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OH/IR Maser
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The gas cloud from a Mira-type star
can give rise to hydoxyl ions (OH) and SiO (silicon monoxide). The dust
formed from silicon monoxide heats up and excites the hydroxyl gving
rise to an inverted population of elevated molecular states. This
inverted population can form a natural maser resulting in very strong
emission at radio frequencies (1612 [strong] and 1667 [weaker] MHz).
Many of these sources show silicate emission while others exhibit
silicate absorption. They may also show pulsations. An
OH/IR signal is
evidence against a possible Dyson sphere. For more information
on astronomical masers see the Hartebeesthoek
radio telescope site.
The image on the left of a starfish preplanetary
nebula IRAS 19024+0044 was made with the
NICMOS infrared camera on the Hubble. Note that the field of view is
very small compared to a normal IRAS image. The object was also imaged
using radiometric interference at Owens Valley and optically with Mount
Palomar and Keck. This spectacular tour de force was carried out by R.
Sahai , C. Contreras , and M. Morris and
is reported in "A Starfish Preplanetary Nebula:1 IRAS
19024+0044", Astrophysical Journal, V620, 948, (2005).
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