Spitzer/IRAC photometry of the η Chameleontis association

We present IRAC 3.6, 4.5, 5.8, and 8 μm photometry for the 17 A-, K-, and M-type members of the η Chameleontis association. These data show infrared excesses toward six of the 15 K and M stars, indicating the presence of circumstellar disks around 40% of the stars with masses of 0.1-1 M _⊙. The two A stars show no infrared excesses. The excess emission around one of the stars is comparable to the median excess for classical T Tauri stars in the Taurus association; the remaining five show comparatively weak excess emission. Taking into account published Ha spectroscopy that shows that five of the six stars are accreting, we argue that the disks with weak mid-infrared excesses are disks in which the inner disks have been largely depleted of small grains by grain growth or, in one case, the small grains have settled to the midplane. This suggests that η Cha has a much higher fraction of disks caught in the act of transitioning into optically thin disks than that measured in younger clusters and associations.