Late-type giant variables in NGC6522, LMC and SMC: How do they differ?

Abstract

Complete samples of 2MASS stars in 3 fields of differing metallicities (and possibly differing age distributions) have been extracted and cross-correlated with MACHO and ISO data to determine their variability and mass-loss properties. In each M_K histogram a fall-off in numbers is seen above the tip of the RGB. The luminosity of the tip increases with metallicity as does the average J-K at given M_K. The proportion of stars with high H-K increases at lower metallicities. This results from the increasing proportion of C stars. All 3 fields contain similar types of variables, mainly short-period, Mira-like and double-period (having long and short periods). The proportion of variable stars decreases at lower metallicities and the minimum period associated with a given amplitude gets longer. Various trends are seen in the K, log P diagrams of each field. The Magellanic Cloud fields largely resemble each other but the Bulge field is noticeably different. The stars that follow the 'A' K,log P relation in the Bulge hardly extend beyond the M giant tip and the other groups also appear truncated in M_K relative to their Magellanic Cloud counterparts. In the Clouds there are many stars with periods around 200-300d that follow the 'C' or Mira relation but have relatively small amplitudes. The mid-IR sample detected by ISOCAM during the Magellanic Cloud Mini-Survey appears to be reliable and complete for sources with M_K more luminous than -7 mag, i.e., for those close to the top of the AGB. The various colour-colour and colour-magnitude diagrams reflect the increasing dominance of C stars at low metallicity. Mira magnitude vs log P relations exist up to at least 7 microns. Mass-loss from longer-period and double-period SRVs occurs at similar rates in each field, despite differences in metallicity.