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RA: 20h 02m 59.3s
Dec: +11° 15′ 34″
Ch: MSA:1242, U2:208, SA:16
Ref: NGC/IC, DAML02, Archinal&Hynes (2003)
Type: open cluster
Mag: B=?, V=10
NGC 6858 is listed in the NGC as a cluster of stars recorded by John Herschel (1829 July 29) from Slough, England. In the General Catalogue (GC 4537) it is placed at RA 19:56:23.6, NPD 79:07:24.4 (1860.0; one observation) which precesses to RA 20:03.1, Dec +11° 16 (2000.0). The GC description is "Cl; cL; E; pRi; st 13" (cluster of stars, considerably large [> 15'], elongated, pretty rich, stars 13th magnitude).
2. Modern investigations
SIMBAD indexes only one paper referencing NGC 6858 (Krone-Martins et al., 2010), and it is not listed in the WEBDA database.
NGC 6858 was flagged as "not a cluster" by Jack Sulentic (Sulentic & Tifft, 1973) based on his visual inspection of photographic plates.
Archinal & Hynes (2003) list it at the coordinates above, giving the angular diameter as 10', involving 12 stars with the brightest star of 10th magnitude.
Krone-Martins et al. (2010) studied open clusters that occurred within the Bordeaux PM2000 catalogue (declination zone +11° to +18°, complete to V=15.4). For the 261 stars around the reported position of NGC 6858 they found a "poor" proper motion fit, suggesting there is no cluster present.
NGC 6858 is also referenced in Tadross (2011), who used 2MASS data to study a sample of NGC clusters. They extracted 2MASS data via Vizier centred on the cluster positions. Constructing radial density profiles and colour-magnitude diagrams they derived basic parameters, finding for NGC 6858 a diameter = 10', log t = 9.40 years, E_B-V = 0.13 mags, and distance = 1310 pc.
NGC 6858 is listed in DAML02 (version 3.2, 2012 January 26) at RA 20:02:56, Dec. +11°15'30" with a 10' diameter, repeating the parameters given in Tadross (2011).
3. Visual examination
The figures below were extracted from the SERC-QV (Quick-V) Survey, retreived from the STScI Digitized Sky Survey.
Figure 1(a) shows a 30'x30' field around the position listed in DAML02. Figure 1(b) has the fainter stars removed. Visual inspection shows a north-south elongated region (8.3' x 2.7') centred on RA 20:02:57, Dec +11°14.3', with an apparent over-density of stars. The 9.4 mag star TYC 1080-00811-1 lies north-east of the groupings centre.
This essentially confirms the observation made by Herschel in 1829. Furthermore, deep sky observer Steve Gottlieb reported on this cluster in his 1995 review of the NGC as follows:
"NGC 6858 (20 02 59 +11 15.6) 17.5-inch (9/23/95): this is an elongated group of 35 stars in a 10'x4' group oriented N-S which precedes a mag 9 star at 20 03 16 +11 16.4 (2000). Fairly uniform in brightness and distribution with no dense spots but includes a couple of nice doubles. Most stars are mag 12-13 with a scattering of fainter stars."
Visually, NGC 6858 is an open cluster, visible in modest-sized telescopes as an elongated scattering of faint stars.
Astrophysically, its reality is controversial. One proper motion study does not find a convincing grouping, while results based on a 2MASS CMD suggest a real cluster.
The 6.9 mag star HD 190070 lies 19.6' south of the centre as determined in Section 3. As Figure 2 shows, 10 or so stars appear to form a loose grouping with HD 190070 at the south-western tip.
Archinal, B.A. & Hynes, S. J. (2003) Star Clusters. Willmann-Bell.
Krone-Martins, A. et al. (2010) Kinematic parameters and membership probabilities of open clusters in the Bordeaux PM2000 catalogue. A&A, 516, A3.
Sulentic, J. W. & Tifft, W. G. (1973) The revised new catalogue of nonstellar astronomical objects. University of Arizona Press: Tucson.
Tadross, A. K. (2011) A catalog of 120 NGC open star clusters. J. Korean Ast. Soc., 44, 1-11.
The RNGC (Sulentic and Tifft 1973) notes that this is a nonexistent object. Their coded description reads NOCL S.
16-inch f/10 SCT (127x 290x)
This one is tricky. There is no grouping at the indicated position of NGC 6858. However at the position as stated above I found a dome like string of stars open towards the west with two brighter stars cuddle inside. This was the most outstanding group in an area of around 48 arc minutes. The brightest star in the group is HD 190070. In the northern part of the field a wide tri-angle of stars is dominated.
The Messier objects
The Bennett objects
The Caldwell list
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