2015ApJ...811...45S

We observed a sample of 20 M giants with the Infrared Spectrograph on the Spitzer Space Telescope. Most show absorption structure at 6.6-6.8 µm which we identify as water vapor, and in some cases, the absorption extends from 6.4 µm into the SiO band at 7.5 µm. Variable stars show stronger H₂ O absorption. While the strength of the SiO fundamental at 8 µm increases monotonically from spectral class K0 to K5, the dependence on spectral class weakens in the M giants. As with previously studied samples, the M giants show considerable scatter in SiO band strength within a given spectral class. All of the stars in our sample also show OH band absorption, most noticeably in the 14-17 µm region. The OH bands behave much like the SiO bands, increasing in strength in the K giants but showing weaker dependence on spectral class in the M giants, and with considerable scatter. An examination of the photometric properties reveals that the color may be a better indicator of molecular band strength than the spectral class. The transformation from Tycho colors to Johnson color is double-valued, and neither nor color increases monotonically with spectral class in the M giants like they do in the K giants.