The dark (AV ≥ 10), cold (Tkin = 10 K), dense (nH2 ≥ 104 cm-3) molecular gas along the Taurus Molecular Cloud ridge (TMC-1) represents a quiescent astrophysical environment where chemical kinetic models consistently reproduce the observed chemical composition and physical structure. Toward the peak in the cyanopolyyne emission (TMC-1 CP), the emission line spectra of cyanoacetylene and methanol reveal chemical and physical heterogeneity on very small (&lt; 0.1 pc) scales. We generate 9-D grid structured of homogeneous rate law solutions using a three-phase rate equations approach to obtain all solutions spanning the physical conditions traced by cyanoacetylene and methanol emission along this line of sight. We compare the agreement between observed abundances and the kinetic models using both an unweighted fit method, which quantifies the average factor of agreement between a modeled and observed abundance, and a weighted fit method, which expresses the mean deviation between a modeled and observed abundance in light of the uncertainties in the modeled emission spectrum. The image shows mean log abundance differences A(t), with unit value line A(t) = 1 in solid black for group S over a restricted grid space. The linestyle corresponds to the cosmic ray ionization: solid (ζ = 10-17 s-1), dashed (ζ = 10-16.5 s-1), dot-dashed (ζ = 10-16 s-1) while the color indicates the value of density: blue (n = 105 cm-3), cyan (n = 104.5 cm-3), and red (n = 104 cm-3). For each method, we identify trends in the best fit solution sets over the free parameter ranges and determine the consistency among the three groups of molecular components: a composite group of all compounds present in both the observations and our chemical network G = {NH2 , CH3OH, c-C3H, l-C3H, l-C3H2 , c-C3H2 , CH3CCH, H2CCN, CH3CN, H2CCO, CH3CHO, HCS+ , H2CS, C4H2 , C3N, HCCNC, C3O, HC3NH+ , CH2CHCN, C5H, CH3C4H, CH3C3N, C3S, C6H, HC3N, HC5N, HC7N, HC9N}, the cyanopolyynes C = {HC3N, HC5N, HC7N, HC9N}, and the saturated species methanol and acetaldehyde S = {CH3OH, CH3CHO}. Furthermore, we characterize the dominant chemical pathways throughout the grids and discuss the competition among various mechanisms. Because each grid model naturally benchmarks the chemical network over each combination of free parameters spanned by the grid dimensions, we present an interactive tool that enables visualization and real-time fitting of each homogeneous model conatained in each of the grid data structures.

Astrochemical kinetic models of dark molecular clouds: Statistical methods and TMC-1

255th National Meeting (2018)

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