Silicon Burning I: Neutronization and the Physics of Quasi-Equilibrium

As the ultimate stage of stellar nucleosynthesis, and the source of the iron peak nuclei, silicon burning is important to our understanding of the evolution of massive stars and supernovae. Our reexamination of silicon burning, using results gleaned from simulation work done with a large nuclear network (299 nuclei and more than 3000 reactions) and from independent calculations of equilibrium abundance distributions, offers new insights into the quasi-equilibrium mechanism and the approach to nuclear statistical equilibrium. We find that the degree to which the matter has been neutronized is of great importance, not only to the final products but also to the rate of energy generation and the membership of the quasi-equilibrium groups. A small increase in the global neutronization results in much larger free neutron fluences, increasing the abundances of more neutron-rich nuclei. As a result, incomplete silicon burning results in neutron richness among the isotopes of the iron peak much larger than the global neutronization would indicate. Finally, we briefly discuss the limitations and pitfalls of models for silicon burning currently employed within hydrodynamic models. In a forthcoming paper we will present a new approximation to the full nuclear network which preserves the most important features of the large nuclear network calculations at a significant improvement in computational speed. Such improved methods are ideally suited for hydrodynamic calculations which involve the production of iron peak nuclei, where the larger network calculation proves unmanageable.Comment: 44 pages of TeX with 25 Postscript figures, uses psfig.sty, To appear in the The Astrophysical Journal, April 1 1996. Complete PostScript version of the paper is also available from http://tycho.as.utexas.edu/~raph/Publications.htm

Current Flow and Pair Creation at Low Altitude in Rotation Powered Pulsars' Force-Free Magnetospheres: Space-Charge Limited Flow

(shortened) We report the results of an investigation of particle acceleration and electron-positron plasma generation at low altitude in the polar magnetic flux tubes of Rotation Powered Pulsars, when the stellar surface is free to emit whatever charges and currents are demanded by the force-free magnetosphere. We observe novel behavior. a) When the current density is less than the Goldreich-Julian (GJ) value (0<j/j_{GJ}<1), space charge limited acceleration of the current carrying beam is mild, with the full GJ charge density being comprised of the charge density of the beam, co-existing with a cloud of electrically trapped particles with the same sign of charge as the beam. The voltage drops are on the order of mc^2/e, and pair creation is absent. b) When the current density exceeds the GJ value (j/j_{GJ}>1), the system develops high voltage drops, causing emission of gamma rays and intense bursts of pair creation. The bursts exhibit limit cycle behavior, with characteristic time scales somewhat longer than the relativistic fly-by time over distances comparable to the polar cap diameter (microseconds). c) In return current regions, where j/j_{GJ}<0, the system develops similar bursts of pair creation. In cases b) and c), the intermittently generated pairs allow the system to simultaneously carry the magnetospherically prescribed currents and adjust the charge density and average electric field to force-free conditions. We also elucidate the conditions for pair creating beam flow to be steady, finding that such steady flows can occupy only a small fraction of the current density parameter space of the force-free magnetospheric model. The generic polar flow dynamics and pair creation is strongly time dependent. The model has an essential difference from almost all previous quantitative studies, in that we sought the accelerating voltage as a function of the applied current.Comment: 35 pages, 29 figures. Accepted for publication in MNRAS. Added new appendix, several minor changes in the tex

Assessing composition gradients in multifilamentary superconductors by means of magnetometry methods

We present two magnetometry-based methods suitable for assessing gradients in the critical temperature and hence the composition of multifilamentary superconductors: AC magnetometry and Scanning Hall Probe Microscopy. The novelty of the former technique lies in the iterative evaluation procedure we developed, whereas the strength of the latter is the direct visualization of the temperature dependent penetration of a magnetic field into the superconductor. Using the example of a PIT Nb3Sn wire, we demonstrate the application of these techniques, and compare the respective results to each other and to EDX measurements of the Sn distribution within the sub-elements of the wire.Comment: 7 pages, 8 figures; broken hyperlinks are due to a problem with arXi

Cold hardening and dehardening in Salix

The variation in cold hardiness in Salix in the autumn was investigated using clones of different geographic origins. In late growing season, the variation was small and inversely related to a phenotypic variation in potential growth rate. When growth had stopped in response to the reduction in daylength, however, large differences in cold hardiness developed. Northern/continental clones started cold hardening up to two months earlier and showed up to three times higher inherent rates of cold hardening than the southern/maritime ones. The two components of cold hardening, the timing of onset and the inherent rate, seemed to be separately inherited traits, as judged from analyses of the prodigy of a crossing between an early-and-rapidly hardening clone and a late-and-slowly hardening one. This suggests that cold hardiness can be improved without adversely affecting growth by selecting for a late onset of cold hardening combined with a rapid rate. Also, in the early stages, cold hardening was more sensitive to low, non-freezing temperatures in the southern/maritime clones than in the northern/continental ones. Cold hardening of stems in the autumn could be monitored from the accumulation of sugars, most predominantly sucrose, raffinose and stachyose. The accumulation of sucrose started already with the cessation of growth, whilst the accumulation of raffinose and stachyose started later and was stimulated by cool temperatures. Multivariate models using sugar data could explain 76% of the variation in cold hardiness in the early stages of hardening. Changes in levels of sugars and other compounds during cold hardening could be assessed non-intrusively from the visible and infrared reflectance spectra of stems. Multivariate models using spectral data could predict up to 96% of the variation in cold hardiness. This technique is expected to greatly facilitate breeding for improved cold hardiness by allowing rapid screening of large populations. The variation in cold hardiness in spring was also investigated. Loss of cold hardiness in spring was closely related to the bursting of buds. A relatively large genetic variation in the temperature requirement for bud burst was demonstrated indicating that this might be modified in sensitive clones to improve their cold hardiness in spring

Workshop on Solar Activity, Solar Wind, Terrestrial Effects, and Solar Acceleration

A summary of the proceedings from the workshop are presented. The areas covered were solar activity, solar wind, terrestrial effects, and solar acceleration. Specific topics addressed include: (1) solar cycle manifestations, both large and small scale, as well as long-term and short-term changes, including transients such as flares; (2) sources of solar wind, as identified by interplanetary observations including coronal mass ejections (CME's) or x-ray bright points, and the theory for and evolution of large-scale and small-scale structures; (3) magnetosphere responses, as observed by spacecraft, to variable solar wind and transient energetic particle emissions; and (4) origin and propagation of solar cosmic rays as related to solar activity and terrestrial effects, and solar wind coronal-hole relationships and dynamics

An Empirical Model for the Radio Emission from Pulsars

A model for slow radio pulsars is proposed which involves the entire magnetosphere in the production of the observed radio emission. It is argued that observations of pulsar profiles suggest that a feedback mechanism exists between the star surface and the null charge surface, requiring particle flow in both directions. In their flow to and from the surface the particles execute an azimuthal drift around the magnetic pole, thereby creating a ring of discrete `emission nodes' close to the surface. Motion of the nodes is observed as the well-known subpulse `drift', but is interpreted here as a small residual component of the real particle drift. The nodes can therefore move in either direction, or even remain stationary. A precise fit is found for the pulsar PSR0943+10. Azimuthal interactions between different regions of the magnetosphere depend on the angle between the magnetic and rotation axes and influence the conal type, as observed. The requirement of intermittent weak pair-production in an outergap suggests a natural evolutionary link between radio and gamma-ray pulsars.Comment: 17 pages 8 figure

High-Energy Emission From Millisecond Pulsars

The X-ray and gamma-ray spectrum of rotation-powered millisecond pulsars is investigated in a model for acceleration and pair cascades on open field lines above the polar caps. Although these pulsars have low surface magnetic fields, their short periods allow them to have large magnetospheric potential drops, but the majority do not produce sufficient pairs to completely screen the accelerating electric field. The accelerating particles maintain high Lorentz factors and undergo cyclotron resonant absorption of radio emission, that produces and maintains a large pitch angle, resulting in a strong synchrotron component. The resulting spectra consist of several distinct components: curvature radiation from primary electrons dominating from 1 - 100 GeV, synchrotron radiation from primary and secondary electrons dominating up to about 100 MeV, and much weaker inverse-Compton radiation from primary electrons at 0.1 - 1 TeV. We find that the relative size of these components depends on pulsar period, period derivative, and neutron star mass and radius with the level of the synchrotron component also depending sensitively on the radio emission properties. This model is successful in describing the observed X-ray and gamma-ray spectrum of PSR J0218+4232 as synchrotron radiation, peaking around 100 MeV and extending up to a turnover around several GeV. The predicted curvature radiation components from a number of millisecond pulsars, as well as the collective emission from the millisecond pulsars in globular clusters, should be detectable with AGILE and GLAST. We also discuss a hidden population of X-ray-quiet and radio-quiet millisecond pulsars which have evolved below the pair death line, some of which may be detectable by telescopes sensitive above 1 GeV.Comment: 34 pages, 6 figures, accepted for publication in Astrophysical Journa

Cosmology with a dynamically screened scalar interaction in the dark sector

Motivated in part by string theory, we consider a modification of the LambdaCDM cosmological model in which the dark matter has a long-range scalar force screened by light particles. Scalar forces can have interesting effects on structure formation: the main example presented here is the expulsion of dark matter halos from low density regions, or voids, in the galaxy distribution.Comment: 26 pages, 3 figure

Effective Screening due to Minihalos During the Epoch of Reionization

We show that the gaseous halos of collapsed objects introduce a substantial cumulative opacity to ionizing radiation, even after the smoothly distributed hydrogen in the intergalactic medium has been fully reionized. This opacity causes a delay of around unity in redshift between the time of the overlap of ionized bubbles in the intergalactic medium and the lifting of complete Gunn-Peterson Lyman alpha absorption. The minihalos responsible for this screening effect are not resolved by existing numerical simulations of reionization.Comment: 24 pages, 5 figures, submitted to Ap