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Spectra of the hydroxyl maser from an OH/IR Star

In Mira-type 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 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. A number of these masers were discovered with the 26m Hartebeesthoek telescope, including the one shown below.

The intensity of the maser follows the changing brightness of the star as it pulsates. This is shown in the image above.

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. In this case the expansion velocity is about 21 km/s.

The changing brightness of the left and right maser peaks seen in the spectra above over a ten year period are shown below.

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. In this case the time difference is more than a month. As radio waves travel at the speed of light (300000 km/s), this means that the hydroxyl masers around this star occur in a shell with a diameter about 100 times that of the solar system (taken as Pluto's orbit).