By Alan H. Batten (auth.), Elias G. Mariolopoulos, Pericles S. Theocaris, L. N. Mavridis (eds.)
When we first approached a few colleagues allover the realm to sound them a few quantity devoted to Professor John Xanthakis at the get together of finishing twenty-five years of clinical actions as fellow of the nationwide Academy of Athens, any attainable doubts as to the feasibility of the undertaking have been speedy dispelled via their hot and inspiring reaction. very quickly 50 authors from 15 nations, coming from quite a lot of Professor Xanthakis' quick colleagues, scholars and buddies joined to supply the 36 contributions incorporated during this quantity. a few of those that the place initially approached came across themselves un capable of give a contribution, as a result time-limit inevitably imposed. Happi ly, they have been simply few in quantity, and we should always wish to list our grat itude to them for his or her stable needs for the good fortune of the enterprise. Their hot phrases have been among the resources of inspiring encouragement prolonged to us.
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Extra info for Compendium in Astronomy: A Volume Dedicated to Professor John Xanthakis on the Occasion of Completing Twenty-five Years of Scientific Activities as Fellow of the National Academy of Athens
0 We have experimented with some values of ~o but we studied in detail the case ~ =90 0 . When the motion of the pendulum becomes circular it can be direct 8r retrograde, namely the angle ~ is continuously increasing or decreasing. The results are similar and we confine our discussion to the direct cases. Table III represents a typical example of the changes of the energy and of the adiabatic invariant. 44. In the examples of Table III, £ takes its final c8nstant value during the eighth period of motion.
The grain will experience a repulsive force due to the radiation pressure P d of the star S and we may write, per unit volume: ra dP rad K (1 ) F dr c RADIATION PRESSURE EFFECT ON DUST 45 here K is a weighted mean extinction coefficient of the dust and the energy flux is equal to L F when we neglect the extinction of the radiation before it reaches the grain, which is justified for a cloud of sufficiently low density. The monochromatic coefficient KA is related to the cross-section for the radiative interaction of the grains and to the dust grain density nd through the product KA 0A n d · = Taking the mean value K of KA , weighted by the fluxes, we get straightforwardly the force acting on a single grain f rad LA 6A is given by the spectral type of S in th~ interval 6A around A and 0A is usually written in the form = TI rd qA r being the radius of a dust grain, assumed spherical, while qA is an ef~iciency factor depending on rdiA in the framework of the Mie theory (Wicramasinghe, 1967).
When we turn to the reflection nebulae associated with the bright stars of the Pleiades cluster in order to detect some possible parabolic profiles, we are faced with several observational difficulties; nevertheless, in the case of the Merope nebula, we do notice a distinctly parabolic boundary on its eastern side and this confirms indirectly the existence of dust in the close neighbourhood of star 23 Tau. 1. INTRODUCTION Interstellar matter associated with the Pleiades star cluster exhibits two distinct features; first, bright nebulosities of somewhat striated appearance around each of the brightest stars and secondly, ray-like parallel filaments lying in east-west direction, apparently uncorrelated with the bright stars, and the general picture suggested (Arny, 1977) is that of an interstellar cloud travelling through the cluster from east to west.