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 x 1034 erg s-1, the total energy of accelerated cosmic rays of W p < 9.3 x 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.