General relativistic simulations of passive-magneto-rotational core collapse with microphysics

Abstract

This paper presents results from axisymmetric simulations of magneto-rotational stellar core collapse to neutron stars in general relativity using the passive field approximation for the magnetic field. These simulations are performed using a new general relativistic numerical code specifically designed to study this astrophysical scenario. The code is an extension of an existing (and thoroughly tested) hydrodynamics code, which has been applied in the recent past to study relativistic rotational core collapse. It is based on the conformally-flat approximation of Einstein's field equations and conservative formulations of the magneto-hydrodynamics equations. The code has been recently upgraded to incorporate a tabulated, microphysical equation of state and an approximate deleptonization scheme. This allows us to perform the most realistic simulations of magneto-rotational core collapse to date, which are compared with simulations employing a simplified (hybrid) equation of state, widely used in the relativistic core collapse community. Furthermore, state-of-the-art (unmagnetized) initial models from stellar evolution are used. In general, stellar evolution models predict weak magnetic fields in the progenitors, which justifies our simplification of performing the computations under the approach that we call the passive field approximation for the magnetic field. Our results show that for the core collapse models with microphysics the saturation of the magnetic field cannot be reached within dynamical time scales by winding up the poloidal magnetic field into a toroidal one. We estimate the effect of other amplification mechanisms including the magneto-rotational instability (MRI) and several types of dynamos. We conclude that for progenitors with astrophysically expected (i.e. weak) magnetic fields, the MRI is the only mechanism that could amplify the magnetic field on dynamical time scales. The uncertainties about the strength of the magnetic field at which the MRI saturates are discussed. All our microphysical models exhibit post-bounce convective overturn in regions surrounding the inner part of the proto-neutron star. Since this has a potential impact on enhancing the MRI, it deserves further investigation with more accurate neutrino treatment or alternative microphysical equations of state.

Special Relativistic Simulations of Magnetically Dominated Jets in Collapsing Massive Stars

“…As we mention below, these studies are complimentary in the sense that the different epochs are focused on, with the different initial conditions for the numerical modeling being taken. In addition to the elaborate studies in the conventional supernova context (see recent reviews for Kotake et al 2006;Janka et al 2007), much attention has been paid recently to the roles of rapid rotation and magnetic fields for studying the formation of magnetars and its possible application to the collapsars (Yamada & Sawai 2004;Takiwaki et al 2004;Kotake et al 2004b;Sawai et al 2005;Matt et al 2006;Moiseenko et al 2006;Obergaulinger et al 2006;Nishimura et al 2006;Burrows et al 2007;Cerdá-Durán et al 2007;Scheidegger et al 2007;Komissarov & Barkov 2007). After the failed or weak explosion, the accretion to the central objects may lead to the formation of a BH (phase 2).…”

Magnetic field amplification and magnetically supported explosions of collapsing, non-rotating stellar cores

“…Magneto-rotational explosions, theoretically discussed by Meier et al (1976), have been studied in various approximations, e.g. by Bisnovatyi-Kogan et al (1976); Symbalisty (1984); Akiyama et al (2003); Kotake et al (2004); Thompson et al (2005); Moiseenko et al (2006); Obergaulinger et al (2006); Dessart et al (2007); Burrows et al (2007); Cerdá-Durán et al (2007); Obergaulinger et al (2009) ;Masada et al (2012). Recent magnetohydrodynamic (MHD) simulations employing (detailed) microphysics (neutrinos and a sophisticated high-density equation of state) have been performed by Burrows et al (2007) and by Scheidegger et al (2008); Winteler et al (2012) and Mösta et al (2014).…”

Numerical Hydrodynamics and Magnetohydrodynamics in General Relativity

“…Cerdá-Durán et al [75]: The GRMHD code developed by [75] has been designed to study magneto-rotational, relativistic, stellar core collapse [76]. It is an extension of the axisymmetric hydrodynamics code developed by [95] (whose 3D extension constitutes the CoCoNuT code described above), in which magnetic fields are included following the approach laid out in [24].…”

“…On the other hand, to further improve the realism of core collapse simulations in general relativity, the incorporation of magnetic fields in numerical codes via solving the MHD equations is also currently being undertaken [362, 75, 76]. The work of [362], which is based on the conservative formulation of the GRMHD equations discussed in Section 3.1.2, is focused on the collapse of initially strongly magnetized cores (∼ 10 12 −10 13 G).…”