Nebulae mainly in Corona Australis

[AusGuy]

Generally, for personal casual viewing, I set my camera to bin 2 mode (effectively creating 9.26 square pixels) and set a fairly aggressive gain of 400+ so as to get quick results – usually with a maximum exposure time of 1 minute. But because I wanted to get as much detail for this field's fainter objects as realistically possible I reverted back to bin 1 mode (4.63 square pixels), upped the exposure time to 2 minutes and cut back the gain to 270.

Image 1 is a view of the full field.

Image 2 is the same but with the brighter objects labelled.

Image 3 is a zoomed in view of the diffuse nebulae.

And Image 4 is further zoomed in to show the fainter objects in more detail.

Bernes 157

The elephant in the room is clearly the dark nebula which swoops down from top-right to hook up and left in the centre of the frame. Most sources put its distance at 420ly away from us. This molecular cloud is a region of cold gas and dust that is so dense that its central part almost totally obscures the background stars of the Sagittarius arm of the Milky Way some 4,500ly behind it. We can see that the darkest parts fade away quite abruptly into a fringe of less dense clouds that generally impart a brownish hue to the background stars. At the location where the dark nebula hooks upwards its width including the fringe seems to span at most ¾ degree which at 420ly distance equates to about 5ly. Without knowing how deep the clouds are there it's not unreasonable to assume that the depth is also about 5ly. Overall, this dark nebula provides a dramatic backdrop to the other actors on the stage.

The naming of this dark nebula has a fascinating history which you can find at www.surastronomico.com/sec-58-shedding-light-on-bernes-157.html Stretching across almost 8 degrees of sky it is mostly out of frame off to the upper right and here we see only one end of it. Its total mass is equivalent to about 50 times that of our Sun. If you look at images that show the full extent of Bernes 157 you can see why it has the nickname “The Anteater Nebula”. The two patches of blue reflection nebulae form its eyes and the curved bit at the end is its long snout. Here we see only its head and shoulders.

IC 4812

This is an easy object that you can't miss. Being so obviously sited in front of Bernes 157 it looks at first sight like it may simply be part of the nearer edge of that dark nebula. But is that really so? It seems fairly clear that this reflection nebula is caused by light emitted by one or both of the two bright stars associated with it (HR 7169 and HR 7170 - but also collectively known as BSO 14). Whether it is just one of of those stars or both must depend on how far away they are. Finding a sensible answer by searching the internet has proved to be extremely problematic but I have finally relied on Cartes du Ciel which says 139ly and 252ly respectively. That means that although they are separated by only 13 arc seconds they cannot be gravitationally bound and thus they are only an optical double. They have almost the same visual magnitudes (6.4 and 6.6). But sources agree that they are of different spectral types. HR 7169 is B9V and HR 7170 is B8V-IV. Thus HR 7170 is the brighter of the two which figures as it is 113ly further back. It seems that each is itself a multiple star system consisting of 2 and 3 members. Since both stars seem to be well in front of the 5ly deep Bernes 157 behind them it looks like the blue nebulosity is not in fact part of that dark nebula - which runs counter to most descriptions that I've read. And there's still the question of whether just one is creating the nebula or both. A puzzle for solution by someone who is better able to cope than I am.

NGCs 6726 and 6727

Also easily seen is the spidery blue reflection nebula created by these two objects. Although only just hinted at in our view, long exposure images confirm that this nebula forms a continuum with IC 4812 and overlies the hook part of dark Bernes 157. Again, the question arises as to whether this nebula simply illuminates the nearer part of the dark molecular cloud or whether it is closer to us, like IC 4812. Its two sources seem to be distinct stars (clusters?) embedded in bright nebulosity.

NGC 6729

This designation is given to quite a large region to the right of the blue reflection nebulae. It includes the bright fan shaped emission/reflection nebula which is quite obvious in our screen view. It also contains various Herbig Harro objects (see below). The following description of that brighter part is clipped from a NASA article. “Caldwell 68, also known as NGC 6729, is located roughly 400 light-years away in the constellation Corona Australis, making it one of the closest star-forming regions to Earth. It is a small part of the larger Corona Australis Molecular Cloud. It was discovered by the German astronomer Johann Friedrich Julius Schmidt in 1861 and has an apparent magnitude of 9.5. The fan-shaped nebula appears to extend out from the star R Coronae Australis ... toward the star T Coronae Australis. The star that illuminates Caldwell 68 is known as a T Tauri star - a type of young star whose brightness fluctuates over time. Often, T Tauri stars have circumstellar disks. These debris disks surround some young stars and could develop into solar systems much like our own.”

Another description of R CrA from Universe Today is “Here we have another unusual one – a “Herbig Ae/Be” pre-main sequence star. The star is an irregular variable with more frequent outbursts during times of greater average brightness, but it also has a long-term periodic variation of about 1,500 days and about 1/2 magnitude that may be linked to changes in its circumstellar shell, rather than to stellar pulsations. Although R Coronae Australis is 40 times brighter than Sol, and about 2 to 10 times larger, most of its stellar luminosity is obscured because the star is still accreting matter.”

HH 100

A real challenge for EEVA! This Herbig Haro object is really small and faint. Nevertheless it can just be seen on screen as an upside-down and back-to-front comma. You'll notice that I've also labelled a tiny red patch of nebulosity to its upper left as “!!”. Apparently that patch was the object first named Bernes 157 before the name was transferred to the dark molecular cloud. It may now be labelled HH 101? It seems that both objects are associated with R CrA and a detailed view and explanation can be seen at https://www.eso.org/public/news/eso1109/ - you will need to turn the image there upside-down to match our view.

NGC 6723

This globular is much, much further away than the other objects in our view. In fact it's so far away that it is located below the bulge of the Milky Way's central core. Even so it is bright and presents a fine sight in our on-screen view although it is just below naked eye visibility at mag. 6.8. As seen it spans about 9 arc minutes but Simbad quotes it as being somewhat larger at 11½ arc minutes. At 28,400ly distant this gives a diameter of 98ly. In our view the inner part is well overexposed so it's not easy to see that in fact, as globulars go, this is a fairly open one with little evidence of core concentration. It is of intermediate metallicity meaning that it contains many younger stars with higher metallicity content. If you enlarge our image you can see that the outer regions of this globular contain many deep blue stars.

Epsilon CrA.

At only 98ly away this star dominates the upper left part of the frame. It is a main sequence class F4V yellow-white dwarf star with a mass of 1.23 times that of the Sun. In fact it is an eclipsing binary varying in brightness between mags. 4.74 and 5 over a period of 14 hours. Two very similar eclipses take place within each orbital period. First a faint star passes in front of the bright one that gives us 95 or so percent of the light, and then the bright one passes in front of the fainter.

HD 175714

It's not surprising that this single star is dimmer at mag. 7.7. It is 592ly distant putting it quite a bit further away than Bernes 157. So it is shining through that dark nebula's fringe. That is unless the nebula itself is receding away from us there. Also it is a class K5 star so quite a bit dimmer and redder than the Sun which probably explains the colour that we see.

HD 176617

According to Simbad this star's class is M3III, putting it well into the red dwarf end of the main sequence, and its magnitude is 8.2. The only distance value that I can find is 250ly +/- 240ly! So it's probably closer than Bernes 157.

HD 176423

Being of spectral class A1 this star is intrinsically very bright but has a visual magnitude of only 9.5, even dimmer than HD 176617. Thus it must be further away and probably lies behind the fringe of Bernes 157 although it does not seem to have been reddened appreciably by that dark nebula.

In conclusion, it seems that EEVA is viable for this target region, even for the dimmer objects and I'll look forward to sharing it with visitors at some time in the future when we are released from Covid19 lock-down.

Vixen R200SS; SW AZ-EQ6 Pro; ZWO ASI294MC Pro; Baader MPCC III; Bortle 4.5

SharpCap livestack using 2 minutes exposures at 270 gain.

Edited August 8, 2021 by AusGuy
Correcting image placement problem

[JeremyS]

Excellent 👍🏻

[Marvin Jenkins]

What a brilliant write up. Fascinating stuff. Thank you.

Marv

[Martin Meredith]

I agree, a wonderful write-up and a lot of links I'd like to follow up when I have more time. It goes to show how much fascinating stuff there is in some fields.

As an aside, I've often found myself wondering about why sometimes in EAA we spend so little time on each field (trying to differentiate EAA from AP, I suppose). Even with my small FOV there are too many objects to properly appreciate in a short time, and with the larger FOVs that are becoming more widely-used in EAA one could easily spend an hour or more on each field.

Martin

[Mike JW]

Martin, you make a good point. 

Mike

[AusGuy]

Thank you very much for your responses. There is so much about this region that I don't really understand. For example, why the blue reflection nebula surrounding NGCs 6726 and 6727 takes on the shape that it does. Reflection nebulae are caused by light bouncing back in our direction off dust particles. The wispy filaments seem to radiate away in all directions from the two central concentrations and it looks like the dust there seems to be preferentially organised along those filaments. But why? Is it because they reveal some local magnetic lines of force? A puzzle.