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 HI 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 $\sim10$ 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/HI bubbles at $V_\mathrm{LSR} = -82$-$-59$ 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 $\sim60$ K,
suggesting that shock-heating occurred. The HI absorption studies imply that
G346.6$-$0.2 is located on the far side 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$-$0.2 was likely produced by adiabatic cooling. With our estimates of
the interstellar proton density 280 cm$^{-3}$ and gamma-ray luminosity $< 5.8
\times 10^{34}$ erg s$^{-1}$, the total energy of accelerated cosmic rays
$W_{\rm p} < 9.3 \times 10^{47}$ erg is obtained. A comparison of the
age-$W_{\rm p}$ relation to other SNRs suggests that most of the accelerated
cosmic rays in G346.6$-$0.2 have been escaped from the SNR shell.
