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Register a new userOne-, Two-, and Three-dimensional Simulations of Oxygen Shell Burning Just Before the Core-Collapse of Massive Stars
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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.
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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.
It has been suggested based on analytic theory that even in non-rotating supernova progenitors stochastic spin-up by internal gravity waves (IGWs) during the late burning stages can impart enough angular momentum to the core to result in neutron star birth spin periods below 100ms, and a relatively firm upper limit of 500ms for the spin period. We here investigate this process using a 3D simulation of oxygen shell burning in a $3M_odot$ He star. Our model indicates that stochastic spin-up by IGWs is less efficient than previously thought. We find that the stochastic angular momentum flux carried by waves excited at the shell boundary is significantly smaller for a given convective luminosity and turnover time than would be expected from simple dimensional analysis. This can be explained by noting that the waves launched by overshooting convective plumes contain modes of opposite angular wave number with similar amplitudes, so that the net angular momentum of excited wave packets almost cancels. We find that the wave-mediated angular momentum flux from the oxygen shell follows a random walk, but again dimensional analysis overestimates the random walk amplitudes since the correlation time is only a fraction of the convective turnover time. Extrapolating our findings over the entire life time of the last burning stages prior to collapse, we predict that the core angular momentum from stochastic spin-up would translate into long birth spin periods of several seconds for low-mass progenitors and no less than 100ms even for high-mass progenitors.
We perform for the first time a 3D hydrodynamics simulation of the evolution of the last minutes pre-collapse of the oxygen shell of a fast-rotating massive star. This star has an initial mass of 38 M$_odot$, a metallicity of $sim$1/50 Z$_odot$, an initial rotational velocity of 600 km s$^{-1}$, and experiences chemically homogeneous evolution. It has a silicon- and oxygen-rich (Si/O) convective layer at (4.7-17)$times 10^{8}$ cm, where oxygen-shell burning takes place. The power spectrum analysis of the turbulent velocity indicates the dominance of the large-scale mode ($ell sim 3$), which has also been seen in non-rotating stars that have a wide Si/O layer. Spiral arm structures of density and silicon-enriched material produced by oxygen-shell burning appear in the equatorial plane of the Si/O shell. Non-axisymmetric, large-scale ($m le 3$) modes are dominant in these structures. The spiral arm structures have not been identified in previous non-rotating 3D pre-supernova models. Governed by such a convection pattern, the angle-averaged specific angular momentum becomes constant in the Si/O convective layer, which is not considered in spherically symmetrical stellar evolution models. Such spiral arms and constant specific angular momentum might affect the ensuing explosion or implosion of the star.
We present 3D hydrodynamics simulations of shell burning in two progenitors with zero-age main-sequence masses of 22 and 27 $M_{odot}$ for $sim$65 and 200 s up to the onset of gravitational collapse, respectively. The 22 and 27 $M_{odot}$ stars are selected from a suite of 1D progenitors. The former and the latter have an extended Si- and O-rich layer with a width of $sim$10$^9$ cm and $sim$5$times 10^9$ cm, respectively. Our 3D results show that turbulent mixing occurs in both of the progenitors with the angle-averaged turbulent Mach number exceeding $sim$0.1 at the maximum. We observe that an episodic burning of O and Ne, which takes place underneath the convection bases, enhances the turbulent mixing in the 22 and 27 $M_odot$ models, respectively. The distribution of nucleosynthetic yields is significantly different from that in 1D simulations, namely, in 3D more homogeneous and inhomogeneous in the radial and angular direction, respectively. By performing a spectrum analysis, we investigate the growth of turbulence and its role of material mixing in the convective layers. We also present a scalar spherical harmonics mode analysis of the turbulent Mach number. This analytical formula would be helpful for supernova modelers to implement the precollapse perturbations in core-collapse supernova simulations. Based on the results, we discuss implications for the possible onset of the perturbation-aided neutrino-driven supernova explosion.
We summarize the results of core collapse supernova theory from one-, two-, and three-dimensional models and provide a snapshot of the field at this time. We also present results from the Oak Ridge group in this context. Studies in both one and two spatial dimensions define the necessary} physics that must be included in core collapse supernova models: a general relativistic treatment of gravity (at least an approximate one), spectral neutrino transport, including relativistic effects such as gravitational redshift, and a complete set of neutrino weak interactions that includes state-of-the-art electron capture on nuclei and energy-exchanging scattering on electrons and nucleons. Whether or not the necessarily approximate treatment of this physics in current models that include it is sufficient remains to be determined in the context of future models that remove the approximations. We summarize the results of the Oak Ridge groups two-dimensional supernova models. In particular, we demonstrate that robust neutrino-driven explosions can be obtained. We also demonstrate that our predictions of the explosion energies and remnant neutron star masses are in agreement with observations, although a much larger number of models must be developed before more confident conclusions can be made. We provide preliminary results from our ongoing three dimensional model with the same physics. Finally, we speculate on future outcomes and directions.
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