2025AJ....170..305Z

Since the physical association between supernova remnant (SNR) G40.5-0.5 and the nearby bright gamma-ray source MGRO J1908+06 is yet unclear, it remains valuable to further investigate the remnant's molecular environment and establish its reliable distance despite previous studies. We perform a new inspection of the CO emission in the region covering G40.5-0.5 to search for evidence of interaction between the remnant and environmental molecular clouds (MCs). The SNR, which appears to blow out toward the southeast in radio, is found to be largely embedded within a molecular gas cavity at a local-standard-of-rest velocity range of ∼+33-+41 km s^–1 and bounded on the northern side by dense gas. In this velocity range, the MCs show a blueshifted line broadening in the northeast and southwest of the remnant. A high ¹²CO J = 3-2/J = 1-0 line ratio is also found on the northeastern edge of the remnant. Therefore, we argue that the SNR interacts with the MCs at V_LSR ∼ +33 to +41 km s^–1, rather than the previously suggested velocity ∼+55 km s^–1. The H I self-absorption features also support this argument. The systemic velocity of the MCs places the remnant at a kinematic distance of ∼2.3 kpc. With this distance, we have derived the fundamental physical parameters of the associated molecular gas, including the column density and volume density. The mass of the progenitor star of the SNR is inferred to be ≳15 M_⊙. The remnant is considered to be in the radiative phase, with an age no larger than ∼1.9 × 10⁴ yr.