Near-Infrared H 2 and Associated O 0 and C + Emission from Dense Photon-dominated Regions

Details Near-Infrared H 2 and Associated O 0 and C + Emission from Dense Photon-dominated Regions Near-Infrared H 2 and Associated O 0 and C + Emission from Dense Photon-dominated Regions

Jun 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...482..298l&link_type=abstract

Astrophysical Journal v.482, p.298

Astronomy and Astrophysics

Astronomy

18

Infrared: Ism: Lines And Bands, Ism: Clouds, Ism: Individual Name: Orion, Ism: Molecules, Molecular Processes

Scientific paper

We have observed near-infrared H2 vibrational line emission from dense (n >~ 104 cm-3) photon-dominated regions (PDRs) along the Orion A molecular cloud "ridge" and toward the Orion B molecular cloud near NGC 2024. Specifically, we have observed the 1.601 mu m 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 H2. To assess the physical state of the PDRs, we combine our H2 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 lines of sight. We use our data to study how the character of the H2 emission depends on the PDR conditions. For the range of densities (104--105 cm-3) and incident far-ultraviolet fluxes (103--105 times the mean interstellar field) in the PDRs, we find that the relative H2 line strengths are characteristic of both low gas density and high gas density UV-excited emission. In Orion, we observe direct evidence of the transition from the (low-density) regime in which the H2 vibrational cascade is dominated by radiative transitions to the (high-density) regime in which collisions influence the vibrational cascade. Our observations show that, for Orion-type abundances and physical conditions, collisions begin to influence the H2 ro-vibrational line ratios near a gas density of 5 x 104 cm-3.

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