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RA: 18h 08m 20s
Dec: −23° 25′ 57″
Ch: MSA:1392, U2:339, SA:22
Type: reflection nebula
Mag: B=?, V=?
Size: 3′ x 3′
IC 4681 is probably the star that ESO and I have chosen. Barnard says only that "There is a conspicuous small nebula or nebulous star in position 1860.0 18h 00m 35s +- -23d 26m +- [this is I4684 which is a double star with faint nebulosity around it] and another in 1860.0 17h 59m 46s +- -23d 25m +-." This second star has no nebulosity around it, and there are no others nearby that Barnard could have meant (the star that Wolfgang chose is closer to Barnard's nominal position, but is not nearly as bright; nor does it have the background of Milky Way stars that might masquarade as nebulosity on a deeply exposed plate).
Whatever happened, Dreyer dropped the +- signs when he picked up these for the second IC. We could have used them, I think.
ESO also suggests that I4681 might be identical to I4684, but this is not possible as Barnard almost certainly saw the two nebulae on the same plate. However, for a case of confusion from his same paper in AN 4239, see IC 4690. So, is it possible that he made some error here, too?
This nebula was discovered photographically by Barnard. He described it as "a small nebula or a nebulous star."
Ced 154a (IC 4681)
Position (1900): RA 18 2.2, Dec - 23 27
Star: -23 13954 (Mp=9.64, V=9.55, SpT=B9)
Spectrum of nebula: continuous spectrum (inferred from sp.t. of illuminating star)
Classification: Neb associated with mainly one star (which may be multiple) - star surrounded by a neb envelope without conspicuous structure (eg. lambda Scorpii)
Size: (not given)
Notes: "Ced 154 The group of nebulous stars following M 8. R. There has been some confusion in the literature as to the proper identification of these objects. Ced 154 a = IC 4681 = -23 13954 = HD 165705 nebulous. Disc. Barnard 1908 (85). (194, 482, 486, 715)."
Gum, C.S. (1955) A survey of southern HII regions. Mem.RAS, 67. [1955MmRAS..67..155G]
Gum notes that Gum 75 is "Almost joined to Gum 72 (M8). The separation from M8 appears to be due to a lane of foreground obscuration. A number of 'nebulous' stars and reference to large faint patches of nebulosity corresponding to bright condensations in this region are given by IC 1274, 1275, 4681, 4684 and 4685 (Nos. 154b, c, d, e and f in the Catalogue of Cederblad)."
= IC 4681, Ced 154a
Pos (1950.0) 18:05.2, −23:27
Size: 3x3 (blue), (not measured on red).
Beverly Lynds (Astrophysical Journal Supplement, No 105, 1965) in her Catalogue of Bright Nebulae, notes that this nebula is in an isolated region, is barely detectable, brightest on the blue POSS plate and has a maximum size of 3' x 3'.
= DG 147, IC4681, Ced 154a
Class: C (reflection neb)
The Messier objects
The Bennett objects
The Caldwell list
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