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Running Chicken Nebula

IC 2944, Ced 118, lambda Centauri Nebula, Caldwell 100, Running Chicken Nebula

RA: 11h 35m 47.32s
Dec: −63° 01′ 10.9″

Con: Centaurus
Ch: MSA:1003, U2:450, SA:25

Ref: Corwin (2004), DAML02

(reference key)

Type: bright nebula (HII region)

Mag: B=4.62, V=4.5

Size: 40′ x 20′
PA: ?

Image gallery

Photos  (5)

Select a photo and click the button to view

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

IC 2944 and IC 2948. Gregg Thompson in Deep Sky No. XX, XXX, 198X (I've lost the reference, so will have to dig it out again later), raises an interesting question concerning the identity of IC 2948. While it is shown in Sky Atlas 2000.0 as a star cluster, its original IC description "eeL" (extremely, extremely large) makes no mention of its being a cluster. This would suggest that Gregg's identification of the nebula as IC 2948 is correct. Digging just a bit further, we find that the object is indeed listed as a bright nebula in Sky Catalogue 2000.0, along with IC 2944. Both of these objects are listed as clusters in an emission nebula complex in the ESO/Uppsala catalogue, which is probably also correct. But let's look more closely at the story behind the present confusion over the identification of IC 2944 and 2948.

The objects are two of those discovered by Royal H. Frost on photographic plates taken in Peru at the Arequipa Station of Harvard Observatory in the early 1900's. For IC 2944, Frost's published notes read, "Nebula around AGC 15848 (lambda Centauri), extending from 11h 30m to 11h 31m, and from -62 14' to -62 40'." For IC 2948, he writes "Nebulous patch extending from 11h 30.6m to 11h 38.1m, and from -62 28' to -63 14'." (The positions are for the equinox 1900). It's obvious from this that Gregg has indeed picked up the correct object.

The two nebulae look, on DSS2R images, as if they are simply parts of a single, much larger complex of nebulae extending over several degrees. It is still possible to make the case, as Frost did, that they are separate objects. In that case, IC 2944 would be somewhat more extensive than Frost measured it: my own estimates make it something like 40 arcmin by 20 arcmin. I2948 is much closer to the size that Frost measured: 45 arcmin by 40 arcmin.

The particular plate that Frost found these nebulae on is plate 6715 taken on 5 May 1904 with the 24-inch Bruce refractor. This telescope is a short-focus instrument capable of taking very wide field photographs. Indeed, the field size is almost exactly that (6.4 x 6.4 degrees) of the modern 1.2-m Schmidt telescopes at Palomar Mountain and at Siding Spring which have given us our definitive twentieth-century optical sky surveys.

Oddly enough, Gregg has also uncovered IC 2948's other common -- perhaps mistaken -- identification in the astronomical literature as a star cluster. B.A. Gould was the first to see it this way in 1897 on plates taken at Cordoba Observatory in Argentina. He counted 236 stars in the area, gave photographic magnitudes for them, and noted the proximity to Lambda Centauri.

In his 1930 book Star Clusters, Harlow Shapley lists IC 2948 in the catalogue of open clusters with an angular diameter of 15 arcmin, but with only 25 stars. This discrepancy with Gould's description is unfortunately not unusual in the early catalogues of clusters and nebulae. These catalogues were usually little more than finding lists and descriptions, though Shapley was among the first to attempt to quantify the study of deep sky objects. He gave a distance of 660 parsecs for IC 2948, from which he calculated an intrinsic linear diameter of 2.9 parsecs.

After another quarter century of obscurity, IC 2948 was again noted in the literature, this time by Colin Gumm in his exploration of the vast, glowing clouds of ionized hydrogen in the southern Milky Way. He entered it as number 42 in his "Survey of Southern H II Regions" found on wide field photographs taken at Mt. Stromlo in the early 1950's.

David Thackery in 1964 was the first to note IC 2948's probable true nature: it is a cluster of brilliant young stars in an H II region. He also noticed the neighboring IC 2944 with its retinue of bright blue giant stars. Together with IC 2948, Thackery described the region as "containing one of the biggest concentrations of (spectral type) O stars in the sky." This has made it interesting to astronomers as a birthplace of stars, and only its far southern location has kept it from assuming an important role in recent studies of stellar evolution.

In 1986, however, Charles Perry and Arlo Landolt of Louisiana State University, working at Cerro Telolo Observatory in Chile, have found that the "cluster" associated with IC 2944 is apparently a chance superposition of O and B type stars at different distances along our line of sight. Is it possible that IC 2948 is similarly an illusion? Gregg's description certainly bears this out, though the appearance of the nebula on the UK Schmidt Southern Sky Survey photograph is that of a typical young association of a gaseous nebula with its superimposed dark Bok globules, and with bright stars buried in the heart of the nebula. It reminds me quite a bit of the similar nebulae M16, M17, and M20 with their associated clusters -- and of course of that spectacular prototype of the stellar nursery, M42, the Orion Nebula. I suspect that if it were further north, IC 2948 would be nearly as famous as any of these.

Well. Back to the identifications. For IC 2944, I've adopted the position for lambda Cen; and for I2948 the approximate geometrical center of the large nebula that Frost describes. As I noted above, ESO-B adopted positions for the clusters rather than the nebulae, so I've taken the ESO-B positions out of the position table.

Remarks

Modern catalogues view IC 2944 as an open cluster associated with the nebula IC 2948. As an open cluster, it is also known as Collinder 249. It measure 14' across, consists of 30 stars (including Lambda Cen) and has a combined magnitude of 4.5. Its Trumpler class describes it as detached from background, no concentration towards the centre, large range in magnitudes and moderately rich, with associated nebulosity (IC 2948).

Burnham describes the open cluster incorrectly as IC 2948. He calls the cluster 15' in diameter with about 25 stars, including the multiple Innes 422. The latter star has three components, closely situated, of magnitudes 7.5, 10 and 12.

Historical observations

NGC/IC Dreyer (1888, 1895, 1908)

This object was discovered photographically by R.H. Frost on plates taken with the 24-inch Bruce refractor at the Arequipa station of Harvard Observatory. It is described in the NGC as a star of magnitude 3.4 (Lambda Cen) within an extremely large nebula.

Published comments

Hinks, A.R. (1911)

On the Galactic Distribution of Gaseous Nebulae and of Star Clusters

(p.696, footnote)

"The large nebula about lambda Centauri, discovered by Frost at Arequipa, Harvard Nebulae 788-91, and described by Innes and Wood, Transvaal Circular 4, has been omitted by an oversight from my figure."

Cederblad, S. (1946) [VII/231]

Ced 118 (IC 2944)

Position (1900): RA 11 31.2, Dec - 62 28

Star: 62 2127 (Mp=3.32, V=3.34, 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 with conspicuous structure (eg. IC 5146)

Size: 66'x36'

Notes: "IC 2944. Disc. Frost 1908 (852). FA 18. (194, 828). R. IC 2944 = {lam} 118 Centauri with nebula. IC 2948 = The cluster following {lam} Centauri. This star (= CPD -62 2127 = HD 100841 = Boss 15899) is probably alone responsible for the nebular light. The cluster is probably situated behind the nebula."

Hoffleit, D. (1953)

1953AnHar.119...37H

A Preliminary Survey of Nebulosities and Associated B-Stars in Carina.

p43.

Thackeray, A.D. (1956)

"IC 2944 - An O-type Association" MNASSA, 15, 20.

Concludes that "IC 2944 [stars and nebulosity] represents an O type association and that its distance probably exceeds 2 kpc, consdierably greater than previous estimates. Such associations are commonly regarded as 'tracers' for spiral arms within the galaxy. This result emphasises the need for accurate classifications and absolute magnitudes before reliable tracings of arms can be carried out."

Mentions dark nebulae, and photos of these

Sher, D. (1965)

"Structure of the Milky Way in Carina" QJRAS, v 6, p 299-320. "IC 2944/2948 is a large H-alpha region containing one of the biggest concentration of O stars in the sky, 'exceeding even the well-known 'elephant trunk' nebula and cluster NGC 6611' which it resembles in some respects. The stars are to loosely grouping within the nebula to be easily recognizable as a cluster on Uppsala Schmidt plates.

Hogg, A.R. (1965)

"Cat. of Open Cl. south of -45 Decl.", Mem. 17 Mnt Stromlo Obs.

Laustsen, S., Madsen, C. & West, R.M. (1987)

Exploring the Southern Sky: A pictorial atlas from the European Southern Observatory. Springer-Verlag.

Scanned image on disk. [1987EtSS.........0L], plate 145.

Photo index

by Jim Lucyk: Sky&Tel. 4/78 p296, Vehrenberg's Atlas of DS Splendors (3ed) p111, Deep Sky #22 p7.

Modern observations

ASV Journal (1971)

Vol 24 No 3 June 1971: "dispersed cluster in 2-inch 64x."

[amastro] discussion

Copied below is a discussion by Brent Archinal of the identifications for some southern cluster/nebulae designations around lambda Centauri which follows from an inquiry by Steve O'Meara. As is usual for this sort of thing, Brent goes back to source material rather than relying on modern publications, which, as in this case, are often wrong. As per O'Meara's subject line, the already infamous Caldwell list has yet more goofs.

Brian

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

Subject: caldwell 100 hell

To: someara@interpac.net

Date: Tue, 08 Jun 1999 17:27:35 EDT

Cc: bwilson2@ix.netcom.com (Barbara M. Wilson),

larrymit@ix.netcom.com (Larry Mitchell), bas@lowell.edu (Brian Skiff),

baa@casa.usno.navy.mil (Brent A. Archinal)

] Okay,

] Here's what I've discovered about Caldwell 100:

] IC 2944, as brent and Brian say, is the nebula surrounding the Lambda

] Centauri star cluster. And that the cluster is Collindar 249. My question

] is...where is the cluster. Also here are some discrepencies:

] * Tirion's sky atlas shows the nebula IC 2944 to be immediately surrounding

] lambda and flowing off the the northwest. IC 2948 it shows as a cluster to

] the southeast (in the position of Collindar 249, I believe).

] * Uranometria shows IC 2944 as a cluster of stars immediately to the east

] (and including) Lambda. IC 2948 is a nebula surrounding lambda but flowing

] to the south and southeast.

] * The DSFG lists IC 2944 as a mag 4.5 cluster southeast of lambda and IC

] 2948 a nebula around this region.

] * And the Millennium star atlas has IC 2948 being a nebula surrounding

] Lambda and the regions to the south and southeast. It does not show a

] cluster anywhere.

] * The Index Catalogue says 2944 is a cluster and nebula centered on Lambda

] * And the Index Catalogue says 2948 is a nebula southeast of Lambda.

] * And Sky Cat 2000 says ngc 2944 is a 15' cluster centered on Lambda -- not

] southeast, as Collindar 249 is.

] So, there's my problem. Any help would be appreciated.

] Stephen and Donna O'Meara

Steve -

I decided to look at this situation in detail, and from scratch, in order to try to make sense of it. The first thing to do of course is to check the original data, which I list below for IC 2944, IC 2948, and Cr 249, along with descriptions of what is at the given positions on the DSS.

=> IC 2944 - The IC lists this as "F. 789", at (1860.0) 11 29 20 NPD 152 14.7, with description "* 3.4 in eL neb". "F. 789" is actually "Frost 789", listed in the Harvard Annals, v. 60, p. 179 (table VII). It is listed there as at (1900.0) 11 31.1 -62 28, and with description "Neb. around A.G.C. 15848, ext. from 11h 30m to 11h 31m, and from -62d 14' to -62' 40'." The position updates to (2000.0) 11 35.7 -63 01, and the range updates to approximately (2000.0) from 11h 35m to 11h 36m, and from -62d 47' to -63d 13'. The IC position updates to (2000.0) 11 35 47 -63 01.1, agreeing with Frost's position.

The DSS shows at this position a very bright star (Lambda Cen, V=3.10, at 2000.0 11 35 46.88 -63 01 11.4, SIMBAD), and faint nebulosity around it that is mostly overpowered by the star. There is also a very rich background of stars here, which may be a cluster.

=> IC 2948 - The IC lists this as "F. 790", at (1860.0) 11 32 15 NPD 152 45, with description "eeL". The position updates to (2000.0) 11 38 46 -63 32. "F. 790" is actually "Frost 790", listed in the Harvard Annals, v. 60, p. 179 (table VII). It is listed there as at (1900.0) 11 34.1 -62 58, and with description "Neb. patch ext. from 11h 30.6m to 11h 38.1m, and from -62d 28' to -63d 14'." The position updates to (2000.0) 11 38.8 -63 31 and the range updates to approximately (2000.0) from 11h 35m to 11h 43m and from -63 01 to -63 47.

At the precisely given position, the DSS shows nothing unusual (using a 600 pixel, default resolution image). However, with histogram equalization, there are concentrations of nebulosity to the NW, NNE, and NE. A 1 degree DSS image shows nearly the full 1 degree square filled with nebulosity, with the patches mentioned above being the brightest portions. Clearly, Frost had the whole area in mind and not just the brighter concentrations. It is also clear that this is the (larger) nebulosity to the SE of IC 2944 and is not IC 1944 itself.

=> Cr 249 - Collinder 249 is listed by Collinder (1931, p. B12) as "IC2944" (more comments on this follow). He gives a position of (1900.0) 11 32 -62 28 and other info (Collinder) as 40' x 15' in size, 2.7 total magnitude, and 25 stars total, and (Lundmark) as 65' x 40' in size, 2.9 total magnitude, and 40 stars total. It is called a "Neb. cl.", so the involved nebulosity was clearly recognized. Collinder's comments (ibid, p. B34) read:

249. IC 2944 [lambda] Centauri cl. - Class: bbaaa. - e: 0.79, 0.93. - Lk: A replica in Southern Sky of Pleiades. Cr: This is a chain of bright stars with nebulosity. Areal density of fainter stars the same as in surroundings (from star-counts).

Collinder goes on to present a _photograph_ (his plate 5) of this cluster, copied from the Franklin-Adams charts, showing several bright stars in nebulosity. I couldn't easily match this photo to a DSS image of the area, so I checked the Franklin-Adams chart (no. 18) themselves. I was surprised to find that Cr 249 in fact is a very large object, consisting of all of the stars around IC 2944 _AND_ IC 2948, and then some! Generally it the whole group of stars extending from Lambda Cen (and IC 2944) on the NW, down to a pair of stars at 11 41.8 -63 49, thus filling a oval about 1 degree in length! Lundmark's size estimate of 65' x 40' thus matches well.

Thus, Cr 249 is NOT IC 2944. Frost mentioned nothing about a star cluster in his descriptions of either IC 2944 or IC 2948. And more importantly, the group of stars is centered on IC 2948 and NOT IC 2944 as Collinder's identification and (incorrect) position implies. These two errors by Collinder are obviously the source of all the later confusion about the objects here.

Anyway, I hope this clears things up once and for all. Here are some final comments on the specific references you cite.

] * Tirion's sky atlas shows the nebula IC 2944 to be immediately surrounding

] lambda and flowing off the the northwest. IC 2948 it shows as a cluster to

] the southeast (in the position of Collindar 249, I believe).

IC 2944 is immediately surrounding Lamda, but both the DSS and the F-A chart show it pretty much concentric on Lamda. IC 2948 is the nebulosity to the SE, while the the whole area is the cluster Cr 249.

]

] * Uranometria shows IC 2944 as a cluster of stars immediately to the east

] (and including) Lambda. IC 2948 is a nebula surrounding lambda but flowing

] to the south and southeast.

Again, IC 2944 is only the nebula around Lamda. The IC 2948 nebulosity is south and southeast of Lamda but does not include it.

]

] * The DSFG lists IC 2944 as a mag 4.5 cluster southeast of lambda and IC

] 2948 a nebula around this region.

Once again, IC 2944 is only the nebula around Lamda. The cluster Cr 249 lies mostly southeast of Lamda but includes it. IC 2948 is the nebula as described.

]

] * And the Millennium star atlas has IC 2948 being a nebula surrounding

] Lambda and the regions to the south and southeast. It does not show a

] cluster anywhere.

IC 2948 does not surround Lamda, but does lie to the south and southeast.

]

] * The Index Catalogue says 2944 is a cluster and nebula centered on Lambda

No it doesn't. As noted above it says "* 3.4 in eL neb".

] * And the Index Catalogue says 2948 is a nebula southeast of Lambda.

That's essentially correct.

] * And Sky Cat 2000 says ngc 2944 is a 15' cluster centered on Lambda -- not

] southeast, as Collindar 249 is.

There apparently is no cluster centered on Lamda, although it does have a rich background of Milky Way stars.

Anyway, I hope all this information helps. Let me know if there are any questions or comments. I'm sending copies of this to Barbara and Larry (as with your original message) and to Brian who originally noted that IC 2944 was the nebulosity around Lamda.

Oh, and Steve, I'll try to get back to you tomorrow about the NGC 6882 and NGC 6685 business.

- Brent

Brian Skiff

15cm - no cl at spot immed E of lambda Cen as per U2000; cl & neb are S&E as is obvious on the atlas. cl is elong string of * m7-8+. about 50 *s vis @ 80x in 20'x4' area located on the E flank of the W portion of a V-shaped section of nebula (got that?). neb best viewed @ 30x/1.6-deg fld and at 50x/1.2-deg fld. it responds best to UHC, but differently to [OIII], and it is not as lg or as br w/[OIII]. neb in two parts: E arm elong N-S, 35'x10'; this is really what forms the V w/the cl W. the Wrn part of the neb is merely oval, 20'x10', and parallels the cl on the W side. BS, 23Feb1990, LCO.

Contemporary observations

Auke Slotegraaf

1997 March 24

1997-03-24, Monday. Jonkershoek. 11x80 tripod. Full Moon. Not found.

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