2026/07/29 by Miya Duffy, Ryan A. Loomis, Martin S. Holdren +9 · 1 voice
Physics and Astronomy · #astro-ph.GA
arxiv created 2026/07/29 · arxiv updated 2026/07/31
The formation history of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium remains a topic of active debate, with proposed mechanisms ranging from high-temperature stellar ejecta processes to low-temperature chemistry within molecular clouds. Recently, the identification of small PAHs in the cold, dark cloud TMC-1 has provided some circumstantial evidence of the latter. If these are formed in situ, their isotopic ratios, particularly 12C/13C, should reflect the local, bulk material of the molecular cloud; in contrast, PAHs formed in the circumstellar envelopes of evolved stars may show enhanced 13C abundances. The expected radio signals of these 13C substituted species will be faint, and thus we conducted a detailed proof-of-concept analysis examining whether spectral line stacking and matched filtering techniques can robustly retrieve signal from multiple singly substituted 13C isotopologues in aggregate. We find that while retrieved signal decreases in the limit of spectral line confusion, false-positive detections are exceedingly improbable at the adopted 5σ matched filter response detection threshold. We then demonstrate the technique on actual observational data of isotopologues of HC9N toward TMC-1. Finally, we show using synthetic data that if laboratory rotational spectra of all singly substituted 13C isotopologues of cyanonaphthalene and cyanopyrene isomers existed, current observations of TMC-1 would be sensitive enough to discriminate between a local, bulk 12C/13C ratio and one with an enhanced 13C abundance suggestive of inheritance.