(IAAC) The interesting question of the Christmas Tree and the Cone

A recent short thread on IAAC centered around the best designation to use in
refering to the strange, somewhat challenging Cone Nebula in NW Monoceros. It
is very close to the lovely, visually *stunning* open cluster commonly known
as the Christmas Tree cluster. The OC is clearly included in entry "2264" of
Dreyer's New General Catalog, and the question naturally arose whether the
Cone should also be associated with it, as a part of an "NGC 2264 complex".

(The Cone also seems to have its own designation as LDN 1613, just as the OC
and bright nebula have their special designations: Collinder 112, Melotte 49,
S Monocerotis cluster, Sharpless 2-273, Lynd's Catalog Bright Nebula 76, etc.
Many thanks to Jeff Bondono's 'dObjects' database, as always, for this info.
BTW, I also raised the question of why the Cone was not included in the large
Dark Nebula catalog published by E. E. Barnard. But that is another topic!)

In response to this thread, I received a private communication from a very
well respected source in deep-sky observing and identification, the gist of
which was, "Given that NGC 2264 is half a degree away from the Cone, and the
Cone has nothing to do with the Christmas tree cluster/asterism, obviously
the NGC designation has nothing to do with the Cone." This was both welcome
and helpful information, and yet it got me wondering...

The original Dreyer description of N2264 (so far as I can glean it from other
sources) seems to contradict this statement, as it both describes NGC2264 as
actually consisting of both a Cluster and Nebulosity (C+N), and further, says
the associated nebula stretches WELL beyond - actually engulfing - the Cone:

>Type: C+N.
>Description: eL neb, 3deg diam, densest 12' sp 15 Mon

On the other hand, the NGC/IC Project database lists the cluster's size at
just 20' - falling well short of the "top" (S vertex) of the Christmas Tree,
and therefore of the Cone. This certainly seems to corroborate the source!

What is more, the NGC/IC database entry (below) makes no mention of a member
of the Lynd Dark Nebula Catalog (LDN) being involved with NGC 2264:

>Cluster w/nebulosity
> ...
>LBN 911, Mel 49, OCL-495, Lund 246, Sh2-273,
>H VIII-5, H V-27, h 401, GC 1440

(Thanks to the folks of http://ngcic.org for that wonderful resource, BTW!)

And yet, an examination of the Digitized Sky Survey (First Generation) adds
to the confusion: a 60'-wide image centered on the Christmas Tree, clearly
shows it embedded in (or at least visually suffused by) a large complex of
nebulosity, just as Dreyer's description would indicate... And sure enough,
impinging on this suffusing nebulosity, without any visible break, one sees
the deep, dark cut of the Cone Nebula: it *appears* to be clearly involved.

In the final analysis, it is possible a professional may choose to conclude
that the dark nebula is in the foreground, and superimposed on the complex of
bright nebulosity and cluster stars that are part of NGC 2264. (But is this
actually the case? My own inexpert search for Abstracts turned up one or two
papers which discuss the Cone as part of N2264, e.g. "1987A&A...181..112P".
Then again, other references turned up by the same search were more vague.)

But in any case, the NGC does double duty: as a referent catalog used in
professional research papers, and as observing guide for amateur deep-sky
observers and imagers the world over. From the amateur's perspective, it
seems very tempting to include the Cone (LDN1613) with the Cluster (N2264).

I welcome comments from others, especially the many folks on our list more
experienced both in observing, and in catalog history and identification...
Luckily, we Northern Hemispherians will have many long Winter months ahead,
to enjoyably debate and consider such fine points of identification. :)

Clear skies, and THANKS to all who contributed to this thread so far!
Lew Gramer

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