Editorial: Seeing that which remains hidden: tracer and proxy species in astrochemistry

Molecular abundances are crucial for obtaining information about astronomical objects, spanning from molecular clouds to star-forming regions and planetary atmospheres (Herbst and van Dishoeck 2009;Tielens 2013). In this context, the importance of tracers and proxy species is unquestionable. For example, the study of species with distinct isotopic composition (also known as isotopologues), enabling the determination of key isotopic ratios such as D/H or 14 N/ 15 N, provides deep insights into key properties of the interstellar medium (ISM), such as molecular depletion and the influence of the stellar radiation field. Moreover they are also a diagnostic tool to follow the formation of planetary systems (Caselli and Ceccarelli 2012; Langer and Penzias 1990).Further knowledge about interstellar chemistry can also be obtained indirectly, by inferring the abundances of molecules that are unobservable via rotational spectroscopy through the observation of their dipolar