2002MNRAS.337..519G

A `complete' sample of 174 M giants classified in the mid-1980s by Blanco and later than subtype M0 in the NGC 6522 Baade's Window clear field has been investigated to establish some general properties of cool bulge stars. Photometric information has been obtained from the MACHO data base to search for variability and, where possible, to determine periods. Near- and mid-infrared magnitudes have been extracted from DENIS and ISOGAL. 46 semiregular variables (SRVs) and two irregular variables were found amongst the 174. Many M5 and all stars M6 or later show variation, whereas earlier subtypes (M1-M4) do not.

The DENIS I - J and J - K_Scolours and the luminosities of the M stars increase with M subclass. K tends to increase with log P among the M-type SRVs.

Almost all the variables were detected at 7 µ m during the ISOGAL programme. Excess radiation at 15 µ m, indicative of heavy mass loss, is associated with high luminosity and late spectral type. The limit of sensitivity of the ISOGAL survey was such that the non-variables were not detected. Four probable M stars not listed by Blanco, two of which are semiregular variables, were detected by ISOGAL.

In the case of doubly periodic SRVs, the longer periods have K magnitudes that place them close to the `D' line of Wood in a K -log P diagram. The unusual MACHO light curve of one particular star, Blanco 26, shows the commencement of a long-period variation with an anomalously short and sharp event and appears to rule out a pulsational model for this phenomenon.