Accurate numerical solutions of the equations of hydrodynamics play an ever more important role in many fields of astrophysics. In this work, we reinvestigate the accuracy of the moving-mesh code ...arepo and show how its convergence order can be improved for general problems. In particular, we clarify that for certain problems arepo only reaches first-order convergence for its original formulation. This can be rectified by simple modifications we propose to the time integration scheme and the spatial gradient estimates of the code, both improving the accuracy of the code. We demonstrate that the new implementation is indeed second-order accurate under the L
1 norm, and in particular substantially improves conservation of angular momentum. Interestingly, whereas these improvements can significantly change the results of smooth test problems, we also find that cosmological simulations of galaxy
formation are unaffected, demonstrating that the numerical errors eliminated by the new formulation do not impact these simulations. In contrast, simulations of binary stars followed over a large number of orbital times are strongly affected, as here it is particularly crucial to avoid a long-term build up of errors in angular momentum conservation.
Sub-Chandrasekhar mass white dwarfs accreting a helium shell on a carbon-oxygen core are potential progenitors of normal Type Ia supernovae. This work focuses on the details of the onset of the ...carbon detonation in the double detonation sub-Chandrasekhar model. In order to simulate the influence of core-shell mixing on the carbon ignition mechanism, the helium shell and its detonation are followed with an increased resolution compared to the rest of the star treating the propagation of the detonation wave more accurately. This significantly improves the predictions of the nucleosynthetic yields from the helium burning. The simulations were carried out with the A
REPO
code. A carbon-oxygen core with a helium shell was set up in one dimension and mapped to three dimensions. We ensured the stability of the white dwarf with a relaxation step before the hydrodynamic detonation simulation started. Synthetic observables were calculated with the radiative transfer code A
RTIS
. An ignition mechanism of the carbon detonation was observed, which received little attention before. In this “scissors mechanism”, the impact the helium detonation wave has on unburnt material when converging opposite to its ignition spot is strong enough to ignite a carbon detonation. This is possible in a carbon enriched transition region between the core and shell. The detonation mechanism is found to be sensitive to details of the core-shell transition and our models illustrate the need to consider core-shell mixing taking place during the accretion process. Even though the detonation ignition mechanism differs form the converging shock mechanism, the differences in the synthetic observables are not significant. Though they do not fit observations better than previous simulations, they illustrate the need for multi-dimensional simulations.
During the common envelope (CE) phase, a giant star in a binary system overflows its Roche lobe and unstable mass transfer leads to a spiral-in of the companion, resulting in a close binary system or ...in a merger of the stellar cores. Dynamo processes during the CE phase have been proposed as a mechanism to generate magnetic fields that are important for forming magnetic white dwarfs (MWDs) and for shaping planetary nebulae. Here, we present the first magnetohydrodynamics simulations of the dynamical spiral-in during a CE phase. We find that magnetic fields are strongly amplified in the accretion stream around the 1 M⊙ companion as it spirals into the envelope of a 2 M⊙ RG. This leads to field strengths of 10–100 kG throughout the envelope after 120 d. The magnetic field amplification is consistent with being driven by the magnetorotational instability. The field strengths reached in our simulation make the magnetic field interesting for diagnostic purposes, but they are dynamically irrelevant. They are also too small to explain the formation of the highest fields found in MWDs, but may be relevant for luminous red novae, and detecting magnetic fields in these events would support the scenario as proposed here.
Hydrodynamical simulations of stellar interactions require stable models of stars as initial conditions. Such initial models, however, are difficult to construct for giant stars because of the wide ...range in spatial scales of the hydrostatic equilibrium and in dynamical timescales between the core and the envelope of the giant. They are needed for, e.g., modeling the common envelope phase where a giant envelope encompasses both the giant core and a companion star. Here, we present a new method of approximating and reconstructing giant profiles from a stellar evolution code to produce stable models for multi-dimensional hydrodynamical simulations. We determine typical stellar stratification profiles with the one-dimensional stellar evolution code mesa. After an appropriate mapping, hydrodynamical simulations are conducted using the moving-mesh code arepo. The giant profiles are approximated by replacing the core of the giant with a point mass and by constructing a suitable continuation of the profile to the center. Different reconstruction methods are tested that can specifically control the convective behaviour of the model. After mapping to a grid, a relaxation procedure that includes damping of spurious velocities yields stable models in three-dimensional hydrodynamical simulations. Initially convectively stable configurations lead to stable hydrodynamical models while for stratifications that are convectively unstable in the stellar evolution code, simulations recover the convective behaviour of the initial model and show large convective plumes with Mach numbers up to 0.8. Examples are shown for a 2 M⊙ red giant and a 0.67 M⊙ asymptotic giant branch star. A detailed analysis shows that the improved method reliably provides stable models of giant envelopes that can be used as initial conditions for subsequent hydrodynamical simulations of stellar interactions involving giant stars.
About ten per cent of 'massive' stars (those of more than 1.5 solar masses) have strong, large-scale surface magnetic fields
. It has been suggested that merging of main-sequence and ...pre-main-sequence stars could produce such strong fields
, and the predicted fraction of merged massive stars is also about ten per cent
. The merger hypothesis is further supported by a lack of magnetic stars in close binaries
, which is as expected if mergers produce magnetic stars. Here we report three-dimensional magnetohydrodynamical simulations of the coalescence of two massive stars and follow the evolution of the merged product. Strong magnetic fields are produced in the simulations, and the merged star rejuvenates such that it appears younger and bluer than other coeval stars. This can explain the properties of the magnetic 'blue straggler' star τ Sco in the Upper Scorpius association that has an observationally inferred, apparent age of less than five million years, which is less than half the age of its birth association
. Such massive blue straggler stars seem likely to be progenitors of magnetars, perhaps giving rise to some of the enigmatic fast radio bursts observed
, and their supernovae may be affected by their strong magnetic fields
.
ABSTRACT The common envelope (CE) phase is an important stage in binary stellar evolution. It is needed to explain many close binary stellar systems, such as cataclysmic variables, SN Ia progenitors, ...or X-ray binaries. To form the resulting close binary, the initial orbit has to shrink, thereby transferring energy to the primary giant's envelope that is hence ejected. The details of this interaction, however, are still not understood. Here, we present new hydrodynamic simulations of the dynamical spiral-in forming a CE system. We apply the moving-mesh code arepo to follow the interaction of a compact star with a red giant possessing a core. The nearly Lagrangian scheme combines advantages of smoothed particle hydrodynamics and traditional grid-based hydrodynamic codes and allows us to capture also small flow features at high spatial resolution. Our simulations reproduce the initial transfer of energy and angular momentum from the binary core to the envelope by spiral shocks seen in previous studies, but after about 20 orbits a new phenomenon is observed. Large-scale flow instabilities are triggered by shear flows between adjacent shock layers. These indicate the onset of turbulent convection in the CE, thus altering the transport of energy on longer timescales. At the end of our simulation, only 8% of the envelope mass is ejected. The failure to unbind the envelope completely may be caused by processes on thermal timescales or unresolved microphysics.
The gravitationally confined detonation (GCD) model has been proposed as a possible explosion mechanism for Type Ia supernovae in the single-degenerate evolution channel. It starts with ignition of a ...deflagration in a single off-centre bubble in a near-Chandrasekhar-mass white dwarf. Driven by buoyancy, the deflagration flame rises in a narrow cone towards the surface. For the most part, the main component of the flow of the expanding ashes remains radial, but upon reaching the outer, low-pressure layers of the white dwarf, an additional lateral component develops. This causes the deflagration ashes to converge again at the opposite side, where the compression heats fuel and a detonation may be launched. We first performed five three-dimensional hydrodynamic simulations of the deflagration phase in 1.4 M⊙ carbon/oxygen white dwarfs at intermediate-resolution (2563 computational zones). We confirm that the closer the initial deflagration is ignited to the centre, the slower the buoyant rise and the longer the deflagration ashes takes to break out and close in on the opposite pole to collide. To test the GCD explosion model, we then performed a high-resolution (5123 computational zones) simulation for a model with an ignition spot offset near the upper limit of what is still justifiable, 200 km. This high-resolution simulation met our deliberately optimistic detonation criteria, and we initiated a detonation. The detonation burned through the white dwarf and led to its complete disruption. For this model, we determined detailed nucleosynthetic yields by post-processing 106 tracer particles with a 384 nuclide reaction network, and we present multi-band light curves and time-dependent optical spectra. We find that our synthetic observables show a prominent viewing-angle sensitivity in ultraviolet and blue wavelength bands, which contradicts observed SNe Ia. The strong dependence on the viewing angle is caused by the asymmetric distribution of the deflagration ashes in the outer ejecta layers. Finally, we compared our model to SN 1991T. The overall flux level of the model is slightly too low, and the model predicts pre-maximum light spectral features due to Ca, S, and Si that are too strong. Furthermore, the model chemical abundance stratification qualitatively disagrees with recent abundance tomography results in two key areas: our model lacks low-velocity stable Fe and instead has copious amounts of high-velocity 56Ni and stable Fe. We therefore do not find good agreement of the model with SN 1991T.
Common-envelope phases are decisive for the evolution of many binary systems. Cases with asymptotic giant branch (AGB) primary stars are of particular interest because they are thought to be ...progenitors of various astrophysical transients. In three-dimensional hydrodynamic simulations with the moving-mesh code
AREPO
, we study the common-envelope evolution of a 1.0
M
⊙
early-AGB star with companions of different masses. Although the stellar envelope of an AGB star is less tightly bound than that of a red giant, we find that the release of orbital energy of the core binary is insufficient to eject more than about twenty percent of the envelope mass. Ionization energy that is released in the expanding envelope, however, can lead to complete envelope ejection. Because recombination proceeds largely at high optical depths in our simulations, it is likely that this effect indeed plays a significant role in the considered systems. The efficiency of mass loss and the final orbital separation of the core binary system depend on the mass ratio between the companion and the primary star. Our results suggest a linear relation between the ratio of final to initial orbital separation and this parameter.
Collisions of red giants in galactic nuclei Ryu, Taeho; Amaro Seoane, Pau; Taylor, Andrew M ...
Monthly notices of the Royal Astronomical Society,
02/2024, Letnik:
528, Številka:
4
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
In stellar-dense environments, stars can collide with each other. For collisions close to a supermassive black hole (SMBH), the collisional kinetic energy can be so large that the colliding ...stars can be destroyed, potentially releasing an amount of energy comparable to that of a supernova. These black hole-driven disruptive collisions have been examined mostly analytically, with the non-linear hydrodynamical effects being left largely unstudied. Using the moving-mesh hydrodynamics code arepo, we investigate high-velocity (>103 km s−1) collisions between 1 M⊙ giants with varying radii, impact parameters, and initial approaching velocities, and estimate their observables. Very strong shocks across the collision surface efficiently convert ${\gtrsim} 10~{{\ \rm per\ cent}}$ of the initial kinetic energy into radiation energy. The outcome is a gas cloud expanding supersonically, homologously, and quasi-spherically, generating a flare with a peak luminosity ≃1041–1044 erg s−1 in the extreme ultraviolet band (≃10 eV). The luminosity decreases approximately following a power law of t−0.7 initially, then t−0.4 after t ≃ 10 d at which point it would be bright in the optical band (≲1eV). Subsequent, and possibly even brighter, emission would be generated due to the accretion of the gas cloud on to the nearby SMBH, possibly lasting up to multiyear time-scales. This inevitable BH–collision product interaction can contribute to the growth of BHs at all mass scales, in particular, seed BHs at high redshifts. Furthermore, the proximity of the events to the central BH makes them a potential tool for probing the existence of dormant BHs, even very massive ones which cannot be probed by tidal disruption events.
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