Non-Equilibrium Abundances Treated Holistically (NEATH): the molecular composition of star-forming clouds

Abstract

Much of what we know about molecular clouds, and by extension star formation, comes from molecular line observations. Interpreting these correctly requires knowledge of the underlying molecular abundances. Simulations of molecular clouds typically only model species that are important for the gas thermodynamics, which tend to be poor tracers of the denser material where stars form. We construct a framework for post-processing these simulations with a full time-dependent chemical network, allowing us to model the behaviour of observationally-important species not present in the reduced network used for the thermodynamics. We use this to investigate the chemical evolution of molecular gas under realistic physical conditions. We find that molecules can be divided into those which reach peak abundances at moderate densities ($10^3\mathrm{c}{\mathrm{m}}^{-3}$) and decline sharply thereafter (such as CO and HCN), and those which peak at higher densities and then remain roughly constant (e.g. NH${}_3$, N${}_2$H${}^{+}$). Evolving the chemistry with physical properties held constant at their final values results in a significant overestimation of gas-phase abundances for all molecules, and does not capture the drastic variations in abundance caused by different evolutionary histories. The dynamical evolution of molecular gas cannot be neglected when modelling its chemistry.

arXiv comment14 pages, 13 figures. MNRAS accepted

References in corpus (58)

• E pur si muove: Galiliean-invariant cosmological hydrodynamical simulations on a moving mesh
• Observations of the Icy Universe
• Cold Dark Clouds: The Initial Conditions for Star Formation
• The UMIST database for astrochemistry 2012
• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• The Dark Molecular Gas
• Systematic Molecular Differentiation in Starless Cores
• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• The SILCC (SImulating the LifeCycle of molecular Clouds) project: I. Chemical evolution of the supernova-driven ISM
• Simulating the formation of molecular clouds. I. Slow formation by gravitational collapse from static initial conditions
• On the Relationship Between Molecular Hydrogen and Carbon Monoxide Abundances in Molecular Clouds
• The Spatial Distribution of Complex Organic Molecules in the L1544 Pre-stellar Core
• Molecular Evolution in Collapsing Prestellar Cores III: Contraction of A Bonnor-Ebert Sphere
• The anatomy of the Orion B Giant Molecular Cloud: A local template for studies of nearby galaxies
• Modeling the Warm Ionized Interstellar Medium and Its Impact on Elemental Abundance Studies
• TreeCol: a novel approach to estimating column densities in astrophysical simulations
• Following the flow: tracer particles in astrophysical fluid simulations
• UCLCHEM: A Gas-Grain Chemical Code
• SILCC-Zoom: The dynamical and chemical evolution of molecular clouds
• Molecular Line Emission as a Tool for Galaxy Observations (LEGO). I. HCN as a tracer of moderate gas densities in molecular clouds and galaxies
• How long does it take to form a molecular cloud?
• A Simple and Accurate Network for Hydrogen and Carbon Chemistry in the ISM
• The Cloud Factory I: Generating resolved filamentary molecular clouds from galactic-scale forces
• Tracing the formation of molecular clouds via [CII], [CI] and CO emission
• Simulations of the star-forming molecular gas in an interacting M51-like galaxy
• Approximations for modelling CO chemistry in GMCs: a comparison of approaches
• Prevalence of Complex Organic Molecules in Starless and Prestellar Cores within the Taurus Molecular Cloud
• Gas phase Elemental abundances in Molecular cloudS (GEMS). II. On the quest for the sulphur reservoir in molecular clouds: the $H_{2}S$ case
• The Central 1000 au of a Pre-stellar Core Revealed with ALMA. II. Almost Complete Freeze-out
• Line Profiles of Cores within Clusters: I. The Anatomy of a Filament
• On column density thresholds and the star formation rate
• Line Profiles of Cores within Clusters: II Signatures of Dynamical Collapse during High Mass Star Formation
• Molecular Line Observations of Infrared Dark Clouds: Seeking the Precursors to Intermediate and Massive Star Formation
• Why does ammonia not freeze out in the center of pre-stellar cores?
• The chemistry of protoplanetary fragments formed via gravitational instabilities
• LEGO II: A 3 mm molecular line study covering 100 pc of one of the most actively star-forming portions within the Milky Way Disc
• Fragmentation induced starvation in Population III star formation: a resolution study
• Characterizing the line emission from molecular clouds. Stratified random sampling of the Perseus cloud
• The complex organic molecular content in the L1498 starless core
• Hydrodynamics with gas-grain chemistry and radiative transfer: comparing dynamical and static models
• Detection of Complex Organic Molecules in Young Starless Core L1521E
• The complex organic molecular content in the L1517B starless core
• A Survey of CH2DOH Towards Starless and Prestellar Cores in the Taurus Molecular Cloud
• Do we need non-ideal magnetohydrodynamics to model protostellar discs?
• On the role of ambient environments in the collapse of Bonnor-Ebert spheres
• Modeling chemistry during star formation: Water deuteration in dynamic star-forming regions
• A 3D physico-chemical model of a pre-stellar core. I. Environmental and structural impact on the distribution of CH${}_3$OH and $c$-C${}_3$H${}_2$
• Timescales for Low-Mass Star Formation in Extragalactic Environments: Implications for the Stellar IMF
• Influence of galactic arm scale dynamics on the molecular composition of the cold and dense ISM II. Molecular oxygen abundance
• Influence of galactic arm scale dynamics on the molecular composition of the cold and dense ISM I. Observed abundance gradients in dense clouds
• Chemical post-processing of magneto-hydrodynamical simulations of star-forming regions: robustness and pitfalls
• The evolution of HCO${}^{+}$ in molecular clouds using a novel chemical post-processing algorithm
• Towards the impact of GMC collisions on the star formation rate
• History-independent tracers: Forgetful molecular probes of the physical conditions of the dense interstellar medium
• Do simulated molecular clouds look like real ones?
• Influence of galactic arm scale dynamics on the molecular composition of the cold and dense ISM III. Elemental depletion and shortcomings of the current physico-chemical models
• Differences in chemical evolution between isolated and embedded prestellar cores
• Different molecular filament widths as tracers of accretion onto filaments

Cited by in corpus (1)

• Does the HCN/CO ratio trace the star-forming fraction of gas? II. Variations in CO and HCN Emissivity