N91 Steve Gottlieb

Steve Gottlieb’s Observations

NGC 1770 = LMC-N91 = ESO 056-35 = S-L 130 = IC 2117 = LH 12 = DEM L39

04 57 17.0 -68 24 39; Dor
Size 3’x2′

25″ (4/4/19 – OzSky): at 244x; very impressive, bright nebulous cluster/association (LH 12) 5′ in diameter. Unfiltered, over two dozen stars are resolved from mag 11-14 including a few pairs. Most of the stars are on the north and west side. It contains an 11th mag star (B2-type HD 268804) near the center and a mag 11.5 star (B2 supergiant HD 268798) on the west side. The western component of a 9″ pair of mag 12.6/13.2 stars on the north side is a Wolf-Rayet (Brey 10 = HD 32402). Two very bright emission knots with a total size of 2′ are on the south side; the southernmost knot is IC 2117 and the northern knot is N91B.

The entire field (located along the northern rim of the super-giant shell LMC 6) displayed a very strong response to a NPB filter with the nebulosity irregular and patchy and the two main nebulous patches on south side intensely bright. With careful viewing Brey 10 was surrounded by a very thin, dim arc (Wolf-Rayet bubble) of perhaps 150°, extending NW to SE of Brey 10 (opening to the NE) with a diameter of 1.5′. The SW edge of the arc was just north of HD 268804. The arc was subtle and visible only intermittently.

18″ (7/9/02 – Magellan Observatory, Australia): this LMC object is an interesting mix of a star cluster (OB-association LH 12) and an associated emission nebula (N91A = IC 2117), with the nebulosity mostly on the south side. A mag 10 star is near the center, along with a number of mag 11-12 and fainter stars. The stars are mostly on the north side of the nebulosity with several in small clumps.

Notes: James Dunlop discovered NGC 1770 = D 169 = D 168 = h2715 on 3 Aug 1826. His description of D 169 reads, “pretty bright pretty large nebula, of an irregular round figure, 5′ diameter; a little brighter in the middle.” His mean position (2 observations) is just off the SW side. Dunlop’s D 168, from 3 Oct 1826 appears to be a duplicate observation. It was found along with D 167 = NGC 1755, but his positions were offset by 7′.

John Herschel described this nebulous cluster on 23 Nov 1834 (sweep 512) as a “cluster of 6th class; pretty rich, large, irregular figure, brighter in the middle; stars of 11..18th mag; fine object.” On 30 Dec 1836 (sweep 759), he wrote “A star of 9th mag, the chief of a cluster of 6th class, 4′ diameter; nebulous. The most condensed part is 1′ south of the stars.”

Brey 10 Nebula = Bat99-11 Nebula = HD 32402 Nebula

04 57 24.1 -68 23 57; Dor

25″ (4/4/19 – OzSky): Brey 10 is located on the N side of the bright nebulous cluster NGC 1770. It is the brighter SW component of a 9″ pair of mag 12.6/13.3 stars and is surrounded by partial Wolf-Rayet bubble with a diameter of 1.5′ (spanning 40 light years). NGC 1770 is along the northern rim of the supergiant shell SGS LMC 6.

Using 244x; NGC 1770/IC 2117 displayed a very strong response to a NPB filter with the nebulosity irregular and patchy, and the two main nebulous patches on south side intensely bright. With careful viewing Brey 10 was surrounded by a very thin, dim arc (Wolf-Rayet bubble) of perhaps 150°, extending NW to SE of Brey 10 (opening to the NE) with a diameter of 1.5′. The SW edge of the arc was just north of HD 268804. The arc was quite subtle and visible only intermittently. Andrew Murrell confirmed the observation.

Notes: Williamina Fleming classified HD 32402 = Brey 10 as an emission line star in a 1901 table of objects with a peculiar spectrum, found on objective-prism plates for the Henry Draper survey. This table included several IC discoveries (Harvard College Observatory Circular 60). Annie Jump Cannon classified HD 32402 as an 0-class star in a list of all 32 known O-class stars in the LMC in Harvard College Observatory Bulletin 801 (1924BHarO.801….1C). She noted it was “a double star, of which the south preceding component is slightly brighter and was assumed to be the Class O star.” It was classified as spectral type Oa in the Henry Draper catalogue.