Query regarding NGC 2257

[Coen]

Hello,

I stumbled across this object and was curious what people's observations/experiences of this might be.

I have seen varying sources quote varying things for this thing, the main comments seems to be it is a LMC globular cluster that covers about 3.5 arc minutes at around magnitude 13.5

The object is located in Dor, not far from the LMC and on the border with Pic.

Now I was scanning the sky in the area with my little 70mm refractor and thought I saw a patch of sky that was somewhat brighter than anywhere else and it also appeared largish centred on where NGC 2257 is to be. Subsequent consultation with the star atlas and various data sources indicates that the phrase "tell him he is dreaming" seems appropriate as it likely I did not see NGC 2257 itself and it appears that it is not situated in a particularly rich patch of fainter background stars either.

Does anyone have some notes or recollections they might wish to share on this object?

Thanks.

[ngcles]

Hi Coen & All,

I have two observations of this cluster, both with 25cm one from suburban Engadine (but at my old house) and the latter from a dark sky site nr Mudgee. Both are from the mid-90s.

x138 21' TF. Mag 12.5 Size 2.1'. E!/Faint, pretty sizable cluster for LMC. Diffuse and gossamery. Hardly there at all. Maybe 2' diameter, found to the S of centre of a long crooked line of mag 11.5 and 12.5 *s. running NW-SE. Cluster found near *s 3&4. No cc, est cc 12. No resolution. V!/LSB but I guess from suburbia better than mag 13.5 would indicate.

x181 17' TF. Mag 13.5 Size 2.1'. Fairly unimpressive G.C with very little central condensation, just to the SW of a mag 11 * by 2', 1-1.5' diameter round LSB cottony ball est cc 10 with a broad slight concentration to the centre. No resolution at all. Not a hint.

It is a rare occasion that I can actually remember an observation that far back -- but the latter one at Mudgee I can actually remember quite well because one of my mentors in astronomy, the great variable star observer Peter Williams was observing with me. It was a searingly hot day that day with temps peaking just over 44 deg C and being assaulted constantly by a legion of bush-flies. We spent most of that day bobing around in the public pool at Mudgee both to cool down and also to escape the flys -- they were that thick.

The RNGC provides a V mag for NGC 2257 of 13.5 and a diameter of 3.4 arc-mins. I have a feeling the former is a bit of an under-estimate and it is more likely about the V mag is 12.5-ish. The discoverer was William Herschel with the 18.7-inch speculum metal reflector which is a rough equivalent in light-gathering power of a modern 15 or 16-inch.

His note was "F, cL, R, vglbM, r, 17.0 sec d" -- (Translated) Faint, Cluster, very gradually little brighter in the middle, not well resolved, 17 arc-seconds diameter.

Steve Gottlieb of the USA observed NGC 2257 from Costa Rica with a 13" Newtonian and there are his notes:

"fairly faint, fairly large, round, 1.5'-2' diameter, broad weak concentration, no resolution. An elongated group of a half dozen mag 10-11 stars oriented NW-SE passes just N of the cluster. This is one of the oldest LMC globular located at the north-east periphery of the cloud, and is comparable in age to galactic globulars, ~10 billion years old."

You can check the above on the NGC/IC project page at

http://www.ngcic.org/pubdb.htm

I've had a bit of dig around through the SIMBAD interface (have I mentioned before how much I hate the SIMBAD interface??) The last sentence in Steve Gottlieb's description as to age of this object was discussed in a paper here:

http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1989AJ.....98.2086W& data_type=PDF_HIGH&whole_paper=YES& type=PRINTER&filetype=.pdf

And it seems from the paper that NGC 2257 is a "true" globular system of the LMC. So many LMC "globulars" aren't globulars at all but are instead "globular-sized", populous and rich open clusters -- ie they are metal rich and relatively young. NGC 2257 seems a classical GC with an age about 11 GYR, a short main-sequence, low turn-off point, a well populated red-giant branch and most importantly a well populated horizontal branch with a large number of "cluster variables" ie RR Lyrae types. It is a true GC having a lot in common with classical Milky Way clusters like say M13 or M3.

From the C-M diagram in the paper, it seems the very brightest stars (just a few) are below 16th mag with a bunch of the brightest giants about mag 17.5. The horizontal-branch stars are about 19th magnitude. Resolution would need a huge telescope to say the least.

Actually, re-considering the magnitude issue again after looking at the C-M diagram, maybe 13.5 is reasonable.

So is it possible to see it with a 70mm refractor? Well I don't know your site or how good it is but I'd most likely concur with you that it wasn't NGC 2257 -- just too faint I think. But that's only my opnion and as I've said before, I could be wrong!

There's no obvious "other" candidate in the area to match the observation, so who knows?

Maybe it was Nibiru ?? ;);):rofl:

Hoping it was of some help.


Best,

Les D

[Coen]

Thanks for the excellent information. I had a look at the RNGC before I posted the above query. Your first entry (138x) with "Diffuse and gossamery. Hardly there at all." sums up what I thought I saw through the eyepiece. I'll translate the hardly there to not there in my case.

My observation was from the backyard in the far northern suburbs of Adelaide. While I routinely see down to magnitude 11.8 around that part of the sky, I will not claim I have seen this particular object and put it on my list of things to look at with (a) a dark site and (b) larger telescope.

Thanks again for your excellent notes.