The peculiar nebula Simeis 57: II. Distance, nature and excitation
  Simeis 57 (HS 191) is an optically bright nebula in the Cygnus X region with
a peculiar appearance that suggests an outflow from a rotating source. Newly
obtained observations and archival data reveal Simeis 57 as a low-density
($n_{e}\,\sim\,100$ cm$^{-3}$) nebula with an east-to-west excitation gradient.
The extinction of the nebula is $A_{V}\,\leq$ 2 mag. The nebula is recognizable
but not prominent in mid- and far-infrared images. In its direction, half a
dozen small CO clouds have been identified at $V_{LSR}$ = + 5 km s$^{-1}$. One
of these coincides with both the optical nebula and a second CO cloud at the
nebular velocity $V_{LSR}\,\approx$ -10 km $^{-1}$. No luminous stars are
embedded in these molecular clouds, nor are any obscured by them and no
sufficiently luminous stars are found in the immediate vicinity of the nebula.
Instead, all available data points to the evolved star HD 193793 = WR 140 (an
O4-5 supergiant and WC7 Wolf-Rayet binary) as the source of excitation,
notwithstanding its large separation of $50'$, about 25 pc at the stellar
distance of 1.7 kpc. Simeis 57 appears to be a part of a larger structure
surrounding the HI void centered on HD 193793.
