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The peculiar nebula Simeis 57. II. Distance, nature and excitation

L. H. T. Oudshoorn, F. P. Israel, J. Brinchmann, M. B. C. Kloppenburg, A. G. A. Brown, J. Bally, T. Gull, P. T. Boyd

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 (ne ~ 100 cm−3) nebula with an east-to-west excitation gradient. The extinction of the nebula is AV ≤ 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 VLSR = +5 km s−1. One of these coincides with both the optical nebula and a second CO cloud at the nebular velocity VLSR ≈−10 km s−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.

ISM: structure; HII regions; ISM: individual objects: Simeis 57; Astrophysics - Astrophysics of Galaxies

Astronomy & Astrophysics
Volume 650, Article Number A171, Number of pages 13
2021 June

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia