2 - The distances of the Clouds

Summary

Cepheids, RR Lyrae stars, Mira variables, clusters and OB stars have dominated in the attempts to determine accurate distances to the Magellanic Clouds in recent years. The subject is of vital importance not only for Magellanic Cloud research but also for the determination of extragalactic distances (see Feast 1988, van den Bergh 1989, de Vaucouleurs 1993). With the accurate photometry possible nowadays the uncertainties in the distance determinations arise mainly through their dependence on the metallicities and ages of the objects involved and on the interstellar absorption in the line of sight, be it in the Galaxy or in the Clouds themselves.

The reddening, foreground and internal

The interstellar reddening affects virtually all studies of the Clouds. It is frequently difficult to determine its magnitude from the material available and mean values found in the literature are then, by necessity, applied. This may occasionally lead to erroneous results, in particular as the objects used to determine the mean may not be representative of the whole galaxy. In the following, some investigations will be referred to either as giving general ideas about the mean reddening or as examples of typical deviations.

The galactic foreground colour excess towards the Magellanic Clouds has been investigated by Schwering and Israel (1991) on a scale of 48 arcmin from galactic HI maps, using E_B-v = 0.17 10⁻²¹ N(HI) H cm⁻².