The vibration-rotation fundamental transition of H2 contributes spectral lines in cool giants. In the coolest giants blending with lines of other molecules makes detection of the sparse H2 spectrum difficult. The exception is the "S-type" giant spectra where there are few blending features and detection is easier. In all cool Miras there is evidence that strong H2 lines are present in the spectrum; for the cool S-type Miras these lines are obvious spectral features in some cases reaching central depths of 80% with FWHM of 30 km s-1. The strength, width, and velocity of the line is attributable to line formation over a great range of the stellar atmosphere. The large column density of H2 required by these strong lines implies a large H2 Rayleigh opacity in the blue.
In Mira variables, where the strongest H2 lines are observed, the column density can be large. Assuming a stellar radius of 300 and a similar thickness for the stellar atmosphere, an appreciable fraction of the stellar mass is suspended in the stellar atmosphere. The H2 lines are at most half the strength in SRa variables as in Mira variables. Assuming that stellar masses and surface temperatures are the same for similar spectral type SR and Mira variables, the H2 line strength reflects the density and extent of the stellar atmosphere. A possible implication is that SR variables represent a higher pulsation mode than the Miras where less energy is transferred to supporting a highly extended stellar atmosphere. The transfer of energy to the stellar atmosphere occurs through the stellar pulsation. HHR found that the outward momentum flux associated with a Mira pulsation exceeds L/c by about a factor of 1000. The estimated mass and extent of the S-type Mira atmospheres requires an energy input approaching that of the stellar luminosity to support it against gravitational collapse.
Hydrogen (atomic and molecular) is the spectral diagnostic of choice in evaluating stellar atmospheres and eventually in determining the abundances. Our identification of the velocity shifted line noted by Tsuji (1983) as H2 puts current oxygen rich giant model atmospheres in reasonable agreement with observation for the non-variable M giants. Lambert et al. (1986) reached a similar conclusion for the carbon stars. However, our results also show that even in the non-variable M and C stars extended atmospheres are required to explain the velocity of the observed H2 line core.
Since Miras are at the final stage in evolution before ejection of the outer envelope, the abundances are of considerable significance. The complexity of computing realistic model atmospheres for the Miras is well known (Bertschinger & Chevalier 1985; Bessell et al. 1989). The H2 line profiles demonstrate that extended, spherical model atmospheres and resonant scattering are essential in a detailed understanding the observed spectrum.
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000