Showing papers in "Monthly Notices of the Royal Astronomical Society in 1987"

Recombination-line intensities for hydrogenic ions – I. Case B calculations for H I and He II

Abstract: Assuming Case B of Baker and Menzel (1938), the relative intensities of H I and He II recombination lines are determined for a larger range of temperature and density than previously considered and with the upper principle quantum number up to 50 and the lower number up to 29. Full collisional effects are included, and new collision strengths for the n = 1, 2, and 3 states of He(+) are presented. Case B theory is found to be invalidated for collisional excitation of the n = 3 levels from both n = 1 and n = 2 states under certain conditions which depend upon electron density and the Lyman-alpha escape probability, and the regimes of temperature and density for which Case B is valid are also considered.

The statistics of cosmic background radiation fluctuations

Abstract: Calculs des fonctions de correlations du rayonnement et des spectres de puissance angulaire pour des anisotropies de fond de microondes, attendues dans des univers avec une matiere sombre froide dominante (Ω=1)

A viscosity prescription for a self-gravitating accretion disc⋆

Abstract: A model for treating the transfer of angular momentum within a gaseous differentially rotating disc subject to gravitational instability is discussed in terms of an effective kinematic viscosity. It is assumed that even when matter in the disc is subject to self-gravitation, the instability does not necessarily lead directly to condensation of parts of the disc into self-gravitating bodies. Conditions under which the present model permits a similarity solution are discussed, and it is shown that the general solution tends to the similarity solution at large times.

Absolute magnitudes of cataclysmic variables

Abstract: On derive les magnitudes absolues Mv des disques d'accretion d'etoiles dans les principales classes de variables cataclysmiques, a partir de plusieurs techniques

The evolution of optically selected QSOs – II

Abstract: Une nouvelle determination de la relation nombre-luminosite et de la fonction de luminosite des quasars est presentee, sur la base d'un catalogue de 420 quasars selectionnes par un faible exces UV et pour lesquels une identification spectroscopique et les redshifts ont ete obtenus en utilisant le systeme a fibres optiques au telescope anglo-australien

Ageing and speeds in a representative sample of 21 classical double radio sources

Abstract: On a utilise des observations a quatre frequences, effectuees avec le Tres Grand Reseau et le telescope de 5 km pour determiner les variations spectrales a l'interieur des lobes d'un echantillon representatif de 21 radiosources doubles classiques 3CR

On magnetic braking of late-type stars

Abstract: This report deals with the rotational history of late-type stars subsequent to their reaching the main sequence. The question of relevance to this Symposium is whether detailed comparison of magnetic braking theory with observations of young stellar clusters will yield any constraints on dynamo activity, in particular on the relation between the surface field strength B s and the angular velocity Ω.

A completely analytical family of anisotropic Plummer models

Abstract: On rapelle les formules permettant de relier la dynamique interne aux quantites observees puis on presente de facon fort detaillee une famille de modeles spheriques, basee sur le modele de Plummer

Physics of modes in a differentially rotating system – analysis of the shearing sheet

Abstract: We analyse the linear non-vortical modes of the shearing sheet, a model compressible two-dimensional fluid system with constant density, constant shear, and Coriolis force. This model has several features found in differentially rotating systems of interest in astrophysics, such as disc galaxies, accretion tori, planetary rings, protostellar nebulae, and possibly even rotating stars.

Magnetic fields and non-uniform rotation in stellar radiative zones

Abstract: Effets des instabilites dynamiques et resistives sur la redistribution magnetique du moment angulaire a l'interieur d'une etoile

Dissection of an N-body bar

Abstract: Etude detaillee d'une barre a deux dimensions formee par suite de l'instabilite globale du disque. La barre est stable et tres robuste

Evolution of N-body open clusters

Abstract: On discute de simulations realistes a r corps de l'evolution dynamique d'amas ouverts et on les compare aux observations. La plupart comportent 1000 corps dont les masses initiales suivent une loi de puissance de pente α=−2,75 et dont la valeur moyenne est 0,5 MO

The chromospheres and coronae of five G-K main-sequence stars

Abstract: Previous data and high and low dispersion IUE observations of Chi 1 Ori, Alpha Cen A, Xi Boo A, Alpha Cen B, and Epsilon Eri are used to model the atmospheres of these stars from the high chromosphere to the corona. The energy lost by radiation and transferred by thermal conduction is investigated to establish the heating requirements, and results similar to those found for the solar atmosphere are found. The observed emission measurement distribution below 100,000 K can be described by a model in which Alfven wave energy input, observed through nonthermal line broadening, is balanced by radiation losses. Comparison of the coronal, transition region, and chromospheric pressures shows scaling relations which are compatible with previous flux correlations.