H-alpha in Miras

The shock wave which runs through the atmosphere of Mira stars, due to radial pulsations, gives rise to (amongst others) emission lines of Hydrogen. In Miras of spectral type M (oxygen-rich, i.e. Oxygen is more abundant than Carbon in the atmosphere) the opacity due to the TiO molecule is very large in the red part of the optical spectrum, but less in the blue part. Therefore, M-type Miras show strong emission at H-delta and H-gamma, but not at H-alpha (because the emission region is shielded from view due to the high opacity).

However, in C-type Miras (Carbon more abundant than Oxygen) it is the other way around: the CN, C2 and SiC absorption is less strong in the red, and therefore the H-alpha emission can be observed.

Also several S/SC stars with weak TiO show H-alpha emission.

Some initial results may be found in the talk I gave for the BAV

(see here: "Tagungsberichte" -> Bericht zum BAV-Treffen in Hartha 2017 -> Spectra of Miras and Semi-Regulars

or directly here [pdf] )