The dynamo model of solar magnetic field generation assumes that magnetic flux is retained at the dynamo site for times of the order of the solar-cycle period. However, flux tubes in the solar convection zone are expected to be buoyant, rising to the surface on timescales much shorter than the solar cycle. Since the initial 1955 paper by Parker on

We study the effect of microturbulence in the line formation calculations of H and He lines, in the parameter range typical for O and early B stars. We are specially interested in its effect on the determination of stellar parameters: Teff, log g and specially on the He abundance. We first analyze the behaviour of H and He model lines between 4 000

We present near-infrared spectroscopy and imaging of the compact steep-spectrum radio source 3C 318, which shows it to be a quasar at redshift z=1.574 (the z=0.752 value previously reported is incorrect). 3C 318 is an IRAS, ISO and SCUBA source so its new redshift makes it the most intrinsically luminous far-infrared (FIR) source in the 3C

We report on a large Hubble Space Telescope imaging survey of BL Lac objects, at spatial resolution >~10 times better than previous ground-based surveys. We focus on data reduction and analysis, describing the procedures used to model the host galaxy surface brightness radial profiles. A total of 69 host galaxies were resolved out of 110 objects

At sufficiently low frequencies, no ground-based radio array will be able to produce high resolution images while looking through the ionosphere. A space-based array will be needed to explore the objects and processes which dominate the sky at the lowest radio frequencies. An imaging radio interferometer based on a large number of small

The errors associated with a segmented primary mirror limit the image quality obtainable with those astronomical telescopes which employ them. It is necessary to derive the effects of segmentation errors on image quality in order to determine realistic error budgets for such telescopes. The main segmentation errors are misalignment and piston of