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.