The first interstellar molecule to be discovered at radio wave¬lengths, the OH RADICAL, is still the most difficult to understand. Although in some dark clouds the OH molecule behaves in a simple fashion, there are many locations in the Galaxy where the OH emission shows characteristics quite unlike those of a normal inter¬stellar molecule. The OH EMISSION LINES in these sources consist of a group of very narrow spectral lines instead of a single broad line (figure 13.14); these narrow lines are usually polarized and vary over a period of months. The emission appears to come from very small volumes; a typical OH source displays emission from a dozen or more ‘hot spots’, each 5—10 AU across and about 1000 AU apart, whereas a normal molecular cloud is something like a million AU across (figure 13.15). Finally, the strengths of these lines are mil¬lions of times stronger than would be expected if the OH molecules in an interstellar cloud behaved normally. The implication of these observations is that the OH lines are being amplified by some means. The process must be akin to that in a MASER (an acronym for Microwave Amplification by Stimulated Emission of Radiation). By a process that is not yet fully understood, the OH molecules absorb energy, probably from collisions, or from infrared or ultraviolet radiation fields, and convert some of it into radiation at a wavelength of 18 cm. Theoretical calculations show that this masering process can take place only under special conditions of density, temperature, motion and magnetic field, though astronomers differ in their opinions as to just what these conditions are.

The original OH MASERS were found at a wavelength of 18cm. Subsequently maser emission from OH molecules has been found at other, shorter, wavelengths. Several other molecules are now known to exhibit maser action as well as OH; the most notable of these are water vapour, H20, and silicon monoxide, SiO. Water-vapour masers are even more powerful than OH masers; the source called W49, for example, emits as much power in the single spectral transition at 1.35cm wavelength as does the Sun at all wavelengths. Hydroxyl, water-vapour and silicon-monoxide masers are usually found close to each other in the sky, but are not actually coincident.

There appear to be two main types of OH maser source, namely those associated with late-type stars and those associated with H+ regions. Only certain late-type stars are QH sources, namely some M supergiants such as NML Cygnus and VY CMa, and some Mira variables such as R Gas. These OH stars are all cool, red objects with oxygen-rich atmospheres. Almost all have infrared emission which comes from a shell of hot dust around the star. Two common characteristics of this type of maser are that they regularly vary hi strength with a period of about a year, and that the OH lines are double, with the two components separated by a frequency which corresponds to a Doppler shift of 20-40 km s”1. The reason for this splitting is still very much a mystery, though it must be somehow related to the expansion or rotation of the cloud of OH molecules surrounding the star.

The OH and H20 masers that are found in H+ regions appear to be connected with an early stage of star formation. They are almost always coincident with either a very compact H+ region, a strong infrared source, or both. The infrared sources are probably the hot central regions of protostars, which are clouds in the process of collapsing to form new stars. Eventually, the study of these masers may give us a great deal of information about the velocities, temperatures and densities in a protostar, but at the moment our lack of understanding of the maser process itself greatly hinders such an interpretation of the data.

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