In most papers dealing with random motions and diffusion of small magnetic elements in the photosphere, the convective flows and in particular the granulation are considered as drivers of these motions. The results of these works have been discussed in terms of the fractal dimension of the granulation as seen in intensity pictures. So far neither a

Measurement of pattern speeds has been an observational imperative in the last years since a clear determination of the corotation resonance radius will give some insight in the nature of density waves and in the mechanism of their maintenance. In this paper we analyse in detail the practical application to the grand design spiral NGC 4321 of two

Not Available

With the use of data from COBE, IRAS and the Two-micron Galactic Survey (TMGS), we examine the effect on star counts and surface brightness maps of the position of the Sun above the Galactic plane and of a possible tilt between the b=0 deg plane and the plane of the old disc (to a distance of 3 or 4 kpc from the Sun). We find that the Sun is 15.5+/

New grids of stellar evolutionary models are presented for a wide range of initial hydrogen content around the solar composition. These compositions are (X, Z) = (0.80, 0.02), (0.70, 0.02) and (0.60, 0.02). Recent OPAL opacities with LS coupling and Grevesse composition were adopted, and Alexander's tables were used for lower temperatures. We

Not Available