Spitzer observations of stars in M33

Each epoch covered nearly a square degree at 3.6, 4.5, and 8.0um. The point-source catalog from the full data set contains 37650 stars.

The stars have luminosities characteristic of the asymptotic giant branch and can be separated into oxygen-rich and carbon-rich populations by their [3.6]-[4.5] colors. The [3.6]-[8.0] colors indicate that over 80% of the stars detected at 8.0um have dust shells. Photometric comparison of epochs using conservative criteria yields a catalog of 2923 variable stars.