Powder- and single crystal Mössbauer spectroscopy on synthetic fayalite

Mössbauer measurements on synthetic iron orthosilicate Fe₂SiO₄ (fayalite) were carried out in the antiferromagnetic spin state below T _N 65 K. The Mössbauer parameters isomer shift , inner magnetic field H(0), angle between H(0) and the z-component of the electric field gradient (efg), quadrupole splitting QS and asymmetry parameter were determined as a function of temperature. These parameters could be attributed to the two crystallographic sites M1 and M2.The smaller isomer shift on M1 with respect to M2 displays the more covalent character of the Fe-O bond on M1, which is supported by previous neutron diffraction experiments. H(0) shows a Brillouin-type behaviour with different fields on the two crystallographic sites (stronger on M1) and a small discontinuity at T = 23 K which corresponds with previous magnetic measurements. The quadrupole splitting is equal on both sites within error bars, in agreement with previous theoretical results and in contradiction to previous Mössbauer refinements published elsewhere.     

Room temperature solid-state detectors for Gamma ray burst spectroscopy

We have evaluated several solid state detectors which offer excellent energy resolution at room temperature for soft X-rays. For soft X-rays (< 1 keV to 20 keV), silicon P-intrinsic-N (PIN) and avalanche-mode photodiodes (APD's) have been studied. Using commercially available charge sensitive pre-amplifiers, these photodiodes provide 1 keV resolution without cooling. Their detection efficiencies are limited to about 20 keV and 15 keV, respectively. To overcome this constraint, we have studied thick (1.5 mm) PIN detectors made by Micron Semiconductor Ltd., U.K., as well as compound semiconducting materials with high Z constituents such as CZT and PbI₂. PbI₂ allows high detection efficiencies of photons up to 100 keV with detectors 100–300 microns thick. These new detectors offer the capability to study the low-energy spectral evolution of Gamma ray bursts (GRBs). A matrix of these detectors could be used as an image plane detector with moderate spatial resolution for imaging.     

The effect of an optically thin region on stability of an accretion disk around a supermassive black hole

We analyse conditions of the innermost portion of an accretion disk and establish a set of equations for this region. A stable innermost region may exist, which can probably explain the observed UV and X-ray spectra, avoiding the unstable emission. We then discuss the detailed radial structure of a disk around a black hole for typical AGN parameters and obtain different kinds of- relationships for different regions of a disk. On the basis of this, we discuss the stability. A new type of cycle is present, which we call a double S shaped cycle. In this cycle, the extent of accretion rate variability is much larger than that in dwarf nova cycles. This probably solves the problem of violent variability of AGN. In the meantime, the very high accretion rate at the hottest state in limit cycles in the unstable region may provide continuous injection of matter to the jet and power the relativistic motion of the jet.     

On the internal structure and magnetic fields of

First, a sequence of four-zone models for the interior of Venus is constructed under the assumption of hydrostatic equilibrium. While the equation of state for each zone is taken to be the Bullen's relation with its coefficients consistent with the PREM Earth model (Dziewonski and Anderson, 1981), the position of core-mantle boundary is determined by matching solutions of the Emden's equation in different regions. The results of hydrostatic models indicate the presence of a reasonably large molten iron core in Venus, broadly similar to the Earth. It is also found that the position of the core-mantle interface is nearly model-independent. Second, we focus on the question why Venus does not possess a significant global magnetic field and on what we can learn from this fact. Solutions of magnetohydrodynamic equations appropriate for the molten core of Venus are discussed. It is argued that, because the Elsasser number measuring the relative importance of Coriolis and Lorentz forces satisfies 1, equations for the problem of thermal convection in the Venusian fluid core must be nearly uncoupled with the dynamo equation. The existence of a global magnetic field, though small, then suggests that the size of the magnetic Reynolds numberR _m must beR _m =O(10), sustaining a dynamo action near its marginal state but not an active dynamo in the Venusian molten core. On the basis of asymptotic relations for finite amplitude convection, a useful constraint on important physical parameters for the liquid core of Venus is derived and discussed.     

Colliding winds model of X-ray radiation in SS 433: analysis of X-ray eclipses

Interpretation of X-ray eclipses in SS 433 binary system in the framework of wind-wind collision model yields high value of mass ratioq =m _x /m _v > 1 which implies high mass of relativistic objectm _x > 8M .     

A possible P-L relation for RR-Lyrae variables in globular clusters

A study of the mean photographic magnitudes of RR Lyrae variables in galactic globular clusters has shown that the luminosity of RR-ab variables decreases with period. It is found that RR-ab variables, in the cluster Centauri, whose distances are greater than 8 arc min from the centre of the cluster, are less luminous than such variables seen in the inner region. The possibility of two transition periods for RR-ab variables, one near 0 _. ^d 4 and the other near 0 _. ^d 9, is suggested to explain the sharp boundaries of the instability gap and of the periods of RR Lyrae variables. It is also shown that the inner RR Lyrae variables in Centauri are more massive than the outer ones.     

Accretion spin-up of low-magnetic neutron stars

The present paper is concerned with the spin-up of low-magnetic neutron stars by the accretion of matter onto the star. Calculations have been made for the evolution of the rotation of a neutron star and applied to different stellar models. It is shown that the existence of a millisecond pulsar imposes no restriction on any of the equations of state considered. However, constraints would arise with the possible discovery of third-octave pulsars (with frequencies in excess of 1000 Hz). Predictions are made as to the distribution of bursters over the orbital periods of neutron stars (about half of these having similar orbital periods). It is demonstrated that in the case of continued accretion onto a star, after it has acquired the critical angular frequency allowing no diviation from axial symmetry, specific accretion disks can be formed with a smooth transition into a star. The specific angular momentum is computed for a neutron star for the instant of the attainment of the Oppenheimer-Volkoff limit.     

Comet Halley's dust drag perturbing the Giotto spacecraft

Hypervelocity impact sputter causes impulses substantially greater than the initial momenta of micro-grains of comet Halley's dust coma, the effective factor being discontinuous at the dust mass (0.2 g) that just penetrates the spacecraft bumper shield. Marginally non-penetrating grains determine the net drag and torque, calculated here for the Giotto shield and exposed components. The torque due to asymmetries induces a precession of the spacecraft axis, whose amplitude is solved for passage through the model dust coma, to find slowly damped oscillations of significant (1°) amplitude.     

Ammonia photolysis and the greenhouse effect in the primordial atmosphere of the earth

Photochemical calculations indicate that in the prebiotic atmosphere of the Earth ammonia would have been irreversibly converted to N₂ in less than 40 years if the ammonia surface mixing ratio were ≤ 10^?4. However, if a continuous outgassing of ammonia were maintained, radiative equilibrium calculations indicate that a surface mixing ratio of ammonia of 10^?5 or greater would provide a sufficient greenhouse effect to keep the surface temperature above freezing. With a 10^?4 mixing ratio of ammonia, 60 to 70% of the present day solar luminosity would be adequate to maintain surface temperatures above freezing. A lower limit to the time constant for accumulation of an amount of nitrogen equivalent to the present day value is 10 my if the outgassing were such as to provide a continuous surface mixing ratio of ammonia ≥ 10^?5.     

Solution of the equation of transfer for Rayleigh's phase function by the principle of invariance

In this paper we shall construct the solution of the equation of transfer in a semi-infinite atmosphere with no incident radiation for Rayleigh's phase function by the method of the Principles of Invariance and using the law of diffuse reflection. The solution will then be applied to find the laws of darkening for Rayleigh's phase function.