CO Observations of the Type Ia Supernova Remnant 3C 397 by the Nobeyama 45-m Radio Telescope: Possible Evidence for the Single-Degenerated Explosion

We present a new CO observation toward the Type Ia supernova remnant (SNR) 3C 397 using the Nobeyama 45-m radio telescope at an unprecedent angular resolution of ~18''. We newly found that the CO cloud at VLSR = 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 post-supernova feedback. By considering the positions of Galactic spiral arms and the X-ray/HI 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 VLSR ~30 km s⁻¹. Given that the previously measured pre-shock density is ~2-5 cm⁻³, the expanding motion of the 60 km s⁻¹ cloud was likely formed by the pre-supernova feedback known as optically thick wind. The scenario is consistent with that 3C 397 exploded inside a wind-blown bubble as a single degenerate system.