2000A&A...363.1065H

Up to seven lines in the S branch of molecular hydrogen have been detected in the 2µm infrared absorption line spectra of long period variables. The H₂ lines show very strong phase dependent changes in velocity, equivalent width, and line profile, with the strongest lines near minimum light. The strength of the H₂ lines also depends on variable type with Mira variables having deep lines (at minimum light), SRa variables generally having detectable lines, and the SRb and non-variables having weak to non-detectable lines. In addition H₂ line strengths depend on period and on C/O abundance, with long period (i.e. cool), pure S-type Miras having H₂ lines with central depths of up to 80%. The appearance of H₂ lines in the spectra of cool giants is linked with geometrical extension of the atmosphere. The greatly increased strength of the H₂ lines in Mira variables is observational evidence of the role of stellar pulsation in greatly extending these stellar atmospheres. Weak emission is seen in H₂ near maximum light in Miras. We suggest this is resonant scattering in the spherically extended stellar atmosphere. The significant role of H₂ opacity in the blue is noted.