Dynamical effects of dark matter in systems of galaxies

SeveralN-body experiments were performed in order to simulate the dynamical behaviour of systems of galaxies gravitationally dominated by a massive dark background.We discuss mass estimates from the dynamics of the luminous component (M _VT) under the influence of such a background, assuming a constant dark/luminous mass ratio (M _D/M _L) and plausible physical conditions. We extend in this way previous studies (Smith, 1980, 1984) about the dependence ofM _VT on the relative distributions of dark and luminous matter (Limber, 1959). We found that the observed ratio of the virial theorem mass to luminosity (M _VT/L) in systems of galaxies of different sizes could be the result of different stages of their post-virialisation evolution as was previously suggested by White and Rees (1978) and Barnes (1983). This evolution is mainly the result of the dynamical friction that dark matter exerts on the luminous component. Thus our results give support to the idea that compact groups of galaxies are dynamically more evolved than large clusters, which is expected from the hierarchical clustering picture for the formation of such structures.     

Possible mass limits for quantum double galaxies

When cosmic quantum mechanics is applied to a double galaxy, the result is mass limits in order for the two galaxies to form a quantum binary system. For a non-relativistic theory (based on the Schrödinger wave equation), the mass limits are: (m _g)_max 10¹² M and (m _g)_min 10¹⁰ M . One possible consequence appears to be a Newtonian gravitational constant that varies with cosmic time, with its value larger in the cosmic past.     

Discrete Alfvén waves in planar flux tubes

Properties of discrete Alfvén wave modes are derived, at frequencies up to the ion-cyclotron frequency, for current-carrying plasma slabs with non-uniform densities. It is shown that the essential features of the dispersion relations can be derived by examining the dominant terms in the potential function, when the wave equation is cast in the Schrödinger equation form. Analytical predictions for a class of mass and current density profiles are compared with numerically calculated dispersion relations and wavefields for particular profiles.     

Problems with the interpretation of the 220 nm interstellar feature

The bump in the ultraviolet part of the interstellar extinction curve provides a great challenge in the modelling of interstellar dust. Its shape can be well approximated by a classical dispersion profile with a total halfwidth of 48 nm centred at 217 nm. Apart from few slightly deviating cases the parameters of the band seem to be surprisingly constant in the solar neighbourhood.The equivalent widthW of the 217 nm band shows a very tight correlation with the colour excessE(B-V). Studies of correlations with the strength of diffuse interstellar bands gave no conclusive results as to the nature of the band.The most common interpretation of the 217 nm feature as originating from small graphite grians meets several difficulties. No final decision on the carrier can be made at present.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.     

Development of a real-time algorithm for detection of solar wind discontinuities

A study is presented of an algorithm, based on the statistical analysis of interplanetary magnetic field data, for the real-time detection of discontinuities in the solar wind. The analysis is based on the application of a sliding or searching algorithm together with a minimum variance treatment. A demonstration is given for a Pioneer data set from 29 August, 1966. This particular data set was chosen because it has been extensively studied (in terms of the fast forward MHD shock wave) by previous groups who utilized the three-dimensional Rankine-Hugoniot shock equations after identifying the shock on a subjective basis. Our procedure differs in that it is perfectly objective because of its searching technique. For this particular data set we identify the original shock with excellent agreement with the earlier studies of its normal vector, as well as an additional shock and tangential discontinuity, neither of which had been identified previously.     

An estimate of the contribution of high nergy cosmic-ray protons to the absorbed dose inventory of a cometary nucleus

We discuss the contribution of cosmic-ray protons at all energies above 1 MeV to the absorbed doses of the surface layers of a comet. Since there exists no calculation which takes into account proton energy losses by means of losses to electrons and nuclear collisions (in a cascade process), and losses due to the low energy end-products of the cascade, we have made a rough estimate of all of these contributions. An analytical formula is proposed that allows a rapid estimate of ranges and the dose absorbed at any depth. We give dose-depth curves for two extreme values of the energy at which nuclear collisions begin to dominate the slowing-down process, and for an intermediate value we display the dose-depth curve down to 20 m from the surface. The relevance of these findings to dosimetry in comets and some alterations of cometary material are considered. The need for improving the analytical expression proposed is stressed.     

Bipolar nebulae and jets from young stars: Contributions from calar alto

The topic is reviewed with emphasis on observations in the optical and near infrared spectral range.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.     

Converging magnetogasdynamic cylindrical shock waves in a uniform atmosphere

A self-similar solution to the problem of the implosion of a cylindrical shock wave in the presence of a magnetic field has been investigated. A strong shock wave in a cylindrically-symmetric flow travels to the axis of symmetry through a gas of uniform initial density ₀ and zero-pressure. A comparative study has been made between the results obtained in ordinary gasdynamics and magnetogasdynamics with transverse and axial components of the magnetic field. The value of similarity exponent has been assigned from that found in the paper of Whitham (1958).     

A high-resolution bragg spectrometer for the observation of the hot interstellar medium X-ray emission

We present a high-resolution Bragg spectrometer designed for the observation of the soft X-ray cosmic diffuse background. The instrument concept is derived from the de Broglie geometry for the study of extended sources. It consists in a mosaïc of spherical TlAP crystals associated with position sensitive detectors located on the focussing surface. The spectral resolution and its variation with the field of view is estimated by Monte-Carlo simulations for different X-ray energies chosen among the most intense lines emitted by an astrophysical plasma in the temperature range 1–4×10⁶K. The estimated sensitivity and the simulations of actual space observations show that the instrument is capable to separate the strongest lines emitted by the most abundant ions (OVII,OVIII, FeXVII, NeIX, etc.) and to map the whole sky during a six month mission.     

Variations in the HCN/HNC abundance ratio in the Orion molecular cloud

We have used observations of the rare isotopes of HCN and HNC to determine the relative abundance of these two chemical isomers along the central ridge of the Orion molecular cloud. The abundance ratio [HCN]/[HNC] decreases by more than an order of magnitude from the relatively warm plateau and hot core sources toward the KL nebula to the colder, more quiescent clouds to the north and south. Even in the cooler regions, however, the ratio is an order of magnitude larger than that found in previous investigations of cold dark clouds. We determine the kinetic temperature in the regions we have studied from new observations of methylacetylene (CH3CCH), together with other recent estimates of the gas temperature near KL. The results suggest that the warmer portions of the cloud are dominated by different chemical pathways than those in the general interstellar cloud material.