The hydrogen Balmer,Paschen, and Brackett series lines in quiescent prominences

To calculate the Balmer, Paschen, and Brackett line intensities, we solved the statistical equilibrium equations for a twenty level plus continuum atom of hydrogen.From the temperature, ionization, and the first three level populations of the prominence models deduced in a previous work we have calculated the populations of the twenty bound levels and the integrated intensities corresponding to the series mentioned above. The method was also applied to the Heasley and Mihalas theoretical models.Since the Lyman series are optically thick, we have worked out two different options: (a) assume radiative balance in these lines and (b) correct the radiative rates by multiplying them with an integro-exponential function EI2 which depends on the line optical depth at the thread center. The first option is shown to be the more consistent.The integrated line intensities from the Balmer series have been compared with the observations and a clear difference was noted between quiescent and active prominences in the sense that the active prominence case can not be well fitted with the available models.To evaluate the influence of the pressure, the temperature, the thermal conduction coefficient and the turbulence, velocity on the spectrum we have compared the results from different models.From this study we conclude that only the lines arising from the lower levels up to 8–10 can give us information about the physical parameters that characterized the solar prominences since the intensities from the higher members of the series depend only on atomic properties because of the small departures from LTE of the upper levels involved.Member of the Carrera del Investigador, CONICET, Argentina.     

Gravitational instability problem of nonuniform media

An instability criterion for perturbations of the gravitational potential in an inhomogeneous, large gas cloud is derived. We assume that perturbations propagate through the central area of the cloud, along the basic state density gradients. The instability criterion obtained in this way represents generalization of the Jeans's criterion to the case when the system is inhomogeneous in the basic state.     

Comparison between the distribution of PGE in black shales from the Bohemian Massif (CSFR) and other black shale occurrences

Selected metal-rich and normal black shales of the Bohemian Massif have been analysed for PGE ^* and the results have been compared with other anoxic sediments of the world. It has been discussed and partly proved that these rocks may represent an important and promising source of platinoids.     

An Analysis of the Light Curves of the Early-Type Binary BF Aurigae

The UBV observations of the massive binary BF Aur were made at the Ankara University Observatory during 1988, 1989 and 1996. Asymmetry of the light curves, arising from unequal height of successive maxima, indicates that the system is active. By analysing these observations in the framework of the Roche model (including the presence of bright regions on the components) one obtains a semidetached configuration of the system, with the cooler secondary component filling its Roche lobe. The analysis of the light curves yields consistent solutions for mass ratio q = m₂/m₁ somewhat less than one. The influence of the mass transfer on the change of the system-orbital-period is relatively small. The upward parabolic character of the O–C diagram (Zhang et al., 1993) indicates a mass transfer from the less massive secondary to the more massive primary. This inturn requires the less massive secondary to fill its Roche lobe. This is consistent with our solution. Based on these facts we introduced the following working hypothesis. At the place where the gas stream from the secondary falls on the primary, relatively small in size but a high temperature contrast active hot-spot (hs) region is formed. As a result of the heating effect caused by the irradiation of the hot-spot region, on the secondary's side facing the hot spot a bright-spot (bs) region is formed. The bright-spot region is larger in size but with significantly lower temperature than the hot spot. This region can be treated as a ‘reflection cap’. By analysing the light curves in the framework of this working hypothesis the basic parameters of the system and the active regions are estimated. The problem is solved in two stages: by obtaining a synthetic light curve in the case when the parameters of the corresponding Close Binary (CB) Roche model (Djurašević, 1992a) are given a priori (the direct problem) and by determining the parameters of the given model for which the best fit between the synthetic light curve and the observations is achieved (the inverse problem) (Djurašević, 1992b). This revised version was published online in July 2006 with corrections to the Cover Date.     

Strike-slip faults parallel to crustal spreading axes: data from Iceland and the Afar Depression

Slickenside studies in regions of crustal spreading such as Iceland and the Afar Depression, East Africa, reveal that a significant number of faults parallel and close to rift axes are strike-slip rather than normal. Therefore, the pattern of brittle deformation in these regions does not conform to the classic two-dimensional schemes of oceanic tectonics and pre-oceanic rifting. Dip-slip and strike-slip faulting presumably alternated along or in the vicinity of spreading axes, indicate a varying stress field and a combination of transverse and longitudinal movements. In Iceland, strike-slip faults parallel to rifts are observed both west and east of the rift system as well as in a median area between overlapping rifts; the mechanisms proposed for their origin include accommodation of oblique convergence or divergence of crustal sections due to variations of spreading directions along axis and the interaction of overlapping rifts. In the Afar Depression this kind of fault is recorded west of the rift of Asal and can be imputed to reflect an interaction among rifts in the vicinity of the Afar triple junction. Rift-parallel strike-slip faults cannot however be assumed to be a feature of all crustal spreading axes due to the peculiarity of the examined regions: both of them are hot-spot areas and the Afar Depression lies at a triple junction.     

Periodicities in the green corona for the sun as a star

A coronal index (CI) derived from the limb observations of the 530.3 nm emission corona (green corona) over 1964–1987 was analyzed by the Fourier transform technique (FTT) to find periodicity in this layer of solar atmosphere. As expected, two pronounced periods were indicated: the rotational, about 27 d, and the activity cycle length, 11 years. Beside these there are seen other periodicities of less significancies, namely of about 5,2.2,1 and 0.5 year. The values of these periodicities in individual cycles 20 and 21 slightly differs that could be related to different activity zone depths beneath the photosphere.     

Intrinsic polarization and extinction features of early-type stars with emission lines due to X-ray irradiation

The explanation of intrinsic polarization and extinction of early-type stars with emission due to X-ray irradiation of silicate grains is given.     

The flares of April 1980

We discuss the spatial and temporal characteristics of X-ray flares occurring in the active region NOAA2372 from April 6 to 13, 1980. The flares are seen to extend in most cases across the whole active complex, involving several magnetic features. They originate in an intermediate bipole, between the two main sunspots of the active region, where high magnetic shear was detected. A rapid expansion is seen in some cases, in conjunction with the start of the impulsive hard X-ray bursts. We also detect, in the late phases of some of the events, a large soft X-ray structure overlying the whole active region, which also shows up as a noise storm region at metric wavelengths. These large loops cool by heat conduction but, in some cases, Hα condensations seem to appear, probably as a result of magnetic compression and a condensation mode of the thermal instability. The topological aspects of the field configuration are discussed, in the context of flare models invoking magnetic reconnection at the site of the primary energy release. In such a model, the intermediate bipole is the natural site of initial magnetic reconnection, particle acceleration and heating. In one particular case of a flare observed at the limb, we find possible evidence of particle acceleration in a neutral sheet at the boundary between two clearly defined magnetic structures.