How Mira Variables Change Visual Light by a Thousand-fold

Abstract

Mira variables change visual light by up to 8 magnitudes over their roughly yearly cycle. Here we present a simple explanation for the extremely large amplitudes of light curves of oxygen-rich Mira variables. Metallic oxides, such as TiO, form throughout the stellar atmosphere as the star cools when approaching minimum light. When this happens, the visual light can be almost completely absorbed at large radii, extending the visual photosphere to nearly twice its nominal size. At these large radii, temperatures can fall to approximately 1400 K and essentially all of the star's radiation emerges in the infrared. Since almost no optical light is emitted at these low temperatures, Mira variables can decrease their visual light by more than a thousand-fold and almost "disappear" to the human eye.