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Steve Gottlieb
August 2nd, 2015, 07:08 PM
Object of the Week, August 2, 2015 – The Unusual Planetary Sh 2-71 in Aquila

Sharpless 2-71 (Sh 2-71) = PN G035.9-01.1 = PK 36-1.1 = LBN 103 = M 1-90
RA: 19 01 59.3
Dec: +02 09 18
Constellation: Aquila

Type: Planetary Nebula
Class: 3b(3)
Size: 124" x 75"
Magnitude: ~12.3V
Central Star Magnitude: 13.5-14 or ?

This planetary has a strange morphology and certainly deserves to be better known! It is one of my favorite large "obscure" summer planetaries and I return to it every year. Interestingly, I seem to describe it slightly differently each time, probably due to its subtle structure.

1746

Sh 2-71 was discovered in 1946 by Rudolph Minkowski and included in a table of "Diffuse and Peculiar Nebulosities (http://cdsads.u-strasbg.fr/full/1946PASP...58..305M)" (designation M 1-90). Stewart Sharpless included it as object #71 in his 1959 paper "Catalogue of H II Regions (http://cdsads.u-strasbg.fr/full/1959ApJS....4..257S)". Sharpless added the note "May be planetary".

It's been long assumed the 14th magnitude star near its center in this Gemini North image is the CS. In 1979 Czech astronomer Luboš Kohoutek announced (http://cdsads.u-strasbg.fr/full/1979IBVS.1672....1K) that this "central star" was variable (period = 68 days). His photometry showed a V magnitude range of 13.2 - 14.0. But he also noted "The observed central star of Sh 2-71 cannot be responsible for the radiation of the surrounding nebula." as the central star is not hot enough based on its B8 spectral type. It has been assumed the variability is caused by the orbiting hot companion.

In 2008, Aussie astronomers David Frew and Quentin Parker noticed a much fainter hot star in the geometric center of the PN with a magnitude consistent with it being the central star. If this is the REAL central star, the star that Kohoutek studied back in 1979 could be a very peculiar and rare chance alignment of a PN and a short period binary. More details are available on this Gemini Observatory announcement (http://www.gemini.edu/node/11815).

So, what can you see? In a dark sky, an 8-inch may reveal the original "central star" and the nebula, though a larger aperture may be necessary to see structure. A challenging, close pair of faint stars is just to its north and my 18-inch marginally splits this pair. As far as details, here is a sampling of my notes --

24" (7/20/12): at 200x and NPB filter, this large relatively bright planetary is elongated ~3:2 N-S and extends ~1.8'x1.3'. The northern portion of the planetary is clearly brighter with the south side having a lower surface brightness and more irregular. With careful viewing, a faint extension or irregular filament extends south on the west side. The filament increases the N-S direction to 2.2' along the west side.

18" (8/1/08): easily picked up unfiltered at 175x surrounding a 14th magnitude central star. Also an extremely faint star was resolved just north of the brighter central star. Good contrast gain using an OIII filter. The planetary is large and elongated N-S or NNW-SSE, ~3:2. The outline is not sharply defined but it does have a better defined linear edge (N-S) on the east side. The northern half is brighter without the filter, but less so using the filter and the southeast corner is slightly brighter.

18" (7/14/07): viewed at 174x with an OIII filter as a roughly rectangular glow, elongated 3:2 N-S with a size of roughly 1.8'x1.2'. Appears slightly brighter along the eastern edge with a small knot or brightening at the southeast corner. The west and southwest edges are slightly weaker and less defined with perhaps indentations or a scalloped edge.

18" (8/25/06): very impressive non-NGC/IC planetary at 140x and UHC filter. With this combination, Sh 2-71 appeared fairly faint, fairly large, elongated at least 3:2 N-S, ~1.6'x1.0'. Appears sharply defined with a straight border along the eastern edge that runs N-S. The south side has a lower, irregular surface brightness and is the faintest section but appears to extend just as far as the north end. Without a filter the 14th magnitude central star was easy and a fainter star was close north of the central star, appearing to be a double. A trio of mag 10.5-12 stars extending beyond the planetary are off the western side. Located 6' E of a mag 9.5 star.

18" (6/20/04): large, faint, irregular glow at 115x and OIII filter. Sh 2-71 appeared elongated ~5:3 roughly N-S, ~1.8'x1.1'. The eastern edge is brighter and better defined. At the southeast corner is a brighter knot that bulges out slightly from the main glow.

18" (8/17/01): interesting, large PN at 140x using an OIII filter. The outline is subtle but irregular. Initially it appeared roughly circular, but with concentration a faint extension sometimes appeared detached at the south side, giving a N-S elongation. The rim is brightest along the east side and curving around to the north giving a partial crescent impression.

18" (8/21/98): immediately picked up at 100x with an OIII filter. This object is an unusually prominent obscure planetary! The best view was at 140x using an OIII filter. Appears fairly large, elongated N-S and brightest along the east side. Seems mottled with a noticeably irregular surface brightness.

13" (7/20/85): at 79x with OIII filter; fairly faint, fairly large, oval N-S, can hold steadily with direct vision. Appears relatively bright for an obscure planetary. Located 6' E of a mag 10 star and several mag 11-12 stars are nearby.

If you track down this object later this month, also try for the challenging, highly obscured globular cluster NGC 6749 just 50' east-southeast (worth its own OOTW!). And what about the huge (roughly 25' in size), low surface brightness HII region Sh 2-72? Is this a visual object?

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As always, "GIVE IT A GO AND LET US KNOW!"

Uwe Glahn
August 2nd, 2015, 08:41 PM
How funny Steve. I just discuss the PN with a friend privately. Very nice choice and a new fact beside the CS. Thanks for the info and the link for the high resolution photo.

I just have one observation with my older 16" (2007). Similar to you I saw a brighter rim to the north, west and east. Without filter I could see two star within the PN. I could not separate the fainter northern star. I made a composite drawing (stars without filter, nebula with [OIII]).
16", 257x, no filter/[OIII], NELM 7m+
1747

Howard B
August 3rd, 2015, 08:16 PM
I also have only observation of Sh 2-71, from a couple years ago at GSSP:

"An interesting PN that looks more like a horseshoe or a blown apart ring. There are two stars in the interior with a few fainter ones strewn about the nebula. Responds about equally well with the UHC and OIII filters. 408x, 21.46 SQM."

1748 1749

I didn't realize that NGC 6749 and Sh 2-72 were so nearby, so that's a good enough reason to go back for another look next week while I'm at the Oregon Star Party.

Steve Gottlieb
August 4th, 2015, 01:00 AM
Thanks for both of you posting the excellent sketches, which capture the visual appearance of Sh 2-71

@Uwe - nice coincidence, though on several occasions when I've observed at Jimi's, he'll pick some little-known VII Zw object (or whatever) and that will turn out to be the next object on my own observing list!

@Howard - let us know how you fare at the Oregon Star Party and good luck with the weather. I'm hoping to observe at Grandview in the White Mountains.

hajuem
August 7th, 2015, 12:32 PM
Hello Steve
I watched the object until last month to observe at very good conditions with 16"!
1750
240x -/OIII
fst: 6m5+; 1100m; Seeing 1-2

I could see the northern arch, each with a western and eastern, brighter knots.
East of the arch is longer than the western part.
Two stars I see centrally. The northern star is significantly less bright as the Southern Star.

CS Hajü
www.astromerk.de

Ivan Maly
August 8th, 2015, 06:51 AM
Fine observations here. I noticed the asymmetry of the shell and the second central star. Cool info, Steve, I didn't realize this was the real one. NGC 6749 is nothing more than an elongated patch with a star on one edge and a pair opposite. 20", 200x, SQM 21.1.

wvreeven
August 18th, 2015, 08:27 AM
I had a look at this nebula last night from SQM 20.3 skies with my 12" telescope:

Sh 2-71 already is visible at 48x without filters as a faint sphere. UHC enhances the view. It shows that the sphere isn't equally illuminated. 188x proved too much, even with filters. At 88x I see a group of stars resembling Ursa Major with a bright star at the tip of the tail. On the other side I see the nebula. Without filter I see a faint star and with OIII I see the nebula but the nebula doesn't seem to be centered at the star. It is also visible that the sphere isn't equally illuminated but there is a brighter, central, elongated part with a faint halo. I'll revisit this nebula with my 20" telescope.