Object Note

Using the determination from Harold Corwin that IC2559 is the southern lobe of NGC3324 seen in the Harvard plates and the NGC3324 is the entire nebula and cluster as originally seen by John Herschel, the position used is the central star HD 92207

Harold Corwin

IC 2599 is part of the emission nebula NGC 3324. But just which part has not been quite clear. (Wolfgang caught this independently, and urged me to write this note.)

Dreyer, in the second IC, credits Pickering (in HA 26, 1897) with the discovery of a second nebula here (John Herschel found the first, NGC 3324, on 1 April 1834, in his seventh sweep after he set up his telescope at Feldhausen in the Cape of Good Hope). But Bailey also includes the second nebula in his 1908 HA 60 list of bright and large nebulae and clusters. Since Dreyer includes other of Bailey's objects in the second IC, it makes sense to at least let Bailey share the credit for this nebula as well.

However, Pickering and Bailey describe it differently. Pickering says

The position of NGC 3324 is nearly that of AGC 14528 ["AGC" is Benjamin Gould's Argentine General Catalogue of 1885; the star, HD 92207, is at 10 37 27.5, -58 43 59 (J2000, precessed from the AGC)]. The [new] nebula a little above it ... around AGC 14525 [= HD 92206A at 10 37 22.4, -58 37 22 (also J2000)], has not received a separate number in Dreyer's [New General] Catalogue.

So, Pickering places IC 2599 north of NGC 3324. Bailey, however, has this to say

There appear to be two nebulae near the position of NGC 3324, one around the triple star AGC 14524 [= CD -57 3378 at 10 37 18.6, -58 37 36], [1452]5, and [1452]6 [= HD 92206B at 10 37 23.2, -58 37 23], and the other near AGC 14528. The former is probably the object seen by [John Herschel], since he refers to a double star; but since he only gives one nebula, he may have included both objects as one, especially since they are really joined by nebulosity. In the present catalogue, they are given as separate objects, and the nebulosity near the star AGC 14528, magnitude 5.57, is entered without a number. Both of these nebulae appear to be merely condensations of the faint extension of the Great Nebula [eta Carinae].

So, Pickering makes the northern object I2599, while Bailey makes it the southern object. So, which is it?

John Herschel's position for N3324 is 10 37 23, -58 37.4 (again J2000), virtually identical with the AGC positions for HD 92206 which must be the double star he noted as involved in the nebula. He goes on to say, "[The nebula] extends to a * 6.7 m half a field distant southwards, and almost as far north ..." The "* 6.7" is AGC 14258, so Pickering is incorrect in saying that N3324's position is "nearly that" of that star.

Unfortunately, Bailey's positions for the two objects are too far north by about 7-8 arcminutes. He does, however, have the correct offset between them.

Another confusion comes from Dreyer's note in IC2 "correcting" the position of N3324 based on Pickering's comment that "the new nebula is a little above [i.e. north] it [John Herschel's star 6.7 mag]. This effectively swaps the positions of the two nebulae. But we know this can't be right since John Herschel specifically mentions the double star, and the southern star is not double.

So, since Bailey's description is clear and correct, and his supposition about John Herschel's observation is also correct, I am going to accept his identification of the southern lobe as the correct identification for IC 2599.

In any case, it's clear that John Herschel saw all of the nebulae that Pickering and Bailey found on their plates, not just one or another of its lobes. The two parts of it show clearly on the original DSS V-plate, and must reflect the appearance in John Herschel's telescope pretty well.

ESO has made the two numbers identical. I suppose this is strictly true, but the Harvard observers made the distinction between the two, so we should do the same -- noting, however, that the IC object is, in fact, simply the southern part of the NGC object, all of which John Herschel saw and described.

Finally, I thank Brian Skiff for digging out the AGC positions for me. He has also contributed a few words about the astrophysics of the nebula:

I would consider the pair AGC 14525/6 to be the principal illuminating (fluorescing) stars of NGC 3324, and thus as good a "center" as any for the nebula. ... [Here are] spectral types

    14524 = LS 1695 O8V + O9.7V
    14525 = HD 92206 A   O6V((f))
    14526 = HD 92206 B   O6V((f))
    14528 = HD 92207     A0Ia
    ["LS" = Luminous Star; see Stephenson and Sanduleak,
     Publ. Warner & Swasey Obs. 1, Part 1, page 1, 1971,
     "Luminous Stars in the Southern Milky Way."]

...the last star could be related to the star-forming region, but is too cool to fluoresce the nebula. So it sounds as though IC 2599 is simply the southern part of NGC 3324 that is near the bright A-type supergiant.

This is another argument for considering IC 2599 to be simply the southern part of NGC 3324.

― IC Notes by Harold Corwin

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