Object of the Week November 17, 2024 - Abell PN 6 and HFG1

Abell PN 6; A 6; PK 136+04.1; PN G136.1+04.9; A55 5; ARO 207
Constellation: Cassiopeia
Type: Planetary Nebula
Morphology: D (a ring of variable thickness)
RA (J2000): 02h 58m 42s
Dec(J2000): +64° 30’ 06”
Mag(v): 15.00
Size: 3.1’ x 2.9’

HFG1 (Heckathorn-Fesen-Gull 1); PN G136.3+05.5; PK 136+05.1
Constellation: Cassiopeia
Type: Planetary Nebula
Morphology: bow shaped shell surrounding the central star and a long collimated tail
RA (J2000): 03h 03m 49s
Dec(J2000): +64° 53’ 28”
Mag(v): 14.56
Size: 8.3’ x 8.3’

This week we feature two planetary nebulae as Object of the Week, both of which I expect you will find quite challenging. So haul out your largest aperture scope, have your OIII filter ready to go, and locate some very dark skies, like Bortle 1, 2 or 3, and give it a go!

First up is the planetary nebula Abell 6. This planetary nebula was discovered in 1955 by the American astronomer George Abell on the photo plates of the Palomar Observatory Sky Survey (POSS). In 1955 he published a first list of 13 globular clusters and the positions of 73 planetary nebulae. In 1966 Abell published a completed list including the size and description of the 86 planetary nebulae discovered on the POSS photo plates which known as the Abell Catalog of Planetary Nebulae, not to be confused with his galaxy cluster catalog, known simply as the Abell Catalog. The most common alternate designation for the planetary nebula Abell 6 is PK 136+4.1. This designation comes from the two Czechoslovak astronomers Luboš Perek and Luboš Kohoutek, who in 1967 compiled a catalog of all the planetary nebulae of the Milky Way known at the time.

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If Abell PN 6 isn’t challenging enough, next up is its nearby neighbor, a rather unusual and extremely challenging planetary nebula, HFG1.

Heckathorn-Fesen-Gull 1 (HFG1, PK136+05) is a very old (~100,000 year old) and extremely faint planetary nebula. It was discovered in deep OIII images by by Joy Heckathorn, Robert Fesen & Theodore Gull in 1982 using the emission line survey of Parker et al (1979). The central star is a precataclysmic binary variable pair V664 Cas (13.7 magnitude). This binary system is comprised of a white dwarf, and a large red giant star. The two stars are very close and rotate about each other in just 14 hours.

HFG1 is the first well observed planetary nebula which reveals a cometary-like structure. Its main morphological features consist of a bow shaped shell, which surrounds the central star, accompanied by a long collimated tail. As HFG1 moves through the interstellar medium and V664 ejects material, a bluish bow shock is formed and a red long trail of emissions is left in its wake.

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Credit T.A. Rector (University of Alaska Anchorage) and H. Schweiker (WIYN and NOIRLab/NSF/AURA)


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Are you able to see this Abell? How about HFG1? Give it a go and let us know.

I'm surprised no one has replied as several observations of HFG 1 were posted on this thread in 2015 including a 4" description by Uwe. Abell 6 is a much tougher target. Have you observed it, Mark?

My first two observations of HFG 1 were made in 1986 just 4 years after discovery (paper).

17.5" (1/16/02): At 64x (31mm Nagler) and OIII filter an extremely faint, huge glow was visible, perhaps 8' in diameter. Generally the hazy region was just an ill-defined "stain" on the sky, but at moments the view sharpened up into a fairly well-defined disc. Several stars are superimposed. Located 7' NE of mag 9.2 HD 18611. This star lies at the NE end of a string of stars heading to the SW and roughly collinear with HFG 1.

17.5" (11/1/86): At 83x and OIII filter appears very large, very diffuse, requires averted vision, at least 7' diameter. Mag 9 SAO 12574 is off the SW side, ~7' from the center of the planetary.

13" (10/10/86): At 62x and OIII filter appears a very faint, extremely large low surface brightness glow requiring averted vision. Can hold steadily at least 80% of time. Several faint stars are superimposed, and a mag 9 star is off the SW edge.

I will follow Steve's observation here.

HFG 1 is sad the more easily object and don't need large aperture. I copy my observations in:

4", 56x, [OIII], NELM 6m5+
round glow; easy to hold with averted vision

8", 47x, [OIII], NELM 6m5+
faint glow suspected even without filter; with [OIII] faint, but direct vision object; concentrated to its middle; round, diffuse glow

16", 67x-100x, [OIII], NELM 6m5+
direct vision with [OIII], centre 2:3 elongated

27", 113x, [OIII], NELM 6m5+
flashy big glow with [OIII]; bright; slightly elongated centre; to the NE and SW two separated plobs, NE plob a little brighter but further away; could not see the [OIII] arc in the SE
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As Steve said, Abell 6 is much more challenging but also doable with smaller aperture when transparency is good enough.

8", 80x, [OIII], NELM 6m5+
excellent transparency; the magnification of 47x (4,2mm EP) don't work to separate the glow of the PN from a near star group at the east and the 6,6mag star SW; with 80x a very faint round glow is popping in and out of view but can be hold with averted vision for several seconds each; glow stands between two 10mag stars NW, the star group at the east and the bright 6,6mag star SW; a rough sketch confirms the exact position and the right size which was not known before; edges not hardly defined

14", 80x, [OIII], NELM 7m0
faint and perfectly circular disc with sharply defined edges; difficult but sure observation even when the PN is not steadily visible with averted vision; co-observer confirms observation and position of the glow; disturbing star group at the east

27", 172x, [OIII], NELM 6m5+
6,6mag star 7' SW disturbs observation of the faint PN; PN itself is easy to detect and steadily visible with averted vision; exactly round with good defined edges
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I didn't know that HFG is near Abell 6. I'll go look for it the next time I go out.

However, here is one observation of Abell 6 about 20-25 years ago with a 22" reflector under NELM 6.5 skies.

The planetary nebula is an extremely faint crescent shaped smudge. A 6.5 magnitude star lies 6’ SW of the nebula, making it way more difficult. The crescent shape is about 120” long by 30” wide and
pointing to the northeast. Central star not spotted.


It was evident that I picked up only part of it.

I've only logged PN Abell 6 through John Tatarchuk's 25" telescope back in the early part of the last decade.

My rough sketch shows a circular glow with some sensation of mottling.

I happened to look at it last month with my 18", but I mis-observed it – I definitely caught some glow, but I overestimated its diameter, perhaps tying it to the glow of a nearby asterism. I need to take another look.

As for HFG 1, I'll just link my response on the HFG 1 thread here