Stars and stellar evolution

arXiv:2510.27631v1 Announce Type: new Abstract: Pre-explosion mass loss in supernova (SN) progenitors is a crucial unknown factor in stellar evolution, yet has been illuminated recently by the diverse zoo of interacting transients. We present SN2024cld, a transitional core-collapse SN at a distance of 39 Mpc, straddling the boundary between SN II and SN IIn, showing persistent interaction with circumstellar material (CSM) similar to H-rich SN1998S and PTF11iqb. The SN was discovered and classified just 12h post-explosion via the GOTO-FAST high-cadence program. Optical spectroscopy, photometry, and polarimetry over 220d chart the complex, long-lived interaction in this transient. Early evolution is dominated by CSM interaction, showing a 14d rise to a peak absolute magnitude of g=-17.6 mag, with clear flash-ionisation signatures. SN2024cld also shows a marked double-plateau light curve powered by CSM interaction, with high-velocity (6000 km/s) shoulders on a strong multi-component H-alpha profile. Dense polarimetric coverage reveals marked evolution in the photospheric geometry -- peaking at p=2% 10 days post-explosion, and rotating approx. 60 deg as the ejecta sweep more distant CSM. We observe a narrow 60 km/s H-alpha P Cygni feature throughout, associated with pre-shock CSM. SN2024cld represents among the best-observed 98S-like SNe to date, revealing a multi-component CSM structure: a dense, inner aspherical envelope, CSM disk/torus, and tenuous, extended wind. We propose this SN arose from an evolved supergiant progenitor experiencing multiple mass loss episodes in its terminal years, with binary interaction plausibly generating the CSM disk. SN2024cld constrains the progenitors and mass-loss paradigms of 98S-like SNe, unveiling the chaotic ends of evolved supergiant stars from afar.

arXiv:2504.06720v2 Announce Type: replace Abstract: RR Lyrae stars have long been considered reliable tracers of old, metal-poor populations, primarily due to their prevalence in globular clusters and the Galactic halo. However, the discovery of a metal-rich subpopulation in the Galactic disc, kinematically colder and more rotationally supported, challenges this classical view. Understanding the age of these metal-rich RR Lyrae stars is crucial for constraining their formation pathways and assessing what Galactic populations they are tracing. In this work, we leverage the unprecedented astrometric precision of Gaia DR3 to infer the age distribution of metal-rich RR Lyrae stars through a kinematic comparison with O-rich Mira variables. Mira variables, with their well-established period-age relation, serve as a natural clock, allowing us to transfer age information to RR Lyrae stars via their phase-space properties. By applying this approach across different metallicity bins, we find that the most metal-rich RR Lyrae stars ($[\rm Fe/H] > -0.5$) exhibit kinematics consistent with long-period ($\rm{period}\approx 150\,\rm{days}$), young Mira variable population; its age corresponds to $\sim 6-7$ Gyr (adopting the period-age relation in Zhang & Sanders 2023) that is significantly younger than typically assumed for RR Lyrae stars. In contrast, those with $-1 < [\rm Fe/H] < -0.5$ show properties more aligned with older ($\approx 9-11$ Gyr) populations. Interestingly we also find evidence of a possible double age populations for the most metal-rich RR Lyrae, one younger with ages between 4 and 6 Gyr, and another one older ranging from 8 to 9 Gyr. These results provide strong evidence that metal-rich RR Lyrae stars in the Galactic field do not exclusively trace ancient populations. This finding challenges the current model of RR Lyrae formation and supports alternative formation scenarios, such as binary evolution.

arXiv:2507.18785v2 Announce Type: replace Abstract: We present and analyze panchromatic (0.35--14 $\mu$m) spectroscopy of the Type II supernova 2023ixf, including near- and mid-infrared spectra obtained 33.6 days after explosion during the plateau-phase, with the James Webb Space Telescope (JWST). This is the first in a series of papers examining the evolution of SN 2023ixf with JWST spanning the initial 1000 days after explosion, monitoring the formation and growth of molecules and dust in ejecta and surrounding environment. The JWST infrared spectra are overwhelmingly dominated by H lines, whose profiles reveal ejecta structures, including flat tops, blue notches, and red shoulders, unseen in the optical spectra. We characterize the nature of these structures, concluding that they likely result from a combination of ejecta geometry, viewing angle, and opacity effects. We find no evidence for the formation of dust precursor molecules such as carbon-monoxide (CO), nor do we observe an infrared excess attributable to dust. These observations imply that the detections of molecules and dust in SN 2023ixf at later epochs arise either from freshly synthesized material within the ejecta or circumstellar material at radii not yet heated by the supernova at this epoch.

arXiv:2510.13954v1 Announce Type: cross Abstract: We present a fully Bayesian, data-driven framework for identifying quasinormal modes in high-accuracy Cauchy-Characteristic Evolution (CCE) gravitational waveforms. Applying this to a public catalog, we identify QNM overtones, retrograde modes, and nonlinear modes up to cubic order in the ringdown. The ringdown mode content is tabulated across a wide range of start times for all available simulations, providing a systematic reference for theoretical and observational studies. We also search for late-time power-law tails, which are, as expected, absent from the CCE waveforms.

arXiv:2510.11783v1 Announce Type: cross Abstract: Numerical relativity (NR) enables the study of physics in strong and dynamical gravitational fields and provides predictions for the gravitational-wave signals produced by merging black holes. Despite the impressive accuracy of modern codes, the resulting waveforms inevitably contain numerical uncertainties. Quantifying these uncertainties is important, especially for studies probing subdominant or nonlinear effects around the merger and ringdown. This paper describes a flexible Gaussian-process model for the numerical uncertainties in all the spherical-harmonic waveform modes across a state-of-the-art catalog of NR waveforms and a highly efficient procedure for sampling the posteriors of quasinormal mode models without the need for expensive Markov chain Monte Carlo. The Gaussian-process model is used to define a likelihood function which allows many Bayesian data analysis techniques - already widely used in the analysis of experimental gravitational wave data - to be applied to NR waveforms as well. The efficacy of this approach is demonstrated by applying it to the analysis of quasinormal modes in Cauchy-characteristic evolved waveforms.

arXiv:2510.12601v1 Announce Type: new Abstract: The MIRI Excesses Around Degenerates Survey is a Cycle 2 James Webb Space Telescope (JWST) Survey program designed to image nearby white dwarfs in the mid-IR with the MIRI imaging mode. Only a handful of white dwarfs have previously been observed beyond 8~\micron. This survey gathered observations for 56 white dwarfs within 25~pc at 10 and 15~\micron, probing each white dwarf for unresolved IR excesses, IR flux deficits indicative of collision induced absorption, or resolved substellar companions. We present in this paper observations of our first target, 2MASS J09424023-4637176, (also UCAC4 217-039132), henceforth called MEAD 62. It is a magnetic DA white dwarf with an estimated age of $7.6^{+1.7}_{-2.2}$\,Gyr. A red candidate companion, MEAD 62B, about 2 magnitudes fainter at 15\,$\mu$m than the white dwarf is detected at an apparent separation of 1.95". If confirmed, MEAD 62B, would be a $0.014^{+0.002}_{-0.003}$\,\Msun\, brown dwarf with T$_{\rm eff} = 343^{+7}_{-11}$\,K, according to ATMO2020 evolutionary models. Although its red F1000W$-$F1500W color is similar to background galaxies, MEAD 62B, is consistent with being an unresolved point-source from empirical PSF fitting. A false positive analysis yields an expectation number of 0.66 red (F1000W$-$F1500$ \geq +0.80$\,mag) unresolved sources within the same separation (r$\leq2$ arcsec) for the entire MEAD survey. Thus, this candidate companion as likely to be an actual companion as a false-positive unresolved background galaxy. Additional observations to measure common proper motion or sample the SED are warranted to confirm the nature of MEAD 62B. A deep near-infrared imaging detection is achievable from the ground while JWST is needed at longer infrared wavelengths.

arXiv:2510.11799v1 Announce Type: new Abstract: We present X-shooter observations of a sample of 21 hydrogen-poor superluminous supernovae (SLSNe-I), spanning a redshift range of z=0.13-0.95, aimed at searching for shells of circumstellar material (CSM). Specifically, we focus on identifying broad Mg II absorption features that are blueshifted by several thousand kilometers per second and have previously been interpreted as arising from resonance line scattering of the SLSN continuum by rapidly expanding CSM ejected shortly before explosion. Utilizing high-quality spectra, we model the region around 2800A to characterize the Mg II line profiles, enabling us to either confirm their presence or place constraints on undetected CSM shells. We identify five objects in our sample that show broad Mg II absorption features consistent with the presence of CSM. While SN2018ibb, SN2020xga and SN2022xgc have been previously reported, we identify previously undiscovered CSM shells in DES15S2nr and DES16C3ggu. These shells were likely expelled approximately two and three months, respectively, before the explosion of their associated SNe, timescales consistent with late-stage mass-loss episodes. We do not find any correlations between the shell properties and the SN properties, except for a marginal correlation between the light curve decline time scale and the shell velocities. We further demonstrate that CSM configurations similar to the majority of the detected shells would have been observable in spectra with signal-to-noise >8 per resolution element, and that the lines from a shell are in general detectable except in the cases where the shell is either very geometrically and/or optically thin. Therefore, we conclude that the detection of CSM shells is not a selection effect, but may instead point to the existence of a subclass of SLSNe-I undergoing late-stage shell ejections shortly before explosion.

arXiv:2510.06320v1 Announce Type: new Abstract: Using high-precision observations from the space-based \textit{Gaia} and \textit{TESS} missions, complemented by ground-based spectroscopic data and multi-band photometric surveys, we perform a detailed investigation of the Galactic open cluster NGC~2506. We present a new analysis of the intermediate-age open cluster NGC~2506, using joint fits to the radial velocities (RVs) and spectral energy distributions (SEDs) of five double-lined binary systems, including two eclipsing binaries. The analysis yields self-consistent estimates of the cluster's age, distance, and extinction, based on 18 free parameters: 10 stellar masses, 5 orbital inclinations, and common values for age, distance, and $A_V$. The SED fitting incorporates stellar isochrones, and the resulting parameters are examined through HR diagrams (R--$T_{\rm eff}$, R--M, and M--$T_{\rm eff}$) to assess evolutionary consistency. The age we derive for the cluster is $1.94 \pm 0.03$ Gyr for an assumed [Fe/H] = -0.30, and a fitting formula is given for extrapolation to other metallicities. The distance we find from the SED fitting is $3189 \pm 53$ pc, and this is to be compared with our own inference from the Gaia data which is $3105 \pm 75$ pc, based on 919 stars identified as cluster members. Our results demonstrate the power of binary systems in tightly constraining cluster-wide age and distance at this evolutionary stage. This approach represents one of the most accurate characterizations of an intermediate-age open cluster using multiple binary systems.

arXiv:2506.02118v2 Announce Type: replace Abstract: Type Iax supernovae (SNe Iax) are one of the most common subclasses of thermonuclear supernova and yet their sample size, particularly those observed shortly after explosion, remains relatively small. In this paper we present photometric and spectroscopic observations of two SNe Iax discovered shortly after explosion, SN 2024bfu and SN 2025qe. Both SNe were observed by multiple all-sky surveys, enabling tight constraints on the moment of first light and the shape of the early light curve. Our observations of SN 2025qe begin <2d after the estimated time of first light and represent some of the earliest observations of any SN Iax. Spectra show features consistent with carbon absorption throughout the evolution of SN 2025qe, potentially indicating the presence of unburned material throughout the ejecta. We gather a sample of SNe Iax observed by ATLAS, GOTO, and ZTF shortly after explosion and measure their rise times and early light curve power-law rise indices. We compare our results to a sample of normal SNe Ia and find indications that SNe Iax show systematically shorter rise times, consistent with previous work. We also find some indication that SNe Iax show systematically lower rise indices than normal SNe Ia. The low rise indices observed among SNe Iax are qualitatively consistent with extended $^{56}$Ni distributions and more thoroughly-mixed ejecta compared to normal SNe Ia, similar to predictions from pure deflagration explosions.

arXiv:2502.18651v3 Announce Type: replace Abstract: In this second paper on our variability survey of central stars of planetary nebulae (CSPNe) using ZTF, we report 11 long-timescale variables with variability timescales ranging from months to years. We also present preliminary analyses based on spectroscopic and/or photometric follow-up observations for six of them. Among them is NGC 6833, which shows a $980$ day periodic variability with strange characteristics: `triangle-shaped' brightening in $r$, $i$, and WISE bands but almost coincidental shallow dips in the $g$-band. The most plausible explanation is a wide binary with the photometric period being the orbital period. Long-period near-sinusoidal variability was detected in two other systems, NGC 6905 and Kn 26, with periods of $700$ days and $230$ days, respectively, making them additional wide-binary candidates. The latter also shows a short period at $1.18$ hours. We then present CTSS 2 and K 3-5, which show brightening and significant reddening over the whole ZTF baseline. A stellar model fit to the optical spectrum of CTSS 2 reveals it to be one of the youngest post-AGB CSPNe known. Both show high-density emission-line cores. We propose these to be late-thermal-pulse candidates, currently evolving towards the AGB phase. We then present recent HST/COS ultraviolet spectroscopy of the known wide-binary candidate LoTr 1, showing that the hot star is a spectroscopic twin of the extremely hot white dwarf in UCAC2 46706450. Similar to this object, LoTr 1 also has a fast-rotating wide subgiant companion. We suggest that the long photometric period of 11 years is the binary orbital period. Finally, we briefly discuss the ZTF light curves of the remaining variables, namely Tan 2, K 3-20, WHTZ 3, Kn J1857+3931, and IPHAS J1927+0814. With these examples, we present the effectiveness of the von Neumann statistics and Pearson Skew-based metric space in searching for long-timescale variables.

arXiv:2505.23889v2 Announce Type: replace Abstract: Active galactic nuclei (AGNs) consist of a central supermassive black hole (SMBH) embedded in a region with both high gas and stellar densities: the gas is present as a thin accretion disc that fuels the central SMBH, while the stars form a dense, roughly isotropic nuclear star cluster. The binaries present in such a cluster could be considered naturally as triples, with the SMBH as a third object, and their dynamics also depend on the interaction with the gas-rich disc. In this paper, we study the evolution of such a binary on an inclined orbit with respect to the disc. The binary experiences both eccentricity excitation via the von Zeipel-Lidov-Kozai (ZLK) effect and drag forces from each time it penetrates the disc. We find that, as the outer orbital inclination decreases, the evolution of inner orbital separation can transition from a regime of gradual hardening to a regime of rapid softening. As such binaries grow wider, their minimum pericentre distances (during ZLK oscillations) decrease. We show that a simple geometric condition, modulated by the complex ZLK evolution, dictates whether a binary expands or contracts due to the interactions with the AGN disc. Our results suggest that the interaction with gas-rich accretion disc could enhance the rate of stellar mergers and formation of gravitational wave sources, as well as other transients. The treatment introduced here is general and could apply, with the proper modifications, to hierarchical triples in other gas-rich systems.

arXiv:2509.12488v1 Announce Type: new Abstract: At high metallicity, a majority of massive stars have at least one close stellar companion. The evolution of such binaries is subject to strong interaction processes, heavily impacting the characteristics of their life-ending supernova and compact remnants. For the low-metallicity environments of high-redshift galaxies constraints on the multiplicity properties of massive stars over the separation range leading to binary interaction are crucially missing. Here we show that the presence of massive stars in close binaries is ubiquitous, even at low metallicity. Using the Very Large Telescope, we obtained multi-epoch radial velocity measurements of a representative sample of 139 massive O-type stars across the Small Magellanic Cloud, which has a metal content of about one fifth of the solar value. We find that 45% of them show radial velocity variations which demonstrate that they are members of close binary systems, and predominantly have orbital periods shorter than one year. Correcting for observational biases indicates that at least 70[+11:-6]% of the O stars in our sample are in close binaries, and that at least 68[+7:-8]% of all O stars interact with a companion star during their lifetime. We found no evidence supporting a statistically significant trend of the multiplicity properties with metallicity. Our results indicate that multiplicity and binary interactions govern the evolution of massive stars and determine their cosmic feedback and explosive fates.

arXiv:2509.04659v1 Announce Type: new Abstract: As sites of some of the most efficient star formation in the Universe, globular clusters (GCs) have long been hypothesized to be the building blocks of young galaxies. Within the Milky Way, our best tracers of the contribution of GCs to the proto-Galaxy are stars with such anomalous overabundance in nitrogen and depletion in oxygen ("high-[N/O] stars") that they can be identified as having originated in a cluster long after they have escaped. We identify associations between these high-[N/O] field stars and GCs using integrals of motion and metallicities and compare to chemically typical halo stars to quantify any excess association, enabling a population-level exploration of the formation sites of the nitrogen-enhanced stars in the field. Relative to the halo as a whole, high-[N/O] stars show stronger associations with the most initially massive, inner Galaxy GCs, suggesting that many nitrogen-rich stars formed in these environments. However, when compared to a sample matched in orbital energy, the excess largely disappears: high-[N/O] stars are, on average, no more associated with surviving GCs than energy-matched halo stars, despite their [N/O] abundances indicating GC origins, consistent with a scenario in which a substantial fraction of low-energy inner-halo stars originate in GCs, so an energy-matched control dilutes any differential excess. We argue that associations between high-[N/O] stars and their parent GCs are further weakened because dynamical friction and the Galactic bar have altered integrals of motion, limiting the reliability of precise present-day associations and, especially, individual star-to-cluster tagging.

arXiv:2509.03412v1 Announce Type: new Abstract: Hydrogen-deficient binary stars comprise one star which has been stripped of its hydrogen through mass transfer to a binary companion. Observations show that the companion is able to accrete several solar masses without spinning up to critical rotation, and so there must be a mechanism to drain spin angular momentum from the accretor. We test magnetically coupled winds and magnetic star-disc coupling as possible mechanisms and find that, while the disc coupling is negligible, the winds are sufficient to allow the accretor to gain mass without spinning up to critical rotation. However, in order to fully replicate observations, time-dependent scalings of the dynamo-generated magnetic field are needed.

arXiv:2509.02825v1 Announce Type: new Abstract: We present long-baseline observations of the Fomalhaut outer debris disk at 223 GHz (1.3 mm) from ALMA Cycle 5, which we use along with archival short-baseline observations to produce a 0".57 resolution mosaic of the disk at a sensitivity of 7 $\mu$Jy/bm. We use radial profiles to measure the disk at the ansae and find that the southeast (SE) side of the disk is 4 AU wider than the northwest (NW) side as observed by ALMA. We also find that the peak brightness of the NW ansa is $21\pm1\%$ brighter than the SE ansa. We perform MCMC fits of the ALMA visibilities using two analytical, eccentric disk models. Our results suggest that the model including a dispersion parameter for the proper eccentricity ($\sigma_{e_p}$), which accounts for additional scatter in the eccentricity of individual orbits, is preferred over the model without one. Such a model implies that self-gravitation, particle collisions, and close-packing could play a role in shaping the overall structure of the Fomalhaut disk as is seen in eccentric planetary rings. Crucially, neither model can reproduce the brightness or width asymmetry near the NW ansa. No emission from the Intermediate Belt is detected, allowing us to place a 3-$\sigma$ upper limit of 396 $\mu$Jy at 1.3 mm. We also discover a spectral line in archival Cycle 3 data centered at $\nu_{\rm obs}\approx230.25$ GHz at the location of the ``Great Dust Cloud," whose redshift from the expected CO line for Fomalhaut confirms the source is a background galaxy.

arXiv:2508.19369v1 Announce Type: cross Abstract: We present a novel theoretical framework for analysing the stability of rotating black hole spacetimes through the conformal properties of the Weyl tensor. By introducing a new conformal invariant constructed from the electric and magnetic parts of the Weyl tensor, we derive a master equation governing perturbations that unifies the Teukolsky and Regge-Wheeler- Zerilli formalisms. Our approach reveals previously unrecognised relationships between quasinormal mode frequencies and the conformal structure of the spacetime.We prove two fundamental theorems: (i) the conformal stability criterion, which relates mode stability to the sign-definiteness of our conformal invariant, and (ii) the isospectrality theorem for conformally related black hole spacetimes. Numerical calculations for Kerr black holes demonstrate that our formalism predicts new branches in the quasinormal mode spectrum, with frequencies differing from standard predictions by up to 3.7% in the near-extremal regime. These results have significant implications for gravitational wave astronomy and tests of general relativity in the strong-field regime.

arXiv:2505.04610v3 Announce Type: replace Abstract: We present ultraviolet (UV) through near-infrared (NIR) photometric and spectroscopic observations of the nearby SN 2024pxl, the third Type Ia supernova (SN Ia) in NGC 6384. SN 2024pxl is a Type Iax supernova (SN Iax) with an intermediate luminosity ($M_r = -16.99\pm0.32$ mag) and an average SN Iax light curve decline rate. SN 2024pxl was discovered $\sim$3 days after first light, and the rising light curve follows a single power law that is inconsistent with significant interaction with a companion star or circumstellar material. Our extensive NIR photometric coverage is comparable to that of the well-observed SNe Iax 2005hk and 2012Z, and we demonstrate that the $J-H$ colors of SNe Iax differ from normal SNe Ia and appear to be more homogeneous as a class. Spectroscopically, we report the earliest-ever NIR spectrum of a SN Iax as measured from maximum light ($t\approx-9$ days): a featureless continuum with similarities to a $\sim$9,000 K blackbody, and the line velocities are consistent with a mixed-ejecta structure, with C, Si, and Fe having similar velocities and velocity evolutions. We find a tentative correlation between the $H$-band break Co II velocity $\sim$20 days post-peak and absolute magnitude, with more luminous SNe Iax showing faster Co II velocities. Our observations suggest that SN 2024pxl resulted from the thermonuclear disruption of a CO white dwarf star that undergoes deflagration burning.

arXiv:2508.13263v2 Announce Type: replace Abstract: In recent years, multiple Type Ia supernovae (SNe Ia) have been observed with "bumps" in their rising light curves shortly after explosion. Here, we present SN 2021qvo: a 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 $\lambda$5972 pseudo-equivalent 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 pre-peak data to reveal a rising light-curve bump, and all four have bump detections. Host-galaxy analysis reveals that SN 2021qvo exploded in a low-mass galaxy ${\rm log}(M_{\ast}/M_{\odot}) = 7.83^{+0.17}_{-0.24}$, also consistent with other members of this class. We investigate the validity of the leading early-bump 2003fg-like SN Ia progenitor model, an interaction between the circumstellar material (CSM) and the SN ejecta, 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 of $\log_{10}(M_\mathrm{CSM}/M_{\odot}) = -2.33^{+0.26}_{-0.15}$. 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.

arXiv:2505.04610v2 Announce Type: replace Abstract: We present ultraviolet (UV) through near-infrared (NIR) photometric and spectroscopic observations of the nearby SN 2024pxl, the third Type Ia supernova (SN Ia) in NGC 6384. SN 2024pxl is a Type Iax supernova (SN Iax) with an intermediate luminosity ($M_r = -16.99\pm0.32$ mag) and an average SN Iax light curve decline rate. SN 2024pxl was discovered $\sim$3 days after first light, and the rising light curve follows a single power law that is inconsistent with significant interaction with a companion star or circumstellar material. Our extensive NIR photometric coverage is comparable to that of the well-observed SNe Iax 2005hk and 2012Z, and we demonstrate that the $J-H$ colors of SNe Iax differ from normal SNe Ia and appear to be more homogeneous as a class. Spectroscopically, we report the earliest-ever NIR spectrum of a SN Iax as measured from maximum light ($t\approx-9$ days): a featureless continuum with similarities to a $\sim$9,000 K blackbody, and the line velocities are consistent with a mixed-ejecta structure, with C, Si, and Fe having similar velocities and velocity evolutions. We find a tentative correlation between the $H$-band break Co II velocity $\sim$20 days post-peak and absolute magnitude, with more luminous SNe Iax showing faster Co II velocities. Our observations suggest that SN 2024pxl resulted from the thermonuclear disruption of a CO white dwarf star that undergoes deflagration burning.

arXiv:2502.17556v2 Announce Type: replace Abstract: We present optical-to-near-infrared (NIR) photometry and spectroscopy of the Type Ia supernova (SN Ia) 2024epr, including NIR spectra observed within two days of first light. The early-time optical spectra show strong, high-velocity Ca and Si features near rarely-observed velocities at $\sim$0.1$c$, and the NIR spectra show a C I "knee." Despite early-time, high-velocity features, SN 2024epr evolves into a normal SN Ia, albeit with stronger peak-light Ca absorption than other SNe Ia with the same light curve shape. Although we infer a normal decline rate, $\Delta m_{15}(B)=1.09\pm0.12$ mag, from the light-curve rise, SN 2024epr is a Branch "cool" object and has red early-time colors ($g-r\approx0.15$ mag at $-10$ days). The high velocities point to a density enhancement in the outer layers of the explosion, predicted by some models, but thick-shell He-detonation models do not match the smoothly rising light curve or apparent lack of He in our early-time NIR spectra. No current models (e.g., delayed detonation or thin He shell double detonation) appear to reproduce all observed properties, particularly the unusual early-time colors. Such constraints are only possible for SN 2024epr from the earliest optical and NIR observations, highlighting their importance for constraining SN Ia models. Finally, we identify several literature SNe Ia with intermediate mass elements at $\sim$30\,000 km s$^{-1}$ within days after the explosion that evolve into otherwise normal SNe Ia at peak light, suggesting the early-time spectra of SNe Ia may hide a broad diversity of observational characteristics.