Object of the Week, May 12, 2024 - Abell 35, a planetary nebula imposter

[Steve Gottlieb]

Name: Abell 35
Aliases: Sh 2-313 = PK 303+40.1 = PN G303.6+40.0
Type: Emission Nebula
RA: 12h 53m 32.9s DEC: -22° 52' 22"
Constellation: Hydra
Diameter: 938" x 636"
Central Star: Companion to mag 9.6 LW Hydrae = BD-22°3467
Distance: ~ 400 light years

Abell 35 was discovered on the Palomar Sky Survey plates and first listed by George Abell in his 1955 paper (#24) "Globular Clusters and Planetary Nebulae Discovered on the National Geographic Society-Palomar Observatory Sky Survey". A few years later Stewart Sharpless independently discovered it and included it as the last object (#313) in his 1959 "A Catalogue of H II Regions". When Abell produced produced his final list in 1966, it was renumbered as Abell 35. He considered it a homogeneous disk PN with an angular size between 636" and 938" -- likely a huge, ancient planetary.

Initially, the bright mag 9.6 offset from center was taken as the central star, (a rapidly rotating G8III-IV type) but in 1988 Grewing and Bianchi discovered the true central star was a hot companion and suggested it was a DAO-type white dwarf. Although the G8 star dominates the optical light, the white dwarf dominates the UV flux.

The morphology of the nebula is quite unusual for a planetary with a highly ionized arc of material ("bow shock") around the CS as it moves through the interstellar medium (ISM) at a speed of 125 km/sec. In a 2012 paper by Ziegler et al "BD–22°3467, a DAO-type star exciting the nebula Abell 35", the authors conclude "BD-22° 3467 may not have been massive enough to ascend the asymptotic giant branch and may have evolved directly from the extended horizontal branch to the white dwarf state. This would explain why it is not surrounded by a planetary nebula. However, the star, ionizes the ambient interstellar matter mimicking a planetary nebula."

Abell 35 (Salman).jpg

More recently, a 2020 paper by Lobling et al: "First discovery of trans-iron elements in a DAO-type white dwarf (BD-22°3467)" writes that
"The peculiar ionized nebula around BD-22°3467 is not a real PN but, nevertheless, it also cannot be excluded, that the nebula material is in some way connected to the evolution of the central star. Assuming that it ejected a PN as an AGB star, the original PN might have already dispersed. The star has a high proper motion and, thus, might now be passing through the edges of the ejected former nebula material or another dense ISM region while ionizing the surrounding material. The classification of Abell 35 as a bow shock nebula in a photoionized Stromgren sphere in the ambient ISM (Frew & Parker 2010) does not necessarily include a PN but also, does not rule out the post-AGB nature of BD-22°3467."

Using 73x in my 18-inch with an OIII filter, Abell 35 appeared as a fairly faint, huge oval WSW-ENE, ~6'x4'. Although the surface brightness is low it was easily visible with the filter and displayed an irregular surface brightness. The unusually bright mag 9.6 star (with the white-dwarf companion) is slightly offset west of center. A second mag 11.5 star is involved on the east side (2.7' following the central star). The planetary is brightest surrounding the central star and just south, with an extension to the east encompassing the mag 11.5 star.

Abell 35 (Marek).jpg

As always,

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

[Raul Leon]

Hi, here's my observation from 4/8/2016: Abell 35 is a planetary nebula in Hydra ; mag: 13.3 ; size: 15' x 11' ; faint, very large and oblong in shape ; used an Astronomik OIII filter with my 21mm Ethos at 115x with my 14.5 StarStructure f/4.3
Scan0488.jpg

[FaintFuzzies]

My observing notes as follows with my 22" – Featureless faint slightly oval disk (1.5:1) with a two stars embedded in the nebula. The 9.6 magnitude central star is off-center towards the west. The brightest part of the nebula is slightly elongated at about 2’ by 1.5’ centered on the central star. An 11.3 magnitude star is towards the east. The O-III filter shows it a quite a bit better than the UHC, but both filters show this 100% of the time with averted vision. Estimated size is about 6’ across with position angle of about 90º. PA of the bright part is also 90º.


Abell35.png
22" @ 155x

[lamperti]

With a 20" and an O-III filter at 73x: "Below Spica. Easy to find but needed filter to catch glimpse of nebulosity. Had to tap the telescope to see it."

[Steve Gottlieb]

I though I'd add that the first visual observation I'm aware of was made by Dana Patchick back in April 1982, just using an 8" f/6 reflector in southern California. I assume he was using a UHC or similar filter.

So, don't be hesitant to give it a try with a smaller scope!

[akarsh]

My notes on this object are meager: I have only one observation from Texas Star Party 2014 when it was found and shown to me by young amateur astronomer Santhosh Surendra. I didn't even log whether it was using my 6" Orion XT or my Obsession 18", but I suspect the latter. My sketch illustrates a roundish lopsided glow around a star, with a stronger glow on the western side and maximum nebulosity on the south east of the star. The star seems to be offset to the west from the center of the glow. A second star lies due east outside the visual region of the glow.

[Uwe Glahn]

I copy my observing notes from my Abell project.

8", 47x, [OIII], NELM 6m5+
nothing visible without filter; with [OIII] filter easy and direct vision glow around bright central star; north and east two other faint stars at the edge of the PN are visible through the filter; glow seems to be round with diffuse edges; no other details visible

16", 51x-100x, [OIII], NELM 6m4
PN visible as a faint and mid size glow around 9,5mag central star LW Hyd; with [OIII] filter the 5'-6' large glow appears much smaller than photographically (which also images the outer red parts); somewhat brighter east-west elongated bar at the southern edge which seems to be cut of to the south; overlooked 16,7bmag galaxy ESO 507-38 (4,7' north of the central star)
Abell35.jpg
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27", 113x, NELM 6m5+
even visible without a filter as a diffuse, 10' large glow around the bright central star; the Hß filter slightly improves the sighting, without showing any clear structure; the UHC filter again improves the visibility and changes the illumination of the glow by showing an east-west elongated edge at the southern end; best results are given with the [OIII] filter - the edge at the south is now easy to see and the "bow shock" structure is clearly indicated by showing a dark structure reaching from the east directly north of a 11mag star to the central star, but the edges of the structure itself are not clearly blanked out

48", 195x, [OIII], NELM 7m0+
the whole only 14' large field is wildly structured and filled with nebula; with filter, the brightest segment of the bow shock is easily visible, starting at the central star to the N and bend to the NE with good defined edges; directly around the central star and to the SE the bow shock is more difficult to see, because of the disturbance of the central star and the 2.6' distant 9.6mag star to the E
Abell35_48.jpg
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