2023PASJ...75..546D

PeVatrons are the most powerful naturally occurring particle accelerators in the Universe. The identification of counterparts associated to astrophysical objects such as dying massive stars, molecular gas, star-forming regions, and star clusters is essential to clarify the underlying nature of the PeV emission, i.e., hadronic or leptonic. We present ^12,13CO (J = 2→1) observations made with the 1.85 m radio-telescope of the Osaka Prefecture University toward the Cygnus OB7 molecular cloud, which contains the PeVatron candidate LHAASO J2108+5157. We investigate the nature of the sub-PeV (gamma-ray) emission by studying the nucleon density determined from the content of H I and H₂, derived from the CO observations. In addition to MML[2017]4607, detected via the observations of the optically thick ¹²CO (J = 1→0) emission, we infer the presence of an optically thin molecular cloud, named [FKT-MC]2022, whose angular size is 1${_{.}^{\circ}}$1 ± 0${_{.}^{\circ}}$2. We propose this cloud as a new candidate to produce the sub-PeV emission observed in LHAASO J2108+5157. Considering a distance of 1.7 kpc, we estimate a nucleon (H I + H₂) density of 37 ± 14 cm^–3, and a total nucleon mass(H I + H₂) of 1.5 ± 0.6 × 10⁴ M_⊙. On the other hand, we confirm that Kronberger 82 is a molecular clump with an angular size of 0${_{.}^{\circ}}$1, a nucleon density ∼10³ cm^–3, and a mass ∼10³ M_⊙. Although Kronberger 82 hosts the physical conditions to produce the observed emission of LHAASO J2108+5157, [FKT-MC]2022 is located closer to it, suggesting that the latter could be the one associated to the sub-PeV emission. Under this scenario, our results favour a hadronic origin for the emission.