HYDROGEN MOLECULES IN QUASAR BROAD-LINE REGIONS

We reconsider the question of molecule formation deep within quasar broad-line-region (BLR) clouds. Previous studies have shown that the transition from H to H2 is essentially complete at column densities N(gas) approximately 10(23) cm-2, a result that is consistent with numerous analyses of BLR atomic line spectra. However, earlier work ignored the rapid photodetachment of H- by the intense radiation flux between 0.75 and 13.6 eV. The negative hydrogen ion participates in the H-2 formation pathway that is most important in the BLR environment, and our calculations reveal an order-of-magnitude decrease in the H-2 abundance at large optical depths when the photodetachment process is considered. Therefore we first explore the contribution of collisions between excited and ground-state atomic hydrogen to molecule formation, because Lyman-line trapping greatly enhances the excited-state populations. Neither the radiative association of H(n = 1) and H*(n = 2), nor the associative ionization of H(n = 1) and H*(n = 2) or H*(n = 3), yields a significant enhancement of the total H-2 abundance in the neutral zone. We then explore the likely importance of small amounts of dust (which is efficient in shielding the H-) in BLR clouds. The H-to-H-2 conversion again can be essentially completed at gas column densities approximately 10(23) cm-2 if a dust-to-gas abundance only 1/10 of that characteristic of our Galaxy's interstellar matter exists in BLR clouds.