Showing papers in "Astronomy and Astrophysics in 1982"

A comparative study of computational methods in cosmic gas dynamics

Abstract: In search of reliable computational methods for cosmic flow problems, we apply several commonly used algorithms and one new algorithm, to a representative problem in galactic gas dynamics. A careful choice of the algorithm used in a calculation is found to be of the utmost importance in obtaining reliable results. Two methods most commonly employed in astronomy (the Beam scheme and FCT methods) prove to be highly unsuitable for our test problem. The penalty in programming effort and computer time per grid point required for the best second-order accurate codes tested is more than offset by the improvement in accuracy obtained and the possibility to reduce the number of points in a grid.

Model calculations of the molecular composition of interstellar grain mantles

Abstract: A chemical reaction scheme comprising gas phase as well as grain surface reactions is used for the numerical calculation of the chemical composition of mantles accreting on interstellar grains. The composition of the growing grain mantle is determined on the basis of the relative accretion rates of the gas phase molecules and diffusion-controlled surface reactions. Results show that grain mantles generally consist of the molecules H2O, H2CO, N2, O2, CO, CO2, H2O2, and NH3. A detailed examination of the formation of H2 on grain surfaces leads to the conclusion that it proceeds through hydrogen abstraction from such molecules as H2CO, H2S, N2H2, and N2H4. The IR characteristics of the calculated grain mantles are discussed with emphasis on the observed ice band at the 3250/cm wavelength.

Measurements of solar transition zone velocities and line broadening using the ultraviolet spectrometer and polarimeter on the Solar Maximum Mission

Abstract: The UVSP instrument on SMM is able to observe solar regions at two wavelengths in the same line with a band-pass of 03 A Intensity and Doppler velocity maps are derived It is shown that the numerical values are sensitive to the adopted Doppler width and the range of velocities is limited to within 30 km/sec A method called Double Dopplergram Determination (DDD) is described for deriving both the Doppler width and the velocity (up to 80 km/sec), and the main sources of uncertainties are discussed To illustrate the method, a set of C IV 1548 A observations is analyzed according to this procedure The mean C IV Doppler width measured (015 A) is comparable to previous determinations A relation is found between bright regions and down-flows Large Doppler widths correspond to strong velocity gradients

Long Periodic Terms in the Solar System

Abstract: The long period variations of the first eight planets in the solar system are studied First, the Lagrangian solution is calculated and then the long period terms with fourth order eccentricities and inclinations are introduced into the perturbation function A second approximation was made taking into account the short period terms' contribution, namely the perturbations of first order with respect to the masses Special attention was paid to the determination of the integration constants The relative importance of the different contributions is shown It is useless, for example, to introduce the long period terms of fifth order if no account has been taken of the short period terms Meanwhile, the terms that have been neglected would not introduce large changes in the integration constants Even so, the calculation should be repeated with higher order short period terms and fifth order long periods

Orientation of the JPL Ephemerides, DE 200/LE 200, to the dynamical equinox of J 2000

Abstract: The lunar and planetary ephemerides, DE 200/LE 200 have been recently produced at JPL. They will form the basis of the ephemerides in the 'Astronomical Almanac' starting in the year 1984. The origin has been referenced to the J 2000 dynamical equinox of the ephemerides themselves. The procedure for this orientation is described here in detail. Analyses of the ephemerides also provide comparisons with the equinox of the FK 4 and with presently adopted values for the earth's obliquity.

Extragalactic gamma radiation: Use of galaxy counts as a galactic tracer

Abstract: A derivation of the extragalactic diffuse gamma radiation with energies above 35 MeV was carried out using galaxy counts as a tracer of galactic matter. The extragalactic radiation has a differential photon number spectrum which may be expressed as a power law with index 2.35 (+0.4, -0.3) and an intensity above 35 MeV of (5.5 + or - 1.3) 0.00001 photons sq cm/s/ster, consistent with previous derivations. Use of a 1/sin of the absolute value of b expression of the galactic component produces a poorer fit, suggesting that the high-latitude galactic gamma-ray production may be dominated by cosmic ray interactions with matter rather than by Compton interactions of cosmic rays with photon fields.

Impulsive and gradual hard X-ray sources in a solar flare

Abstract: The paper reports recent observations of a relatively large hard X-ray burst with well developed impulsive and gradual components, that was associated with a relatively modest optical flare on Aug. 14, 1979. The impulsive X-ray source has a much higher ion density and the electron injection in the phase is shorter in duration consisting of fewer energetic electrons than in the gradual phase. The gradual X-ray source seems to consist of a lower density region, probably bounded by a magnetic arch in which the energetic electrons are perfectly trapped. The different temporal development of the impulsive and gradual hard X-ray components suggest large differences in the physical parameters of the ambient plasma.

Radio imaging of solar flares using the very large array - New insights into flare process

Abstract: An interpretation of VLA observations of microwave bursts is presented in an attempt to distinguish between certain models of flares. The VLA observations provide information about the pre-flare magnetic field topology and the existence of mildly relativistic electrons accelerated during flares. Examples are shown of changes in magnetic field topology in the hour before flares. In one case, new bipolar loops appear to emerge, which is an essential component of the model developed by Heyvaerts et al. (1977). In another case, a quadrupole structure, suggestive of two juxtaposed bipolar loops, appears to trigger the flare. Because of the observed diversity of magnetic field topologies in microwave bursts, it is believed that the magnetic energy must be dissipated in more than one way. The VLA observations are clearly providing means for sorting out the diverse flare models.

Production of CS and S in Comet Bradfield /1979 X/

Abstract: High and low resolution ultraviolet spectra of carbon monosulfide (CS) in Comet Bradfield (1979 X) were obtained with the IUE satellite. The high resolution rotational profile of the (0, 0) band at 257.5 nm can be fitted with a theoretical profile derived assuming a Boltzmann temperature of 70 K. Spatial plots of the low resolution data for both S and CS show that these emissions are concentrated toward the cometary nucleus. The results that have been obtained are consistent with a Haser model for CS and S where the parent molecule is CS2. A very rapid variation of CS brightness with heliocentric distance is found.

The importance of plasma effects on electron-cyclotron maser-emission from flaring loops

Abstract: Electron cyclotron maser instability has been suggested as the cause of the observed short (10-20 msec), intense (an approximate brightness temperature of 10 to the 15th K) and up to 100% polarized microwave solar emission. It is shown that plasma effects and thermal cyclotron damping, ignored in previous theories, play an important role in controlling the frequency range of the emission. The radio emission is suppressed for ratios of the plasma frequency to the cyclotron frequency smaller than 0.4. An examination of the cyclotron damping, reveals that the maser action is suppressed unless a large fraction (i.e., over 10%) of the accelerated electrons participates in the emission process.

Visible light observations of a dense plasmoid associated with a moving Type IV solar radio burst

Abstract: A coronal transient rising above the eastern limb of the sun was observed simultaneously on April 27, 1980 by the Coronagraph/Polarimeter (C/P) aboard the Solar Maximum Mission spacecraft and by the Culgoora radioheliograph (CRH). The C/P observed an outward-moving loop transient followed by a plasmoid of dense ionized material, while the CRH observed several types of radio bursts, including a moving Type IV burst at 80 MHz whose position coincided with the bright plasmoid. An estimate of the reasonable lower limit of the electron density in the moving Type IV burst indicates that this burst could have resulted from emission at either the fundamental or the second harmonic of the plasma frequency. This is a new result because in the past it was thought that the density in the moving Type IV source region would be too low for plasma radiation.

On the transport and propagation of relativistic electrons in galaxies The effect of adiabatic deceleration in a galactic wind for the steady state case

Abstract: We present analytic solutions to the time-dependent equation describing the transport of relativistic electrons, which are being convected and diffused and may be either gaining or losing energy. The solutions allow for (i) a spatially varying convection velocity; (ii) a diffusion coefficient which is both energy and spatially dependent; (iii) adiabatic decleration effects; (iv) a term describing both synchrotron loss and /or Fermi acceleration which is also spatially dependent; (v) an impulsive source, a steady-state source, or time variable injection source for the electrons; (vi) a point (spatial) injection source or a spatially more diffuse source; (vii) arbitrary energy dependence of the injection source; (viii) spatial and energy ''boundaries'' where the spatial or energy form of the convection, diffusion, and energy supply terms in the basic equation can change character.