AAVSO: American Association of Variable Star Observers

Low Resolution Spectroscopy of Miras 2—R Octantis

Volume 49 number 1 (2021)

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Bill Rea
6A Bygrave Place, Bishopdale, Christchurch 8053, New Zealand; rea.william@gmail.com
John C. Martin
Henry R. Barber Observatory, University of Illinois at Springfield

Abstract

Low resolution spectroscopy with a filter wheel grating (R ~ 50–200) is an effective tool to monitor Mira type variables around their cycle. In early type Mira variables changes in spectral type are reflected in the depth of TiO absorption bands relative to nearby quasi-continuum points. However, those indices lose sensitivity in spectral types later/cooler than M6 or M7. Flux ratios between quasi-continuum points are better suited to monitor a cooler Mira like R Oct at low spectral resolution through its entire cycle. We identify flux ratios between quasi-continuum points in low resolution spectra that are sensitive enough to distinguish between comparison stars of different spectral types and find cycle-to-cycle differences in the behavior of R Oct around peak brightness. Those ratios are also much more sensitive than TiO to changes from mid to minimum in the cycle even when rendered less effective by lower signal to noise levels when R Oct approached minimum brightness.