Mid-infrared period-magnitude relations for AGB stars

Asymptotic giant branch (AGB) variables are found to obey period-luminosity relations in the mid-infrared (mid-IR) similar to those seen at K-S (2.14 mu m), even at 24 mu m where emission from circumstellar dust is expected to be dominant. Their loci in the M, log P diagrams are essentially the same for the Large Magellanic Cloud (LMC) and for NGC 6522 in spite of different ages and metallicities. There is no systematic trend of slope with wavelength. The offsets of the apparent magnitude versus log P relations imply a difference between the two fields of 3.8 in distance modulus. The colours of the variables confirm that a principal period with log P > 1.75 is a necessary condition for detectable mass loss. At the longest observed wavelength, 24 mu m, many semiregular variables have dust shells comparable in luminosity to those around Miras. There is a clear bifurcation in LMC colour-magnitude diagrams involving 24 mu m magnitudes.