Long-period variables in the Sgr I field of the Galactic Bulge

We report 580 sets of JHKL observations of 63 long-period Mira-like variables in the Sgr I clear field located in the Bulge of our Galaxy at 3° from the Centre. These include 8 IRAS sources whose periods are given for the first time. No variable shows a period in excess of 700 d. The sample appears to be close to complete and it is demonstrated that the apparent period distribution from a given survey is highly dependent on the wavelength used. The colours and other properties of the Sgr I Miras are compared with those in the LMC and the solar neighbourhood. Although the J-K colours of the shorter-period Sgr I Miras are similar to those of their counterparts in the LMC, showing that they have similar temperatures at a given period, differences exist in their J-H and H-K colours. At longer periods, the Sgr I Miras appear redder in all colour indices than the LMC relations and their extrapolations. The apparentK-log P and mbol-log P relations for Miras in the Sgr I field are derived. The scatter about these relations is higher in Sgr I than for the LMC because of the significant depth of the Bulge in the line of sight. In conjunction with a simple model of the Bulge, the K-log P and Mbol-log P relations for LMC Miras are used to discuss the distance of the Sgr I field, the best value of which is found to be 8.7 ± 0.7 kpc (taking the distance modulus of the LMC to be 18.55 with s.e. 0.1 - 0.15). The value of R0, the distance to the Galactic Centre, is 0.2 kpc greater if the Sgr I field is part of a bar lying in the Galactic Plane and inclined at 45° to the line of sight.