Discovery of a Wind-blown Bubble Associated with the Supernova Remnant G346.6-0.2: A Hint for the Origin of Recombining Plasma

We report on CO and H I studies of the mixed-morphology supernova remnant (SNR) G346.6-0.2. We find a wind-blown bubble along the radio continuum shell with an expansion velocity of ~10 km s^-1, which was likely formed by strong stellar winds from the high-mass progenitor of the SNR. The radial velocities of the CO/H I bubbles at V _LSR = -82 to -59 km s^-1 are also consistent with those of shock-excited 1720 MHz OH masers. The molecular cloud in the northeastern shell shows a high kinetic temperature of ~60 K, suggesting that shock heating occurred. The H I absorption studies imply that G346.6-0.2 is located on the farside of the Galactic center from us, and the kinematic distance of the SNR is derived to be ${11.1}_{-0.3}^{+0.5}$ kpc. We find that the CO line intensity has no specific correlation with the electron temperature of recombining plasma, implying that the recombining plasma in G346.6-0.2 was likely produced by adiabatic cooling. With our estimates of the interstellar proton density of 280 cm^-3 and gamma-ray luminosity <5.8 × 10³⁴ erg s^-1, the total energy of accelerated cosmic rays of W _p < 9.3 × 10⁴⁷ erg is obtained. A comparison of the age-W _p relation to other SNRs suggests that most of the accelerated cosmic rays in G346.6-0.2 have escaped from the SNR shell.