H-2 in the 2 micron infrared spectra of long period variables I. Observations

Up to seven lines in the S branch of molecular hydrogen have been detected in the 2 mum infrared absorption line spectra of long period variables. The H-2 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 H-2 lines also depends on variable type with Mira variables having deep lines (at minimum light), SRa variables generally having detec table lines, and the SRb and non-variables having weak to non-detectable li nes. In addition H-2 line strengths depend on period and on C/O abundance, with long period (i.e. cool), pure S-type Miras having H-2 lines with centr al depths of up to 80%. The appearance of H-2 lines in the spectra of cool giants is linked with geometrical extension of the atmosphere. The greatly increased strength of the H-2 lines in Mira variables is observational evid ence of the role of stellar pulsation in greatly extending these stellar at mospheres. Weak emission is seen in H-2 near maximum light in Miras. We sug gest this is resonant scattering in the spherically extended stellar atmosp here. The significant role of H-2 opacity in the blue is noted.