Seeing that Which Remains Hidden: Tracer and Proxy Species in Astrochemistry

Molecular abundances are crucial for obtaining information about interstellar objects, spanning from molecular clouds to star-forming regions and planetary atmospheres. In this context, the importance of tracers and proxy species is undisputed. For example, the study of isotopic species, which permits the determination of critical isotopic ratios such as D/H or 14N/15N, offers deep insights into the level of molecular depletion and/or the role of stellar radiation field, as these effects act directly on the chemistry of the interstellar medium (ISM).

Further knowledge about interstellar chemistry can be obtained also indirectly, for instance by inferring the abundances of molecules unobservable via rotational spectroscopy through the observation of their dipolar derivatives, like in the case of N2H+ for molecular nitrogen or benzonitrile for aromatic benzene. The investigation of molecular systems that can act as a proxy for the presence of other, related species in astronomical environments will certainly lead to important insights within astrochemistry.

These species represent meaningful proxies for the physico-chemical conditions of astronomical objects including their density, temperature, and evolutionary stage. Given their importance in astrochemistry, this Research Topic aims to collect contributions about the study of various astronomical environments through the use of any molecular stand-in such as isotopic species and polar proxies of molecules, among many others, from any field of astrochemistry or astrophysics such as:

• Observational and theoretical study of the ISM of the Milky Way and of external galaxies
• IR spectroscopy including polycyclic aromatic hydrocarbons (PAHs)
• Experimental and theoretical spectroscopy
• Thermodynamic aspects relevant to interstellar chemistry
• Reactive and non-reactive potential energy surface explored by computational and experimental methods

Contributions are expected to explore either the current state of this topic or what will be learned thanks to space facilities, e.g. James Webb Space Telescope (JWST), next-generation astronomical facilities, e.g. Square Kilometre Array (SKA), next generation Very Large Array (ngVLA), and high-performance laboratory technologies (e.g., broadband spectrometers, quantum-computing, reaction techniques emulating the harsh conditions of the ISM).

Keywords: proxy species, tracer species, chemistry, interstellar medium, astrochemistry, spectroscopy, computational methods, experimental methods, isotopic species, PAHs

Important note: All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.