Geometricity for derived categories of algebraic stacks

We prove that the dg category of perfect complexes on a smooth, proper Deligne-Mumford stack over a field of characteristic zero is geometric in the sense of Orlov, and in particular smooth and proper. On the level of triangulated categories, this means that the derived category of perfect complexes embeds as an admissible subcategory into the bounded derived category of coherent sheaves on a smooth, projective variety. The same holds for a smooth, projective, tame Artin stack over an arbitrary field.Comment: 31 page

Early prediction of ripening and storage quality of pear fruit in South Africa

The original publication is available at http://www.actahort.org/books/671/671_11.htmMaturity index records for the principal pear cultivars in the two most important pear producing areas in South Africa were compiled to study the fluctuation in total soluble solids (TSS), fruit firmness and starch breakdown during consecutive seasons. Temperatures (heat units, hu), sunlight hours (ssh), fruit size (diameter), days after bloom (dafb) and soil type were used to calculate the rate of change in TSS levels, firmness and starch breakdown during the last five to six weeks before the picking date. These variables were shown to have an effect on the internal fruit quality. High temperatures were found to result in a faster drop in firmness levels and increased sunlight hours improved the TSS levels. Orchards on sandy soils showed consistently lower TSS levels and firmness when different orchards were compared during consecutive seasons. Equations were fitted to the data for each cultivar and area to develop models for the prediction of the different indices. The predicted rates of change of the different indices were compared with the actual values to test the accuracy and proved to explain more than 80% of the variation during consecutive seasons. The results also showed that the rate of change for these indices could be estimated as early as the middle of December. These models are considered important tools to assist growers, pack houses and marketers in identifying seasons when fruit will have a short shelf life. Picking dates, cold storage and marketing can be adapted to ensure the best possible fruit quality

Decompositions of derived categories of gerbes and of families of Brauer-Severi varieties

It is well known that the category of quasi-coherent sheaves on a gerbe banded by a diagonalizable group decomposes according to the characters of the group. We establish the corresponding decomposition of the unbounded derived category of complexes of sheaves with quasi-coherent cohomology. This generalizes earlier work by Lieblich for gerbes over schemes whereas our gerbes may live over arbitrary algebraic stacks. \par By combining this decomposition with the semi-orthogonal decomposition for a projectivized vector bundle, we deduce a semi-orthogonal decomposition of the derived category of a family of Brauer-Severi varieties whose components can be described in terms of twisted sheaves on the base. This reproves and generalizes a result of Bernardara

Non-commutative desingularization of determinantal varieties, I

We show that determinantal varieties defined by maximal minors of a generic matrix have a non-commutative desingularization, in that we construct a maximal Cohen-Macaulay module over such a variety whose endomorphism ring is Cohen-Macaulay and has finite global dimension. In the case of the determinant of a square matrix, this gives a non-commutative crepant resolution.Comment: 52 pages, 3 figures, all comments welcom

Updated Information on the Local Group

The present note updates the information published in my recent monograph on \underline{The Galaxies of the Local Group}. Highlights include (1) the addition of the newly discovered Cetus dwarf spheroidal as a certain member of the Local Group, (2) an improved distance for SagDIG, which now places this object very close to the edge of the Local Group zero-velocity surface, (3) more information on the evolutionary histories of some individual Local Group members, and (4) improved distance determinations to, and luminosities for, a number of Local Group members. These data increase the number of certain (or probable) Local Group members to 36. The spatial distribution of these galaxies supports Hubble's claim that the Local Group ``is isolated in the general field.'' Presently available evidence suggests that star formation continued much longer in many dwarf spheroidals than it did in the main body of the Galactic halo. It is suggested that ``young'' globular clusters, such as Ruprecht 106, might have formed in now defunct dwarf spheroidals. Assuming SagDIG, which is the most remote Local Group galaxy, to lie on, or just inside, the zero-velocity surface of the Local Group yields a dynamical age \gtrsim 17.9 \pm 2.7 Gyr.Comment: 19 pages, 1 figure, to be published in the April 2000 issue of PAS

Evaluation of candidate probiotic strains for gilthead sea bream larvae (Sparus aurata) using an in vivo approach

Aims: The aim of this study was to evaluate the effect of six bacterial strains on gilthead sea bream larvae (Sparus aurata). Methods and Results: Six bacterial strains isolated from well-performing live food cultures were identified by sequencing fragments of their 16s rDNA genome to the genus level as Cytophaga sp., Roseobacter sp., Ruergeria sp., Paracoccus sp., Aeromonas sp. and Shewanella sp. Survival rates of gilthead sea bream larvae transferred to seawater added these bacterial strains at concentrations of 6 +/- 0.3 x 10(5) bacteria ml(-1) were similar to those of larvae transferred to sterilized seawater and showed an average of 86% at 9 days after hatching, whereas, survival rates of larvae transferred to filtered seawater were lower (P < 0.05), and showed an average of 39%, 9 days after hatching. Conclusion: Several bacterial strains isolated from well-performing live food cultures showed a positive effect for sea bream larvae when compared with filtered seawater. Significance and Impact of the Study: The approach used in this study could be applied as an in vivo evaluation method of candidate probiotic strains used in the rearing of marine fish larvae

The Discovery of an Anomalous X-ray Pulsar in the Supernova Remnant Kes 73

We report the discovery of pulsed X-ray emission from the compact source 1E 1841-045, using data obtained with the Advanced Satellite for Cosmology and Astrophysics. The X-ray source is located in the center of the small-diameter supernova remnant (SNR) Kes 73 and is very likely to be the compact stellar-remnant of the supernova which formed Kes 73. The X-rays are pulsed with a period of ~ 11.8 s, and a sinusoidal modulation of roughly 30 %. We interpret this modulation to be the rotation period of an embedded neutron star, and as such would be the longest spin period for an isolated neutron star to-date. This is especially remarkable since the surrounding SNR is very young, at ~ 2000 yr old. We suggest that the observed characteristics of this object are best understood within the framework of a neutron star with an enormous dipolar magnetic field, B ~ 8x10^14 G

Gas Physics, Disk Fragmentation, and Bulge Formation in Young Galaxies

We investigate the evolution of star-forming gas-rich disks, using a 3D chemodynamical model including a dark halo, stars, and a two-phase interstellar medium with feedback processes from the stars. We show that galaxy evolution proceeds along very different routes depending on whether it is the gas disk or the stellar disk which first becomes unstable, as measured by the respective Q-parameters. This in turn depends on the uncertain efficiency of energy dissipation of the cold cloud component from which stars form. When the cold gas cools efficiently and drives the instability, the galactic disk fragments and forms a number of massive clumps of stars and gas. The clumps spiral to the center of the galaxy in a few dynamical times and merge there to form a central bulge component in a strong starburst. When the kinetic energy of the cold clouds is dissipated at a lower rate, stars form from the gas in a more quiescent mode, and an instability only sets in at later times, when the surface density of the stellar disk has grown sufficiently high. The system then forms a stellar bar, which channels gas into the center, evolves, and forms a bulge whose stars are the result of a more extended star formation history. We investigate the stability of the gas-stellar disks in both regimes, as well as the star formation rates and element enrichment. We study the morphology of the evolving disks, calculating spatially resolved colours from the distribution of stars in age and metallicity, including dust absorption. We then discuss morphological observations such as clumpy structures and chain galaxies at high redshift as possible signatures of fragmenting, gas-rich disks. Finally, we investigate abundance ratio distributions as a means to distinguish the different scenarios for bulge formation.Comment: 16 pages, Latex, 14 figures, to appear in Astronomy and Astrophysics, Version with high quality images available at http://www.astro.unibas.ch/leute/ai.shtm

Magnetic Surfaces in Stationary Axisymmetric General Relativity

In this paper a new method is derived for constructing electromagnetic surface sources for stationary axisymmetric electrovac spacetimes endowed with non-smooth or even discontinuous Ernst potentials. This can be viewed as a generalization of some classical potential theory results, since lack of continuity of the potential is related to dipole density and lack of smoothness, to monopole density. In particular this approach is useful for constructing the dipole source for the magnetic field. This formalism involves solving a linear elliptic differential equation with boundary conditions at infinity. As an example, two different models of surface densities for the Kerr-Newman electrovac spacetime are derived.Comment: 15 page