Infrared spectral properties of M giants

G.C. Sloan (Cornell), C. Goes (Cornell), R.M. Ramirez (Cornell), K.E. Kraemer (Boston Coll.), C.W. Engelke (Boston Coll.)

2015, ApJ, 811, 45

Full manuscript available locally (PDF), or from the arXiv (1508.05165).

The 20 IRS spectra described in this paper are available on this website.

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 H₂O 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 much weaker dependence on spectral class in the M giants, and with considerable scatter. An examination of the photometric properties reveals that the V-K color may be a better indicator of molecular band strength than the spectral class. The transformation from Tycho colors to Johnson B-V color is double-valued, and neither B-V nor B_T-V_T color increases monotonically with spectral class in the M giants like they do with the K giants.

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Last modified 19 September, 2015. © Gregory C. Sloan and others.