What’s The Nature and Excitation Source of Galactic Nebula, Simeis 57? (Astronomy)

A team of international astronomers investigating the Galactic nebula, Simeis 57, identified its distance and revealed, its nature and the source of its excitation. Their study recently appeared on Arxiv.

The Galactic nebula Simeis 57, also known as HS 191 is a very bright emission nebula of peculiar shape in the constellation of Cygnus. Often referred to as the Propeller Nebula, this nebula is a popular object for amateur astrophotographers. Its major features are two curved nebulosities (DWB 111 and DWB 119). The center of this nebula is cut by a long dust filament that continues adjacent to the nebular patch DWB 118. Other nearby nebulae are DWB 126 to the north and DWB 108 and DWB 107 to the south; the latter has the appearance of a bright rim. At a Galactic longitude of 80.3°, Simeis 57 is inside the solar circle but its actual distance is unknown. In this direction, the Galactic line of sight is tangential to the Orion-Cygnus spiral arm and several kiloparsecs long. The relatively high Galactic latitude of +4.7° and the large angular extent (∼ 200 ) suggest, however, that it is not very distant.

In an earlier paper, Israel and colleagues presented highresolution radio continuum maps from which they concluded that the radio emission of Simeis 57 is free-free thermal emission originating in a gas of moderate electron densities with S and N2 spectra that extend a modest foreground extinction. All of this points to excitation by a star that is sufficiently bright to be easily identified, but the lack of an obvious candidate meant that the nature of Simeis 57 was left a mystery. Now, Boyd and colleagues investigated the ionized gas, dust, and stars in the field of Simeis 57 in a further attempt to identify its nature and the source of its excitation.

They obtained new observations and archival data which revealed that, 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.

Fig 1: Fully sampled J=2-1 13CO maps in the field centered on Simeis 57 (offset relative to α = 20h16m08.2s, δ = 43°41’02.4″) smoothed to a resolution of 30”. At left is the emission at +5 km¯1 (averaged over a 6 km s¯1velocity interval), in the center the emission at -10 km s¯1 (averaged over a 10 km s¯1 interval), and at right the position-velocity distribution. © Boyd et al.

They also found that, there is no major CO cloud complex but small (4-8 pc) CO clouds, all at VLSR ≈ +5 km s¯1, are scattered across the field. One of these clouds coincides with the nebula and another fragmentary cloud at the nebular velocity of VLSR ≈ −10 km s¯1. They do not appear to be physically connected and both are smaller than the ionized nebula. They have substantial substructure.

While, from CO data, they suggested, molecular gas densities of 1000 cm¯3 and 100 cm¯3 and modest column densities N(H2) = 1-2 × 1021 cm¯2. These data clearly establish that Simeis 57 is not part of a larger star-forming complex, but is an isolated object in a larger field filled with fragmentary gas and dust clouds. No luminous stars are embedded in the dust nor are any hidden by it; there are no central objects.

Fig. 2. Identification chart of bright stars near Simeis 57. The red cross denotes star No. 16 (HD 193793/WR140 aka the spectroscopic binary SBC9 1232, which is the most likely candidate for the exciting star of Simeis 57. © Boyd et al.

In addition, the larger field surrounding the nebula revealed only one probable excitation source. This is the evolved binary HD 193793 consisting of an O4-5 supergiant and a WC7 Wolf-Rayet star (WR 140) at a well-established distance of D = 1.71 kpc and with a projected distance of 23 pc to the nebula. According to authors, both its luminosity and the hardness of its UV radiation appear sufficient to explain the excitation of Simeis 57. Moreover, its location to the east of the nebula fits well with the observed [OIII] emission asymmetry.

“It thus appears that the nebula Simeis 57 consists of separate filaments and diffuse emission that together only fortuitously produce the remarkably coherent appearance of an outflow object.”

— concluded authors of the study

Featured image: Simeis 57 – Propeller Nebula © Jenafan et al.

Reference: L.H.T. Oudshoorn, F.P. Israel, J. Brinchmann, M.B.C. Kloppenburg, A.G.A. Brown, J. Bally, T.R. Gull, P.T. Boyd, “The peculiar nebula Simeis 57: II. Distance, nature and excitation”, Arxiv, pp. 1-13, 2021. https://arxiv.org/abs/2104.14585

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