Late-type giant variables in NGC 6522, LMC and SMC. How do they differ?

¹ CNRS UMR 6091, Observatoire de Besançon, BP 1615, 25010 Besançon Cedex, France e-mail: mathias@obs-besancon.fr
² CNRS UMR 7095, Institut d'Astrophysique de Paris, 98bis Bld. Arago, 75014 Paris, France
³ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
⁴ European Southern Observatory, Karl-Schwarschild-Str. 2, 85748 Garching bei München, Germany

Received: 11 June 2004
Accepted: 3 August 2004

Abstract

Complete samples of 2MASS stars in three 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  at a given . These quantities have been compared with the data for galactic globular clusters obtained by Ferraro et al. ([CITE]). In the , colour–colour diagrams, the increasing proportion of stars with high  values is conspicuous at lower metallicities. This is well-known to result from the increasing proportion of carbon stars.
All three fields contain similar types of variables, mainly short-period (usually tens of days), Mira like (long-period, large amplitude) and double-period (having both long and short periods). The proportion of stars that vary 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 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 200–300 d 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 (MCMS) 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 carbon stars at low metallicity. Mira magnitude vs. log P relations exist at least up to 7 μm. Mass-loss from longer-period and double-period SRVs occurs at similar rates in each field, in spite of the metallicity differences.