A refined search for high-velocity gas in the Cygnus Loop supernova remnant

Abstract

We present the results of a sensitive search for high-velocity gas in interstellar absorption lines associated with the Cygnus Loop supernova remnant (SNR). We examine high-resolution high signal-to-noise ratio optical spectra of six stars in the Cygnus Loop region with distances greater than ∼700 pc. All stars show low-velocity Na i and Ca ii absorption. However, only one star, HD 198301, exhibits high-velocity Ca ii absorption components, at velocities of +62, +82, and +96 km s−1. The distance to this star of ∼870 pc helps to constrain the distance to the receding edge of the Cygnus Loop’s expanding shock front. One of our targets, HD 335334, was previously thought to exhibit high positive and high negative velocity interstellar Na i and Ca ii absorption. This was one factor leading Fesen et al. to derive a distance to the Cygnus Loop of 725 ± 15 pc. However, we find that HD 335334 is in fact a double-line spectroscopic binary and shows no evidence of high-velocity interstellar absorption. As such, the distance to HD 335334 cannot be used to constrain the distance to the Cygnus Loop. Our detection of Ca ii absorption approaching 100 km s−1 toward HD 198301 is the first conclusive detection of high-velocity absorption from a low-ionization species associated with the Cygnus Loop SNR. A large jump in the Na i column density toward BD+31 4218, a star located beyond the northwestern boundary of the Cygnus Loop, helps to constrain the distance to a large molecular cloud complex with which the Cygnus Loop is evidently interacting.

The Distance to the S147 Supernova Remnant

“…Interestingly, the highvelocity absorption features are also rapidly time variable in strength and wavelength (e.g., Kameswara Rao et al 2020 most recently). The only other case where this approach has been deliberately used for distance determination is for the Cygnus Loop (Fesen et al 2018(Fesen et al , 2021Ritchey et al 2024).…”