Object of the Week, December 25, 2022 – Abell 7 + neighbors

[Steve Gottlieb]

Abell 7, PK 215-30.1, PN G215.5-30.8, [Abell55] 6
Type: Planetary Nebula
Constellation: Lepus
RA: 05h 03m 07.5s
DEC: -15° 36’ 13"
Size: 790" x 776"
Central Star: 15.5

Our last OOTW for 2022 is the huge, ancient planetary Abell 7 (and its neighbors), which lies in Lepus. With dimensions 14'x13', a 1996 study by Tweedy and Kwitter ("An Atlas of Ancient Planetary Nebulae and their interaction with the Interstellar Medium") had this to say about Abell 7:

"Of those planetary nebulae that do not interact with the interstellar medium (ISM), this is the largest. There are a number of asymmetric enhancements that reveal a sharp drop in ionization, none of which appear on the perimeter of the nebula. Although these could be interaction features that happen to lie on the face of the nebula, an H-alpha image obtained at the KPNO 4 m by G. H. Jacoby (1995, personal communication) has convinced us that these are embedded within the nebula and are not due to the ISM."

The "asymmetric enhancements" show up well in this image by Peter Goodhew using H-alpha and O III filters.

Abell 7.jpg

Distance measures vary widely (common to PNe), depending on the measure used, but a 2021 paper using Gaia’s Early Data Release 3 (EDR3) reports ~832 light years.

The first visual observation I'm aware of was made by Dana Patchick in 1985 using a 17.5-inch. I looked for it about 3 months after Dana with my 17.5-inch at 83x with an O III filter from the Sierra foothills. Unfortunately, conditions were poor in terms of transparency and seeing. Nevertheless, I logged it as...

Very faint, extremely large, surrounding a group of six stars, requires averted vision. Appears as a hazy region of very low surface brightness and the edges were ill-defined"

I took another look in 2004 with my 18-inch at 73x, again with an OIII filter and also with poor seeing and fair transparency (such as conditions often are in the winter).

Extremely large, ghostly glow, perhaps 6' in diameter. Although the surface brightness is very low, it was visible as an irregularly-shaped, hazy patch, with 5 or 6 stars involved on the south side. The edge of the planetary was better defined on the southern periphery and it appeared to fade out on the north side, so I may not have seen the full extent. Once identified, though, it wasn't difficult with averted vision.

Abell 7 - Macon.jpg

Extra 1: The small edge-on in the images is MCG -03-13-058 = PGC 16611 and it's situated ~10' NW of center. It appeared in my 18-inch as an extremely faint, low surface brightness, extended patch.

Extra 2: If you shift 12' further northwest, you'll run into HCG 32A = MCG -03-13-53, which is the brightest member of the Hickson Compact Group 32. This quartet lies at a distance of 560-580 million l.y. In my 24-inch, galaxies 32A and 32B were easy, with a mag 13.5 attached on the west side of 32A. HCG 32C was visible but quite faint, and I missed 32D. Legacy Survey shows HCG 32C is quite distorted with long tidal tails and a possible bridge to 32A.

Extra 3: About 1.2° northwest of Abell 7 is R Leporis, perhaps better known as Hind's Crimson Star, though truth be told it was first discovered by William Herschel in 1785 and called a "bright garnet star" (he identified all deep red stars as "garnets"). In any case, R Lep is a beauty, so make sure to check it out if you haven't observed it before.

HCG 32.jpg

HCG 32b.jpg


Happy Holidays!

"Give it a go and let us know!
Good luck and great viewing!"

[wvreeven]

With my 12" in October 2015 from a moderately light polluted area, I observed

At 88x with OIII I occasionally see a part of the nebula. There are two resonably bright stars on the east side and one of the west side. In between these stars I see a group of faint stars. Just north of the eastern part of this group I see a smudge occasionally. This one I need to revisit with my 20 telescope.

[Howard B]

I have an observation from twenty years ago with my old 20-inch Obsession, and it was only in passing because I was really observing Hickson 32. My note is in parentheses because I added it on the end of my Hickson 30 notes:

"(Huge, very faint planetary PK 215-30.1 / Abell7 is in the same low power fov. The OIII filter shows a gentle brightening at the indicated spot so I guess that's it.)"

I was able to see Hickson 32A, B, C and D but my notes only indicate I saw them, and that the entire group was "pretty darn faint overall" at 413x. This was my last observation of the night (5:01am!) from the best high altitude dark sky site I can drive to - Steens Mountain in southern Oregon.

[ScottH]

Back on October 4th of this year, I could just make out its sizeable disk with an O-III filter and 39x in my 6-inch reflector. It is quite the challenge object for single-digit telescopes considering its southern declination!

Scott H.

[Uwe Glahn]

I recorded several positive observations and write down four with different apertures each. (details in my project)
Summarising I could detect the planetary up from a 4" binocular telescope with difficulties but clear positive sighting. The galaxy was recorded with a 16-inch and 27-inch. The shells were indicated with my old 16-inch but clearly visible with my 27-inch. Unfortunately no sketch recorded yet.

I was not aware of the small distance of HCG 32. I found only one observation because of the low position in the sky with a rented 20-inch from La Palma.

sketch: 20", 321x, NELM 7m0+, Seeing III
HCG32.jpg
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[akarsh]

Here's my rough sketch of Abell 7 from October 13th 2015, observed from near Pontotoc, TX with my 18" f/4.5, using a 20mm Pentax XW with an OIII filter:
Abell7_Sketch.png

My log reads: "Very faint object, but almost definitely visible as a mottled circular glow. Seemed more donut shaped than uniform. The sketch [above] was made without careful perusal of the DSS. DSS seems to suggest that the glow is even bigger [than the sketch] and I seem to have seen some sort of inner boundary." Then after a short break, "Went to eyepiece again. Some hints of glow extending to DSS sizes, beyond the star marked *, to the point marked as d?. Repeated sensation of a void in the marked manner."

I have one other observation of Abell 7 with John Tatarchuk's 25" telescope, presumably also from the central Texas hill country. However, that's not accessible to me now since I'm away from my logbooks in India. I also have an observation of HCG 32, also inaccessible at the moment.