Itamar Lichtenstadt - Academia.edu (original) (raw)

Papers by Itamar Lichtenstadt

The Astrophysical Journal Supplement Series, 1999

We have developed a time-dependent, multi-energy-group, and multi-angle (Sn) Boltzmann transport ... more We have developed a time-dependent, multi-energy-group, and multi-angle (Sn) Boltzmann transport scheme for radiation hydrodynamics simulations, in one and two spatial dimensions. The implicit transport is coupled to both 1D (sphericallysymmetric) and 2D (axially-symmetric) versions of the explicit Newtonian hydrodynamics code VULCAN. The 2D variant, VULCAN/2D, can be operated in general structured or unstructured grids and though the code can address many problems in astrophysics it was constructed specifically to study the core-collapse supernova problem. Furthermore, VULCAN/2D can simulate the radiation/hydrodynamic evolution of differentially rotating bodies. We summarize the equations solved and methods incorporated into the algorithm and present results of a time-dependent 2D test calculation. A more complete description of the algorithm is postponed to another paper. We highlight a 2D test run that follows for 22 milliseconds the immediate post-bounce evolution of a collapsed...

The Astrophysical Journal, 1987

Lecture Notes in Physics, 1988

We continue our investigation of the effects of the magnetorotational instability (MRI) on collap... more We continue our investigation of the effects of the magnetorotational instability (MRI) on collapsing, rotating iron cores. A weak seed field will be exponentially amplified with a growth time t ~ 1/Ω to produce a magnetic field with strength 1014-1016 G within 300 msec after bounce. We follow the collapse with a one-dimensional flux-limited diffusion numerical code. In new work, we explore cylindrical symmetry for angular velocity profiles and invoke a centrifugal quasi-potential to allow somewhat higher initial angular rotation to be explored. We investigate the effect of convection in the core collapse environment to enhance or inhibit the growth of magnetic field, and also explore the feedback effects of the magnetic pressure and viscous drag. The rapid growth of the magnetic field may promote the formation of MHD jets up the rotation axis and, ultimately, a supernova explosion. This research is supported by NASA Grant NAG5-10766 and NSF Grant AST-0098644.

We investigate the effects of the magnetorotational instability (MRI) on collapsing, rotating iro... more We investigate the effects of the magnetorotational instability (MRI) on collapsing, rotating iron cores. We assume that a weak seed field is exponentially amplified with a growth time t ~ 1/Omega and sustained by MRI dynamo action. This process should dominate the linear growth of field due to winding of an initial seed field. We examine a variety of initial rotational profiles applied to the core of a 15 Msun star. We follow the collapse with a one-dimensional flux-limited diffusion numerical code. The assumption that the specific angular momentum is conserved then yields an estimate of the angular velocity profile in the collapsed core that undergoes strong differential rotation. Distortion due to rotation and pressure and torques due to the magnetic field are neglected in this preliminary study. The magnetic field attains a strength of 1014}-10{16G within 50 msec after bounce, and peaks at the boundary of the proto-neutron star where the shear is the strongest. The ratio of magn...

The authors report on the collapse of a number of more-or-less realistic iron cores, using the Il... more The authors report on the collapse of a number of more-or-less realistic iron cores, using the Illinois equation of state (LLPR) for warm nuclear matter below nuclear saturation density. They assume Newtonian gravity and complete neutrino trapping at all times (adiabatic hydrodynamics). Because neutrino losses can only increase the deleptonization during collapse and the shock dissipation during rebound, there is little purpose in including the dilatorious effects of neutrino transport until after an explosion is obtained in the adiabatic case. The authors are thus using state-of-the-art initial core configurations, equations of state and hydrodynamics to find necessary (but not sufficient) conditions for supernova explosions.

The Astrophysical Journal, 1982

ABSTRACT One-zone calculations of deleptonization during stellar collapse are reported using the ... more ABSTRACT One-zone calculations of deleptonization during stellar collapse are reported using the Lamb, et al (1978, 1981) equation of state and electron capture rates, incorporating neutron shell blockage and neutrino trapping. With a simple neutrino leakage scheme, a high final lepton fraction, of 0.37 is found, and the formation of a large homologous core and strong shock is predicted. Other recent hydrodynamic calculations are compared critically with those presented and reasons are suggested as to why they failed to produce a supernova explosion. The onset of homologous collapse in a many-zone hydrodynamic calculation is also demonstrated, and, following Bethe et al., (1979), it is suggested that the one-zone calculation gives a good description of the evolution of the edge of the homologous core.

The Astrophysical Journal, 1980

ABSTRACT The paper considers the adiabatic hydrodynamics of a collapsing stellar core, neglecting... more ABSTRACT The paper considers the adiabatic hydrodynamics of a collapsing stellar core, neglecting neutrino losses, so that hydrodynamic shocks are the only mechanism for entropy generation and possible mass ejection. Two mechanisms are distinguished depending on the thermal stiffness of the outer core and the strength of the reflected shock: (1) for reasonable values of the thermal stiffness of the outer core, shock heating can produce thermal pressure sufficient for mass ejection, and (2) if the thermal stiffness of the outer core is insufficient, mass ejection requires a large pressure deficit and an inner core soft enough to be deeply compressed, violently rebound, and oscillate. The efficiency of the shock heating mechanism depends on the low density incompressibility, the initial central density, and the density at which the cold equation of state stiffens; a soft outer core requires a large thermal stiffness to thermalize the rapid infall, but then leads to large mass ejection by shock heating.

The Astrophysical Journal, 2005

Using the 2D multi-group, flux-limited diffusion version of the code VUL-CAN/2D, that also incorp... more Using the 2D multi-group, flux-limited diffusion version of the code VUL-CAN/2D, that also incorporates rotation, we have calculated the collapse, bounce, shock formation, and early post-bounce evolutionary phases of a corecollapse supernova for a variety of initial rotation rates. This is the first series of such multi-group calculations undertaken in supernova theory with fully multi-D tools. We find that though rotation generates pole-to-equator angular anisotropies in the neutrino radiation fields, the magnitude of the asymmetries is not as large as previously estimated. The finite width of the neutrino decoupling surfaces and the significant emissivity above the τ = 2/3 surface moderate the angular contrast. Moreover, we find that the radiation field is always more spherically symmetric than the matter distribution, with its plumes and convective eddies. The radiation field at a point is an integral over many sources from the different contributing directions. As such, its distribution is much smoother than that of the matter and has very little power at high spatial frequencies. We present the dependence of the angular anisotropy of the neutrino fields on neutrino species, neutrino energy, and initial rotation rate. Only for our most rapidly rotating model do we start to see qualitatively different hydrodynamics, but for the lower rates consistent with the pre-collapse rotational profiles derived in the literature the anisotropies, though interesting, are modest. This does not mean -2that rotation does not play a key role in supernova dynamics. The decrease in the effective gravity due to the centripetal effect can be quite important. Rather, it means that when a realistic mapping between initial and final rotational profiles and 2D multi-group radiation-hydrodynamics are incorporated into collapse simulations the anisotropy of the radiation fields may be only a secondary, not a pivotal factor, in the supernova mechanism.

The Astrophysical Journal, 2003

We investigate the action of the magnetorotational instability (MRI) in the context of iron-core ... more We investigate the action of the magnetorotational instability (MRI) in the context of iron-core collapse. Exponential growth of the field on the time scale Ω −1 by the MRI will dominate the linear growth process of field line "wrapping" with the same characteristic time. We examine a variety of initial rotation states, with solid body rotation or a gradient in rotational velocity, that correspond to models in the literature. A relatively modest value of the initial rotation, a period of ∼ 10 s, will give a very rapidly rotating PNS and hence strong differential rotation with respect to the infalling matter. We assume conservation of angular momentum on spherical shells. Rotational distortion and the dynamic feedback of the magnetic field are neglected in the subsequent calculation of rotational velocities. In our rotating and collapsing conditions, a seed field is expected to be amplified by the MRI and to grow exponentially to a saturation field. Results are discussed for two examples of saturation fields, a fiducial field that corresponds to v A = rΩ and a field that corresponds to the maximum growing mode of the MRI. We find, as expected, that the shear is strong at the boundary of the newly formed protoneutron star, and, unexpectedly, that the region within the stalled shock can be subject to strong MHD activity. Modest initial rotation velocities of the iron core result in sub-Keplerian rotation and a sub-equipartition magnetic field that nevertheless produce substantial MHD luminosity and hoop stresses : saturation fields of order 10 14 -10 16 G can develop ∼ 300 msec after bounce with an associated MHD luminosity of ∼ 10 52 erg s −1 . Bi-polar flows driven by this MHD power can affect or even cause the explosions associated with core-collapse supernovae.

The Astrophysical Journal, 1984

ABSTRACT The adiabatic collapse of 1.7 and 1.5 solar mass iron cores is investigated using the LL... more ABSTRACT The adiabatic collapse of 1.7 and 1.5 solar mass iron cores is investigated using the LLPR equation of state and Fermi gas electron capture rates, and assuming complete neutrino trapping. For a variety of initial configurations, infall deleptonization leaves a homologous core of only 1 solar mass; the large overlay mass that the shock must penetrate and dissociate then prevents significant ejection of mass and kinetic energy. If all electron capture is artificially suppressed, ejection is obtained of 0.1 solar mass, with 5 x 10 to the 50th and 2 x 10 to the 52nd ergs kinetic energy for the 1.7 and 1.5 solar mass initial cores, respectively. In stars of these masses, neutrino processes are not responsible for supernova explosion but instead kill the otherwise efficient thermal stiffening mechanism. The initial iron core configuration needed for a supernova explosion must be cooler, and therefore lighter and more isentropic, than those cores heretofore considered. Such a cooler presupernova configuration can evolve if hydrostatic electron capture leads to greater neutrino cooling before the contraction becomes dynamic.

The Astrophysical Journal, 1987

The Astrophysical Journal, 1978

The Astrophysical Journal, 2004

We have developed a time-dependent, multi-energy-group, and multi-angle (S n ) Boltzmann transpor... more We have developed a time-dependent, multi-energy-group, and multi-angle (S n ) Boltzmann transport scheme for radiation hydrodynamics simulations, in one and two spatial dimensions. The implicit transport is coupled to both 1D (sphericallysymmetric) and 2D (axially-symmetric) versions of the explicit Newtonian hydrodynamics code VULCAN. The 2D variant, VULCAN/2D, can be operated in general structured or unstructured grids and though the code can address many problems in astrophysics it was constructed specifically to study the core-collapse supernova problem. Furthermore, VULCAN/2D can simulate the radiation/hydrodynamic evolution of differentially rotating bodies. We summarize the equations solved and methods incorporated into the algorithm and present results of a time-dependent 2D test calculation. A more complete description of the algorithm is postponed to another paper. We highlight a 2D test run that follows for 22 milliseconds the immediate post-bounce evolution of a collapsed core. We present the relationship between the anisotropies of the overturning matter field and the distribution of the corresponding flux vectors, as a function of energy group. This is the first 2D multi-group, multi-angle, time-dependent radiation/hydro calculation ever performed in core collapse studies. Though the transport module of the code is not gray and does not use flux limiters (however, there is a fluxlimited variant of VULCAN/2D), it still does not include energy redistribution and most velocity-dependent terms.

Physical Review Letters, 1980

ABSTRACT Consideration is given to the thermal stiffness of warm nuclear matter as a factor gover... more ABSTRACT Consideration is given to the thermal stiffness of warm nuclear matter as a factor governing the shock heating mechanism of supernova explosions. Equations of state for nuclear matter obtained by integrating the Lamb-Lattimer-Pethick-Ravenhall curves for the adiabatic coefficient at a fixed lepton fraction of 0.30 are used to derive a thermal stiffness of 1.25 at density of 10 to the 11th g/cu cm increasing to 1.55 at 10 to the 14th g/cu cm in collapsing stellar cores. At these values, it has been shown that mass can be ejected by the shock heating mechanism. The effects of general relativity, which have not been included in the calculation, are considered to increase the mass and kinetic energy ejected.

Nuclear Physics A, 1979

ABSTRACT Excitation functions of the elastic and the inelastic proton scattering leading to the g... more ABSTRACT Excitation functions of the elastic and the inelastic proton scattering leading to the ground state and to the 2+ (3.833 MeV), 3- (4.506 MeV) and 5- (5.73 MeV) excited states in 48Ca were measured at 90°, 125°, 140° and 160° at bombarding energies from 5.55 to 8.4 MeV. Nine resonances were identified and the angular distributions of the inelastically scattered protons were measured at bombarding energies corresponding to the resonances observed in the excitation curves. The spins of the resonances and their partial widths for decay to the ground state and to the excited states in 48Ca were determined. The results indicate that six positive parity states and three negative parity states may be interpreted as having a large component of a single particle state coupled respectively to the 3- and 2+ excited states of 48Ca. The center of gravity of the parent analogue states in 49Ca, which were interpreted as having a large |48Ca(3-)psin(p3/2)>J component in their configuration, fits with the prediction of the particle-core weak coupling model. Present address: Physics Division, Argonne National Laboratory, Argonne, Illinois, USA

Astrophysics and Space Science, 1980

Time-dependent neutrino transport out of an optically thick neutronized stellar core is calculate... more Time-dependent neutrino transport out of an optically thick neutronized stellar core is calculated to study the effects of neutrino degeneracy and of source depletion. Neutrino trapping inhibits further neutrino emission until neutrinos peel out of the outer zones of the core, exposing successively inner zones. This inwardly propagating neutrino rarefaction wave can lead toe-+p?v+n oscillations in chemical composition. The effect of neutrino Fermi statistics is to retard considrably and disperse neutrino leakage out of the core, making neutrino transport insignificant during fast stages of core collapse.

Astrophysical Journal Supplement Series, 1999

We examine the roles the presence of hyperons in the cores of neutron stars may play in determini... more We examine the roles the presence of hyperons in the cores of neutron stars may play in determining global properties of these stars. The study is based on estimates that hyperons appear in neutron star matter at about twice the nuclear saturation density, and emphasis is placed on effects that can be attributed to the general multispecies composition of the

The Astrophysical Journal Supplement Series, 1999

The Astrophysical Journal, 1987

Lecture Notes in Physics, 1988

The Astrophysical Journal, 1982

The Astrophysical Journal, 1980

The Astrophysical Journal, 2005

The Astrophysical Journal, 2003

The Astrophysical Journal, 1984

The Astrophysical Journal, 1987

The Astrophysical Journal, 1978

The Astrophysical Journal, 2004

Physical Review Letters, 1980

Nuclear Physics A, 1979

Astrophysics and Space Science, 1980

Astrophysical Journal Supplement Series, 1999