Accurate estimates of internal red-giant rotation rates are a crucial ingredient for constraining and improving current models of stellar rotation. Asteroseismic rotational inversions are a method to estimate these internal rotation rates. In this work, we focus on the observed differences in the rotationally-induced frequency shifts between prograde and retrograde modes, which were ignored in previous works when estimating internal rotation rates of red giants using inversions. We systematically study the limits of applicability of linear rotational inversions as a function of the evolution on the red-giant branch and the underlying rotation rates. We solve for the oscillation mode frequencies in the presence of rotation in the lowest-order perturbative approach. This enables a description of the differences between prograde and retrograde modes through the coupling of multiple mixed modes. We compute synthetic rotational splittings taking these near-degeneracy effects into account. We use red-giant models with one solar mass, a large frequency separation between 16 and 9 microhertz and core rotation rates between 500 and 1500 nHz covering the regime of observed parameters of Kepler red-giant stars. Finally, we use these synthetic data to quantify the systematic errors of internal rotation rates estimated by means of rotational inversions in the presence of near-degeneracy effects. We show that the systematic errors in the estimated rotation rates introduced by near-degeneracy effects surpass observational uncertainties for more evolved and faster rotating stars. The estimated rotation rates of some of the previously analysed red giants suffer from significant systematic errors that have not been taken into account yet. Notwithstanding, reliable analyses with existing inversion methods are feasible for a number of red giants within the parameter ranges determined here.

Enhanced emission in the months to years preceding explosion has been detected for several core-collapse supernovae (SNe). Though the physical mechanisms driving the emission remain hotly debated, the light curves of detected events show long-lived ($\geq$50 days), plateau-like behavior, suggesting hydrogen recombination may significantly contribute to the total energy budget. The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will provide a decade-long photometric baseline to search for this emission, both in binned pre-explosion observations after an SN is detected and in single-visit observations prior to the SN explosion. In anticipation of these searches, we simulate a range of eruptive precursor models to core-collapse SNe and forecast the discovery rates of these phenomena in LSST data. We find a detection rate of ~40-130 yr$^{-1}$ for SN IIP/IIL precursors and ~110 yr$^{-1}$ for SN IIn precursors in single-epoch photometry. Considering the first three years of observations with the effects of rolling and observing triplets included, this number grows to a total of 150-400 in binned photometry, with the highest number recovered when binning in 100-day bins for 2020tlf-like precursors and in 20-day bins for other recombination-driven models from the literature. We quantify the impact of using templates contaminated by residual light (from either long-lived or separate precursor emission) on these detection rates, and explore strategies for estimating baseline flux to mitigate these issues. Spectroscopic follow-up of the eruptions preceding core-collapse SNe and detected with LSST will offer important clues to the underlying drivers of terminal-stage mass loss in massive stars.

Stellar mergers are responsible for a large variety of astrophysical phenomena. They form blue straggler stars, give rise to spectacular transients, and produce some of the most massive stars in the Universe. Here, we focus on mergers from binary evolution and stellar collisions but do not cover mergers involving compact objects. We review how mergers come about, explain the physics and outcome of the merger process, discuss the evolution and ultimate fates of merged stars, and relate to observations. Our main conclusions are: (i) Mergers of main-sequence stars often fully rejuvenate and have interior structures similar to genuine single stars. (ii) Contrarily, mergers involving post-main-sequence stars can have interior structures that cannot be achieved by single-star evolution. Such merged stars may become long-lived blue supergiants that can explode in SN1987A-like events, interacting and superluminous supernovae, ultra-long gamma-ray bursts or collapse into very massive black holes. These black holes may even populate the pair-instability-supernova black-hole mass gap. (iii) Strong magnetic fields are produced in stellar mergers. Merged stars may thus be at the origin of some magnetic OBA stars and their descendants, highly magnetic white dwarfs and neutron stars. (iv) Initially, stellar merger products rotate rapidly, but there are several mechanisms that can quickly spin them down. Hence, merged stars may be rather slow rotators for most of their evolution.

The latest GWTC-4 release from the LIGO-Virgo-KAGRA (LVK) collaboration nearly doubles the known population of double compact object mergers and reveals a new trimodal structure in the chirp-mass distribution of merging binary black holes (BBHs) below 30 Msun. Recent detailed stellar evolution models show that features in the pre-collapse cores of massive stars produce a bimodal black hole (BH) mass distribution, which naturally extends to a trimodal BBH chirp-mass distribution. Both distributions depend only weakly on metallicity, implying universal structural features which can be tested with LVK observations. Using a new compact-remnant mass prescription derived from these models, we perform rapid population synthesis simulations to test the robustness of the predicted chirp-mass structure against uncertainties in binary evolution and cosmic star formation history, and compare these results with the current observational data. The trimodal chirp-mass distribution emerges as a robust outcome of the new remnant-mass model, persisting across variations in binary and cosmic physics. In contrast, traditional BH formation models lacking a bimodal BH mass spectrum fail to reproduce the observed trimodality. The updated models also predict lower BBH merger rates by a factor of a few, in closer agreement with LVK constraints. Intriguingly, the central chirp-mass peak, dominated by unequal-mass BBHs, originates from a previously underappreciated formation pathway in which strong luminous blue variable winds suppress binary interaction before the first BH forms. If isolated binary evolution dominates BBH formation below 30 Msun, the relative heights of the three chirp-mass peaks offer powerful observational constraints on core collapse, BH formation, binary evolution, and cosmic star formation. These universal structural features may also serve as standard sirens for precision cosmology.

We present 3D radiation hydrodynamics simulations of common-envelope (CE) evolution involving a 12 solar mass red supergiant donor and a 3 solar mass companion. Existing 3D simulations are predominantly adiabatic, focusing strongly on low-mass donors on the red giant and asymptotic giant branches. However, the adiabatic assumption breaks down once the perturbed CE material becomes optically thin or when entering a longer-timescale evolutionary phase after the dynamical plunge-in. This is especially important for high-mass red supergiant donors, which have short thermal timescales, adding significant uncertainty to our understanding of how massive binary stars evolve into gravitational-wave sources, X-ray binaries, stripped-envelope supernovae, and more. We compare our radiation hydrodynamics simulations with an adiabatic simulation from Paper I that is otherwise identical, finding that radiative diffusion strongly inhibits CE ejection. The fraction of ejected mass is roughly half that of the adiabatic case without accounting for recombination energy release. Almost no material is ejected during the dynamical plunge-in, and longer-timescale ejection during the slow spiral-in is suppressed. However, the orbital separation reached at the end of the dynamical plunge-in does not differ significantly. The large amount of remaining bound mass tentatively supports the emerging view that the dynamical plunge-in is followed by a non-adiabatic phase, during which a substantial fraction of the envelope is ejected and the binary orbit may continue to evolve.

The growth of galaxies in the early Universe is driven by accretion of circum- and inter-galactic gas. Simulations predict that steady streams of cold gas penetrate the dark matter halos of galaxies, providing the raw material necessary to sustain star formation. We report a filamentary stream of gas that extends for 100 kiloparsecs and connects to the massive radio galaxy 4C 41.17. The stream is detected using sub-millimeter observations of the [CI] line of atomic carbon, a tracer of neutral atomic or molecular hydrogen gas. The galaxy contains a central gas reservoir that is fueling a vigorous starburst. Our results show that the raw material for star formation can be present in cosmic streams outside galaxies.

Magnetic fields are known to be dynamically important in the interstellar medium of our own Galaxy, and they are ubiquitously observed in diffuse gas in the halos of galaxies and galaxy clusters. Yet, magnetic fields have typically been neglected in studies of the formation of galaxies, leaving their global influence on galaxy formation largely unclear. We extend our MHD implementation in the moving-mesh code Arepo to cosmological problems which include radiative cooling and the formation of stars. In particular, we replace our previously employed divergence cleaning approach with a Powell 8-wave scheme, which turns out to be significantly more stable, even in very dynamic environments. We verify the improved accuracy through simulations of the MRI in accretion disks, that reproduce its correct linear growth rate. Using this new MHD code, we simulate the formation of isolated disk galaxies similar to the Milky Way using idealized initial conditions with and without magnetic fields. We find that the magnetic field is quickly amplified in the initial starburst and the differential rotation of the forming disk until it eventually saturates when it becomes comparable to the thermal pressure. The additional pressure component leads to a lower star formation rate at late times compared to simulations without magnetic fields, and induces changes in the spiral arm structures of the gas disk. In addition, we observe highly magnetized fountain-like outflows from the disk. These results are robust with numerical resolution and are largely independent of the initial magnetic seed field assumed in the initial conditions, as the amplification process is rapid and self-regulated. Our findings suggest an important influence of magnetic fields on galaxy formation and evolution, cautioning against their neglect in theoretical models of structure formation.

Type Ia supernovae (SNe Ia) are runaway thermonuclear explosions in white dwarfs that result in the disruption of the white dwarf star, and possibly its nearby stellar companion. SNe Ia occur over an immense range of stellar population age and host galaxy environments, and play a critical role in the nucleosynthesis of intermediate-mass and iron-group elements, primarily the production of nickel, iron, cobalt, chromium, and manganese. Though the nature of their progenitors is still not well-understood, SNe Ia are unique among stellar explosions in that the majority of them exhibit a systematic lightcurve relation: more luminous supernovae dim more slowly over time than less luminous supernovae in optical light (intrinsically brighter SNe Ia have broader lightcurves). This feature, unique to SNe Ia, is rather remarkable and allows their peak luminosities to be determined with fairly high accuracy out to cosmological distances via measurement of their lightcurve decline. Further, studying SNe Ia gives us important insights into binary star evolution physics, since it is widely agreed that the progenitors of SNe Ia are binary (possibly multiple) star systems. In this review, we give a current update on the different proposed Type Ia supernova progenitors, including descriptions of possible binary star configurations, and their explosion mechanisms, from a theoretical perspective. We additionally give a brief overview of the historical (focusing on the more recent) observational work that has helped the astronomical community to understand the nature of the most important distance indicators in cosmology.

Over the past decades, many explosion scenarios for Type Ia supernovae have been proposed and investigated including various combinations of deflagrations and detonations in white dwarfs of different masses up to the Chandrasekhar mass. One of these is the gravitationally confined detonation model. In this case a weak deflagration burns to the surface, wraps around the bound core, and collides at the antipode. A subsequent detonation is then initiated in the collision area. Since the parameter space for this scenario, that is, varying central densities and ignition geometries, has not been studied in detail, we used pure deflagration models of a previous parameter study dedicated to Type Iax supernovae as initial models to investigate the gravitationally confined detonation scenario. We aim to judge whether this channel can account for one of the many subgroups of Type Ia supernovae, or even normal events. To this end, we employed a comprehensive pipeline for three-dimensional Type Ia supernova modeling that consists of hydrodynamic explosion simulations, nuclear network calculations, and radiative transfer. The observables extracted from the radiative transfer are then compared to observed light curves and spectra. The study produces a wide range in masses of synthesized 56 Ni ranging from 0.257 to 1.057 $M_\odot$ , and, thus, can potentially account for subluminous as well as overluminous Type Ia supernovae in terms of brightness. However, a rough agreement with observed light curves and spectra can only be found for 91T-like objects. Although several discrepancies remain, we conclude that the gravitationally confined detonation model cannot be ruled out as a mechanism to produce 91T-like objects. However, the models do not provide a good explanation for either normal Type Ia supernovae or Type Iax supernovae.

SNe Ia play a key role in the fields of astrophysics and cosmology. It is widely accepted that SNe Ia arise from thermonuclear explosions of WDs in binaries. However, there is no consensus on the fundamental aspects of the nature of SN Ia progenitors and their explosion mechanism. This fundamentally flaws our understanding of these important astrophysical objects. We outline the diversity of SNe Ia and the proposed progenitor models and explosion mechanisms. We discuss the recent theoretical and observational progress in addressing the SN Ia progenitor and explosion mechanism in terms of the observables at various stages of the explosion, including rates and delay times, pre-explosion companion stars, ejecta-companion interaction, early excess emission, early radio/X-ray emission from CSM interaction, surviving companions, late-time spectra and photometry, polarization signals, and SNR properties, etc. Despite the efforts from both the theoretical and observational side, the questions of how the WDs reach an explosive state and what progenitor systems are more likely to produce SNe Ia remain open. No single published model is able to consistently explain all observational features and the full diversity of SNe Ia. This may indicate that either a new progenitor paradigm or the improvement of current models is needed if all SNe Ia arise from the same origin. An alternative scenario is that different progenitor channels and explosion mechanisms contribute to SNe Ia. In the next decade, the ongoing campaigns with the JWST, Gaia and the ZTF, and upcoming extensive projects with the LSST and the SKA will allow us to conduct not only studies of individual SNe Ia in unprecedented detail but also systematic investigations for different subclasses of SNe Ia. This will advance theory and observations of SNe Ia sufficiently far to gain a deeper understanding of their origin and explosion mechanism.

We present JEKYLL, a new code for modelling of supernova (SN) spectra and lightcurves based on Monte-Carlo (MC) techniques for the radiative transfer. The code assumes spherical symmetry, homologous expansion and steady state for the matter, but is otherwise capable of solving the time-dependent radiative transfer problem in non-local-thermodynamic-equilibrium (NLTE). The method used was introduced in a series of papers by Lucy, but the full time-dependent NLTE capabilities of it have never been tested. Here, we have extended the method to include non-thermal excitation and ionization as well as charge-transfer and two-photon processes. Based on earlier work, the non-thermal rates are calculated by solving the Spencer-Fano equation. Using a method previously developed for the SUMO code, macroscopic mixing of the material is taken into account in a statistical sense. In addition, a statistical Markov-chain model is used to sample the emission frequency, and we introduce a method to control the sampling of the radiation field. Except for a description of JEKYLL, we provide comparisons with the ARTIS, SUMO and CMFGEN codes, which show good agreement in the calculated spectra as well as the state of the gas. In particular, the comparison with CMFGEN, which is similar in terms of physics but uses a different technique, shows that the Lucy method does indeed converge in the time-dependent NLTE case. Finally, as an example of the time-dependent NLTE capabilities of JEKYLL, we present a model of a Type IIb SN, taken from a set of models presented and discussed in detail in an accompanying paper. Based on this model we investigate the effects of NLTE, in particular those arising from non-thermal excitation and ionization, and find strong effects even on the bolometric lightcurve. This highlights the need for full NLTE calculations when simulating the spectra and lightcurves of SNe.

The origin of carbon in the Universe remains uncertain. At solar metallicity, binary-stripped massive stars -- stars that lost their envelope through stable interaction with a companion -- have been suggested to produce twice as much carbon as their single-star counterparts. However, understanding the chemical evolution of galaxies over cosmic time requires examining stellar yields across a range of metallicities. Using the stellar evolution code MESA, we compute the carbon yields from wind mass loss and supernova explosions of single and binary-stripped stars across a wide range of initial masses ($10$-$46,M_\odot$), metallicities ($Z = 0.0021$, $0.0047$, $0.0142$), and initial orbital periods ($10$-$5000$ days). We find that metallicity is the dominant factor influencing the carbon yields of massive stars, outweighing the effects of binarity and detailed orbital parameters. Since the chemical yields from binary massive stars are highly sensitive to metallicity, we caution that yields predicted at solar metallicity should not be directly extrapolated to lower metallicities. At sub-solar metallicities, particularly below $1/7$ solar, weak stellar winds and inefficient binary stripping result in carbon yields from binary-stripped stars that closely resemble those of single stars. This suggests that binary-stripped massive stars are unlikely to explain the presence of carbon-enhanced metal-poor stars or the carbon enrichment observed in high-redshift galaxies as probed by the James Webb Space Telescope. Our findings only concern the stripped stars in massive binaries. The impact of other outcomes of binary star evolution, in particular stellar mergers and accretors, remains largely unexplored but will be necessary for a full understanding of the role of massive binaries in nucleosynthesis.

We present a freeze-out approach to the formation of heavy elements in expanding nuclear matter. Applying concepts used in the description of heavy-ion collisions or ternary fission, we determine the abundances of heavy elements taking into account in-medium effects such as Pauli blocking and the Mott effect, which describes the dissolution of nuclei at high densities of nuclear matter. With this approach, we search for a universal primordial distribution in an equilibrium state from which the gross structure of the solar abundances of heavy elements freezes out via radioactive decay of the excited states. The universal primordial state is characterized by the Lagrangian parameters of temperature and chemical potentials of neutrons and protons. We show that such a state exists and determine a temperature of 5.266 MeV, a neutron chemical potential of 940.317 MeV and a proton chemical potential of 845.069 MeV, at a baryon number density of 0.013 fm$^{-3}$ and a proton fraction of 0.13. Heavy neutron-rich nuclei such as the hypothesized double-magic nucleus $^{358}$Sn appear in the primordial distribution and contribute to the observed abundances after fission. We discuss astrophysical scenarios for the realization of this universal primordial distribution for heavy element nucleosynthesis, including supernova explosions, neutron star mergers and the inhomogeneous Big Bang. The latter scenario may be of interest in the light of early massive objects observed with the James Webb Space Telescope and opens new perspectives to explain universality of the observed r-process patterns and the lack of observations of population III stars.

We discuss new methods to integrate the cosmic ray (CR) evolution equations coupled to magneto-hydrodynamics (MHD) on an unstructured moving mesh, as realised in the massively parallel AREPO code for cosmological simulations. We account for diffusive shock acceleration of CRs at resolved shocks and at supernova remnants in the interstellar medium (ISM), and follow the advective CR transport within the magnetised plasma, as well as anisotropic diffusive transport of CRs along the local magnetic field. CR losses are included in terms of Coulomb and hadronic interactions with the thermal plasma. We demonstrate the accuracy of our formalism for CR acceleration at shocks through simulations of plane-parallel shock tubes that are compared to newly derived exact solutions of the Riemann shock tube problem with CR acceleration. We find that the increased compressibility of the post-shock plasma due to the produced CRs decreases the shock speed. However, CR acceleration at spherically expanding blast waves does not significantly break the self-similarity of the Sedov-Taylor solution; the resulting modifications can be approximated by a suitably adjusted, but constant adiabatic index. In first applications of the new CR formalism to simulations of isolated galaxies and cosmic structure formation, we find that CRs add an important pressure component to the ISM that increases the vertical scale height of disk galaxies, and thus reduces the star formation rate. Strong external structure formation shocks inject CRs into the gas, but the relative pressure of this component decreases towards halo centres as adiabatic compression favours the thermal over the CR pressure.

Rotation is an important, yet poorly-modelled phenomenon of stellar structure and evolution. Accurate estimates of internal rotation rates are therefore valuable for constraining stellar evolution models. We aim to assess the accuracy of asteroseismic estimates of internal rotation rates and how these depend on the fundamental stellar parameters. We apply the recently-developed method called extended-MOLA inversions to infer localised estimates of internal rotation rates of synthetic observations of red giants. We search for suitable reference stellar models following a grid-based approach, and assess the robustness of the resulting inferences to the choice of reference model. We find that matching the mixed mode pattern between the observation and the reference model is an important criterion to select suitable reference models. We propose to i) select a set of reference models based on the correlation between the observed rotational splittings and the mode-trapping parameter ii) compute rotation rates for all these models iii) use the mean value obtained across the whole set as the estimate of the internal rotation rates. We find that the effect of a near surface perturbation in the synthetic observations on the rotation rates estimated based on the correlation between the observed rotational splittings and the mode-trapping parameter is negligible. We conclude that when using an ensemble of reference models, constructed based on matching the mixed mode pattern, the input rotation rates can be recovered across a range of fundamental stellar parameters like mass, mixing-length parameter and composition. Further, red-giant rotation rates determined in this way are also independent of a near surface perturbation of stellar structure.

We investigate whether pure deflagration models of Chandrasekhar-mass carbon-oxygen white dwarf stars can account for one or more subclass of the observed population of Type Ia supernova (SN Ia) explosions. We compute a set of 3D full-star hydrodynamic explosion models, in which the deflagration strength is parametrized using the multispot ignition approach. For each model, we calculate detailed nucleosynthesis yields in a post-processing step with a 384 nuclide nuclear network. We also compute synthetic observables with our 3D Monte Carlo radiative transfer code for comparison with observations. For weak and intermediate deflagration strengths (energy release E_nuc <~ 1.1 x 10^51 erg), we find that the explosion leaves behind a bound remnant enriched with 3 to 10 per cent (by mass) of deflagration ashes. However, we do not obtain the large kick velocities recently reported in the literature. We find that weak deflagrations with E_nuc ~ 0.5 x 10^51 erg fit well both the light curves and spectra of 2002cx-like SNe Ia, and models with even lower explosion energies could explain some of the fainter members of this subclass. By comparing our synthetic observables with the properties of SNe Ia, we can exclude the brightest, most vigorously ignited models as candidates for any observed class of SN Ia: their B - V colours deviate significantly from both normal and 2002cx-like SNe Ia and they are too bright to be candidates for other subclasses.

Massive stars mainly form in close binaries, where their mutual interactions can profoundly alter their evolutionary paths. Evolved binaries consisting of a massive OB-type main-sequence star with a stripped helium star or a compact companion represent a crucial stage in the evolution towards double compact objects, whose mergers are (potentially) detectable via gravitational waves. The recent detection of X-ray quiet OB+black hole binaries and OB+stripped helium star binaries has set the stage for discovering more of these systems in the near future. In this work, based on 3670 detailed binary-evolution models and using empirical distributions of initial binary parameters, we compute the expected population of such evolved massive binaries in coeval stellar populations, including stars in star clusters and in galaxies with starburst activities, for ages up to 100 Myr. Our results are vividly illustrated in an animation that shows the evolution of these binaries in the color-magnitude diagram over time. We find that the number of OB+black hole binaries peaks around 10 Myr, and OB+neutron star binaries are most abundant at approximately 20 Myr. Both black holes and neutron stars can potentially be found in populations with ages up to 90 Myr. Additionally, we analyze the properties of such binaries at specific ages. We find that OB+helium stars and OB+black hole binaries are likely to be identifiable as single-lined spectroscopic binaries. Our research serves as a guide for future observational efforts to discover such binaries in young star clusters and starburst environments.

We present an analytic toy model for the radiation produced by the interaction between the cold streams thought to feed massive halos at high redshift and their hot CGM. We begin by deriving cosmologically motivated parameters for the streams as they enter the halo virial radius, $R_{\rm v}$, as a function of halo mass and redshift. For $10^{12}M_{\odot}$ halos at $z=2$, we find the Hydrogen number density in streams to be $n_{\rm H,s}\sim (0.1-5)\times 10^{-2}{\rm cm}^{-3}$, a factor of$\delta \sim (30-300)$ times denser than the hot CGM density, while the stream radii are in the range $R_{\rm s}\sim (0.03-0.50)R_{\rm v}$. As the streams accelerate towards the halo centre, they become denser and narrower. The stream-CGM interaction induces Kelvin-Helmholtz Instability (KHI), which leads to entrainment of CGM mass by the stream and therefore to stream deceleration by momentum conservation. Assuming that the entrainment rates derived by Mandelker et al. 2019 in the absence of gravity can be applied locally at each halocentric radius, we derive equations of motion for the stream in the halo. Using these, we derive the net acceleration, mass growth, and energy dissipation induced by the stream-CGM interaction, as a function of halo mass and redshift, for different CGM density profiles. For the range of model parameters considered, we find that the interaction can induce dissipation luminosities $L_{\rm diss}>10^{42}~{\rm erg~s^{-1}}$within$\le 0.6 R_{\rm v}$ of halos with $M_{\rm v}>10^{12}M_{\odot}$ at $z=2$, with the emission scaling with halo mass and redshift approximately as $\propto M_{\rm v}\,(1+z)^2$. The magnitude and spatial extent of the emission produced in massive halos at high redshift is consistent with observed Ly$\alpha$ blobs, though better treatment of the UV background and self-shielding is needed to solidify this conclusion.