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

Abstract

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 $\sim 10$ 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/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 $\sim 60$ 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 $W_{\mathrm{p}}<9.3\times 10^{47}$ erg is obtained. A comparison of the age-$W_{\mathrm{p}}$ relation to other SNRs suggests that most of the accelerated cosmic rays in G346.6$-$0.2 have been escaped from the SNR shell.

14 pages, 7 figures, 1 table, accepted for publication in The Astrophysical Journal (ApJ)