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.

The death of massive stars is triggered by an infall-induced bounce shock that disrupts the star. How such a shock is launched and propagates through the star is a decade-long puzzle. Some models assume that the shock can be reenergized by absorbing neutrinos, leading to highly aspherical explosions. Other models involve jet-powered shocks that lead to bipolar explosions reflected in the geometry of the shock-breakout emission. We report measurement of the geometry of the shock breakout through unprecedentedly early spectropolarimetry of the nearby type II supernova 2024ggi starting ~1.2 days after the explosion. The measurement indicates a well-defined symmetry axis of the shock breakout, which is also shared by the hydrogen-rich envelope that emerged after the circumstellar matter was engulfed by the ejecta, revealing a persisting and prominent symmetry axis throughout the explosion. These findings suggest that the physical mechanism driving the explosion of massive stars manifests a well-defined axial symmetry and acts on large scales.

Context. Classical Be stars are rapidly rotating B-type stars exhibiting Balmer emission lines originating from circumstellar Keplerian gaseous disks, which likely form through episodic mass-ejection events. The currently favored model for Be star disks is the viscous decretion disk (VDD) model. However, the mechanism behind the mass ejection process during the formation of a VDD is a mystery. Aims. We present peculiar behavior of the Be star KIC 9715425, which exhibited several minor outbursts (MIBs) and one major outburst (MAB) observed as brightenings in broadband photometry, as well as transient H alpha line emission. The variability may provide valuable keys to understanding the nature of Be outbursts. Methods. Based on Kepler, the All-Sky Automated Survey for Supernovae (ASAS-SN) and the Transiting Exoplanet Survey Satellite time-series photometry, the Large Sky Area Multi-Object Fiber Spectroscopic Telescope spectroscopy, multi-wavelength observations in the near-ultraviolet (NUV) from the Galaxy Evolution Explorer (GALEX) and optical from Gaia, Xinglong 2.16 m telescope and Isaac Newton Telescope of KIC 9715425 covering its outbursts, we determined fundamental parameters of the central star and the circumstellar disk. A frequency analysis and spectral modeling were carried out to characterize the events. Results. During the major outburst, we find a 36% flux increase in the GALEX NUV band compared to 25% in the Kepler band, suggesting that whatever produced the flux increase should be hotter than the central B-type star. This is contradictory to the conventional scenario that the VDD should be cooler. In addition, such a high flux increase can only be accounted for by a luminous disk, which should produce a much stronger H alpha emission than observed. The origin of this anomalously hot component remains unexplained. Conclusions. Except for the bluing, the available observations of KIC 9715425 are perfectly compatible with being a normal Be star. If the bluing was physically related to the MAB, the most economical effort to identify the nature of the underlying process could be adaptive NUV and far-ultraviolet monitoring of Be stars with cyclically repeating MABs.

Context. Many classical Be stars acquire their very rapid rotation by mass- and angular-momentum transfer in massive binaries, marking the first phase of the evolutionary chain. Later-stage products, such as Be+subdwarf- and Be+neutron-star binaries (Be X-ray binaries), are also well known, although the search for definitive proof of Be+white dwarf companions is ongoing. Short-lived intermediate-phase objects, that is, binaries past the interaction stage but with a donor star that has not yet reached the end of its evolution or contraction, have only recently been discovered. Aims. The main hallmark of this kind of binary is a system of absorption lines with low width, significant radial-velocity variations, and peculiar relative line strengths. Data archives and the literature can be searched for additional candidates exhibiting this pattern, and follow-up observations can be obtained in order to increase the number of these systems with quantitatively known orbits, providing a basis for an initial statistical investigation and to develop observational strategies for abundance analyses. Methods. We identified 13 candidates at various confidence levels. To verify their nature, we derived orbital elements from new high-quality spectra and interferometric observations where possible. We also performed qualitative analyses of other basic parameters, and preliminarily evaluated indicators of advanced stages of nucleosynthesis. Results. Adding to the two known systems identified as classical Be star+pre-subdwarf binaries (LB-1 and HR 6819), we confirm two more (V742 Cas, HD 44637) with interferometry, with V742 Cas setting a new record for the smallest visually observed angular semi-major axis, at a = 0.663 mas. Two further systems (V447 Sct, V1362 Cyg) are not resolved interferometrically, but other evidence puts them at the same confidence level as LB-1. V2174 Cyg is a candidate with very high confidence, but was not observed interferometrically. The remaining systems are either candidates with varying levels of confidence -mainly due to the lack of available spectroscopic or interferometric observations for comparison with the others and orbit determination- or could be rejected as candidates with the followup observations. Conclusions. Of a mostly magnitude-complete sample of 328 Be stars, 0.5-1% are found to have recently completed the mass overflow that led to their formation. Another 5% are systems with a compact subdwarf companion -that is, they are further evolved after a previous overflow- and a further 2% possibly harbor white dwarfs. All these systems are early B subtypes, but if the original sample is restricted to early subtypes (136 objects), these percentages increase by a factor of about 2.5, while dropping to zero for the mid and late subtypes (together 204 objects). This strongly suggests that early-type versus mid- and late-type Be stars follow differently weighted channels to acquire their rapid rotation, namely binary interaction versus evolutionary spin up.

We present JWST spectral and photometric observations of the Type IIP supernova (SN) 2022acko at  50 days past explosion. These data are the first JWST spectral observations of a core-collapse SN. We identify  30 different H I features, other features associated with products produced from the CNO cycle, and s-process elements such as Sc II and Ba II. By combining the JWST spectra with ground-based optical and NIR spectra, we construct a full Spectral Energy Distribution from 0.4 to 25 microns and find that the JWST spectra are fully consistent with the simultaneous JWST photometry. The data lack signatures of CO formation and we estimate a limit on the CO mass of < 10^-8 solar mass. We demonstrate how the CO fundamental band limits can be used to probe underlying physics during stellar evolution, explosion, and the environment. The observations indicate little mixing between the H envelope and C/O core in the ejecta and show no evidence of dust. The data presented here set a critical baseline for future JWST observations, where possible molecular and dust formation may be seen.

Context. The BRIght Target Explorer (BRITE) mission collects photometric time series in two passbands with the aim of investigating stellar structure and evolution. Since their launches in the years 2013 and 2014, the constellation of five BRITE nano-satellites has observed a total of more than 700 individual bright stars in 64 fields. Some targets have been observed multiple times. Thus, the total time base of the datasets acquired for those stars can be as long as nine years. Aims. Our aim is to provide a complete description of ready-to-use BRITE data, to show the scientific potential of the BRITE-Constellation data by identifying the most interesting targets, and to demonstrate how scientists can use these data in their research, and encourage them to do so. Methods. We applied a decorrelation process to the automatically reduced BRITE-Constellation data to correct for instrumental effects. We performed a statistical analysis of the light curves obtained for the 300 stars observed in the first 14 fields during the first ~2.5 yr of the mission. We also performed cross-identification with the International Variable Star Index. Results. We present the data obtained by the BRITE-Constellation mission in the first 14 fields it observed, from November 2013 to April 2016. We also describe the properties of the data for these fields and the 300 stars observed in them. Using these data, we detect variability in 64% of the presented sample of stars. Sixty-four stars, or 21.3% of the sample, have not yet been identified as variable in the literature and their data have not been analysed in detail. They can therefore provide valuable scientific material for further research. All data are made publicly available through the BRITE Public Data Archive and the Canadian Astronomy Data Centre.

Dust associated with various stellar sources in galaxies at all cosmic epochs remains a controversial topic, particularly whether supernovae (SNe) play an important role in dust production. We report evidence of dust formation in the cold, dense shell behind the ejecta-circumstellar medium (CSM) interaction in the Type Ia-CSM SN 2018evt three years after the explosion, characterized by a rise in the mid-infrared (MIR) emission accompanied by an accelerated decline in the optical radiation of the SN. Such a dust-formation picture is also corroborated by the concurrent evolution of the profiles of the Ha emission line. Our model suggests enhanced CSM dust concentration at increasing distances from the SN as compared to what can be expected from the density profile of the mass loss from a steady stellar wind. By the time of the last MIR observations at day +1041, a total amount of 1.2+-0.2x10^{-2} Msun of new dust has been formed by SN 2018evt, making SN 2018evt one of the most prolific dust factories among SNe with evidence of dust formation. The unprecedented witness of the intense production procedure of dust may shed light on the perceptions of dust formation in cosmic history.

Rapid rotation and nonradial pulsations enable Be stars to build decretion disks, where the characteristic line emission forms. A major but unconstrained fraction of Be stars owe their rapid rotation to mass and angular momentum transfer in a binary. The faint, stripped companions can be helium-burning subdwarf OB-type stars (sdOBs), white dwarfs (WDs), or neutron stars. We present optical/near-infrared Center for High Angular Resolution Astronomy (CHARA) interferometry of 37 Be stars selected for spectroscopic indications of low-mass companions. From multiepoch H - and/or K -band interferometry plus radial velocities and parallaxes collected elsewhere, we constructed 3D orbits and derived flux ratios and absolute dynamical masses of both components for six objects, quadrupling the number of anchor points for evolutionary models. In addition, a new wider companion was identified for the known Be + sdO binary 59 Cyg, while auxiliary Very Large Telescope Interferometer/GRAVITY spectrointerferometry confirmed circumstellar matter around the sdO companion to HR 2142. On the other hand, we failed to detect any companion to the six Be stars with γ Cas–like X-ray emission, with sdOB and main-sequence companions of the expected spectroscopic mass being ruled out for the X-ray-prototypical stars γ Cas and π Aqr, leaving elusive WDs as the most likely companions, as well as a likely explanation of the X-rays. No low-mass main-sequence close companions were identified for the other stars.

We present a JWST mid-infrared spectrum of the under-luminous Type Ia Supernova (SN Ia) 2022xkq. The spectrum was obtained with the medium-resolution spectrometer on the Mid-Infrared Instrument (MIRI) roughly 130 days after explosion. We identify the first MIR lines beyond 14 $\mu$m in SN Ia observations. We also find distinct features unique to under-luminous SNe Ia, including: isolated emission of stable Ni, strong blends of [Ti II], and large ratios of singly ionized to doubly ionized species in both [Ar] and [Co]. Comparisons to normal-luminosity SNe Ia spectra at similar phases show a tentative trend between the width of the [Co III] 11.888 $\mu$m feature and the SN light curve shape. Using non-LTE-multi-dimensional radiation hydro simulations and the observed electron capture elements we constrain the mass of the exploding white dwarf. The best-fitting model shows that SN 2022xkq is consistent with an off-center delayed-detonation explosion of a near-Chandrasekhar mass WD of high-central density ($\rho_{c} \geq 2.0 \times 10^{9}$ g cm$^{-3}$) seen equator on, and produced M($^{58}$Ni) $\geq 0.06$ M$_{\odot}$. The observed line width of various species are consistent with the overall abundance distribution; and the narrow stable Ni lines indicate little to no mixing in the central regions, favoring central ignition of sub-sonic carbon burning followed by an off-center DDT which begins at a single point. Observations at later epochs may further constrain the physics revealing the presence of additional species including Cr and Mn. Our work demonstrates the power of using the full coverage of MIRI in combination with detailed modeling to elucidate the physics of SNe Ia at a level not previously possible.

We present a JWST MIRI medium-resolution spectrometer spectrum (5-27 mu m) of the Type Ia supernova (SN Ia) SN 2021aefx at +415 days past B-band maximum. The spectrum, which was obtained during the iron-dominated nebular phase, has been analyzed in combination with previous JWST observations of SN 2021aefx to provide the first JWST time series analysis of an SN Ia. We find that the temporal evolution of the [Co iii] 11.888 mu m feature directly traces the decay of 56Co. The spectra, line profiles, and their evolution are analyzed with off-center delayed-detonation models. Best fits were obtained with white dwarf (WD) central densities of rho c = 0.9-1.1 x 109 g cm-3, a WD mass of M WD = 1.33-1.35 M circle dot, a WD magnetic field of approximate to 106 G, and an off-center deflagration-to-detonation transition at approximate to 0.5 M circle dot seen opposite to the line of sight of the observer (-30 degrees). The inner electron capture core is dominated by energy deposition from gamma-rays, whereas a broader region is dominated by positron deposition, placing SN 2021aefx at +415 days in the transitional phase of the evolution to the positron-dominated regime. The formerly "flat-tilted" profile at 9 mu m now has a significant contribution from [Ni iv], [Fe ii], and [Fe iii] and less from [Ar iii], which alters the shape of the feature as positrons mostly excite the low-velocity Ar. Overall, the strength of the stable Ni features in the spectrum is dominated by positron transport rather than the Ni mass. Based on multidimensional models, our analysis is consistent with a single-spot, close-to-central ignition with an indication of a preexisting turbulent velocity field and excludes a multiple-spot, off-center ignition.

The explosion processes of supernovae (SNe) are imprinted in their explosion geometries. Here, we study the intrinsic polarization of 15 hydrogen-rich core-collapse SNe and explore the relation with the photometric and spectroscopic properties. Our sample shows diverse properties of the continuum polarization. The polarization of most SNe has a low degree at early phases but shows a sudden rise to $\sim 1$ \% degree at certain points during the photospheric phase as well as a slow decline during the tail phase, with a constant polarization angle. The variation in the timing of peak polarisation values implies diversity in the explosion geometry: some SNe have aspherical structures only in their helium cores, while in other SNe these reach out to a significant part of the outer hydrogen envelope with a common axis from the helium core to the hydrogen envelope. Other SNe show high polarization from early phases and a change of the polarization angle around the middle of the photospheric phase. This implies that the ejecta are significantly aspherical to the outermost layer and have multi-directional aspherical structures. Exceptionally, the Type~IIL SN~2017ahn shows low polarization at both the photospheric and tail phases. Our results show that the timing of the polarization rise in Type~IIP SNe is likely correlated with their brightness, velocity and the amount of radioactive Ni produced: brighter SNe with faster ejecta velocity and a larger $^{56}$Ni mass have more extended-aspherical explosion geometries. In particular, there is a clear correlation between the timing of the polarization rise and the explosion energy, that is, the explosion asphericity is proportional to the explosion energy. This implies that the development of a global aspherical structure, e.g., a jet, might be the key to realising an energetic SN in the mechanism of SN explosions.

ABSTRACT Young star clusters enable us to study the effects of stellar rotation on an ensemble of stars of the same age and across a wide range in stellar mass and are therefore ideal targets for understanding the consequences of rotation on stellar evolution. We combine MUSE spectroscopy with HST photometry to measure the projected rotational velocities (Vsin i) of 2184 stars along the split main sequence and on the main sequence turn-off (MSTO) of the 100 Myr-old massive ($10^5\, {\rm M_{\odot }}$) star cluster NGC 1850 in the Large Magellanic Cloud. At fixed magnitude, we observe a clear correlation between Vsin i and colour, in the sense that fast rotators appear redder. The average Vsin i values for stars on the blue and red branches of the split main sequence are $\sim \! 100\, {\rm km\, s^{-1}}$ and $\sim \! 200\, {\rm km\, s^{-1}}$, respectively. The values correspond to about $25-30{{\ \rm per\ cent}}$ and $50-60{{\ \rm per\ cent}}$ of the critical rotation velocity and imply that rotation rates comparable to those observed in field stars of similar masses can explain the split main sequence. Our spectroscopic sample contains a rich population of ∼200 fast rotating Be stars. The presence of shell features suggests that 23 per cent of them are observed through their decretion discs, corresponding to a disc opening angle of 15 degrees. These shell stars can significantly alter the shape of the MSTO, hence care should be taken when interpreting this photometric feature. Overall, our findings impact our understanding of the evolution of young massive clusters and provide new observational constraints for testing stellar evolutionary models.

The observations of all known major activity phases of the disks around the classical Be stars γ Cas and 59 Cyg with low-mass companions are comprehensively reviewed and purely qualitatively evaluated again, though taking advantage of new insights gained over the past 25 yr into the physics of Be disks. Both stars have exhibited activity cycles in the violet-to-red (V/R) flux ratio of emission lines with two peaks. This activity is indistinguishable from those of the vast majority of Be stars and so probably were caused by one-armed ( m = 1) disk oscillations. The anomalous high-activity phases from 1932 to 1942 in γ Cas and between 1972 and 1976 in 59 Cyg were distinguished from m = 1 density waves by large variations in the separations of pairs of emission peaks. In two consecutive cycles, shell phases during which the emission peaks were maximally separated alternated with single (blended) emission peaks. The amplitude in peak separation of more than a factor of two implies a high-amplitude variation in the disk aspect angle. When the peaks were blended and the disk was viewed closest to face-on, local maxima in visual brightness probably occurred in γ Cas, and the visibility of the stellar absorption lines was reduced, as is expected from increased free-bound emission into the line of sight (there is no time-resolved photometry for 59 Cyg from the event in the 1970s). In y Cas, the pre-event V/R variability (pre-event observations of 59 Cyg do not exist) was practically identical to m = 1 variability. In spite of the subsequent rapid rise in amplitude (up to ~4), the V/R variations connected smoothly in phase but may require an explanation involving the 3D structure of the disk. The phasing of single-peak and shell stages relative to the V/R activity was the same in both cycles of γ Cas, whereas this is not clear for 59 Cyg. During both high-activity cycles of γ Cas, but at different phases, transient additional pairs of emission lines appeared in γ Cas that were much sharper than the main ones and they also had different peak separations and V/R ratios. In the second instance, their velocities were up to ~+500 km s −1 . The extremely rapid excitation of the activity phases and their short duration of only two cycles in both stars may indicate a resonant behavior of an unidentified nature. In both stars, the line emission was strongly developed at the onset of the high-activity phases but it basically disappeared at the end of them, and the disks may have been dynamically destroyed. The atypical disk variations were presumably triggered by enhanced interactions between a disk and companion star. In both systems, there seems to be less evidence for a mass-loss outburst than for a reduced mass-injection rate into the disk. The resulting lower viscous coupling between a disk and star would have facilitated the tilting of the disk.

Accurate distance determination to astrophysical objects is essential for the understanding of their intrinsic brightness and size. The distance to SN 1987A has been previously measured by the expanding photosphere method and by using the angular size of the circumstellar rings with absolute sizes derived from light curves of narrow UV emission lines, with reported distances ranging from 46.77 to 55 kpc. In this study, we independently determined the distance to SN 1987A using photometry and imaging polarimetry observations of AT 2019xis, a light echo of SN 1987A, by adopting a radiative transfer model of the light echo developed in Ding et al. We obtained distances to SN 1987A in the range from 49.09 ± 2.16 kpc to 59.39 ± 3.27 kpc, depending on the interstellar polarization and extinction corrections, which are consistent with the literature values. This study demonstrates the potential of using light echoes as a tool for distance determination to astrophysical objects in the Milky Way, up to kiloparsec level scales.

We report evidence of dust formation in the cold, dense shell behind the ejecta-circumstellar medium (CSM) interaction in the Type Ia (SNIa) SN2018evt three years after the explosion, characterized by a rise in the mid-infrared (MIR) flux accompanied by an accelerated decline in the optical. Such a dust-formation picture is also corroborated by the concurrent evolution of the profiles of the Ha emission lines. Our models suggest enhanced dust concentration at increasing distances from the SN as compared to what can be expected from the density profile of the mass loss from a steady stellar wind. This may indicate an enhanced dust presence at larger distances from the progenitor star. The dust distribution can be modeled in terms of a double-shell which assumes a sudden change of the density profile of the dust or a single-shell model with a flatter radial profile. The inferred mass-loss history of SN2018evt is consistent with a progenitor's mass loss in a binary system containing a C/O white dwarf and a massive asymptotic giant branch star. The grand rebrightening in the MIR after day +310 is attributed to the formation of new dust within the CDS region behind the forward shock. The mass of the newly-formed dust increases rapidly with time and follows a power law with an index of 4. By the time of the last MIR observations at day +1041, a total amount of 1.2+-0.2 x 10^{-2} Msun of dust has been produced, making SN 2018evt one of the most prolific dust factory among SNe with evidence of dust formations.

ABSTRACT A rare class of supernovae (SNe) is characterized by strong interaction between the ejecta and several solar masses of circumstellar matter (CSM) as evidenced by strong Balmer-line emission. Within the first few weeks after the explosion, they may display spectral features similar to overluminous Type Ia SNe, while at later phase their observation properties exhibit remarkable similarities with some extreme case of Type IIn SNe that show strong Balmer lines years after the explosion. We present polarimetric observations of SN 2018evt obtained by the ESO Very Large Telescope from 172 to 219 d after the estimated time of peak luminosity to study the geometry of the CSM. The non-zero continuum polarization decreases over time, suggesting that the mass-loss of the progenitor star is aspherical. The prominent H α emission can be decomposed into a broad, time-evolving component and an intermediate-width, static component. The former shows polarized signals, and it is likely to arise from a cold dense shell (CDS) within the region between the forward and reverse shocks. The latter is significantly unpolarized, and it is likely to arise from shocked, fragmented gas clouds in the H-rich CSM. We infer that SN 2018evt exploded inside a massive and aspherical circumstellar cloud. The symmetry axes of the CSM and the SN appear to be similar. SN 2018evt shows observational properties common to events that display strong interaction between the ejecta and CSM, implying that they share similar circumstellar configurations. Our preliminary estimate also suggests that the circumstellar environment of SN 2018evt has been significantly enriched at a rate of ∼0.1 M⊙ yr−1 over a period of >100 yr.

ABSTRACT Optical spectropolarimetry of the normal thermonuclear supernova (SN) 2019np from −14.5 to +14.5 d relative to B-band maximum detected an intrinsic continuum polarization (pcont) of 0.21 ± 0.09 per cent at the first epoch. Between days −11.5 and +0.5, pcont remained ∼0 and by day +14.5 was again significant at 0.19 ± 0.10 per cent. Not considering the first epoch, the dominant axis of ${\rm Si\, {\small II}}$ λ6355 was roughly constant staying close the continuum until both rotated in opposite directions on day +14.5. Detailed radiation-hydrodynamical simulations produce a very steep density slope in the outermost ejecta so that the low first-epoch pcont ≈ 0.2 per cent nevertheless suggests a separate structure with an axis ratio ∼2 in the outer carbon-rich (3.5–4) × 10−3 M⊙. Large-amplitude fluctuations in the polarization profiles and a flocculent appearance of the polar diagram for the ${\rm Ca\, {\small II}}$ near-infrared triplet (NIR3) may be related by a common origin. The temporal evolution of the polarization spectra agrees with an off-centre delayed detonation. The late-time increase in polarization and the possible change in position angle are also consistent with an aspherical 56Ni core. The pcont and the absorptions due to ${\rm Si\, {\small II}}$ λ6355 and ${\rm Ca\, {\small II}}$ NIR3 form in the same region of the extended photosphere, with an interplay between line occultation and thermalization producing p. Small-scale polarization features may be due to small-scale structures, but many could be related to atomic patterns of the quasi-continuum; they hardly have an equivalent in the total-flux spectra. We compare SN 2019np to other SNe and develop future objectives and strategies for SN Ia spectropolarimetry.

We present six epochs of optical spectropolarimetry of the Type IIP supernova (SN) 2021yja ranging from $\sim$ 25 to 95 days after the explosion. An unusually high continuum linear polarization of $p \sim 0.9\%$ is measured during the early photospheric phase, followed by a steady decrease well before the onset of the nebular phase. This behavior has not been observed before in Type IIP supernovae (SNe IIP). The observed continuum polarization angle does not change significantly during the photospheric phase. We find a pronounced axis of symmetry in the global ejecta that is shared in common with the H$\alpha$ and Ca II near-infrared triplet lines. These observations are consistent with an ellipsoidal geometry. The temporal evolution of the continuum polarization is also compatible with the SN ejecta interacting with aspherical circumstellar matter, although no spectroscopic features that may be associated with strong interaction can be identified. Alternatively, we consider the source of the high polarization to be an extended hydrogen envelope that is indistinguishable from low-density circumstellar matter.

The CRyogenic InfraRed Echelle Spectrograph (CRIRES) Upgrade project CRIRES^+ extended the capabilities of CRIRES. It transformed this VLT instrument into a cross-dispersed spectrograph to increase the wavelength range that is covered simultaneously by up to a factor of ten. In addition, a new detector focal plane array of three Hawaii 2RG detectors with a 5.3 μm cutoff wavelength replaced the existing detectors. Amongst many other improvements, a new spectropolarimetric unit was added and the calibration system has been enhanced. The instrument was installed at the VLT on Unit Telescope 3 at the beginning of 2020 and successfully commissioned and verified for science operations during 2021, partly remotely from Europe due to the COVID-19 pandemic. The instrument was subsequently offered to the community from October 2021 onwards. This article describes the performance and capabilities of the upgraded instrument and presents on sky results.

We present a JWST/MIRI low-resolution mid-infrared (MIR) spectroscopic observation of the normal Type Ia supernova (SN Ia) SN 2021aefx at +323 days past rest-frame B-band maximum light. The spectrum ranges from 4-14 um, and shows many unique qualities including a flat-topped [Ar III] 8.991 um profile, a strongly tilted [Co III] 11.888 um feature, and multiple stable Ni lines. These features provide critical information about the physics of the explosion. The observations are compared to synthetic spectra from detailed NLTE multi-dimensional models. The results of the best-fitting model are used to identify the components of the spectral blends and provide a quantitative comparison to the explosion physics. Emission line profiles and the presence of electron capture (EC) elements are used to constrain the mass of the exploding white dwarf (WD) and the chemical asymmetries in the ejecta. We show that the observations of SN 2021aefx are consistent with an off-center delayed-detonation explosion of a near-Chandrasekhar mass (Mch) WD at a viewing angle of -30 degrees relative to the point of the deflagration-to-detonation transition. From the strength of the stable Ni lines we determine that there is little to no mixing in the central regions of the ejecta. Based on both the presence of stable Ni and the Ar velocity distributions, we obtain a strict lower limit of 1.2 Msun of the initial WD, implying that most sub-Mch explosions models are not viable models for SN 2021aefx. The analysis here shows the crucial importance of MIR spectra for distinguishing between explosion scenarios for SNe Ia.