Hi Renato,
Quote:
Originally Posted by Renato1
I would use the old Herald Bobroff AstroAtlas very detailed charts for checking out the Magellanic Clouds.
The first thing I noticed when I got that atlas was that it had quite a few open clusters marked, where the first edition of Uranometria instead had globular clusters marked for the same objects.
I can't find the reference, but going from memory, I read that some 40 or so objects originally identified as globular clusters in the Magellanic Clouds - because they looked like globular clusters - were subsequently found to not be globular clusters because they were bluer and not quite as round. As such, they weren't really open clusters either.
Regards,
Renato
|
Probably the best reference material/atlas for observing the LMC and SMC are Mati Morel's Visual Atlas of the LMC and second volume for the SMC -- published in 1989. It is out of print and now very difficult to find. Exceptionally detailed.
As you and others have noted, there are a lot of clusters both in the LMC and the SMC that are misidentified as G.Cs in many amateur maps when they are actually extremely large, highly populous open-type clusters. Because of the visual compression effect induced by distance, a great many within the Magellanic Clouds look like globular clusters but photometry and spectroscopy shows them to be far, far too young to be classical G.Cs.
G.C's stars generally have very, very low metallicity and cluster colour-magnitude diagrams show them to be genuinely ancient objects composed only of Population II stars. The canonical age for a cluster to be a genuine G.C is ~10GYRs. Their C-M diagrams clearly show a highly truncated main sequence terminating in K or G-type stars (the cluster is so old all the higher mass stars have already evolved into red-giants and expired), a strong red giant branch and AGB stars + (most importantly) a blue (relatively) horizontal branch containing RR Lyrae-type (so-called "cluster variables") variable stars.
In more recent times, a number of objects (particularly in the Magellanic Clouds) have come to light that share some characteristics of both O.Cs and G.Cs. Cross-over types that have some properties consistent with being globular and others indicating "open", so the dividing line is beginning to get a bit blurry. Until relatively recent times, there hadn't been high res spectroscopy of many objects in both clouds so a lot of objects, particularly in older maps were classed as G.Cs or O.Cs based purely on photographic or visual appearance This explains the change in designation you have observed in the atlases.
This is probably the paper by Paul Hodge from which many of the "designations" on older atlases are probably based, but doesn't include genuine spectroscopy or C.M diagrams. It is based purely on the difference between yellow and blue plates where a "red" appearance is assumed to be a G.C. Since then a lot of these clusters have been whittled away as hi-res spectroscopy and C.M diagrams showed them to be massive and populous open-type clusters with Population I stars.
https://articles.adsabs.harvard.edu/...pJ...131..351H
The best and most recent data will involve trawling through the professional literature on SIMBAD for each individual object to see what the most recent data indicates. (...and yes I know this is both a highly time-consuming and tedious process).
http://simbad.u-strasbg.fr/simbad/sim-fid
There is only one "genuine" G.C in the SMC -- NGC 121 in an adjacent field to 47 Tucanae. You may well find in some maps as many as a dozen G.Cs marked in the SMC (based on appearance) but NGC 121 is the only genuine G.C. It is about 10.8 GYR old and has some of the oldest stars in the SMC. Within a field or two of 47 Tucanae, there are three other SMC clusters that have all, at one time in recent times been considered (or proposed as) G.Cs -- Kron 3, Kron 7 and NGC 152. But we now know NGC 121 is the only one that's the real deal. Kron 3 & 7 and NGC 152 are all visible in moderate apertures given a dark sky.
Similarly in the LMC many are marked as globular but are really (probably) open. I think from memory there are only about 12 or 13 genuine G.Cs in the LMC but I can't remember which ones right at this point. There are a great many others that mimic the appearance of a G.C in the eyepiece, but are really quite young clusters.
Ooo Ooo Oooo ...
I have just found (what appears to be) a (
relatively) simple answer -- there is a Wiki page devoted (some of) the G.C's in the Local Group and lists the definite G.Cs in the LMC: NGCs: 1466, 1754, 1786, 1806, 1818, 1835, 1841, 1846, 1854, 1866, 1868 + Reticulum (ESO 118-31) -- so there seems to be an even dozen in the LMC and just the one in the SMC. There is still some conjecture over whether ESO 118-31 is gravitationally bound to the LMC or the Milky Way ... or an escapee.
You can throw NGC 2257 into that list as well -- it is definitely a GC.
Going through the ages of some of these clusters that are, according to the Wiki page G.Cs, indicates quite young ages. So again were almost back to square one.
As for any others you see marked on maps, you'll have to trawl through SIMBAD to see the state of the current literature.
The Wiki page does not include G.Cs attached to the Andromeda Galaxy (M31) or M33. Best source for that is Paul Hodge's atlas of the Andromeda Galaxy. Andromeda probably has about 250-300 classical G.Cs but very few are visual targets in amateur 'scopes.
https://en.wikipedia.org/wiki/List_of_globular_clusters
As to what is and what really isn't a GC when it comes to the LMC and SMC has for the past 30 years and continues to be an increasingly confusing and muddy area.
This page may also be very helpful in terms of quality maps:
https://cloudsofmagellan.net.au/page-2.htm
Hope this helps in some way. I know I've gone around in circles somewhat but when it comes to definite classification for LMC and SMC clusters, it is a very fluid thing. Probably best in the end to think of all of them as clusters or varying age (where you can find a definite, properly determined age).
Best,
L.