FAR ULTRAVIOLET SPECTROSCOPIC EXPLOREROBSERVATIONS OF KPD 2055+3111, A STAR BEHIND THE CYGNUS LOOP

Abstract

We have observed a star behind the Cygnus Loop supernova remnant using the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite to study the line-of-sight interstellar medium structures toward and through this prototypical remnant. An sdOB star, KPD 2055+3111, was identified from Ultraviolet Imaging Telescope UV images and lies in projection within the bright northeast Cygnus Loop filaments (NGC 6992). This is the first known UV background source for the Cygnus Loop. We have observed this star as well as the directly adjacent emission-line filaments. Although the intrinsic spectrum of the star is complex, a broad O VI λ1032 absorption line due the Cygnus Loop is present in the stellar spectrum, confirming that the star lies beyond the Cygnus Loop. Optical spectroscopy of the star and model fits permits a distance estimate to the star of 576 ± 61 pc, thus providing an independent upper limit on the distance to the Cygnus Loop. Numerous absorption transitions of molecular hydrogen are present in the FUSE spectrum of KPD 2055+3111. Assessment of the properties of the H2 indicates a column density of (3.3 ± 0.6) × 1016 cm–2 and a two-temperature J-level population T(J = 0-1) = 106 ± 40 K and T(J = 2-5) = 850 ± 230 K. There is no direct evidence from line widths, component structure, or velocity displacements that the detected H2 is associated with the Cygnus Loop as opposed to the interstellar gas along the sight line, so either source remains viable. The O VI emission line profiles directly adjacent to line of sight to the star show dramatic variability on small (20'') spatial scales, highlighting how differently the UV-emitting gas can be distributed compared with optical and other wave bands. This impacts the ability to directly compare the emission and absorption components along the sight line.