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In Mira variable stars the outer atmosphere of the stars blows away as a strong wind and can be thrown off at the peaks of the pulsations. The gas in the circumstellar envelope condenses rapidly as it blows away from the star, forming molecules such as water and silicon monoxide. The SiO molecules combine to create grains of dust. The water molecules can be split by ultraviolet light from other stars to form hydroxyl molecules. The heat from the warm dust excites the hydroxyl. The excited hydroxyl molecules give rise to maser action in the lines at 1667 and 1612 MHz.
Miras with short pulsation periods (about one year) and low mass loss rates produce weak masers in the 1667 MHz line. However, in Miras with a high mass loss rate and long pulsation periods (up to six years), the 1612 MHz hydroxyl masers becomes much stronger than the 1667 masers. They produce a characteristic U-shaped emission line shape from the shell of gas expanding away from the star. These stars are known as OH/IR stars for their strong hydroxyl (OH) masers and strong infrared (IR) emission from the shell of warm gas.
The intensity of the maser follows the changing brightness of the star as it pulsates.
The difference in the velocities of the two peaks is equal to twice the velocity with which the shell of gas and dust given off by the star is expanding. A typical expansion velocity is about 20 km s-1. The left peak rises before the right peak and also falls before the right peak. This is a result of the extra time it take the radio waves to cross from the far side of the shell of masers to the near side.