Cool but not Smooth—The Molecular Spectra of Red Stars

Abstract

Formy PhD I had to model the Hα line profile in the spectra of symbiotic stars; these are interacting binary stars in which one of the components is a cool giant often of spectral class M. The continuum in the vicinity of the Hα line was seemingly very ‘noisy’ in one or two of the spectra which I was using and I remember someone at the time commenting that perhaps all the myriad spikes and dips weren’t in fact instrumental noise but the titanium oxide (TiO) bands in the spectrum of the red giant. I recall even then thinking ‘is the continuous spectrum of a red giant such terra incognita that one cannot distinguish the absorption bands from instrumental noise?’ It’s true that in most books which list the spectral classes, the section on classMcan sometimes be almost dismissive with phrases like ‘dominated by bands of titanium oxide and other molecules’. However, if you dig deep enough in the literature you’ll certainly find that much is known about the patterns of these mysterious bands to the extent that even bands due to different isotopes of titanium have been identified; even so it will probably be a struggle to identify individual bands in your spectra.