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

Abstract

We carried out new CO($J$ = 2-1) observations toward the mixed-morphology supernova remnant (SNR) W49B with the Atacama Large Millimeter/submillimeter Array (ALMA). We found that CO clouds at $\sim$10 km s${}^{-1}$ show a good spatial correspondence with synchrotron radio continuum as well as an X-ray deformed shell. The bulk mass of molecular clouds accounts for the western part of the shell, not for the eastern shell where near-infrared H${}_2$ emission is detected. The molecular clouds at $\sim$10 km s${}^{-1}$ show higher kinetic temperature of $\sim$20-60 K, suggesting that modest shock-heating occurred. The expanding motion of the clouds with $\hat{I}”V\sim$6 km s${}^{-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 as the thermal conduction model. The total energy of accelerated cosmic-ray protons $W_{\mathrm{p}}$ is estimated to be $\sim$$2\times 10^{49}$ erg by adopting an averaged gas density of $\sim$$650\pm 200$ cm${}^{-3}$. The SNR age-$W_{\mathrm{p}}$ diagram indicates that W49B shows one of the highest in-situ values of $W_{\mathrm{p}}$ in the gamma-ray bright SNRs.

19 pages, 10 figures, 2 tables, published in The Astrophysical Journal (ApJ)