Showing papers in "Astronomy and Astrophysics in 1970"

The Intrinsic Colours of Stars and Two-Colour Reddening Lines

Abstract: Mean intrinsic colours, on the Johnson UBV and Cape U.BV systems, are obtained for stars of all MK spectral classes by analysis of the data in the Photoelectric Catalogue (Blanco et al., 1968). The intrinsic colours of stars of a given MK class are determined from: (1) the average colours of unreddened stars, for A to M giants, sub-giants, and dwarfs; (2) blue-most envelopes in two-colour and spectral-colour diagrams, for A to M bright giants and supergiants, and also for O and B type stars; and (3) two-colour reddening lines, for O and B type stars. The coefficients of the two-colour reddening line equation, E u = α E v + β E 2 v , are obtained for the early type stars. The mean slope, α, of the reddening line on the UBV system for O9 to B1 main sequence stars is found to be 0.75 ± 0.01 in Cygnus, and 0.70 ± 0.01 in the rest of the sky. The slopes of the reddening lines are dependent on spectral class; the curvature, β, is 0.05 in both regions. On the Cape U c BV system the slope is found to be 0.37 ± 0.01; there is no curvature term and no significant dependence on spectral class.

Absolute measurement of mean lifetimes of excited states using the method of correlated photons in a cascade.

Abstract: Abstract Absolute mean lifetimes were measured using the method of correlated photons in cascade. The mean lifetimes are found to be: Hg 7 3S1 : τ = (8.4 ± 0.4) ns, A II 4p′ 2Do 5 2 : τ = (6.27 ± 0.06) ns. A II A 4p 4Do 7 2 : τ = (5.06 ± 0.12) ns.

Reprint of 1970A&A.....5...84Z. Gravitational instability: an approximate theory for large density perturbations.

Abstract: An approximate solution is given for the problem of the growth of perturbations during the expansion of matter without pressure. The solution is qualitatively correct even when the perturbations are not small. Infinite density is first obtained on disc-like surfaces by unilateral compression. The following layers are compressed first adiabatically and then by a shook wave. Physical conditions in the compressed matter are analysed.

Shell Source Burning Stars with Highly Condensed Cores

Abstract: Simple analytical relations are derived - similar to the homology relations for homogeneous stam - which describe the behaviour of shell source burning models with highly condensed cores. Many features which are known from evolutionary calculations (and partly &om observations) of such stars can be understood as coming essentially from the increase of the mass Ma of the core. Changes of the rdus RE of the core, of the radiation pressure, and of the chemical composition are also conaidered. For e comparison, sequences of numerically calculated equilibrium models are pmnted. They show that the analytical relations give, even quantitatively, good approximations to the numerical results.

On the secular change of the obliquity of the ecliptic

Abstract: Ecliptic obliquity secular change rate apparent discrepancy possibly due to earth-moon barycenter motion