NEAR-INFRARED H-2 AND ASSOCIATED O-0 AND C-DOMINATED REGIONS( EMISSION FROM DENSE PHOTON)

We have observed near-infrared H-2 vibrational line emission from dens e (n greater than or similar to 10(4) cm(-3)) photon-dominated regions (PDRs) along the Orion A molecular cloud ''ridge'' and toward the Ori on B molecular cloud near NGC 2024. Specifically, we have observed the 1.601 pm v = 6-4 Q(1), 2.121 mu m v = 1-0 S(1), and 2.247 mu m v = 2- 1 S(1) lines of H-2. To assess the physical state of the PDRs, we comb ine our H-2 data with observations of the [O I] 63 mu m, [O I] 146 mu m, and [C II] 158 mu m atomic fine-structure lines along the same line s of sight. We use our data to study how the character of the H-2 emis sion depends on the PDR conditions. For the range of densities (10(4)- 10(5) cm(-3)) and incident far-ultraviolet fluxes (10(3)-10(5) times t he mean interstellar field) in the PDRs, we find that the relative H-2 line strengths are characteristic of both low gas density and high ga s density UV-excited emission. In Orion, we observe direct evidence of the transition from the (low-density) regime in which the H-2 vibrati onal cascade is dominated by radiative transitions to the (high-densit y) regime in which collisions influence the vibrational cascade. Our o bservations show that, for Orion-type abundances and physical conditio ns,collisions begin to influence the H-2 ro-vibrational line ratios ne ar a gas density of 5 x 10(4) cm(-3).