The colours of the Sun

We compile a sample of Sun-like stars with accurate effective temperatures, metallicities and colours (from the ultraviolet to the near-infrared). A crucial improvement is that the effective temperature scale of the stars has recently been established as both accurate and precise through direct measurement of angular diameters obtained with stellar interferometers. We fit the colours as a function of effective temperature and metallicity, and derive colour estimates for the Sun in the Johnson–Cousins, Tycho, Strömgren, 2MASS and SDSS photometric systems. For  ( B − V )_⊙  , we favour the 'red' colour 0.64 versus the 'blue' colour 0.62 of other recent papers, but both values are consistent within the errors; we ascribe the difference to the selection of Sun-like stars versus interpolation of wider colour– T _eff–metallicity relations.     

Circumstellar discs in the young σ Orionis cluster

We present new K - and L '-band imaging observations for members of the young (3–5 Myr) σ Orionis cluster, obtained at the United Kingdom Infrared Telescope (UKIRT) with the UKIRT 1–5 micron Imager Spectrometer (UIST). We determine ( K − L ') colour excesses with respect to the photospheres, finding evidence for warm circumstellar dust around 27 out of 83 cluster members that have masses between 0.04 and  1.0 M_⊙  . This indicates a circumstellar disc frequency of at least (33 ± 6) per cent for this cluster, consistent with previous determinations from smaller samples and also consistent with the 3-Myr disc half-life suggested by Haisch et al. There is marginal evidence that the disc frequency declines towards lower masses, but the data are also consistent with no mass dependence at all. There is no evidence for spatial segregation of objects with and without circumstellar discs.     

2D/3D Dust Continuum Radiative Transfer Codes to Analyze and Predict VLTI Observations

Radiative Transfer (RT) codes with image capability are a fundamental tool for preparing interferometric observations and for interpreting visibility data. In view of the upcoming VLTI facilities, we present the first comparison of images/visibilities coming from two 3D codes that use completely different techniques to solve the problem of self-consistent continuum RT. In addition, we focus on the astrophysical case of a disk distorted by tidal interaction with by-passing stars or internal planets and investigate for which parameters the distortion can be best detected in the mid-infrared using the mid-infrared interferometric device MIDI. This revised version was published online in July 2006 with corrections to the Cover Date.     

A comparison of chemical and chemodynamical models

Representative results from a comparison of the chemical evolution of spherical collapse models without and with a intercloud medium are presented. The hot metal-rich gas distributes quickly the metals produced in supernovae throughout the galaxy, thus leading to a more homogeneous chemical evolution and flatter metallicity gradients in the gas and the stars. The stellar population is somewhat less concentrated towards the centre. The strong outflow results in a substantial loss of metals from the galaxy to its surroundings, and a lower effective yield in the galaxy. This revised version was published online in August 2006 with corrections to the Cover Date.     

X-ray, Lyα and Hα emission from simulated disk galaxies

The X-ray properties of the haloes of disk galaxies formed in fully cosmological, hydro/gravity simulations are discussed. The results are found to be consistent with observational X-ray detections and upper limits. Disk galaxy haloes are predicted to be about an order of magnitude brighter at z ∼ 1 than at z=0. The Lyα and Hα surface brightness of an edge-on, Milky Way like model galaxy has been determined. The emission is found to be quite extended, with a scale height of about 600 pc, neglecting dust corrections. This revised version was published online in August 2006 with corrections to the Cover Date.     

Velocity Field of a Complex Sunspot with Light Bridges

For the leading part of sunspot group NOAA 8323, which rapidly changed its complex structure, a time series of the line-of-sight (LOS) component of the velocity field was obtained. With a two-dimensional Fabry–Pérot spectrometer, the magnetically insensitive line Fei 557.6 nm was scanned. The inclination of the LOS (heliographic angle) to the vertical was =28.5°. The umbra of the observed spot was divided by a system of light bridges into several parts. The spatial and temporal velocity field also exhibits a considerable complexity: in one extended umbral area there is a downward flow of 1 km s^–1 relative to other dark sub-umbrae. At the center-side penumbra, with a line-of-sight Evershed outflow of 1.5 km s^–1, a persistent patch, somewhat darker than the average penumbra, has a LOS velocity of 1.3 km s^–1 in opposite direction, probably a downflow. At the limb-side penumbra, a photosphere-like area is interspersed, interrupting the Evershed flow which resumes with typical strength beyond this feature towards the outer penumbral boundary. Most interesting is the behavior of the light bridges, which have a slight blue shift, interrupted by short events of strong blue or red shifts which – within the time resolution of 35 s – instantly affect a considerable part of a light bridge.     

The gaseous Halo of the Virgo Cluster Galaxy 45

Observations of the Virgo Cluster galaxy NGC 4569 in soft X-rays and in Hα reveal both an asymmetrically distributed hot gaseous halo and a giant filament of diffuse emission, respectively, extending from the same side of the disk, the latter one to at least 8.8 kpc. A deep longslit spectrum along the filament shows velocities systematically different from the systemic velocity of NGC 4569. With the inclination of the disk one can determine the Hα spur as a giant outflow in NGC 4569. This revised version was published online in August 2006 with corrections to the Cover Date.