The Distance to Supernova Remnant CTB 109 Deduced from Its Environment

We conducted a study of the environment around the supernova remnant (SNR) CTB 109. We found that the SNR is part of a large complex of H II regions extending over an area of 400 pc along the Galactic plane at a distance of about 3 kpc at the closer edge of the Perseus spiral arm. At this distance, CTB 109 has a diameter of about 24 pc. We demonstrated that including spiral shocks in the distance estimation is an ultimate requirement to determine reliable distances to objects located in the Perseus arm. The most likely explanation for the high concentration of H II regions and SNRs is that the star formation in this part of the Perseus arm is triggered by the spiral shock.