The Algebra of Strand Splitting. I. A Braided Version of Thompson's Group V

We construct a braided version of Thompson's group V.Comment: 27 page

Umklapp scattering of pairs in BCS superconductivity theory

The BCS theory of superconductivity is extended to recognize pairing of electrons by both normal and umklapp scattering. Application of the variational approach shows that coexistence of normal and umklapp scattering frustrates superconductivity.Comment: 9 pages, 5 figures. to be published in Journal of Physics: Condensed Matte

Plans for phase coherent long baseline interferometry for geophysical applications using the Anik-B communications satellite

A pilot project to establish an operational phase stable very long baseline interferometer (VLBI) for geophysical studies is described. Methods for implementation as well as practical applications are presented

A Unified Near Infrared Spectral Classification Scheme for T Dwarfs

A revised near infrared classification scheme for T dwarfs is presented, based on and superseding prior schemes developed by Burgasser et al. and Geballe et al., and defined following the precepts of the MK Process. Drawing from two large spectroscopic libraries of T dwarfs identified largely in the Sloan Digital Sky Survey and the Two Micron All Sky Survey, nine primary spectral standards and five alternate standards spanning spectral types T0 to T8 are identified that match criteria of spectral character, brightness, absence of a resolved companion and accessibility from both northern and southern hemispheres. The classification of T dwarfs is formally made by the direct comparison of near infrared spectral data of equivalent resolution to the spectra of these standards. Alternately, we have redefined five key spectral indices measuring the strengths of the major H$_2$O and CH$_4$ bands in the 1-2.5 micron region that may be used as a proxy to direct spectral comparison. Two methods of determining T spectral type using these indices are outlined and yield equivalent results. These classifications are also equivalent to those from prior schemes, implying that no revision of existing spectral type trends is required. The one-dimensional scheme presented here provides a first step toward the observational characterization of the lowest luminosity brown dwarfs currently known. Future extensions to incorporate spectral variations arising from differences in photospheric dust content, gravity and metallicity are briefly discussed. A compendium of all currently known T dwarfs with updated classifications is presented.Comment: 52 pages, 11 figures; accepted for publication to Ap

Changes in the Seasonal Cycle in the Circumpolar Arctic, 1976-95: Temperature and Precipitation

The Arctic is one region where the effects of global climate change are expected to be easy to observe. This study identifies regions in the circumpolar Arctic that have experienced similar changes in the seasonal cycle of temperature and precipitation over recent decades. Data from Arctic and northern nations have been compiled to form a new data set of over 1200 circumpolar Arctic climate stations. Changes in the seasonal cycle between two decades (1976-85 and 1986-95) are examined for the 247 temperature and 555 precipitation stations that meet specific completeness criteria. Inter-decadal shifts are analyzed using 11-day averages of daily mean temperature and 5-day averages of total daily precipitation. Examined at time-steps finer than annual or monthly means, climatic variations in the region are not consistent either through the seasons or across space. Some areas have demonstrated recent increases in temperature or precipitation, while others have displayed decreases in these elements. Many areas reveal climatic shifts in specific periods of the year that contrast markedly with the trends observed in other periods and other places.L'Arctique est une région où il devrait être relativement facile d'observer les effets du changement climatique mondial. Cette étude inventorie les régions de l'Arctique circumpolaire qui ont connu des changements similaires dans le cycle saisonnier de température et de précipitation au cours des dernières décennies. Des données provenant de nations de l'Arctique et du Nord ont été compilées en vue de créer un nouvel ensemble de données issues de plus de 1200 stations climatiques de l'Arctique circumpolaire. On examine les changements dans le cycle saisonnier entre deux décennies (1976-1985 et 1986-1995) pour les 247 stations de température et les 555 stations hyétométriques qui répondent aux critères spécifiques d'intégralité. On analyse les décalages entre les deux décennies en utilisant les moyennes sur 11 jours de la température quotidienne moyenne, ainsi que les moyennes sur cinq jours de la précipitation quotidienne totale. Si on les examine à des pas de temps plus petits que les moyennes annuelles ou mensuelles, les variations climatiques dans la région ne sont pas cohérentes, que ce soit en fonction des saisons ou de l'espace. Certaines zones ont affiché de récentes hausses de température ou de précipitation, tandis que d'autres ont enregistré des baisses de ces mêmes paramètres. Un grand nombre de régions montrent des changements de climat durant des périodes spécifiques de l'année, changements qui contrastent de façon très nette avec les tendances observées à d'autres moments et en d'autres lieux

Magnetic Fluctuations in a Charge Ordered State of the One-Dimensional Extended Hubbard Model with a Half-Filled Band

Magnetic properties in a charge ordered state are examined for the extended Hubbard model at half-filling. Magnetic excitations, magnetic susceptibilities and a nuclear spin relaxation rate are calculated with taking account of fluctuations around the mean-field solution. The relevance of the present results to the observation in the 1:1 organic conductors, (TTM-TTP)I$_3$, is discussed.Comment: 4 pages, 3 figures, to be published in J. Phys. Soc. Jpn. Vol.71 (2002) No.

The S2 VLBI Correlator: A Correlator for Space VLBI and Geodetic Signal Processing

We describe the design of a correlator system for ground and space-based VLBI. The correlator contains unique signal processing functions: flexible LO frequency switching for bandwidth synthesis; 1 ms dump intervals, multi-rate digital signal-processing techniques to allow correlation of signals at different sample rates; and a digital filter for very high resolution cross-power spectra. It also includes autocorrelation, tone extraction, pulsar gating, signal-statistics accumulation.Comment: 44 pages, 13 figure

Blue Straggler Stars: Early Observations that Failed to Solve the Problem

In this chapter, I describe early ideas on blue stragglers, and various observations (some published, some not) that promised but failed to resolve the question of their origin. I review the data and ideas that were circulating from Allan Sandage's original discovery in 1953 of "anomalous blue stars" in the globular cluster M3, up until about 1992, when what seems to have been the only previous meeting devoted to Blue Straggler Stars (BSSs) was held at the Space Telescope Science Institute.Comment: Chapter 2, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G. Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe