Velocity dispersion of the high rotational levels of H2

We present a study of the high rotational bands (J >= 2) of H-2 toward four early-type Galactic stars: HD 73882, HD 192639, HD 206267, and HD 207538. In each case, the velocity dispersion, characterized by the spectrum fitting parameter b, increases with the level of excitation, a phenomenon that has previously been detected by the Copernicus and IMAPS observatories. In particular, we show with 4 sigma confidence that for HD 192639 it is not possible to fit all J levels with a single b-value and that higher b-values are needed for the higher levels. The amplitude of the line broadening, which can be as high as 10 km s(-1) 1, makes explanations such as inhomogeneous spatial distribution unlikely. We investigate a mechanism in which the broadening is due to the molecules that are rotationally excited through the excess energy acquired after their formation on a grain ( H-2 formation pumping). We show that different dispersions would be a natural consequence of this mechanism. We note, however, that such a process would require a formation rate 10 times higher than what was inferred from other observations. In view of this result, and of the difficulty in accounting for the velocity dispersion as thermal broadening ( T would be around 10,000 K), we conclude then that we are most certainly observing some highly turbulent warm layer associated with the cold diffuse cloud. Embedded in a magnetic field, it could be responsible for the high quantities of CH+ measured in the cold neutral medium.