Object of the Week March 7th 2021 - Abell 31

Abell 31

Sh 2-290

R.A. 08h54m13.2s

Dec. +08°53'53" (2000)

Magnitude: 12.20

Size: 16.8'

Mag CSPN 15.5

Cons Cancer

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Abell 31 in Cancer is one of the brighter of the Abell planetary nebulae, although of course this term is relative.
I am slightly surprised that it has not been covered before in the OOTW series. Abell 31 is thought to be about 2000
light years away and is thus perhaps 10 light years in diameter, making it very large for an object of this class.
The GAIA distance of 504pc would put it perhaps slightly closer and thus smaller. I note however that there do
seem to be some discrepancies between the HST parallax distances and the GAIA ones.

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On the sky this makes it almost as large as the Helix nebula, NGC 7293, making it one of the largest planetary nebulae
on the sky The planetary is also strongly interacting with the local ISM as can be seen from the bow shocks, as seen
in this image http://www.chart32.de/component/k2/o...bula-in-cancer.
The southern limb is seen to be much sharper and the northern limb more diffuse, indicating the direction of travel
through the ISM. There is an interesting collection of images of A31 in different wavelengths at
http://sharplesscatalog.com/sharpless/SH2-290.aspx. The central star is probably off centre as it will not be
slowed by the interactions of the envelope with the ISM. It is probable that A31 belongs to the thick disk
population of our galaxy.
The central star of A31 is still pretty hot however with a temperature of around 94000K and with a mass of
perhaps 0.6 solar masses. It is suspected to be part of a binary system.

The amateur observation history of this object seems to go back a long time with a mention (under its other
name of Sh2-290) in the NSOG where it is suggested that it should be faintly visible in 20cm class instruments
with an OIII filter. I must admit that my observations of it with a 50cm scope and an OIII filter would suggest
that, at least from the UK, it would require a larger telescope. I did however see it as a crescent shaped object
with banded structure. I note that most observations suggest it should be seen as a fairly round object. The size
indicates that you are going to need a low power eyepiece to see the whole of the nebula.

As always give it a go and let us know.

Excellent selection Owen, and an object I've only observed once so far - almost exactly eleven years ago. Here are my notes from March 6, 2010:

"Big, diffuse and relatively easy to see with the OIII filter and 31mm Nagler, and with the UHC and 21mm Ethos. The latter gave the better view and showed more stars too. The PN is brightest in the center and gradually dims toward its perimeter, with a couple of slightly brighter areas along the rim. Still this is all quite subtle - averted vision is needed to appreciate the view at all. 21.25 SQM."

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I copy my observing notes from my still in progress revision of my old (and faulty) Abell PN Project.

PN: Abell 31 (PNG 219.1+31.2, PK 219+31.1, Sh 2-290)

4" Binocular Telescope, 23x, [OIII], NELM 6m5+
with [OIII] filter easy visible as a round and diffuse glow around the 10mag bright GSC 8112166; the 10' large glow seems to be a little bit brighter towards the SW edge but is to small for the given aperture to resolve more structure; the whole glow is steadily visible with averted vision

8", 31x, [OIII], NELM 6m5+
large and easy visible glow with direct vision and [OIII] filter; positioned along an parallelogram of 10mag stars; brightest part seems to be east of the SE star of the parallelogram and is elongated 1:2 north-south; west following another fainter part is visible; the western part runs out very diffuse; all in all the whole nebula is around 14' large

16", 51x, [OIII], NELM 6m5
bright glow with maximum EP and [OIII] filter; positioned around a 10mag star; nebula shows some structures - brightest part to the west to SW of the mentioned star; very diffuse edges; at the NE a second smaller part is visible

sketch: 16", 51x, [OIII], NELM 6m5+
Attachment 4242
home

27", 113x, [OIII], NELM 6m5+
visible even without filter as a faint glow around a 10mag star; positive and good response to [OIII] filter, only moderate response to UHC filter and no response to Hß filter; with maximum EP (and field) plus [OIII] filter the whole glow shows a mottled structure, broken with some difficult to locate dark lanes; brightest part is positioned directly SW of the star; another part is positioned NE and moves along the star; a third part is positioned at the SE end of the PN; the photographic striking H? shell at the SE side is not visible with all filter combination

This is a tricky Abell! Last night I examined this field at 75 and 150x with my OIII filter (20", SQM 21.85, transp. 9/10, seeing 2-4 arcsec, elev. 700 m, object past culmination). There was a readily visible nebulosity that looked pretty much exactly as Uwe depicted it, less the fainter periphery to S and SW that is seen in his sketch. The close double at the center of the more compact ENE part was seen as a starlike object at 75x, at which magnification the said part of the nebulosity was invisible. At 150x, the double star is invisible, probably due to seeing, but the nebulous patch is visible. On PANSTARRS g there is a void there. The brightest, larger part of the nebulosity (WSW of the visual "central star", elongated NNW-SSE) corresponds on PANSTARRS g to a prominent nebulous patch, but also manages to be exactly inscribed in an oblate circlet of subthreshold stars. On the whole, I rated my observation as negative and the PN, as one of the "invisible Abells." That said, blinking the SW part with the filter when the object is on the meridian may be worthwhile in the future.

Hello Owen,

I observed Abell 31 about 3 years ago with my 12" Dobsonian. At 49x magnification without filter I didn't see it at all, but with an O-III filter I saw a large-area low-contrast glow.

Clear skies

Robin

Hi, here's my observation from 3/12/2021 ; Abell 31 planetary nebula in Cancer ; magnitude:12.2 ; size: 16.8' ; large ,faint and roundish, seems like one side is brighter than the other ; I used a 21mm Ethos at 75x with a Lumicon Gen 3 OIII filter Attachment 4258

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The only observation I found was from March 2012 with John Tatarchuk's 25" from central Texas. This one's back on the list, just like Bear-Claw.

I probably hold some sort of record. I saw Abell 31 at 27x in my 130mm reflector this past winter...without a nebula filter. That shocked and awed me for several weeks.

Scott