The peculiar nebula Simeis 57

II. Distance, nature and excitation

¹ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: oudshoorn@strw.leidenuniv.nl
² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrellas, 4150-762 Porto, Portugal
³ Presently: Pels Rijcken, PO Box 11756, 2502 AT Den Haag, The Netherlands
⁴ Center for Astrophysics and Space Astronomy (CASA), University of Colorado, 389 UCB, Boulder, CO 808309, USA
⁵ NASA/GSFC, Mail Code: 667, Greenbelt, MD 20771, USA

Received: 8 September 2020
Accepted: 21 April 2021

Abstract

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 ~ 100 cm⁻³) nebula with an east-to-west excitation gradient. The extinction of the nebula is A_V ≤ 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 ≈−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.