ALMA CO Observations of the Mixed-morphology Supernova Remnant W49B: Efficient Production of Recombining Plasma and Hadronic Gamma Rays via Shock-Cloud Interactions

We carried out new CO(J = 2-1) observations toward the mixed-morphology supernova remnant (SNR) W49B with the Atacama Large Millimeter/submillimeter Array. We found that CO clouds at ~10 km s^-1 show a good spatial correspondence to the synchrotron radio continuum as well as to an X-ray deformed shell. The bulk mass of molecular clouds accounts for the western part of the shell, not the eastern shell, where near-infrared H₂ emission is detected. The molecular clouds at ~10 km s^-1 show higher kinetic temperatures of ~20-60 K, suggesting that modest shock heating occurred. The expanding motion of the clouds with ΔV ~ 6 km s^-1 was formed by strong winds from the progenitor system. We argue that the barrel-like structure of Fe-rich ejecta was possibly formed not only by an asymmetric explosion, but also by interactions with dense molecular clouds. We also found a negative correlation between the CO intensity and the electron temperature of recombining plasma, implying that the origin of the high-temperature recombining plasma in W49B can be understood to be the thermal conduction model. The total energy of accelerated cosmic-ray protons W_p is estimated to be ~2 × 10⁴⁹ erg by adopting an averaged gas density of ~650 ± 200 cm^-3. The SNR age-W_p diagram indicates that W49B shows one of the highest in situ values of W_p among gamma-ray-bright SNRs.