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Object of the Week, August 2, 2015: The Unusual Planetary Sh 2-71 in Aquila
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.
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Sh 2-71 was discovered in 1946 by Rudolph Minkowski and included in a table of "Diffuse and Peculiar Nebulosities" (designation M 1-90). Stewart Sharpless included it as object #71 in his 1959 paper "Catalogue of H II Regions". 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 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.
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!"
