Astrochemical Kinetic Grid Models of Groups of Observed Molecular Abundances: Taurus Molecular Cloud 1 (TMC-1)

TL;DR

This study uses grid-based chemical models to analyze molecular abundances in TMC-1, revealing how physical conditions influence chemical compositions and improving understanding of small-scale heterogeneity in molecular clouds.

Contribution

It introduces a grid of time-dependent chemical models with Bayesian uncertainty analysis to better match observed molecular abundances in TMC-1.

Findings

Best fit times reveal minimal solution space for different molecule groups.

Bayesian uncertainties constrain chemical model solutions.

Physical conditions significantly affect molecular abundance reproduction.

Abstract

The emission line spectra of cyanoacetylene and methanol reveal chemical and physical heterogeneity on very small (< 0.1 pc) scales toward the peak in cyanopolyyne emission in the Taurus Molecular Cloud, TMC-1 (CP). We generate grids of homogeneous chemical models using a three-phase rate equation approach to obtain all time-dependent abundances spanning the physical conditions determined from molecular tracers of compact and extended regions of emission along this line of sight. Each time-dependent abundance is characterized by one of four features: a maximum/minimum, a monotonic increase/decrease, oscillatory behavior, or inertness. We similarly classify the time-dependent agreement between modeled and observed abundances by calculating both the root-mean-square logarithm difference and root-mean-square deviation between the modeled and observed abundances at every point in our grid models for three groups of molecules: (i) a composite group of all species present in both the observations and our chemical network G, (ii) the cyanopolyynes C = {HC$_3$N, HC$_5$N, HC$_7$N, HC$_9$N}, and (iii) the oxygen-containing organic species methanol and acetaldehyde S = {CH$_3$OH, CH$_3$CHO}. We discuss how the Bayesian uncertainties in the observed abundances constrain solutions within the grids of chemical models. The calculated best fit times at each grid point for each group are tabulated to reveal the minimum solution space of the grid models and the effects the Bayesian uncertainties have on the grid model solutions. The results of this approach separate the effect different physical conditions and model-free parameters have on reproducing accurately the abundances of different groups of observed molecular species.