2025ApJ...978..123I

We present a new CO observation toward the Type Ia supernova remnant 3C 397 using the Nobeyama 45 m radio telescope at an unprecedented angular resolution of ∼18″. We newly found that the CO cloud at V_LSR = 55.7-62.2 km s^–1 (60 km s^–1 cloud) shows a good spatial correspondence with the radio continuum shell. We also found an expanding gas motion of the 60 km s^–1 cloud with an expansion velocity of ∼3 km s^–1, which is thought to be formed by the pre- and/or postsupernova feedback. By considering the positions of Galactic spiral arms and the X-ray/H I absorption studies, we concluded that 3C 397 is physically associated with the 60 km s^–1 cloud rather than the previously known CO cloud at V_LSR ∼ 30 km s^–1. Given that the previously measured preshock density is ∼2-5 cm^–3, the expanding motion of the 60 km s^–1 cloud was likely formed by the presupernova feedback known as optically thick wind. The scenario is consistent with the fact that 3C 397 exploded inside a wind-blown bubble as a single degenerate system.