We present CO observations toward three large supernova remnants ( SNRs ) in the third Galactic quadrant using the Purple Mountain Observatory Delingha 13.7 m millimeter-wavelength telescope .
The observations are part of the high-resolution CO survey of the Galactic plane between Galactic longitudes l = -10 ^ { \circ } to 250 ^ { \circ } and latitudes b = -5 ^ { \circ } to 5 ^ { \circ } .
CO emission was detected toward the three SNRs : G205.5 + 0.5 ( Monoceros Nebula ) , G206.9 + 2.3 ( PKS 0646 + 06 ) , and G213.0 - 0.6 .
Both of SNRs G205.5 + 0.5 and G213.0 - 0.6 exhibit the morphological agreement ( or spatial correspondences ) between the remnant and the surrounding molecular clouds ( MCs ) , as well as kinematic signatures of shock perturbation in the molecular gas .
We confirm that the two SNRs are physically associated with their ambient MCs and the shock of SNRs is interacting with the dense , clumpy molecular gas .
SNR G206.9 + 2.3 , which is close to the northeastern edge of the Monoceros Nebula , displays the spatial coincidence with molecular partial shell structures at V _ { LSR } \sim 15 { km } { s } ^ { -1 } .
While no significant line broadening has been detected within or near the remnant , the strong morphological correspondence between the SNR and the molecular cavity implies that SNR G206.9 + 2.3 is probably associated with the CO gas and is evolving in the low-density environment .
The physical features of individual SNRs , together with the relationship between SNRs and their nearby objects , are also discussed .