Red Variables in the Central Bulge of the Galaxy -- I THE PERIOD DISTRIBUTION OF MIRA VARIABLES

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Abstract
Baade's three galactic centre windows in the Sagittarius star cloud have been searched for Mira variables using V and I plates taken with the 1·88-m Radcliffe reflector. Periods, which are of a provisional nature in some cases, have been determined for 121 red variables of large amplitude. Most of these stars are Mira variables of spectral type M. The dependence of luminosity on period is conspicuous, so that the apparent magnitude limit of the observations imposes a cut-off at long periods. The use of V and especially I plates permits the detection of long-period Miras which were not discovered in earlier blue light surveys. There is a rapid decline in the surface density of Mira variables with increasing angular distance from the galactic centre. The rate of decline is in agreement with Innannen's galactic model, which predicts that the stars will extend over a considerable distance in the line of sight. The period distribution of the Miras extends to much longer periods than that of those in globular clusters. Several lines of evidence suggest that the presence of a sufficiently large population of stars of age less than 4 × 109 yr in the central bulge of the Galaxy is unlikely, so that the Miras of all periods are presumably older than this.

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