Formation and destruction rates of interstellar H2.

The interstellar ultraviolet field near 1000 A is calculated at several locations within 200 pc of the Sun. We include the possibility that interstellar clouds are mutually self-shielding in the Lyman bands, and at the Sun the computed H2 photodestruction rate is 2 >c 10-11 1 Also, a value of the H2 formation rate, R, of 10-17cm3 s 1is consistent with observations toward stars in which n(H2) is very low. Using these rates, we find that the total densities of hydrogen nuclei within observed clouds are 20(10- `7/R) and, therefore, "standard clouds" may be 10 percent H2. Our results also apply to HD; and by assuming a deuterium abundance, we can infer that n(H+) < 10-2 . Consequently, the hydrogen ionization rate is probably not much higher thanabout5 x 1. Subject headings: interstellar matter - molecular, interstellar - ultraviolet