Harold Corwin

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 M 16, M 17, and M 20 with their associated clusters -- and of course of that spectacular prototype of the stellar nursery, M 42, 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.

― IC Notes by Harold Corwin

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