Luminous blue variables as the progenitors of supernovae with quasi-periodic radio modulations

The interaction between supernova ejecta and circumstellar matter, arising from previous episodes of mass loss, provides us with a means of constraining the progenitors of supernovae. Radio observations of a number of supernovae show quasi-periodic deviations from a strict power-law decline at late times. Although several possibilities have been put forward to explain these modulations, no single explanation has proven to be entirely satisfactory. Here we suggest that Luminous blue variables undergoing S-Doradus type variations give rise to enhanced phases of mass loss that are imprinted on the immediate environment of the exploding star as a series of density enhancements. The variations in mass loss arise from changes in the ionization balance of Fe, the dominant ion that drives the wind. With this idea, we find that both the recurrence timescale of the variability and the amplitude of the modulations are in line with the observations. Our scenario thus provides a natural, single-star explanation for the observed behaviour that is, in fact, expected on theoretical grounds.

The UMIST Database for Astrochemistry 2006

We present a new version of the UMIST Database for Astrochemistry, the fourth such version to be released to the public. The current version contains some 4573 binary gas-phase reactions, an increase of 10% from the previous (1999) version, among 420 species, of which 23 are new to the database. Major updates have been made to ion-neutral reactions, neutral-neutral reactions, particularly at low temperature, and dissociative recombination reactions. We have included for the first time the interstellar chemistry of fluorine. In addition to the usual database, we have also released a reaction set in which the effects of dipole-enhanced ion-neutral rate coefficients are included. These two reactions sets have been used in a dark cloud model and the results of these models are presented and discussed briefly. The database and associated software are available on the World Wide Web at www.udfa.net. Tables 1, 2, 4 and 9 are only available in electronic form at http://www.aanda.org

An exploratory non-LTE model atmosphere analysis of B-type supergiants in the Small Magellanic Cloud

A preliminary differential non-LTE model atmosphere analysis of moderate resolution (R ~ 5 000) and signal-to-noise ratio spectra of 48 Small Magellanic Cloud B-type supergiants is presented. Standard techniques are adopted, viz. plane-parallel geometry and radiative and hydrostatic equilibrium. Spectroscopic atmospheric parameters (T_eff, log g and v_turb), luminosities and chemical abundances (He, C, N, O, Mg and Si) are estimated. These are compared with those deduced for a comparable sample of Galactic supergiants. The SMC targets appear to have similar atmospheric parameters, luminosities and helium abundances to the Galactic sample. Their magnesium and silicon underabundances are compatible with those found for main sequence SMC objects and there is no evidence for any large variation in their oxygen abundances. By contrast both their carbon and nitrogen lines strengths are inconsistent with single abundances, while their nitrogen to carbon abundance ratios appear to vary by at least as much and probably more than that found in the Galactic sample.

Strömgren uvby photometry of B-type stars from the Palomar-Green Survey

We present Strömgren uvby photometry for a sample of 31 high Galactic latitude stars selected from the Palomar-Green Survey. The data include photometric magnitudes accurate to

Chemistry in the Circumstellar Medium: Unveiling the Dust Formation Zone

The growth of dust grains in the inner regions of late-type stars is shrouded in mystery due to the difficulty of understanding the growth of heterogeneous particles from simple atoms and molecules and the lack of observational data. This article reviews the molecular processes important in circumstellar envelopes and discusses how ALMA might be used to probe the dust formation zone either directly or indirectly.

A New Theory for Perpendicular Transport of Cosmic Rays

We present an improved nonlinear theory for the perpendicular transport of charged particles. This approach is based on an improved nonlinear treatment of field-line random walk in combination with a generalized compound diffusion model. The generalized compound diffusion model employed is more systematic and reliable, in comparison with previous theories. Furthermore, the theory shows remarkably good agreement with test-particle simulations and solar wind observations.