Presupernova Evolution of Differentially Rotating Massive Stars Including Magnetic Fields

As a massive star evolves through multiple stages of nuclear burning on its way to becoming a supernova, a complex, differentially rotating structure is set up. Angular momentum is transported by a variety of classic instabilities, and also by magnetic torques from fields generated by the differential rotation. We present the first stellar evolution calculations to follow the evolution of rotating massive stars including, at least approximately, all these effects, magnetic and non-magnetic, from the zero-age main sequence until the onset of iron-core collapse. The evolution and action of the magnetic fields is as described by Spruit 2002 and a range of uncertain parameters is explored. In general, we find that magnetic torques decrease the final rotation rate of the collapsing iron core by about a factor of 30 to 50 when compared with the non-magnetic counterparts. Angular momentum in that part of the presupernova star destined to become a neutron star is an increasing function of main sequence mass. That is, pulsars derived from more massive stars will rotate faster and rotation will play a more dominant role in the star's explosion. The final angular momentum of the core is determined - to within a factor of two - by the time the star ignites carbon burning. For the lighter stars studied, around 15 solar masses, we predict pulsar periods at birth near 15 ms, though a factor of two range is easily tolerated by the uncertainties. Several mechanisms for additional braking in a young neutron star, especially by fall back, are also explored.Comment: 32 pages, 3 figures (8 eps files), submitted to Ap

Differential Rotation in Neutron Stars: Magnetic Braking and Viscous Damping

Diffferentially rotating stars can support significantly more mass in equilibrium than nonrotating or uniformly rotating stars, according to general relativity. The remnant of a binary neutron star merger may give rise to such a ``hypermassive'' object. While such a star may be dynamically stable against gravitational collapse and bar formation, the radial stabilization due to differential rotation is likely to be temporary. Magnetic braking and viscosity combine to drive the star to uniform rotation, even if the seed magnetic field and the viscosity are small. This process inevitably leads to delayed collapse, which will be accompanied by a delayed gravitational wave burst and, possibly, a gamma-ray burst. We provide a simple, Newtonian, MHD calculation of the braking of differential rotation by magnetic fields and viscosity. The star is idealized as a differentially rotating, infinite cylinder consisting of a homogeneous, incompressible conducting gas. We solve analytically the simplest case in which the gas has no viscosity and the star resides in an exterior vacuum. We treat numerically cases in which the gas has internal viscosity and the star is embedded in an exterior, low-density, conducting medium. Our evolution calculations are presented to stimulate more realistic MHD simulations in full 3+1 general relativity. They serve to identify some of the key physical and numerical parameters, scaling behavior and competing timescales that characterize this important process.Comment: 11 pages. To appear in ApJ (November 20, 2000

Can Extra Mixing in RGB and AGB Stars Be Attributed to Magnetic Mechanisms?

It is known that there must be some weak form of transport (called cool bottom processing, or CBP) acting in low mass RGB and AGB stars, adding nuclei, newly produced near the hydrogen-burning shell, to the convective envelope. We assume that this extra-mixing originates in a stellar dynamo operated by the differential rotation below the envelope, maintaining toroidal magnetic fields near the hydrogen-burning shell. We use a phenomenological approach to the buoyancy of magnetic flux tubes, assuming that they induce matter circulation as needed by CBP models. This establishes requirements on the fields necessary to transport material from zones where some nuclear burning takes place, through the radiative layer, and into the convective envelope. Magnetic field strengths are determined by the transport rates needed by CBP for the model stellar structure of a star of initially 1.5 solar mass, in both the AGB and RGB phases. The field required for the AGB star in the processing zone is B_0 ~ 5x10^6 G; at the base of the convective envelope this yields an intensity B_E < 10^4 G (approximately). For the RGB case, B_0 ~ 5x10^4 to 4x10^5 G, and the corresponding B_E are ~ 450 to 3500 G. These results are consistent with existing observations on AGB stars. They also hint at the basis for high field sources in some planetary nebulae and the very large fields found in some white dwarfs. It is concluded that transport by magnetic buoyancy should be considered as a possible mechanism for extra mixing through the radiative zone, as is required by both stellar observations and the extensive isotopic data on circumstellar condensates found in meteorites.Comment: 26 pages, 4 figures, accepted by Astrophysical Journa

Управление направлениями повышения эффективности отрасли виноградарства

Целью статьи является изучение порядка управления затратами и прибылью с целью повышения экономической эффективности производства винограда и определение важности отрасли виноградарства

Magnetically-dominated jets inside collapsing stars as a model for gamma-ray bursts and supernova explosions

It has been suggested that magnetic fields play a dynamically-important role in core-collapse explosions of massive stars. In particular, they may be important in the collapsar scenario for gamma-ray bursts (GRB), where the central engine is a hyper-accreting black hole or a millisecond magnetar. The present paper is focussed on the magnetar scenario, with a specific emphasis on the interaction of the magnetar magnetosphere with the infalling stellar envelope. First, the ``Pulsar-in-a-Cavity'' problem is introduced as a paradigm for a magnetar inside a collapsing star. The basic set-up of this fundamental plasma-physics problem is described, outlining its main features, and simple estimates are derived for the evolution of the magnetic field. In the context of a collapsing star, it is proposed that, at first, the ram pressure of the infalling plasma acts to confine the magnetosphere, enabling a gradual build-up of the magnetic pressure. At some point, the growing magnetic pressure overtakes the (decreasing) ram pressure of the gas, resulting in a magnetically-driven explosion. The explosion should be highly anisotropic, as the hoop-stress of the toroidal field, confined by the surrounding stellar matter, collimates the magnetically-dominated outflow into two beamed magnetic-tower jets. This creates a clean narrow channel for the escape of energy from the central engine through the star, as required for GRBs. In addition, the delayed onset of the collimated-explosion phase can explain the production of large quantities of Nickel-56, as suggested by the GRB-Supernova connection. Finally, the prospects for numerical simulations of this scenario are discussed.Comment: Invited paper in the "Physics of Plasmas" (May 2007 special issue), based on an invited talk at the 48th Annual Meeting of the APS Division of Plasma Physics (Oct. 30 - Nov. 3, 2006, Philadelphia, PA); 24 pages, 7 figure

The Origin of Solar Activity in the Tachocline

Solar active regions, produced by the emergence of tubes of strong magnetic field in the photosphere, are restricted to within 35 degrees of the solar equator. The nature of the dynamo processes that create and renew these fields, and are therefore responsible for solar magnetic phenomena, are not well understood. We analyze the magneto-rotational stability of the solar tachocline for general field geometry. This thin region of strong radial and latitudinal differential rotation, between the radiative and convective zones, is unstable at latitudes above 37 degrees, yet is stable closer to the equator. We propose that small-scale magneto-rotational turbulence prevents coherent magnetic dynamo action in the tachocline except in the vicinity of the equator, thus explaining the latitudinal restriction of active regions. Tying the magnetic dynamo to the tachocline elucidates the physical conditions and processes relevant to solar magnetism.Comment: 10 pages, 1 figure, accepted for publication in ApJ

Effects of Strong Magnetic Fields on Neutron Star Structure

We study static neutron stars with poloidal magnetic fields and a simple class of electric current distributions consistent with the requirement of stationarity. For this class of electric current distributions, we find that magnetic fields are too large for static configurations to exist when the magnetic force pushes a sufficient amount of mass off-center that the gravitational force points outward near the origin in the equatorial plane. (In our coordinates an outward gravitational force corresponds to $\partial\ln g_{tt}/\partial r>0$, where $t$ and $r$ are respectively time and radial coordinates and $g_{tt}$ is coefficient of $dt^2$ in the line element.) For the equations of state (EOSs) employed in previous work, we obtain configurations of higher mass than had been reported; we also present results with more recent EOSs. For all EOSs studied, we find that the maximum mass among these static configurations with magnetic fields is noticeably larger than the maximum mass attainable by uniform rotation, and that for fixed values of baryon number the maximum mass configurations are all characterized by an off-center density maximum.Comment: Submitted to the Astrophysical Journal. 37 pages, 8 figures, uses aastex macro

A new model for mixing by double-diffusive convection (semi-convection): I. The conditions for layer formation

The process referred to as "semi-convection" in astrophysics and "double-diffusive convection in the diffusive regime" in Earth and planetary sciences, occurs in stellar and planetary interiors in regions which are stable according to the Ledoux criterion but unstable according to the Schwarzschild criterion. In this series of papers, we analyze the results of an extensive suite of 3D numerical simulations of the process, and ultimately propose a new 1D prescription for heat and compositional transport in this regime which can be used in stellar or planetary structure and evolution models. In a preliminary study of the phenomenon, Rosenblum et al. (2011) showed that, after saturation of the primary instability, a system can evolve in one of two possible ways: the induced turbulence either remains homogeneous, with very weak transport properties, or transitions into a thermo-compositional staircase where the transport rate is much larger (albeit still smaller than in standard convection). In this paper, we show that this dichotomous behavior is a robust property of semi-convection across a wide region of parameter space. We propose a simple semi-analytical criterion to determine whether layer formation is expected or not, and at what rate it proceeds, as a function of the background stratification and of the diffusion parameters (viscosity, thermal diffusivity and compositional diffusivity) only. The theoretical criterion matches the outcome of our numerical simulations very adequately in the numerically accessible "planetary" parameter regime, and can easily be extrapolated to the stellar parameter regime. Subsequent papers will address more specifically the question of quantifying transport in the layered case and in the non-layered case.Comment: Submitted to Ap

Laboratory forensics for open science readiness: an investigative approach to research data management

Recently, the topic of research data management has appeared at the forefront of Open Science as a prerequisite for preserving and disseminating research data efficiently. At the same time, scientific laboratories still rely upon digital files that are processed by experimenters to analyze and communicate laboratory results. In this study, we first apply a forensic process to investigate the information quality of digital evidence underlying published results. Furthermore, we use semiotics to describe the quality of information recovered from storage systems with laboratory forensics techniques. Next, we formulate laboratory analytics capabilities based on the results of the forensics analysis. Laboratory forensics and analytics form the basis of research data management. Finally, we propose a conceptual overview of open science readiness, which combines laboratory forensics techniques and laboratory analytics capabilities to help overcome research data management challenges in the near future.Prevention, Population and Disease management (PrePoD)Public Health and primary car