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NGC 6882

NGC 6882, Cl Collinder 416, COCD 482, VIII 22, GC 4557

RA: 20h 11m 48s
Dec: +26° 49′ 0″

Con: Vulpecula
Ch: MSA:1194, U2:163, SA:9

Ref: SIMBAD, Collinder (1931), DAML02

(reference key)

Type: open cluster

Mag: B=?, V=14.1

Size: 8′
PA: ?

History and Accurate Positions for the NGC/IC Objects (Corwin 2004)

NGC 6882 is probably a duplicate observation of NGC 6885. Both clusters were found by WH on subsequent nights (9 and 10 Sept 1784; N6882 is from 10 Sept), were refered to the same star, and have almost identical descriptions: "A cluster of coarsely scattered stars." For NGC 6885, he adds, "... not rich".

There is nothing striking near the position of N6882, but that for N6885 is in the middle of a large scattered cluster also observed by JH. Over the years, there has been considerable speculation about what WH saw. Some observers have made the clusters identical, while others (notably Reinmuth) have pointed at the wide group of three bright (m = 6) stars about 20 arcmin north of N6885. Brent Archinal has suggested that the clump of nine stars at 20 09 51, +26 35.1, including HD xxxxxx (the southernmost of Reinmuth's three stars), is N6882. This is unlikely as the clump is only two arcmin across. Had WH seen this, he would most likely have put it into his 7th class; it certainly is not "coarsely scattered."

Neither of these matches WH's description, so I'm more inclined to the identity of the two NGC objects. This would imply an error of 15 arcmin in WH's declination; the RA's are 12 seconds different, but both are still well within the central part of the cluster (which is over 20 arcmin across).

Adding to my conviction that N6882 = N6885 is the fact that, of the seven objects found by WH on 10 Sept 1784, four have significant offsets in WH's positions (the three besides N6882 are: N6800, -1 minute off in RA; N7720, +40 seconds off in RA; and N7741, +4 arcmin off in Dec). WH was clearly not up to snuff that night, and the +15 arcmin error in the declination of N6882 fits right in with the other problems.

Brent has more about 20th century cataloguers' notions on the identity of these two NGC numbers in his marvelous book with Steve Hynes, "Star Clusters." I've tried to stay with to WH's observations, though, spare as they are: they are the source of the two NGC numbers, so it is primarily to them that I look for a solution.

Historical observations

William Herschel (c.1784)

Synonyms: H VIII-022

Discovered in 1785 by William Herschel with an 18.7-inch f/13 speculum telescope. He called it "a cluster of coarsely scattered stars."

Published comments

Archinal, Brent [amastro] NGC 6882, NGC 6885, Cr 416 identities

Date: Sun, 13 Jun 1999 14:40:52 EDT

Steve -

Just getting back to you (finally) on the NGC 6882, NGC 6885, and Collinder 416 situation, which you've asked about relative to Caldwell 37. I'm sending copies to Harold Corwin and Brian Skiff because of their past interest in these identities.

As with the IC 2944, IC 2948, and Cr 249 situation, I thought it best to study this problem from scratch. Below I include relevant data from the two Herschels, Trumpler, and Collinder. I have not actually copied every bit of data that Trumpler and Collinder give on the objects here, but I think I've included enough for the purposes of identification. If you want more (or want me to fax or mail copies of the pages), let me know.

First I'll consider what's actually here, then go over the discovery information, and then give some conclusions.

"Reality"

========

First, what's really in the sky here? A 48' DSS image shows three groupings of stars that should be considered:

a) A large group of both bright and faint stars around 20 Vul. Diameter is about 20'. Best (2000.0) position is that of 20 Vul (SIMBAD, 20 12 00.70 +26 28 43.7).

b) A group of faint stars NW of 20 Vul, diameter about 10'. Position is 20 11 39 +26 32.2.

c) A ellipse of 8 stars, including one moderately bright one just S of 19 Vul (SIMBAD, 20 10 33.54 +26 54 15.0). Length of ellipse is about 4'. Position is 20 11 57 +26 44.1.

Discovery information and comments

==================================

Next, what's the discovery information for objects here?

=> H.VIII.22 = GC 4557 = NGC 6882 - William Herschel recorded this object on September 10, 1784. He listed it as of class VIII, i.e. a "coarsely scattered cluster of stars", with position 1m12s following and 0d 12' south of 18 Vul. His description is "A Cl. of co. sc. st.". We have for the position:

18 Vul (2000.0) 20 10 33.54 +26 54 15.0

18 Vul (1784.694) 20 01 34.72 +26 16 38.3

H. offset + 1 12 -00 12

NGC 6882 (1784.694) 20 02 47 +25 04.6

NGC 6882 (2000.0) 20 11 53 +25 42.5

Of course, the problem is there really is nothing quite like what Herschel described at his given position. His position matches that of object "c" above, and for some time I was ready to adopt this as the correct object. However, Harold Corwin [e-mail of July 23, 1998] has convinced me otherwise. Clearly "c" is not a class VIII object, and does not really match the description of a "coarsely scattered cluster". It seems more likely that Herschel simply made an approximately 15' mistake in the declination, and in fact this object is group "a". Group "b" does not seem to be a likely candidate, since it is small enough and has faint enough stars to also not match as class VIII object or Herschel's description. So, the identification H.VIII.22 = NGC 6882 = object "a" seems likely, but as Corwin has pointed out, we'll probably never know for sure.

=> H.VIII.20 = h 2071 = GC 4559 = NGC 6885 - William Herschel observed this object on September 9, 1784 (the day before he observed H.VIII.22 above). He listed this also as a class VIII object, with position 1m 0s following and 0d 27' south of 18 Vul. His description is "A Cl. of co. sc. st. not rich.". We have for the position:

18 Vul (2000.0) 20 10 33.54 +26 54 15.0

18 Vul (1784.691) 20 01 34.71 +26 16 38.3

H. offset + 01 00 -00 27

NGC 6885 (1784.691) 20 02 35 +25 49.6

NGC 6885 (2000.0) 20 11 37 +26 27.5

Here the ambiguity is whether Herschel was referring to group "a" or "b". His declination matches that of "a" quite well, but is at the south edge of group "b". His right ascension matches "b" quite well but is on the west side of group "a". So clearly his position does not help resolve this problem. We are left only with his description and classification as a type VIII object. These imply that the larger group "a" is what he had in mind, since it is as large as a class VIII object would imply and is more "considerly scattered" than the "b" group. Therefore, so far it seems quite likely that NGC 6885 = H.VIII.20 = group "a".

However, we also have to consider John Herschel's observation of this object. His "Slough Observations" (1833, p. 466) show that he observed this (his h 2071) in sweep 167, on August 18, 1828. His lists this as H.VIII.20, and his description is "Splendid cluster. More than fills the field; loose and straggling; poor in stars, one = 6.7 m whose place is given; the rest 9, 10, 11. He gives a position of:

h 2071 (1830.0) 20 4 51.7 NPD 64 1 10

h 2071 (2000.0) 20 11 59.3 +26 29 2

This is _precisely_ the modern position of 20 Vul:

20 Vul (2000.0) 20 11 59.3 +26 29 02 (SIMBAD)

showing that this is of course h's "6.7 m" star (SIMBAD, V=5.91). Clearly by position and description, group "a" is a perfect match for this object.

Therefore it seems clear that H.VIII.20 = h 2071 = GC 4559 = NGC 6885 = group "a". Perhaps one could argue that there is a slight amount of ambiguity in William Herschel's observation of H.VIII.20 (that it may be of group "b"). However, since the match with "a" is better, it would seem the more likely identification. One could also argue that the identification with group "a" should just be accepted by historical precedence, since John Herschel's adopted this in the GC (and Dreyer adopted it in the NGC).

=> Trumpler clusters - We now move forward to 1930, when Trumpler (p.175) listed two clusters which he called NGC 6682 and NGC 6885. He notes:

"NGC 6882, 6885 - In the region around 20 Vulpeculae two clusterings seem to be superposed: A loose clustering of 20-30 mostly faint stars, about 8' in diameter, with center at 20h 7m 28s +26d 15.0' (1900) was identified with NGC 6882. The coarse group of a few bright stars clustered around 20 Vulpec, with a diameter of about 22', was identified with NGC 6885. In each of these two clusters a physical relationship of the stars is indicated by the magnitude spectral class diagram, but the two clusters are evidently at different distances."

Trumpler's position for the smaller cluster is:

"NGC 6882" (1900.0) 20 07 28 +26 15.0

"NGC 6882" (1900.0) 20 11 39 +26 32.9

Clearly by this position and description, Trumpler's "NGC 6882" is group "b", while his "NGC 6885" is group "a". His group "a" identification of NGC 6885 matches that adopted above. However, his group "b" is fairly clearly _NOT_ NGC 6882. As noted above, this group does not match Herschel's description or class. Therefore, Trumpler should be credited with the _discovery_ of this cluster, group "b". Unfortunately, he did not name it. And it does not make sense to apply a new Trumpler number to this cluster retroactively.

=> Collinder clusters - The following year (1931), Collinder listed two clusters in this area. His Cr 416 he listed (p. B19) as "NGC 6882", with size 20' x 16' (Lundmark) or 20' x 15' (Collinder), total magnitude 5.6 or 5.8, and 40 or 20 stars. His Cr 417 he listed as "NGC 6885" with size 9' x 6' and 8' x 6', total magnitude 9.1 (and 9.1) and 35 or 40 stars. Positions are:

Cr 416 (1900.0) 20 7.5 +26 15

Cr 417 (1900.0) 20 7.8 +26 11

These are precisely the positions that Trumpler gives (Trumpler, Table 6, p. 175) for his "NGC 6882" and "NGC 6885". These update to:

Cr 416 (2000.0) 20 11.6 +26 33

Cr 417 (2000.0) 20 12.0 +26 29

Clearly, by Collinder's _description only_, Cr 416 is the large group "a", and Cr 417 is the smaller group b". But _by position only_, Cr 416 is the smaller group "b", and Cr 417 is the larger group "a"! What's going on? My guess is that Collinder simply copied Trumpler's position (since they match exactly) for "NGC 6882" and "NGC 6885" not realizing that his identifications were the reverse of Trumpler's. It is admittedly a somewhat arbitrary decision as to which set of Collinder identifications should be adopted for which cluster. However, accepting his identifications by position maintains the right ascension ordering of the Collinder numbering. This seems to be the best way to go, with Cr 416 being group "b" and Cr 417 being group "a".

Conclusions

===========

So, given a few (hopefully) reasonable assumptions as made above, I conclude that:

group "a" = H.VIII.20 = h 2071 = GC 4559 = NGC 6885 = Collinder 417.

This group is also likely (but not definitely) H.VIII.22 = GC 4557 = NGC 6882.

group "b" = Collinder 416. But note this group was apparently discovered by Trumpler.

group "c" is probably just an asterism that happens to be at Herschel's incorrect position for NGC 6882. For cataloguing purposes, I'm tentatively just giving it the designation AH00 J2011.9$+$2644 ("Archinal and Hynes, 2000" catalogue, with a J2000.0 positional name).

Finally, given all this, Caldwell 37's identity is somewhat ambiguous. Moore calls this NGC 6885, lists it at the position of NGC 6885 (group "a") and gives a total magnitude of 5.9 (obviously including 20 Vul from group "a"). However, it gives a size of 7', which is a match only for group "b".

Anyway, this is probably a lot more detailed response than you wanted, but I figured it was worthwhile checking these identities one more time. Let me know if there are any further questions or comments. Regards, - Brent

Trumpler, R.J. (1928)

Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 8' and the class as 1 2 p.

Sulentic & Tifft (1973)

The RNGC (Sulentic and Tifft 1973) notes that this is a 5.5 mag open cluster.

Modern observations

Brian Skiff

NGC 6885

Hoag: 19 Vul V=5.49/1.41, comp SE V=7.60/-0.09. knot in circlet V > 12.5.

8cm - two cl after all: sm one w/few *s res on NW side of 20 Vul. then lgr one of about 15 brtr *s scattered around S side of 20 Vul. BS, 15Sep1982, Anderson Mesa.

15cm - 20 Vul on SE. garland curves from it W & S. 10' hole in garland w/two cen *s. 30 *s. delicate.

- two grps apparent @ 30x/1.6-deg: lgr grp incl 18-19-20 Vul is 50' diam, little detached from fld. 50 *s brtr than m12. 19 Vul is orangish. then a concen grp in ragged 8'-10' circlet w/20 Vul on E side. another 50 *s counted just here to m13 @ 80x. knot of fntr *s in N side of circlet. BS, 13Jul1989, Anderson Mesa.

25cm - surrounds 20 Vul. mod size and pretty loose. much haze. 20 *s seen.

30cm - 75 *s. 20 Vul is on E.

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