Molecular-Cloud-Scale Chemical Composition III: Constraints of Average Physical Properties through Chemical Models

Abstract

It is important to understand the origin of molecular line intensities and chemical composition in the molecular-cloud scale in the Galactic sources because it serves as a benchmark to compare with the chemical compositions of extragalactic sources. Recent observations of the 3-mm spectra averaged over the 10-pc scale show similar spectral pattern among sources for molecular lines HCN, HCO${}^{+}$, CCH, HNC, HNCO, c-C${}_3$H${}_2$, CS, SO, N${}_2$H${}^{+}$, and CN. To constrain the average physical property emitting such spectral pattern, we model molecular spectra using a time-dependent gas-grain chemical model followed by a radiative transfer calculation. We use a grid of physical parameters such as the density $n=3\times 10^2-3\times 10^4$ cm${}^{-3}$, the temperature, $T=10-30$ K, the visual extinction $A_{\mathrm{V}}=2,4,10$ mag, the cosmic-ray ionization rate $\zeta =10^{-17}-10^{-16}$ s${}^{-1}$, and the sulfur elemental abundance $S/H=8\times 10^{-8}-8\times 10^{-7}$. Comparison with the observed spectra indicates that spectra are well reproduced with the relatively low density of $n=(1-3)\times 10^3$cm${}^{-3}$, $T=10$K, $\zeta =10^{-17}$ s${}^{-1}$, and the short chemistry timescale of $10^5$ yrs. This short chemistry timescale may indicate that molecular clouds are constantly affected by the turbulence, and exposed to low-density, low $A_{\mathrm{V}}$ regions that "refreshes" the chemical clock by UV radiation. The relatively low density obtained is orders of magnitude lower than the commonly-quoted critical density in the optically thin case. Meanwhile, this range of density is consistent with results from recent observational analysis of molecular-cloud-scale mapping.

arXiv comment30 pages, 21 figures, accepted for publication in The Astrophysical Journal

References in corpus (40)

• A computer program for fast non-LTE analysis of interstellar line spectra
• From filamentary clouds to prestellar cores to the stellar IMF: Initial highlights from the Herschel Gould Belt survey
• Formation of methyl formate and other organic species in the warm-up phase of hot molecular cores
• The Spitzer ice legacy: Ice evolution from cores to protostars
• Rapid Formation of Molecular Clouds and Stars in the Solar Neighborhood
• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• Our astrochemical heritage
• Non-thermal desorption from interstellar dust grains via exothermic surface reactions
• Polycylcic Aromatic Hydrocarbons (PAH's) in dense cloud chemistry
• The Formation and Destruction of Molecular Clouds and Galactic Star Formation
• Formation of Massive Molecular Cloud Cores by Cloud-cloud Collision
• Chemistry in Disks. IV. Benchmarking gas-grain chemical models with surface reactions
• Molecular Cloud Formation Behind Shock Waves
• 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 in NGC1068 imaged with ALMA: II. The chemistry of the dense molecular gas
• Molecules as tracers of galaxy evolution: an EMIR survey. I. Presentation of the data and first results
• Molecular Line Emission as a Tool for Galaxy Observations (LEGO). I. HCN as a tracer of moderate gas densities in molecular clouds and galaxies
• ALMA Multi-line Imaging of the Nearby Starburst Galaxy NGC 253
• Ambipolar-Diffusion Timescale, Star-Formation Timescale, and the Ages of Molecular Clouds: Is There a Discrepancy?
• Testing the universality of the star formation efficiency in dense molecular gas
• Lambda = 3 mm line survey of nearby active galaxies
• Cold CO in circumstellar disks: On the effects of photodesorption and vertical mixing
• Water deuteration and ortho-to-para nuclear spin ratio of H2 in molecular clouds formed via the accumulation of HI gas
• Multi-molecule ALMA observations towards the Seyfert 1 galaxy NGC 1097
• Chemistry in Cosmic-Ray Dominated Regions (CRDRs)
• Electron Excitation of High Dipole Moment Molecules Reexamined
• Distributions of molecules in the circumnuclear disk and surrounding starburst ring in the Seyfert galaxy NGC 1068 observed with ALMA
• Molecular tracers of PDR-dominated galaxies
• Spectral Line Survey toward Spiral Arm of M51 in the 3 mm and 2 mm Bands
• ALMA Astrochemical Observations of the Infrared-Luminous Merger NGC 3256
• Molecular-Cloud-Scale Chemical Composition I: Mapping Spectral Line Survey toward W51 in the 3 mm Band
• Spectral Line Survey toward Molecular Clouds in the Large Magellanic Cloud
• Water deuterium fractionation in the high-mass star-forming region G34.26+0.15 based on Herschel/HIFI data
• Molecular-Cloud-Scale Chemical Composition II: Mapping Spectral Line Survey toward W3(OH) in the 3 mm Band
• A Multi-Transition Study of Molecules toward NGC 1068 based on High-Resolution Imaging Observations with ALMA
• Molecular transitions as probes of the physical conditions of extragalactic environments
• A Molecular Line Survey toward the Nearby Galaxies NGC 1068, NGC 253, and IC 342 at 3 mm with the Nobeyama 45-m Radio Telescope: Impact of an AGN on 1 kpc Scale Molecular Abundances
• Physical and chemical differentiation of the luminous star-forming region W49A - Results from the JCMT Spectral Legacy Survey
• ALMA observations of the dense and shocked gas in the nuclear region of NGC 4038 (Antennae galaxies)

Cited by in corpus (16)

• Surface Diffusion of Carbon Atoms as a Driver of Interstellar Organic Chemistry
• Enhanced UV radiation and dense clumps in Mrk231's molecular outflow
• A molecular absorption line survey toward the AGN of Hydra-A
• Incorporating Astrochemistry into Molecular Line Modelling via Emulation
• Chemical evolution along the circumnuclear ring of M83
• On the density regime probed by HCN emission
• The MALATANG Survey: Dense Gas and Star Formation from High Transition HCN and HCO+ maps of NGC253
• ALMA observations of CS in NGC 1068: chemistry and excitation
• A 3 mm Spectral Line Survey toward the Barred Spiral Galaxy NGC 3627
• CON-quest II. Spatially and spectrally resolved HCN/HCO+ line ratios in local luminous and ultraluminous infrared galaxies
• A nearly constant CN/HCN line ratio in nearby galaxies: CN as a new tracer of dense gas
• Penetration of Non-energetic Hydrogen Atoms into Amorphous Solid Water and their Reaction with Embedded Benzene and Naphthalene
• UCLCHEMCMC: A MCMC Inference tool for Physical Parameters of Molecular Clouds
• Atomic Carbon in the Central Molecular Zone of the Milky Way : Possible Cosmic-ray Induced Chemistry or Time-Dependent Chemistry Associated with SNR Sagittarius A East
• Surveying the Whirlpool at Arcseconds with NOEMA (SWAN) II: Survey design and observations
• Molecular Gas Density Measured with H${}_2$CO and CS toward a Spiral Arm of M51