High-mass-loss AGB stars in the South Galactic Cap

Observations are presented for 61 Miras in the South Galactic Cap (b <-30°) which were selected on the basis of their IRAS 12/25-μm flux ratios as high-mass-loss candidates. JHKL photometry (over 1500 observations) was obtained for all of the stars, optical spectra for 49 and OH-maser detections for four. 58 of the Miras are oxygen-rich, and three are carbon stars with thick shells. Periods have been determined for all of the stars for which they were not already known-they range from 166 to 700 d. The carbon Mira IZ Peg (AFGL 3099) shows two periods, of 488 and 345 d, respectively. Distances and mass-loss rates have been determined for all 61 stars. Mass-loss rates range from about 10-7 to over 10-5 M⊙ yr-1, they are tightly correlated with K-[12] but not with the 12/25-μm flux ratio. The kinematic behaviour of the Miras is discussed on the basis of the new spectra and of published radio observations of molecular lines. The kinematic properties are dependent on the pulsation period, as is the height above the Galactic plane. These results are interpreted as indicating that there can be only a rather small fractional change to the pulsational period of a Mira during its lifetime as a Mira. A high asymmetric drift is determined for the Galactic Cap Miras in the period range from 250 to 350 d; possible reasons for this are discussed. Evidence is given that supports the view that almost all IRAS sources with a variability index of 9 are Mira variables. It is shown, however, that biases may result if sources selected on the basis of their variability indices are used for Galactic structure studies. The current status of the period-luminosity and period-luminosity-colour relations is briefly reviewed. It is demonstrated that the assumption sometimes made that local Miras are 0.25 mag fainter at K than those with the same period in the LMC is not supported by observations. A comparison of Miras in the Cap with those in the 7° < | b \ < 8° field of the Bulge indicates no obvious differences in period distribution or other properties.