Molecular-cloud-scale Chemical Composition. III. Constraints of Average Physical Properties through Chemical Models

It is important to understand the origin of molecular line intensities and chemical composition at the molecular-cloud scale in Galactic sources because it serves as a benchmark to compare with the chemical compositions of extragalactic sources. Recent observations of 3-mm spectra averaged over a 10 pc scale show similar spectral patterns 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 properties emitting such spectral patterns, 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 × 10 ² -3 × 10 ⁴ cm ^-3 , temperature, T = 10-30 K, visual extinction A _V = 2, 4, 10 mag, cosmic-ray ionization rate ζ = 10 ^-17 -10 ^-16 s ^-1 , and sulfur elemental abundance S/H = 8 ×10 ^-8 -8 × 10 ^-7 . A comparison with observations indicates that spectra are well reproduced with a relatively low density of n = (1-3) × 10 ³ cm ^-3 , T = 10 K, ζ = 10 ^-17 s ^-1 , and a short chemistry timescale of 10 ⁵ yr. This short chemistry timescale may indicate that molecular clouds are constantly affected by turbulence, and exposed to low-density, low-A _V regions that "refresh" 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. This range of density is consistent with results from recent observational analysis of molecular-cloud-scale mapping.