A Distance Estimate to the Cygnus Loop Based on the Distances to Two Stars Located Within the Remnant

Underlying nearly every quantitative discussion of the Cygnus Loop supernova remnant is uncertainty about its distance. Here we present optical images and spectra of nebulosities around two stars whose mass-loss material appears to have interacted with the remnants expanding shock front and thus can be used to estimate the Cygnus Loops distance. Narrow passband images reveal a small emission-line nebula surrounding an M4 red giant near the remnants eastern nebula NGC 6992. Optical spectra of the nebula show it to be shock-heated with significantly higher electron densities than seen in the remnants filaments. This along with a bow-shaped morphology suggests it is likely red giant mass-loss material shocked and accelerated by passage of the Cygnus Loops blast wave. We also identify a B7 V star located along the remnants northwestern limb which also appears to have interacted with the remnants shock wave. It lies within a small arc of nebulosity in an unusually complex region of highly curved and distorted filaments along the remnants northern shock front suggestive of a localized disturbance of the shock front due to the B stars stellar winds. Based on the assumption that these two stars lie inside the remnant, combined with an estimated distance to a molecular cloud situated along the remnants western limb, we propose a distance to the Cygnus Loop of 1.0 +/- 0.2 kpc. Although larger than several recent estimates of 500 - 800 pc, a distance ~1 kpc helps resolve difficulties with the remnants postshock cosmic ray and gas pressure ratio and estimated supernova explosion energy.