Albert (avandonk) recently shared with me his wide field image of Crux. It just framed the main stars very well. On the very edge (south of Beta), near the coal sack there appears to be a red nebulae with a central star (9th mag?). I was surprised that the full data base of Voyager 4 failed to identify it. I don't feel I have the right to post Albert's image, but I estimate it's position as 12hr 50.4m, -61° 35'.
Does anyone have any suggestions or imaged it themselves?
I found something on one of images I took earlier this year with 135mm F5.6 and Canon 400D.
On first image, Beta is at lower left, down at the lower edge is Jewell Box.
The second image is the crop (200% size) with H 7 in the middle.
Just had a look through SIMBAD for those co-ordinates (have I mentioned before how much I hate the SIMBAD interface!) and the best bet is probably Gum 46:
Hi Glen
Gum 46 does sound like a possability but how big is the object in your image. Gum 46 is a tiny object no more than 2-3' across. please let us know.
Andrew
Les, thanks for the SINBAD work. Yes, I have no doubt it is Gum 46.
Andrew, I've had a closer look at Albert's (avandonk) brilliant image and I estimate it to be around 2 arc min in size (about what you expected). The overly enlarged image (my apologies Albert) shows the distinctive Gum 46 shape. The larger field shows it position. The top bright star is Beta Crucis, with the open cluster Tr20 centre right. Gum 46 is in the bottom right corner. The brightish star between the neb and cluster is 5th mag iota Crucis. To give you an idea of the scale Beta and Iota are 1° 20' apart.
Bojan, does this image help you sort out your 'red' star?
Gum 46 seems to have a reasonable surface brightness - has anyone observed it?
Iota Cru is 49 arc-min from both Gum 46 (lower left) and Tr 20 (right).
Gum 46 is low in the sky at present and the mag 10.8 star in it might make it hard to see.
The nebulae is 1.3 degrees to the right of the Jewel Box at 4am.
I have observed it a couple of times now. From and observation during 1999 I have the object listed as small faint nebula surrounding a 10th magnitude star. Responds well to the UHC filter. The nebula is located in a very busy field.
Andrew
"Gum 46 seems to have a reasonable surface brightness - has anyone observed it?"
I haven't, though I'll put it on the list of things to do -- probably a waste of time though (see below).
I had a bit of a poke around in the ESO online DSS which allows access to blue, red and infrared (don't know which channel, might be "K-band" but probably "J-band") plates. They are attached and tagged. All are 5' x 5'. I can't say whether they were all identical length exposures. The blue plate is the same sky-coverage but the image scale is not quite the same.
Gum 46 is essentially invisible on the blue plate, very, very bright and detailed on the red and somewhat dimmer but still visible in what I assume to be J-band.
I see from the original pic by Albert (very nice BTW) that it records well with a cooled camera, short exposure with a fast lens, and appears very, very red. Compare the colour of Gum 46 with the carbon star next to Mimosa (Beta Crucis). ESB-365 is an exceptionally red star ("blood red" visually), but Gum 46 is a somewhat deeper red than ESB-365.
The coal-sack is only about 125-odd pc away, it is therefore a fair assumption that Gum 46 is well in the background, many times further than the coal-sack and is shining through it. For a start, the illuminating star of Gum 46 is an O 9.5 star and if it were < 125 pc (ie in front of the coal-sack), it would be shining in our sky at about 1st magnitude -- if not brighter. The redness of the nebula also corroborates this hypothesis that it is a long way in the background.
Being behind the coal-sack therefore will have the effect of very significantly reddening the nebula -- explaining its brilliant appearance on the red plate and non-recording on the blue one and its photographic appearance with Albert's set-up.
Given our eye's relative insensitivity to deep red at low light-levels and the apparent total absence of blue/green in the images, I'd therefore be very surprised if anyone has seen it in less than a 25" 'scope (if at all in any 'scope). I might be wrong but that's the way I see it (or don't see it in this case). Visual filters (UHC, H-Beta) will almost certainly be of no use because there seems to be no light H-Beta or OIII for them to pass.
If it weren't for the coal-sack, it'd probably be a very nice object in medium-sized 'scopes.
"I have observed it a couple of times now. From and observation during 1999 I have the object listed as small faint nebula surrounding a 10th magnitude star. Responds well to the UHC filter."
Hmmm ... seems you and I were writing at the same time. Wow, so it can be seen in 20"! I don't doubt you at all, but it is surprising and even more so that it brightens to a UHC filter.
Hmmm... have to give it a go then with the 18" come Autumn I guess ...
Bojan, does this image help you sort out your 'red' star?
Yep..
Red star is red star, and Gum 46 is barely visible on my photo (attached)... only the central star is there.
This is no surprise, since my Canon 400D is not modified and the image was taken from Mt Waverley (LP is terrible here and I did not use any filter when taking this).
I may have a good reason now to go outside Melbourne next weekend :-)
