ABSTRACT
CONTEXT: In bright photodissociation regions (PDRs) associated to massive star formation, the presence of dense "clumps" that are immersed in a less dense interclump medium is often proposed to explain the difficulty of models to account for the observed gas emission in high-excitation lines. AIMS: We aim at presenting a comprehensive view of the modeling of the CO rotational ladder in PDRs, including the high-J lines that trace warm molecular gas at PDR interfaces. METHODS: We observed the 12CO and 13CO ladders in two prototypical PDRs, the Orion Bar and NGC 7023 NW using the instruments onboard Herschel. We also considered line emission from key species in the gas cooling of PDRs (C+, O, H2) and other tracers of PDR edges such as OH and CH+. All the intensities are collected from Herschel observations, the literature and the Spitzer archive and are analyzed using the Meudon PDR code. RESULTS: A grid of models was run to explore the parameter space of only two parameters: thermal gas pressure and a global scaling factor that corrects for approximations in the assumed geometry. We conclude that the emission in the high-J CO lines, which were observed up to J up =23 in the Orion Bar (J up =19 in NGC 7023), can only originate from small structures of typical thickness of a few 10-3 pc and at high thermal pressures (Pth ~ 108 K cm-3). CONCLUSIONS: Compiling data from the literature, we found that the gas thermal pressure increases with the intensity of the UV radiation field given by G0, following a trend in line with recent simulations of the photoevaporation of illuminated edges of molecular clouds. This relation can help rationalising the analysis of high-J CO emission in massive star formation and provides an observational constraint for models that study stellar feedback on molecular clouds.
ABSTRACT
The CO+ reactive ion is thought to be a tracer of the boundary between a HII region and the hot molecular gas. In this study, we present the spatial distribution of the CO+ rotational emission toward the Mon R2 star-forming region. The CO+ emission presents a clumpy ring-like morphology, arising from a narrow dense layer around the HII region. We compare the CO+ distribution with other species present in photon-dominated regions (PDR), such as [CII] 158 µm, H2 S(3) rotational line at 9.3 µm, polycyclic aromatic hydrocarbons (PAHs) and HCO+. We find that the CO+ emission is spatially coincident with the PAHs and [CII] emission. This confirms that the CO+ emission arises from a narrow dense layer of the HI/H2 interface. We have determined the CO+ fractional abundance, relative to C+ toward three positions. The abundances range from 0.1 to 1.9 ×10-10 and are in good agreement with previous chemical model, which predicts that the production of CO+ in PDRs only occurs in dense regions with high UV fields. The CO+ linewidth is larger than those found in molecular gas tracers, and their central velocity are blue-shifted with respect to the molecular gas velocity. We interpret this as a hint that the CO+ is probing photo-evaporating clump surfaces.
ABSTRACT
We present results from a comprehensive submillimeter spectral survey toward the source Orion South, based on data obtained with the HIFI instrument aboard the Herschel Space Observatory, covering the frequency range 480 to 1900 GHz. We detect 685 spectral lines with S/N > 3σ, originating from 52 different molecular and atomic species. We model each of the detected species assuming conditions of Local Thermodynamic Equilibrium. This analysis provides an estimate of the physical conditions of Orion South (column density, temperature, source size, & V LSR ). We find evidence for three different cloud components: a cool (T ex ~ 20 - 40 K), spatially extended (> 60â³), and quiescent (ΔVFWHM ~ 4 km s -1) component; a warmer (T ex ~ 80 - 100 K), less spatially extended (~ 30â³), and dynamic (ΔVFWHM ~ 8 km s -1) component, which is likely affected by embedded outflows; and a kinematically distinct region (T ex > 100 K; V LSR ~ 8 km s -1), dominated by emission from species which trace ultraviolet irradiation, likely at the surface of the cloud. We find little evidence for the existence of a chemically distinct "hot core" component, likely due to the small filling factor of the hot core or hot cores within the Herschel beam. We find that the chemical composition of the gas in the cooler, quiescent component of Orion South more closely resembles that of the quiescent ridge in Orion-KL. The gas in the warmer, dynamic component, however, more closely resembles that of the Compact Ridge and Plateau regions of Orion-KL, suggesting that higher temperatures and shocks also have an influence on the overall chemistry of Orion South.
