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Register a new userThree-Dimensional Simulations of Massive Stars: I. Wave Generation and Propagation
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We present the first three-dimensional (3D), hydrodynamic simulations of the core convection zone (CZ) and extended radiative zone spanning from 1% to 90% of the stellar radius of an intermediate mass (3 $mathrm{M}_odot$) star. This allows us to self-consistently follow the generation of internal gravity waves (IGWs) at the convective boundary and their propagation to the surface. We find that convection in the core is dominated by plumes. The frequency spectrum in the CZ and that of IGW generation is a double power law as seen in previous two-dimensional (2D) simulations. The spectrum is significantly flatter than theoretical predictions using excitation through Reynolds stresses induced by convective eddies alone. It is compatible with excitation through plume penetration. An empirically determined distribution of plume frequencies generally matches the one necessary to explain a large part of the observed spectrum. We observe waves propagating in the radiation zone and excited standing modes, which can be identified as gravity and fundamental modes. They show similar frequencies and node patterns to those predicted by the stellar oscillation code GYRE. The continuous part of the spectrum fulfills the IGW dispersion relation. A spectrum of tangential velocity and temperature fluctuations close to the surface is extracted, which are directly related to observable brightness variations in stars. Unlike 2D simulations we do not see the high frequencies associated with wave breaking, likely because these 3D simulations are more heavily damped.
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(Abridged) Stars more massive than $20-30M_{odot}$ are so luminous that the radiation force on the cooler, more opaque outer layers can balance or exceed the force of gravity. These near or super-Eddington outer envelopes represent a long standing challenge for calculating the evolution of massive stars in one dimension, a situation that limits our understanding of the stellar progenitors of some of the most exciting and energetic explosions in the universe. In particular, the proximity to the Eddington limit has been the suspected cause for the variability, large mass loss rate and giant eruptions of an enigmatic class of massive stars: the luminous blue variables (LBVs). When in quiescence, LBVs are usually found on the hot ($T_{eff} approx 2 - 4 times 10^4$ K) S Dor instability strip. While in outburst, most LBVs stay on the cold S Dor instability strip with a $T_{eff} approx 9000$ K. Here we show that physically realistic three dimensional global radiation hydrodynamic simulations of radiation dominated massive stars with the largest supercomputers in the world naturally reproduce many observed properties of LBVs, specifically their location in the Hertzsprung-Russell (HR) diagram and their episodic mass loss with rates of $10^{-7}-10^{-5} M_{odot}/yr$. The helium opacity peak is found to play an important role to determine these properties, which is not realized in the traditional one dimensional models of massive stars. The simulations also predict that convection causes irregular envelope oscillations and 10-30% brightness variations on a typical timescale of a few days. The variability is more prominent in our models that are on the cool part of the S Dor instability. These calculations pave the way to a quantitative understanding of the structure, stability and the dominant mode of mass loss of massive stars.
Non-spherical structure in massive stars at the point of iron core collapse can have a qualitative impact on the properties of the ensuing core-collapse supernova explosions and the multi-messenger signals they produce. Strong perturbations can aid successful explosions by strengthening turbulence in the post-shock region. Here, we report on a set of $4pi$ 3D hydrodynamic simulations of O- and Si-shell burning in massive star models of varied initial masses using MESA and the FLASH simulation framework. We evolve four separate 3D models for roughly the final ten minutes prior to, and including, iron core collapse. We consider initial 1D MESA models with masses of 14-, 20-, and 25 $M_{odot}$ to survey a range of O/Si shell density and compositional configurations. We characterize the convective shells in our 3D models and compare them to the corresponding 1D models. In general, we find that the angle-average convective speeds in our 3D simulations near collapse are three to four times larger than the convective speeds predicted by MESA at the same epoch for our chosen mixing length parameter of $alpha_{rm{MLT}}=1.5$. In three of our simulations, we observe significant power in the spherical harmonic decomposition of the radial velocity field at harmonic indices of $ell=1-3$ near collapse. Our results suggest that large-scale modes are common in massive stars near collapse and should be considered a key aspect of pre-supernova progenitor models.
We perform two- (2D) and three-dimensional (3D) hydrodynamics simulations of convective oxygen shell-burning that takes place deep inside a massive progenitor star of a core-collapse supernova. Using one dimensional (1D) stellar evolution code, we first calculate the evolution of massive stars with an initial mass of 9-40 $M_odot$. Four different overshoot parameters are applied, and CO core mass trend similar to previous works is obtained in the 1D models. Selecting eleven 1D models that have a silicon and oxygen coexisting layer, we perform 2D hydrodynamics simulations of the evolution $sim$100 s until the onset of core-collapse. We find that convection with large-scale eddies and the turbulent Mach number $sim$0.1 is obtained in the models having a Si/O layer with a scale of 10$^8$ cm, whereas most models that have an extended O/Si layer up to a few $times 10^9$ cm exhibit lower turbulent velocity. Our results indicate that the supernova progenitors that possess a thick Si/O layer could provide a preferable condition for perturbation-aided explosions. We perform 3D simulation of a 25 $M_odot$ model, which exhibits large-scale convection in the 2D models. The 3D model develops large ($ell = 2$) convection similar to the 2D model, however, the turbulent velocity is lower. By estimating the neutrino emission properties of the 3D model, we point out that a time modulation of the event rates, if observed in KamLAND and Hyper-Kamiokande, would provide an important information about structural changes in the presupernova convective layer.
The convection that takes place in the innermost shells of massive stars plays an important role in the formation of core-collapse supernova explosions. Upon encountering the supernova shock, additional turbulence is generated, amplifying the explosion. In this work, we study how the convective perturbations evolve during the stellar collapse. Our main aim is to establish their physical properties right before they reach the supernova shock. To this end, we solve the linearized hydrodynamics equations perturbed on a stationary background flow. The latter is approximated by the spherical transonic Bondi accretion, while the convective perturbations are modeled as a combination of entropy and vorticity waves. We follow their evolution from large radii, where convective shells are initially located, down to small radii, where they are expected to encounter the accretion shock above the proto-neutron star. Considering typical vorticity perturbations with a Mach number $sim 0.1$ and entropy perturbations with magnitude $sim 0.05 k_mathrm{b}/mathrm{baryon}$, we find that the advection of these perturbations down to the shock generates acoustic waves with a relative amplitude $delta p/gamma p lesssim 10%$, in agreement with published numerical simulations. The velocity perturbations consist of contributions from acoustic and vorticity waves with values reaching $sim 10%$ of the sound speed ahead of the shock. The perturbation amplitudes decrease with increasing $ell$ and initial radii of the convective shells.
We present two-dimensional simulations of wave propagation in a realistic, non-stationary model of the solar atmosphere. This model shows a granular velocity field and magnetic flux concentrations in the intergranular lanes similar to observed velocity and magnetic structures on the Sun and takes radiative transfer into account. We present three cases of magneto-acoustic wave propagation through the model atmosphere, where we focus on the interaction of different magneto-acoustic wave at the layer of similar sound and Alfven speeds, which we call the equipartition layer. At this layer the acoustic and magnetic mode can exchange energy depending on the angle between the wave vector and the magnetic field vector. Our results show that above the equipartition layer and in all three cases the fast magnetic mode is refracted back into the solar atmosphere. Thus, the magnetic wave shows an evanescent behavior in the chromosphere. The acoustic mode, which travels along the magnetic field in the low plasma-$beta$ regime, can be a direct consequence of an acoustic source within or outside the low-$beta$ regime, or it can result from conversion of the magnetic mode, possibly from several such
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