Radio spectra of the low-luminosity active galactic nucleus NGC 266 at centimetre-to-submillimetre wavelengths

We report multi-frequency and multi-epoch radio continuum observations with multi-spatial resolution for the low-luminosity active galactic nucleus (LLAGN) NGC 266. In the centimetre regime, we find diffuse components with Very Large Array (VLA) observations, and a variable compact core with a rising spectrum with Very Long Baseline Array (VLBA) observations. Although the spectral index of the rising spectrum is consistent with the prediction of the simple advection-dominated accretion flow (ADAF) model, the observed radio power is slightly high compared with that of the model prediction. A spectral break at centimetre-to-millimetre wavelengths is inferred from the upper limits of flux densities from Nobeyama Millimetre Array (NMA) and James Clerk Maxwell Telescope (JCMT) data at millimetre and submillimetre wavelengths, respectively. More complicated considerations are required for the theoretical model to interpret such observed radio properties.     

Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker

The Limousin ophiolite is located at the suture zone between two major thrust sheets in the western French Massif Central. This ophiolitic section comprises mantle‐harzburgite, mantle‐dunite, wehrlites, troctolites and layered gabbros. It has recorded a static metamorphic event transforming the gabbros into undeformed amphibolites and the magmatic ultramafites into serpentinites and/or pargasite‐bearing chloritites. With various thermobarometric methods, it is possible to show that the different varieties of amphibole have registered low‐P (c. 0.2 GPa) conditions with temperature ranging from high‐T, late‐magmatic conditions to greenschist–zeolite metamorphic facies. The abundance of undeformed metamorphic rocks (which is typical of the lower oceanic crust), the occurrence of Ca–Al (–Mg) metasomatism illustrated by the growth of Ca–Al silicates in veins or replacing the primary magmatic minerals, the P–T conditions of the metamorphism and the numerous similarities with oceanic crustal rocks from Ocean Drilling Program and worldwide ophiolites are the main arguments for an ocean‐floor hydrothermal metamorphism in the vicinity of a palaeo‐ridge. Among the West‐European Variscan ophiolites, the Limousin ophiolites constitute an extremely rare occurrence that has not been involved in any HP (subduction‐related) or MP (orogenic) metamorphism as observed in other ophiolite occurrences (i.e. France, Spain and Germany).     

Estimation of dispersion in estuaries by a self-consistent method

Longitudinal and transverse dispersion in estuaries may be determined in principle by measuring the change in time of the concentration field of a water soluble dye. In practice, measurements may take the form of several sampling runs crossing the dye patch. The coefficients of dispersion are usually estimated from the second moments of the concentration field; the use of second moments, in particular, introduces errors because measurements at long times or distance are unduly emphasized.A self-consistent method, free of the disadvantages inherent in the use of higher moments, is described in which the distribution of concentration in space is related to the zeroth spatial moment of concentration. In this formulation, based upon sampling runs across the dye patch, the concentration field in the estuary may be directly and simply related to the dispersion coefficients in both the intercept and gradient of a linear graph.The methods are illustrated by the characterization of a series of experiments in the Inner Estuary of Milford Haven.     

A lower limit to the field strength in magnetic reconnection sites in solar flares inferred from hard X-ray bursts

It is shown how the hard X-ray burst count rate and itse-folding ime can be used to estimate the minimum magnetic fieldB _min required in a flare magnetic reconnection site for the burst to be interpreted in terms of a thick target model. Application of the method to data from the Solar Maximum Mission (HXRBS) indicates absolute minimum fields well in excess of 100 G, and impossibly high values for some reconnection geometries.     

First results from the Gauribidanur Radio heliograph

Observations of the Sun at two frequencies (51 and 77 MHz) using the East-West arm of the Gauribidanur Radio heliograph are presented.     

What Drives Star Formation?

Current theoretical models for what drives star formation (especially low-mass star formation) are: (1) magnetic support of self-gravitating clouds with ambipolar diffusion removing support in cores and triggering collapse and (2) compressible turbulence forming self-gravitating clumps that collapse as soon as the turbulent cascade produces insufficient turbulent support. Observations of magnetic fields can distinguish between these two models because of different predictions in three areas: (1) magnetic field morphology, (2) the scaling of field strength with density and non-thermal velocities, and (3) the mass to magnetic flux ratio, M/Φ. We first discuss the techniques and limitations of methods for observing magnetic fields in star formation regions, then describe results for the L1544 prestellar core as an exemplar of the observational results. Application of the three tests leads to the following conclusions. The observational data show that both magnetic fields and turbulence are important in molecular cloud physics. Field lines are generally regular rather than chaotic, implying strong field strengths. But fields are not aligned with the minor axes of oblate spheroidal clouds, suggesting that turbulence is important. Field strengths appear to scale with non-thermal velocity widths, suggesting a significant turbulent support of clouds. Giant Molecular Clouds (GMCs) require mass accumulation over sufficiently large volumes that they would likely have an approximately critical M/Φ. Yet H I clouds are observed to be highly subcritical. If self-gravitating (molecular) clouds form with the subcritical M/Φ of H I clouds, the molecular clouds will be subcritical. However, the observations of molecular cloud cores suggest that they are approximately critical, with no direct evidence for subcritical molecular clouds or cloud envelopes. Hence, the observations remain inconclusive in deciding between the two extreme-case models of what drives star formation. What is needed to further advance our understanding of the role of magnetic fields in the star formation process are additional high sensitivity surveys of magnetic field strengths and other cloud properties in order to further refine the assessment of the importance of magnetic fields in molecular cores and envelopes.     

The energy-dependent radiation transfer in a homogeneous finite cylinder

Radiation transfer problem in the slowing-down region for cylindrical geometry with diffuse reflectivity and internal source is connected with source-free problem with isotropic boundary condition. Modelled kernels are used to represent the slowing-down kernel. Calculations are performed for the radiation flux at the boundary of the cylinder. Numerical results are obtained using the bi-variation technique.     

Change in cloud-top temperatures over Europe

Long-term measurements from Advanced Very High Resolution Radiometer (AVHRR) onboard the National Oceanic and Atmospheric Administration satellites were evaluated to assess variability in cloud-top temperatures over central and eastern Europe that saw radical infrastructural changes after the fall of the East Bloc in 1989 that has affected the pollution levels and hence cloud albedo. Four years in the late 1980s (1985-1988) and in the late 1990s (1997-2000) were chosen, as these are distinctively marked as episodes of very high and lower air pollution (sulphates and particulate matter). During the late 1980s, low- and medium-level clouds were colder by more than 2 K and convective clouds even by 4 K. Cloud-tops over and around polluted regions are higher, and their temperatures showed stronger variability, suggesting an indirect aerosol effect in the thermal spectral range as well.