Toward a robust physical and chemical characterization of heterogeneous lines of sight: The case of the Horsehead nebula

Abstract

Dense cold molecular cores/filaments are surrounded by an envelope of translucent gas. Some of the low-J emission lines of CO and HCO${}^{+}$ isotopologues are more sensitive to the conditions either in the translucent environment or in the dense cold one. We propose a cloud model composed of three homogeneous slabs of gas along each line of sight (LoS), representing an envelope and a shielded inner layer. IRAM-30m data from the ORION-B large program toward the Horsehead nebula are used to demonstrate the method's capability. We use the non-LTE radiative transfer code RADEX to model the line profiles from the kinetic temperature $T_{kin}$, the volume density $n_{H_2}$, kinematics and chemical properties of the different layers. We then use a maximum likelihood estimator to simultaneously fit the lines of the CO and HCO${}^{+}$ isotopologues. We constrain column density ratios to limit the variance on the estimates. This simple heterogeneous model provides good fits of the fitted lines over a large part of the cloud. The decomposition of the intensity into three layers allows to discuss the distribution of the estimated physical/chemical properties along the LoS. About 80$\%$ the CO integrated intensity comes from the envelope, while $\sim 55\%$ of that of the (1-0) and (2-1) lines of C${}^{18}$O comes from the inner layer. The $N{(}^{13}CO)/N(C^{18}O)$ in the envelope increases with decreasing $A_v$, and reaches $25$ in the pillar outskirts. The envelope $T_{kin}$ varies from 25 to 40 K, that of the inner layer drops to $\sim 15$ K in the western dense core. The inner layer $n_{H_2}$ is $\sim 3\times 10^4{\text{cm}}^{-3}$ toward the filament and it increases by a factor $10$ toward dense cores. The proposed method correctly retrieves the physical/chemical properties of the Horsehead nebula and offers promising prospects for less supervised model fits of wider-field datasets.

References in corpus (27)

• Cold Dark Clouds: The Initial Conditions for Star Formation
• Herschel-Planck dust optical-depth and column-density maps: I. Method description and results for Orion
• The anatomy of the Orion B Giant Molecular Cloud: A local template for studies of nearby galaxies
• Low sulfur depletion in the Horsehead PDR
• Molecular Line Emission as a Tool for Galaxy Observations (LEGO). I. HCN as a tracer of moderate gas densities in molecular clouds and galaxies
• Spatially resolved l-c3h+ emission in the horsehead photodissociation region: Further evidence for a top-down hydrocarbon chemistry
• Formation, fractionation and excitation of carbon monoxide in diffuse clouds
• HCO mapping of the Horsehead : Tracing the illuminated dense molecular cloud surfaces
• First detection of interstellar S2H
• Electron Excitation of High Dipole Moment Molecules Reexamined
• On the chemistry and distribution of HOC+ in M82: More evidence for extensive PDRs
• LEGO II: A 3 mm molecular line study covering 100 pc of one of the most actively star-forming portions within the Milky Way Disc
• [CII] emission from L1630 in the Orion B molecular cloud
• The Relationship Between the Dust and Gas-Phase CO Across the California Molecular Cloud
• Characterizing the line emission from molecular clouds. Stratified random sampling of the Perseus cloud
• SCUBA observations of the Horsehead Nebula - what did the horse swallow?
• Star Formation Occurs in Dense Gas, but What Does "Dense" Mean?
• Ion-molecule reactions involving HCO${}^{+}$ and N${}_2$H${}^{+}$: Isotopologue equilibria from new theoretical calculations and consequences for interstellar isotope fractionation
• A dynamically young, gravitationally stable network of filaments in Orion B
• The chemical structure of young high-mass star-forming clumps: (II) parsec-scale CO depletion and deuterium fraction of $\mathrm{HC}{\mathrm{O}}^{+}$
• Characterizing the line emission from molecular clouds. II. A comparative study of California, Perseus, and Orion A
• A Complete HCN Survey of the Perseus Molecular Cloud
• HCN emission from translucent gas and UV-illuminated cloud edges revealed by wide-field IRAM 30m maps of Orion B GMC: Revisiting its role as tracer of the dense gas reservoir for star formation
• The extremely sharp transition between molecular and ionized gas in the Horsehead nebula
• Deep learning denoising by dimension reduction: Application to the ORION-B line cubes
• Gas kinematics around filamentary structures in the Orion B cloud
• JWST observations of the Horsehead photon-dominated region I. First results from multi-band near- and mid-infrared imaging

Cited by in corpus (2)

• Estimating the dense gas mass of molecular clouds using spatially unresolved 3 mm line observations
• Spatial distribution of organics in the Horsehead nebula: Signposts of chemistry driven by atomic carbon