History-independent tracers: Forgetful molecular probes of the physical conditions of the dense interstellar medium

Abstract

Molecular line emission is a powerful probe of the physical conditions of astrophysical objects but can be complex to model, and it is often unclear which transitions would be the best targets for observers who wish to constrain a given parameter. We therefore produce a list of molecular species for which the gas history can be ignored, removing a major modelling complexity. We then determine the best of these species to observe when attempting to constrain various physical parameters. To achieve this, we use a large set of chemical models with different chemical histories to determine which species have abundances at 1 MYr that are insensitive to the initial conditions. We then use radiative transfer modelling to produce the intensity of every transition of these molecules. We finally compute the mutual information between the physical parameters and all transitions and transition ratios in order to rank their usefulness in determining the value of a given parameter. We find 48 species that are insensitive to the chemical history of the gas, 23 of which have collisional data available. We produce a ranked list of all the transitions and ratios of these species using their mutual information with various gas properties. We show mutual information is an adequate measure of how well a transition can constrain a physical parameter by recovering known probes and demonstrating that random forest regression models become more accurate predictors when high-scoring features are included. Therefore, this list can be used to select target transitions for observations in order to maximize knowledge about those physical parameters.

arXiv comment9 pages, 2 figures. Accepted for publication in A&A. Results table at https://uclchem.github.io/hits

References in corpus (30)

• A computer program for fast non-LTE analysis of interstellar line spectra
• An atomic and molecular database for analysis of submillimetre line observations
• Gas and grain chemical composition in cold cores as predicted by the Nautilus 3-phase model
• On the Formation of CO2 and Other Interstellar Ices
• On the internal structure of starless cores. II. A molecular survey of L1498 and L1517B
• Exploring Molecular Complexity with ALMA (EMoCA): Deuterated complex organic molecules in Sagittarius B2(N2)
• Parametrization of C-shocks. Evolution of the Sputtering of Grains
• SiO emission from the Galactic Center Molecular Clouds
• Molecular line emission in NGC1068 imaged with ALMA: II. The chemistry of the dense molecular gas
• Formation of Complex Organic Molecules in Cold Interstellar Environments through non-diffusive grain-surface and ice-mantle chemistry
• UCLCHEM: A Gas-Grain Chemical Code
• HCN/HNC intensity ratio: a new chemical thermometer for the molecular ISM
• A lambda=3 mm molecular line survey of NGC1068. Chemical signatures of an AGN environment
• Submillimeter-HCN Diagram for an Energy Diagnostics in the Centers of Galaxies
• Dense Gas Tracers in Perseus: Relating the N2H+, NH3, and Dust Continuum Properties of Pre- and Proto-Stellar Cores
• A new look at sulphur chemistry in hot cores and corinos
• Extragalactic CS survey
• Gaseous CO Abundance---An Evolutionary Tracer for Molecular Clouds
• Fire in the Heart: A Characterization of the High Kinetic Temperatures and Heating Sources in the Nucleus of NGC253
• The Distribution and Origin of C${}_2$H in NGC 253 from ALCHEMI
• Kinematics of dense gas in the L1495 filament
• Starburst Energy Feedback Seen Through HCO${}^{+}$/HOC${}^{+}$ Emission in NGC 253 from ALCHEMI
• A new proxy to estimate the cosmic-ray ionisation rate in dense cores
• Chemical modelling of glycolaldehyde and ethylene glycol in star-forming regions
• Sulfur Chemistry in L1157-B1
• Molecular transitions as probes of the physical conditions of extragalactic environments
• HCN $J$=4-3, HNC $J$=1-0, ${\mathrm{H}}^{13}\mathrm{CN}$ $J$=1-0, and $\mathrm{H}{\mathrm{C}}_3\mathrm{N}$ $J$=10-9 Maps of Galactic Center Region I: Spatially-Resolved Measurements of Physical Conditions and Chemical Composition
• Tracers of the ionization fraction in dense and translucent gas: I. Automated exploitation of massive astrochemical model grids
• Diagnosing shock temperature with NH${}_3$ and H${}_2$O profiles
• Dense circum-nuclear molecular gas in starburst galaxies

Cited by in corpus (4)

• PDFchem: A new fast method to determine ISM properties and infer environmental parameters using probability distributions
• Machine learning-accelerated chemistry modeling of protoplanetary disks
• Non-Equilibrium Abundances Treated Holistically (NEATH): the molecular composition of star-forming clouds
• The initial magnetic criticality of prestellar cores