Large-Scale CO and [C I] Emission in the ρ Ophiuchi Molecular Cloud

We present a comprehensive study of the ρ Ophiuchi molecular cloud that addresses aspects of the physical structure and condition of the molecular cloud and its photodissociation region (PDR) by combining far-infrared and submillimeter-wave observations with a wide range of angular scale and resolution. We present 1600 arcmin² maps (2.3 pc²) with 0.1 pc resolution in submillimeter CO (4 → 3) and [C I] (³P₁ → ³P₀) line emission from the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) and pointed observations in the CO (7 → 6) and [C I] ( ³P₂ → ³P₁) lines. Within the large-scale maps, smaller spectral line maps of 3000 AU resolution over ~90 arcmin² (0.2 pc²) of the cloud in CO, CS, HCO⁺, and their rare isotopomers are made at the Heinrich Hertz Telescope (HHT) in Arizona. Comparison of CO, HCO⁺, and [C I] maps with far-infrared observations of atomic and ionic species from the Infrared Space Observatory (ISO) far-infrared and submillimeter continuum emission and near-infrared H₂ emission allows clearer determination of the physical and chemical structure of the ρ Oph PDR, since each species probes a different physical region of the cloud structure. Although a homogeneous plane-parallel PDR model can reproduce many of the observations described here, the excitation conditions needed to produce the observed HCO⁺ and [O I] emission imply inhomogeneous structure. Strong chemical gradients are observed in HCO⁺ and CS; the former is ascribed to a local enhancement in the H₂ ionization rate, and the latter is principally due to shocks. Under the assumption of a simple two-component gas model for the cloud, we find that [C II] and [C I] emission predominantly arises from the lower density envelopes (10³-10⁴ cm^-3) that surround denser cloud condensations, or "clumps." The distribution of [C I] is very similar to that of C¹⁸O and is generally consistent with illumination from the "far" side of the cloud. A notable exception is found at the western edge of the cloud, where UV photons create a PDR viewed "edge-on." The abundance of atomic carbon is accurately modeled using a radiation field that decreases with increasing projected distance from the exciting star HD 147889 and a total gas column density that follows that of C¹⁸O, decreasing toward the edges of the cloud. In contrast to the conclusions of other studies, we find that no nonequilibrium chemistry is needed to enhance the [C I] abundance. Each spectral line is traced to a particular physical component of the cloud and PDR. Although CO rotational line emission originates from both dense condensations and diffuse envelopes, the millimeter-wave transitions mostly find their origins in envelope material, whereas the high-J submillimeter lines stem more from the dense clumps. Submillimeter HCO⁺ and infrared [O I] and [C II] emission indicate clump surface temperatures of 50-200 K, an ultraviolet radiation field with I_UV ≈ 10-90, densities of 10⁵-10⁶ cm^-3, and interior temperatures of ≤20 K. This study highlights the value of large-scale infrared and submillimeter mapping for the interpretation of molecular cloud physical and chemical structure, and important future observations are highlighted.