A Mysterious Ring in Dark Space?
  We report the discovery of a low-surface-brightness (27.42 mag arcsec^(-2) in
g band) nebula, which has a ring-like shape in the Beijing-Arizona Sky Survey
(BASS). Positive detections have been found in multiband data from far
ultraviolet to far infrared, except the z band from BASS and W1, W2 from the
Wide-field Infrared Survey Explorer. The reddening of the nebula E(B - V) ~
0.02 mag is estimated from Infrared Astronomical Satellite (IRAS) 100 micron
intensity and HI column density. With the help of the 3D reddening map from
Pan-STARRS 1, the Two Micron All Sky Survey, and Gaia, the distance to the
nebula of about 500 pc from Earth is derived. Such a low-surface-brightness
nebula whose energy can be interpreted by the diffuse Galactic light could
account for the optical counterpart of the infrared cirrus, which was detected
by IRAS more than 30 yr ago. The ring-like structure might be the ultimate
phase of an evolved planetary nebula, while the central white dwarf star has
been ejected from the nebula for an unclear reason. On the other hand, the ring
structure being a superposition of two close filaments might be another
reasonable explanation. Considering the lack of spectroscopic data and
uncertainty in the distance measurement, these interpretations need to be
checked by future observations.
