How Mira Variables Change Visual Light by a Thousandfold

Mira variables change visual light by up to 8 magnitudes over their roughlyyearly cycle. Here we present a simple explanation for the extremely largeamplitudes of light curves of oxygen-rich Mira variables. Metallic oxides, suchas TiO, form throughout the stellar atmosphere as the star cools whenapproaching minimum light. When this happens, the visual light can be almostcompletely absorbed at large radii, extending the visual photosphere to nearlytwice its nominal size. At these large radii, temperatures can fall toapproximately 1400 K and essentially all of the star's radiation emerges in theinfrared. Since almost no optical light is emitted at these low temperatures,Mira variables can decrease their visual light by more than a thousand-fold andalmost "disappear" to the human eye.Comment: 16 pages including 4 figures To appear in ApJ 568 (April 1,2002