Herschel / HIFI spectral line survey of the Orion Bar - Temperature and density differentiation near the PDR surface

Abstract

Photon Dominated Regions (PDRs) are interfaces between the mainly ionized and mainly molecular material around young massive stars. Analysis of the physical and chemical structure of such regions traces the impact of far-ultraviolet radiation of young massive stars on their environment. We present results on the physical and chemical structure of the prototypical high UV-illumination edge-on Orion Bar PDR from an unbiased spectral line survey with a wide spectral coverage. A spectral scan from 480-1250 GHz and 1410-1910 GHz at 1.1 MHz resolution was obtained by the HIFI instrument onboard the Herschel Space Observatory. For molecules with multiple transitions we used rotational diagrams to obtain excitation temperatures and column densities. For species with a single detected transition we used an optically thin LTE approximation. In case of species with available collisional rates, we also performed a non-LTE analysis to obtain kinetic temperatures, H2 volume densities, and column densities. About 120 lines corresponding to 29 molecules (including isotopologues) have been detected in the Herschel/HIFI line survey, including 11 transitions of CO, 7 transitions of 13CO, 6 transitions of C18O, 10 transitions of H2CO, and 6 transitions of H2O. Most species trace kinetic temperatures in the range between 100 and 150 K and H2 volume densities in the range between 10^5 and 10^6 cm^-3. The species with temperatures and / or densities outside of this range include the H2CO transitions tracing a very high temperature (315 K) and density (1.4x10^6 cm^-3) component and SO corresponding to the lowest temperature (56 K) measured as a part of this line survey. The observed lines/species reveal a range of physical conditions (gas density /temperature) involving structures at high density / high pressure, obsoleting the traditional 'clump / interclump' picture of the Orion Bar.

arXiv commentAccepted for publication in A&A, abstract abridged

References in corpus (34)

• Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy
• A computer program for fast non-LTE analysis of interstellar line spectra
• The distance to the Orion Nebula
• Rotational quenching of CO due to H${}_2$ collisions
• Distance to Orion KL Measured with VERA
• A line confusion limited millimeter survey of Orion KL (I): sulfur carbon chains
• Compression and ablation of the photo-irradiated cloud the Orion Bar
• Herschel observations of EXtra-Ordinary Sources: the present and future of spectral surveys with Herschel/HIFI
• Herschel observations of EXtra-Ordinary Sources: Analysis of the HIFI 1.2 THz Wide Spectral Survey Toward Orion KL I. Methods
• The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR
• Herschel/HIFI observations of [CII] and [13CII] in PDRs
• Rotational quenching of H2CO by molecular hydrogen: cross-sections, rates, pressure broadening
• The chemistry of ions in the Orion Bar I. - CH+, SH+, and CF+: The effect of high electron density and vibrationally excited H2 in a warm PDR surface
• Astrochemistry of Sub-Millimeter Sources in Orion: Studying the Variations of Molecular Tracers with Changing Physical Conditions
• Discovery of Interstellar CF+
• Deuterium chemistry in the Orion Bar PDR - "warm" chemistry starring CH2D+
• Evolution of dust in the Orion Bar with Herschel: I. Radiative transfer modelling
• OH emission from warm and dense gas in the Orion Bar PDR
• The three dimensional dynamic structure of the inner Orion Nebula
• Herschel observations of interstellar chloronium
• Chemical stratification in the Orion Bar: JCMT Spectral Legacy Survey observations
• APEX 1 mm line survey of the Orion Bar
• Spatially extended OH+ emission from the Orion Bar and Ridge
• Modelling clumpy PDRs in 3D - Understanding the Orion Bar stratification
• C${}_2$H observations toward the Orion Bar
• Detection of HF emission from the Orion Bar
• Herschel Search for O2 Toward the Orion Bar
• First NH3 detection of the Orion Bar
• Submillimeter CO Line Emission from Orion
• A non-equilibrium ortho-to-para ratio of water in the Orion PDR
• Analysis of the Herschel/HEXOS Spectral Survey Towards Orion South: A massive protostellar envelope with strong external irradiation
• Physical and chemical differentiation of the luminous star-forming region W49A - Results from the JCMT Spectral Legacy Survey
• The HIFI spectral survey of AFGL 2591 (CHESS). III. Chemical structure of the protostellar envelope
• The effect of sideband ratio on line intensity for Herschel/HIFI

Cited by in corpus (23)

• Photodissociation and X-Ray Dominated Regions
• Structure of photodissociation fronts in star-forming regions revealed by observations of high-J CO emission lines with Herschel
• STATCONT: A statistical continuum level determination method for line-rich sources
• Complex organic molecules in strongly UV-irradiated gas
• FEEDBACK: a SOFIA Legacy Program to Study Stellar Feedback in Regions of Massive Star Formation
• State-to-state chemistry and rotational excitation of CH${}^{+}$ in photon-dominated regions
• Modelling clumpy PDRs in 3D - Understanding the Orion Bar stratification
• Spatially resolved images of reactive ions in the Orion Bar
• Bottlenecks to interstellar sulfur chemistry: Sulfur-bearing hydrides in UV-illuminated gas and grains
• Synthetic observations of star formation and the interstellar medium
• The water line emission and ortho-to-para ratio in the Orion Bar photon-dominated region
• The KOSMA-$\tau$ PDR Model -- I. Recent updates to the numerical model of photo-dissociated regions
• Origin of hydrogen fluoride emission in the Orion Bar: An excellent tracer for CO-dark H2 gas clouds
• OH mid-infrared emission as a diagnostic of H${}_2$O UV photodissociation. II. Application to interstellar PDRs
• The role of highly vibrationally excited H2 initiating the N chemistry: Quantum study and 3-sigma detection of NH emission in the Orion Bar PDR
• PDRs4All. XII. FUV-driven formation of hydrocarbon radicals and their relation with PAHs
• Electron-impact excitation of diatomic hydride cations II: OH${}^{+}$ and SH${}^{+}$
• Merged H/H2 and C+/C/CO transitions in the Orion Bar
• Etching glass in the early Universe: Luminous HF and water emission in a QSO-SMG pair at z=4.7
• CF${}^{+}$ excitation in the interstellar medium
• Herschel and Odin observations of H2O, CO, CH, CH+, and NII in the barred spiral galaxy NGC 1365. Bar-induced activity in the outer and inner circumnuclear tori
• Detection of Dust Condensations in the Orion Bar Photon-dominated Region
• First detection of CF${}^{+}$ in the Large Magellanic Cloud