Infrared spectral properties of M giants

TL;DR

This study analyzes the infrared spectral features of 20 M giants, revealing water vapor, SiO, and OH absorption bands, and explores their dependence on spectral class and photometric properties.

Contribution

It provides new infrared spectral data on M giants, highlighting water vapor presence and the complex relationship between molecular band strength and photometric colors.

Findings

Water vapor absorption at 6.6-6.8 um identified in M giants.

SiO band strength shows weak dependence on spectral class in M giants.

V-K color correlates better with molecular band strength than spectral class.

Abstract

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 um which we identify as water vapor, and in some cases, the absorption extends from 6.4 um into the SiO band at 7.5 um. Variable stars show stronger H2O absorption. While the strength of the SiO fundamental at 8 um 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 um 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 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 BT-VT color increases monotonically with spectral class in the M giants like they do in the K giants.