Fig 14.15.

Hydrogen column density NH as a function of visual extinction AV .Though there is some scatter, there is a good correlation in the two quantities as long as AV is less than one magnitude.

and cool (a situation somewhat similar to the solar corona). It has been noted that if we compare the pressure within a cloud Pc with the pressure in the intercloud medium Pic, we find

Pcl = nclTcl

The pressure in the clouds and intercloud medium is approximately the same. Some theoreticians have proposed a picture of the interstellar medium known as the two-phase model, in which this equality of pressures is not a coincidence, but follows from the ways in which the gas can cool. The two-phase model is now considered overly simplified and has been replaced by more dynamic pictures of the interstellar medium.

For clouds that are near enough to be seen optically, it was found that the 21 cm emission often follows the optical obscuration of the dust. This suggests that the gas and dust are well mixed. This idea was tested in detail by seeing the degree to which the hydrogen column density NH correlates with the visual extinction AV. The results of these studies are shown in Fig. 14.15. There is some scatter in the data, but it is clear that NH and AV are related. The general ratio Nh/Av is approximately 1021 atoms cm~2/1 mag.

This ratio was found to hold as long as the extinction is less than 1 mag. When the extinction becomes higher, the relationship no longer holds. This was a mystery for many years. Two possible solutions were proposed. One was that with a lot of dust, very little radiation can penetrate to heat the cloud. It is possible that the hydrogen is so cold that the emission lines are just very weak. The other possibility is that under the higher extinction conditions, pairs of hydrogen atoms combine to form molecular hydrogen, H2. Molecular hydrogen obviously has a very different structure than atomic hydrogen, and has no equivalent of the 21 cm line. In fact, cold molecular hydrogen has no emission or absorption lines in the radio or visible parts of the spectrum. The recent discovery of a large number of

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