Velocity Dispersion of the High Rotational Levels of H2
We present a study of the high rotational bands (J>=2) ofH2 toward four early-type Galactic stars: HD 73882, HD192639, HD 206267, and HD 207538. In each case, the velocity dispersion,characterized by the spectrum fitting parameter b, increases with thelevel of excitation, a phenomenon that has previously been detected bythe Copernicus and IMAPS observatories. In particular, we show with 4σ confidence that for HD 192639 it is not possible to fit all Jlevels with a single b-value and that higher b-values are needed for thehigher levels. The amplitude of the line broadening, which can be ashigh as 10 km s-1, makes explanations such as inhomogeneousspatial distribution unlikely. We investigate a mechanism in which thebroadening is due to the molecules that are rotationally excited throughthe excess energy acquired after their formation on a grain(H2 formation pumping). We show that different dispersionswould be a natural consequence of this mechanism. We note, however, thatsuch a process would require a formation rate 10 times higher than whatwas inferred from other observations. In view of this result, and of thedifficulty in accounting for the velocity dispersion as thermalbroadening (T would be around 10,000 K), we conclude then that we aremost certainly observing some highly turbulent warm layer associatedwith the cold diffuse cloud. Embedded in a magnetic field, it could beresponsible for the high quantities of CH+ measured in thecold neutral medium.
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