qvo exploded in a low-mass galaxy log ( M * / M ⊙ ) = 7.8 3 − 0.24 + 0.17 , also consistent with other members of this class. The current leading early bump 2003fg-like SN Ia progenitor model involves an interaction between the circumstellar material (CSM) and the SN ejecta. We test the validity of this theory by modeling the early bump and subsequent light-curve evolution of SN 2021qvo with the Modular Open Source Fitter for Transients. We find that the bump can be modeled with a best-fit CSM mass in the range M CSM = 3.31−8.51 × 10 −3 M ⊙ . SN 2021qvo adds to the small but growing number of 2003fg-like SNe Ia with rising light-curve bumps; as the number of these SNe Ia with CSM estimates continues to grow, population-level inferences about the CSM distribution will be able to constrain the progenitor scenario for these SNe Ia.

Abstract [HP99] 159 is remarkable as the first supersoft X-ray source (SSS) identified with an evolved helium star donor. With a likely orbital period of 1.164 d or 2.327 d, the origin of the SSS component is controversial, with the two current models being either steady He-burning on the white dwarf surface, or that it is a helium nova in the decaying phase. To help resolve this issue we present extensive new long-term spectroscopy (with SALT) and photometry (at SAAO and with OGLE) of [HP99] 159 which (a) supports 2.327 d as the orbital period, and (b) finds only a small He ii radial velocity modulation. The latter is surprising as it implies a very low inclination system, whereas our light curve modelling suggests i ∼ 50○, and hence that the He ii must be produced in outflowing material further above, or beyond, the disc. We find that the decaying nova model cannot fit our OGLE light curve and the observed SSS flux level. [HP99] 159 has been essentially constant as an SSS over several decades, implying a sustained high level of mass-transfer from its He star donor, making it the only confirmed single-degenerate scenario SN Ia progenitor. We have updated the known SSS binary parameters and find a clear ∼1.5 mag difference in their MV when compared to the MV − Σ properties of LMXBs, likely due to the larger irradiated areas and more luminous donors.

Abstract SN 2020eyj is the first Type Ia supernova (SN Ia) showing the signature of compact helium-rich circumstellar material (CSM). Such a large amount of CSM is difficult to explain in a single-degenerate scenario where the donor star is a helium star. Here we show that, under certain conditions, it is possible that the transfer of helium leads to a common envelope (CE) engulfing the system, similar to the CE wind model proposed by X. Meng & P. Podsiadlowski. If in such a helium CE wind model the initial white dwarf mass is larger than 1.1 M ⊙ and the helium star is more massive than 1.8 M ⊙ , the mass of a helium CE can be larger than 0.3 M ⊙ prior to supernova explosion. The CE mass heavily depends on the initial parameters of the binary system. A dynamical CE ejection event could occur shortly before the supernova, and then our model may naturally explain the properties of SN 2020eyj, specifically the massive He-rich CSM, and its dim peak brightness, low ejecta velocity, and low birth rate.

The Zwicky Transient Facility Data Release 2 (ZTF DR2) includes a total of 3,628 Type Ia supernovae (SNe Ia), providing the largest and most complete sample of spectroscopically confirmed SNe Ia at low redshift to date. In this paper, we present a photometric and spectroscopic analysis of 124 subluminous SNe Ia, the largest sample of spectroscopically classified subluminous Type Ia supernova observed with a single instrument, comprising 87 91bg-like, 12 86G-like, 18 04gs-like, and 7 02es-like events. We complement the published DR2 light-curve parameters with new parameters obtained using template-based fits from . We measured the expansion velocities and pseudo-equivalent widths (pEW) of key spectral features using . Next, the spectral averages were constructed for each subluminous subtype, binned by phase. We also analyzed the host galaxy environments, both global and local, in terms of g - z color, stellar mass, and directional light radius (d_DLR). We found that all subluminous SNe Ia (except the 02es-like subtype) are intrinsically red. This is made evident when we separate the extrinsic color components from intrinsic ones. Since has not been trained on subluminous SNe Ia, it compensates for their redder colors by inflating the c parameter, thereby extending the luminosity-width relation to negative values of x1. As expected, all subluminous SNe Ia fall within the cool region of the Branch et al. (2006) diagram, with the exception of 02es-like events, which display lower łambda5972 pEW values. All subluminous subtypes tend to occur in more massive, redder host galaxies and in the reddest local environments within their stellar mass bins. Notably, 91bg- and 86G-like SNe Ia explode at significantly larger normalized galactocentric distances. Finally, we identified the pEW of the blended + + absorption feature at 4300 Å, along with s_BV, as robust and sufficient indicators for subclassifying subluminous SNe Ia. SALT2 SNooPy Spextractor SALT2 Si ii Ti ii Si ii Mg ii

Massive short-period binaries involving hot subdwarf stars (sdO/Bs) are rare but very relevant to constraining pathways for binary star evolution. Moreover, some of the most promising candidate progenitor systems leading to Type Ia supernovae (SNe Ia) involve sdO/Bs. LAMOST,J065816.72+094343.1 has been identified to be such a candidate system. To explore the nature and evolutionary future of LAMOST,J065816.72+094343.1, we complemented archival spectroscopic data with additional time series spectra as well as high-resolution spectroscopy of the object. After combining these with photometric data, we determined the orbital parameters of the system and the mass of the companion. We solved the orbit of the system by analyzing 68 low- and medium-resolution spectra using state-of-the-art mixed local thermodynamic equilibrium (LTE) and non-LTE model atmospheres. Additionally, we gathered nine high-resolution spectra to determine atmospheric parameters and the projected rotational velocity of the sdOB. The inclination angle of the system was constrained assuming tidal synchronization of the sdOB, which was verified via analysis of the ellipsoidal variations in the TESS light curve. We determine LAMOST,J065816.72+094343.1 to be a binary consisting of a massive 0.82 ± 0.17 , M _⊙ sdOB component with a 1.30^ +0.31 _ -0.26 , M _⊙ unseen companion. Due to the companion's mass being very close to the Chandrasekhar mass limit and high for a white dwarf, it is unclear whether the compact companion is a white dwarf or a neutron star. We find the system to be in a close orbit, with a period of P=0.31955193 , d and an inclination angle of i = 49.6^ +5.2 _ -4.2 , . While the exact nature of the companion remains unknown, we determine the system to either lead to a SN Ia or an intermediate mass binary pulsar, potentially after a phase as an intermediate-mass X-ray binary. deg

ABSTRACT We present a chemical abundance study of giant stars in the Galactic open cluster NGC 5822, which hosts two barium stars (#002 and #201) and three lithium-enriched giants (#006, #102, and #240). Using high-resolution optical and near-infrared (H and K band) spectra from FEROS and IGRINS, we determine atmospheric parameters and abundances for 23 elements (Li, C, N, O, F, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti, Cr, Fe, Ni, Y, Ce, Nd, Yb, and Pb). This includes species not yet studied in this cluster, such as F, P, K, Yb, and Pb, as well as oxygen isotopic ratios $^{16}$O/$^{17}$O and $^{16}$O/$^{18}$O. Membership was assessed using astrometry and chemical abundances, providing insight into the evolutionary stages of Li-enriched giants and cluster parameters (age, distance, extinction). However, the identification of Ba-stars remains challenging due to their binary nature and less reliable astrometric solutions. The cluster’s abundances are broadly consistent with expectations for the Galactic thin disc. The mean fluorine abundance agrees with chemical evolution models predicting that young clusters (<2 Gyr) exhibit elevated [F/Fe], with production from SN II, SN Ia, AGB, and Wolf–Rayet stars. No distinct chemical or rotational features were found to explain the lithium enrichment, likely occurring either during the red clump phase or near the RGB tip. For the Ba-stars, nucleosynthesis models combined with the cluster’s turn-off mass suggest polluting companion masses of 3.00 and 3.75 ${\rm M}_{\odot }$ for stars #002 and #201. These results highlight the importance of open clusters as laboratories for chemically peculiar stars.

Aims. We investigated the photometric and spectroscopic evolution of SN Ia 2025bvm to perform a detailed classification and provide an independent distance estimate for the host galaxy NGC 4156. Methods. We present UBVRI photometry for a period between days −16 and 80 relative to the B -band maximum. Six optical spectra were taken between days −3 and 28. We used different fitting methods to derive the basic photometric parameters. We determined the expansion velocities from the blueshift of the Si II absorption line and the equivalent widths of the interstellar Na I lines. Results. SN 2015bvm exhibits a photometric evolution typical for Type Ia supernovae; the Δ m 15 parameter and the rise time are both close to their mean values. The rising part of the light curves shows no signs of excess flux. The color curves indicate a significant color excess of E ( B − V ) tot = 0.22 ± 0.04 mag. The interstellar Na I lines in the host galaxy are stronger than expected for this value of dust extinction. SN 2025bvm is notable for its high expansion velocity at maximum light and the presence of a plateau in the velocity temporal evolution. We estimate a distance modulus of μ = 34.84 ± 0.10 mag, consistent with the value derived from the host galaxy redshift.

Abstract The parameter space for mass loss in Type Ia supernova (SN Ia) progenitors is large, with different progenitor scenarios favoring different mass-loss regimes. Here we focus on the impact that uniform and isotropic outflows have on the circumstellar environment of SN Ia progenitors. We vary mass-loss rate, wind velocity, and outflow duration, and evolve supernova remnant (SNR) models in this grid of circumstellar structures in order to compare the bulk properties of these models (ages, radii, and Fe K α centroids and luminosities) to observations. We find that roughly 55% (7/13) of young X-ray-bright Type Ia SNRs in the Milky Way and the Large Magellanic Cloud had progenitors that did not substantially modify their surroundings on ∼parsec scales. This group includes SN Ia with a range of luminosities, and at least one likely product of a double-detonation explosion in a sub-Chandrasekhar white dwarf (WD). The other half of our sample can be divided into two distinct classes. A small subset of SNRs (∼15%, 2/13) have large radii and low Fe K α centroids and are likely expanding into large cavities that might have been excavated by fast (∼1000 km s −1 ), sustained progenitor outflows. The majority of the SNRs that are expanding into a modified medium (∼30%, 4/13) show evidence for dense material, likely associated with slow (∼10 km s −1 ) progenitor outflows, possibly a byproduct of accretion processes in near-Chandrasekhar WDs spawned by younger stellar populations.

Abstract Photospheric and high-velocity features (PVFs and HVFs) of Si ii λ6355 and Ca ii IR3 lines in supernova Ia (SN Ia) spectra provide insights into ejecta structure, energetics, and circumstellar interaction, yet their interplay remains poorly understood. We analyse a representative sample of 145 nearby SNe Ia observed within ±5 days of B-band maximum light, including normal, 91T-, and 91bg-like events with measured light-curve decline rates (Δm15) and Si ii and Ca ii line properties from the literature. We model PVF and HVF velocity distributions using Gaussian Mixture Models, compare Si ii and Ca ii PVF velocity distributions, assess Ca ii HVF properties, and test correlations between Si ii PVF velocities and Δm15, with emphasis on HVF effects. For the first time, we show that the Ca ii PVF velocity distribution, measured for the same events at the same phases as Si ii, is predominantly unimodal, in contrast to the well-known bimodal Si ii PVF distribution that supports the high-velocity/normal-velocity division. This contrast likely reflects a subclass-dependent formation depth of the Ca ii line, as supported by a positive correlation ( > 3.3σ) between Δm15 and the velocity offset between Ca ii and Si ii PVFs, particularly in faster-declining SNe Ia. Importantly, HVFs do not significantly bias PVF velocity distributions. A significant negative correlation ( > 3.3σ) between Si ii PVF velocity and Δm15 is found only for HVF-weak SNe Ia, consistent with more energetic explosions yielding faster ejecta, while this trend vanishes in HVF-strong events, likely due to circumstellar interaction. These results underscore the critical role of HVFs and SN Ia subclass in interpreting ejecta kinematics in both models and observations.

Abstract 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.

Abstract We present photometric and spectroscopic observations of SN 2024gy, a Type Ia supernova (SN Ia) exhibiting high-velocity features (HVFs) in its early-time spectra. This SN reaches a peak B -band magnitude of −19.25 ± 0.29 mag and subsequently declines by Δ m 15 ( B ) ≈ 1.12 mag, consistent with the luminosity–width relation characteristic of normal SNe Ia. Based on the peak thermal luminosity of (1.2 ± 0.3) × 10 43 erg s −1 , we estimate that 0.57 ± 0.14 M ⊙ of 56 Ni was synthesized during the explosion. Our dense early spectral monitoring revealed significant velocity disparities within the ejecta. Notably, absorption features from the Ca ii near-infrared triplet were observed at velocities exceeding 25,000 km s −1 , while the Si ii λ 6355 line velocity at the same epoch was significantly lower at ∼16,000 km s −1 . This velocity disparity likely reflects distinct ionization states of intermediate-mass elements in the outermost layers. The prominent Ca ii HVFs may originate from ionization suppression within the highest-velocity ejecta, potentially indicative of minimal hydrogen mixing in a delayed-detonation explosion scenario. Additionally, the Ni/Fe ratio derived from the nebular spectrum of SN 2024gy provides further support for this model.

Type Ia supernovae (SNe Ia) are among the most precise cosmological distance indicators used to study the expansion history of the Universe. The vast increase in SN Ia data due to large-scale astrophysical surveys has led to the discovery of a wide variety of SN Ia sub-classes, such as transitional and fast-declining SNe Ia. However, their distinct photometric and spectroscopic properties differentiate them from the population of normal SNe Ia such that their use as cosmological tools remains challenged. Here, we present a high-cadenced photometric and spectroscopic dataset of two SNe Ia, SNe 2020ue and 2020nlb, which were discovered in the nearby Virgo cluster of galaxies. Our study shows that SN 2020nlb is a normal SN Ia whose unusually red colour is intrinsic, arising from a lower photospheric temperature rather than interstellar reddening, providing clear evidence that colour diversity among normal SNe Ia can have a physical origin. In contrast, SN 2020ue has photometric properties, such as colour evolution and light curve decay rate, similar to those of transitional SNe. It is hence more spectroscopically aligned with normal SNe Ia. This is evident from spectroscopic indicators such as the pseudo-equivalent width of lines. Thus, such SNe Ia, which lie photometrically at the edge of the standard normal SNe Ia range, may be missed in cosmological SNe Ia samples. Our results highlight that a spectroscopic analysis of SNe Ia around peak brightness is crucial for identifying intrinsic colour variations and constructing a more complete and physically homogeneous SN Ia sample for precision cosmology. Si II

Abstract Type Ia-CSM supernovae (SNe) are a rare and peculiar subclass of thermonuclear SNe characterized by emission lines of hydrogen or helium, indicative of high-density circumstellar medium (CSM). Their implied mass-loss rates of ∼10 −4 –10 −1 M ⊙ yr −1 (assuming ∼100 km s −1 winds) from optical observations are generally in excess of values observed in realistic SN Ia progenitors. In this paper, we present an independent study of CSM densities around a sample of 29 archival Ia-CSM SNe using radio observations with the Karl G. Jansky Very Large Array at 6 GHz. Motivated by the late (∼2 yr) radio detection of the Ia-CSM SN 2020eyj, we observed old (>1 yr) SNe, when we are more likely to see the emergent synchrotron emission that may have been suppressed earlier by free–free absorption by the CSM. We do not detect radio emission down to 3 σ limits of ∼35 μ Jy in our sample. The only radio-detected candidate in our sample, SN 2022esa, was likely misclassified as a Ia-CSM with early spectra, and appears more consistent with a peculiar Ic based on later epochs. Assuming wind-like CSM with temperatures between 2 × 10 4 K and 10 5 K, and a magnetic field-to-shock energy fraction ϵ B = 0.01 − 0.1, the radio upper limits rule out mass-loss rates between ∼10 −4 and 10 −2 M ⊙ yr −1 (100 km s −1 ) −1 . This is somewhat in tension with the estimates from optical observations, and may indicate that more complex CSM geometries and/or lower values of ϵ B may be present.

The relation between Type Ia supernovae (SNe Ia) and the stellar masses of their host galaxy is well documented. In particular, Hubble residuals display a distinct luminosity shift based on host mass. This is known as the mass step. This effect is widely used as an additional correction factor in the standardisation of SN Ia luminosities. We investigate the Hubble residuals and the mass step of normal SNe Ia in the context of velocities based on 277 normal SNe Ia that are near their peak in the second data release (DR2) of the Zwicky Transient Facility (ZTF). We divided the sample into high-velocity (HV) and normal-velocity (NV) SNe Ia, separated at 12,000 ̨ms. This produced a sample of 70 HV and 207 NV objects. We then explored potential environment- and/or progenitor-related effects by investigating the velocities with parameters such as the light-curve stretch x_1, the colour c, and the host galaxy properties. Although we only find a marginal difference between the Hubble residuals of HV and NV SNe Ia, the NV mass step is $0.149 ± 0.024$ mag ($6.3σ$). The HV mass step is smaller, $0.046 ± 0.041$ mag ($1.1σ$), and is consistent with zero. The difference between the NV and HV mass steps is modest, at ∼2.2σ. Moreover, the clearest subtype difference appears for SNe in central regions (d_ DLR <1), where NV SNe Ia show a large mass step, whereas HV SNe Ia are consistent with no step, yielding a difference of $3.1–3.6σ$ between NV and HV SNe Ia. We observe a host-colour step for both subtypes. NV SNe Ia show a step of $0.142 ± 0.024$ mag ($5.9σ$), while HV SNe Ia show a step of $0.158 ± 0.042$ mag ($3.8σ$), where the HV SNe Ia step appears to be larger, but the significance is lower because the sample size is smaller. Overall, the NV and HV colour steps are statistically consistent. HV SNe Ia also show modest (∼2.5–$3σ$) steps in certain subsets, such as those in outer regions (d_ DLR >1), whereas NV SNe display stronger environmental trends. Our results indicate that NV SNe Ia appear to be more environmentally sensitive, particularly in central likely metal-rich and older regions, while HV SNe Ia show weaker and subset-dependent trends. This suggests that applying a universal mass-step correction might introduce biases, and that incorporating refined classifications and/or environment-dependent factors, such as the location within the host, might improve future cosmological analyses beyond the standard x_1 and c cuts.