2023MNRAS.524.5993C

One of the most fundamental hypotheses in astrochemistry and astrobiology states that crucial biotic molecules like glycine (NH₂CH₂COOH) found in meteorites and comets are inherited from early phases of star formation. Most observational searches for glycine in the interstellar medium have focused on warm high-mass molecular cloud sources. However, recent studies suggest that it might be appropriate to shift the observational focus to cold low-mass sources. We aim to detect glycine towards the so-called methanol hotspot in the Barnard 5 dark cloud. The hotspot is a cold source (T_gas ≈ 7.5 K) with yet high abundances of complex organic molecules (COMs) and water in the gas phase. We carried out deep pointed observations with the Onsala 20 m telescope, targeting several transitions of glycine conformers I and II (Gly-I and Gly-II) in the frequency range 70.2-77.9 GHz. No glycine lines are detected towards the targeted position, but we use a line stacking procedure to derive sensitive abundance upper limits w.r.t. H₂ for Gly-I and Gly-II, i.e. ≤(2-5) × 10^–10 and ≤(0.7-3) × 10^–11, respectively. The obtained Gly-II upper limits are the most stringent for a cold source, while the Gly-I upper limits are mostly on the same order as previously measured limits. The measured abundances w.r.t. H₂ of other COMs at the B5 methanol hotspot range from 2 × 10^–10 (acetaldehyde) to 2 × 10^–8 (methanol). Hence, based on a total glycine upper limit of (2-5) × 10^–10, we cannot rule out that glycine is present but undetected.