Object of the Week May 6th 2018 - Abell 36

[obrazell]

Abell 36

Bat Symbol, PN G318.4+41.4, PK 318+41.1, ESO 577-24

Virgo

R.A.: 13h40m41.3s
Dec.: -19°52'55"

Mag 13.1

Breaking with the current run on OOTW as galaxies this week’s challenge is Abell 36 in Virgo. Also known as the Bat Symbol this is a large but relatively faint planetary nebula. It was first discovered by George Abell on POSS plates and included first of all in his 1955 paper and then catalogued as number 36 in the classic 1966 paper on faint old planetary nebula. Abell 36 has a relatively bright central star at a magnitude of 11.3. Although its magnitude and spectral type (sdO) are well known there seems to be a lot of doubt about the central star’s temperature and I have seen estimates as anything between 93000K and 113000K.

a36dss.jpg

DSS Image of Abell 36

The nebula’s distance also seems to be unknown to any accuracy. The mean distance seems to be around 640pc but I have seen distances for it as low as 800 light years (245pc). The early distance measures came from Hipparcos parallaxes but they were very poor as the CS is near the magnitude limit of Hipparcos. I guess with such a bright central star it may be possible to dig a parallax out of GAIA DR2 (Wouter?) (My attempt at this via Aladin gives a parallax for the central star of 2.2902mas which would correspond to a distance of 437pc approximately, if I have got my sums right 😊 ). The nebula itself appears to be a spheroid with various arcs projected on it from a later ejection event that is highly collimated. As these events are tilted it suggested that there is a continually precessing source at the centre so possibly a binary star, although none has been observed. The nebula is surrounded by a very faint large halo (approx. 5 degrees) of ionised material. This does not appear to have come from the PN itself but is ionised local ISM. The halo is also barrel shaped like the nebula and suggests the PN is optically thin to the ionising radiation so it leaks out to ionise the local ISM. The central star in Abell 36 is off centre and this could be due to the interaction of the nebula with the ISM as it ages.



PanSTARRS image of A36

From northern latitudes the nebula never rises very high and my only view of it from home (51N) is as a faint patch with my old 20” Obsession with the telescope pretty much pointing horizontal. It did respond well to an OIII filter. I must admit I have not tried it with the 22" as skies from the UK appear to be getting worse low down. I suspect it may be much easier to see from further south. Its size is variously given as 6’ or 8’. I was surprised to see it on the Astronomical League’s Planetary Nebula list when the brighter PN IC 972 in Virgo is not.

There are some fine amateur images of Abell 36 at

http://www.caelumobservatory.com/gallery/abell36.shtml

and

http://www.capella-observatory.com/I...Ns/Abell36.htm


As always give it a go and let us know.

[Mark McCarthy]

With my 20-inch in SQML 21.3 sky & good seeing from northern California (37N): 205x, "Very faint, need OIII and averted vision; central star seen and an irregularly round small halo with some brightening on the SE rim"

I'd love the chance to see it from southern skies.

[wvreeven]

At first I thought "finding the parallax is easy: just go to Simbad!" but then it turns out that Simbad hasn't finished cross matching all sources yet. So I dug into the Gaia DR2 archive and searched for all sources within 0.01 degree = 36" around the coordinates given by Owen. It resulted in two sources, one with a G magnitude of 20.8 and one with a G magnitude of 11.5. Remember that the Gaia G passband covers a wavelength range of just over 300 nm to just below 1100 nm so the V magnitude is slightly different.

The values for G, Bp and Rp in Gaia DR2 are respectively 11.509609, 11.307274 and 11.805822. Using this formula

V := G + 0.01760 + 0.006860 * (BPg−RPg) + 0.1732 * (BPg−RPg)^2

published by my colleagues Evans et al, I find V = 11.5668378394782528 or 11.567 which is slightly fainter than the value quoted by Owen. The parallax for this source in Gaia DR2 is pi = 2.2901741623622263 mas with an error of pi_err = 0.08316206303447575 mas. However, due to the nature of the parallax measurements and error determination, simply taking d = 1/pi and determining the error in d using 1/(pi + pi_err) and 1/(pi - pi_err) will not result in a reliable distance estimate.

Fortunately yet another Gaia colleague Bailer-Jones has calculated statistically reliable distances for all Gaia DR2 sources. These distances were not part of the official Gaia DR2 data but were released afterwards. The distance obtained by him is d = 431.986626330559 pc with a lower estimate of d_lo = 416.880338287953 pc and a higher estimate of 448.207424221519 pc.

[wvreeven]

By the way, my observations from July 9, 2015, with my 20" telescope from SQM 21.0 skies read: "At 83x invisible without filter. I see a triangle of reasonably bright stars. West of the northern most two stars of these lie three stars in a bent line. The north western most of these is the brightest. A UHC filter reveals a large sphere around that star. The nebula also responds well to OIII. In both cases the nebula is at the edge of visibility."

[obrazell]

Thanks Wouter. Aladin seems to have a GAIA option which had the source you describe plonked on the central star in the nebula which is where I got my data from which is similar to yours. Not sure if that is from SIMBAD or directly from the GAIA database. The GAIA to V transform is very useful though. Is there access to the new GAIA distance data anywhere?

Interesting that we now have three distance measurements. I guess Hipparcos can be thrown out. Will be interested to see how the Frew et al H-Alpha distance scale stacks up for those PN that have GAIAI data.


Owen

[wvreeven]

The Gaia DR2 data are available in Vizier, so I assume that Aladin uses that already. The data just aren't available in Simbad yet but that's foreseen for the end of June.

The Evans' paper, about the photometric properties of Gaia, can be found here.

The Bailer-Jones' paper, about the distances, can be found here. The abstract contains a URL to the so called TAP service at the Astronomischer Rechen Institut (ARI) in Germany where the data are hosted. We are still discussing if the distance data will also be included in the Gaia archive at ESAC. How good is your ADQL?

Please note that the distances published by Bailer-Jones et al is based on so called priors. This means that a certain assumption was made for the shape of the Milky Way around us, which may or may not be correct. For sources with accurately known distances this will not be a problem but for all the other sources this will introduce a certain uncertainty in the calculated distance and distance errors. I haven't read the paper so I am not sure how big the uncertainty will be. Maybe your d = 1/pi = 437 pc distance is better than the one calculated by Bailer-Jones!

[Raul Leon]

In my 14.5 Dob , Abell 36 is large, ovalish, dim planetary , with a fairly bright central star. It responds well to npb filter, but without filter it is virtually undetectable. I used a 21mm Ethos at 75x magnification . It seems to be brighter on the SE side. I attached a sketch of my observation. RaulScan0571.jpg

[lamperti]

I've observed this twice: once with a 20" at 272x where the central star was visible but the nebulosity was very faint, even with a UHC filter. The second time was with a 22" at 337x: The O-III filter was slightly better than the UHC. "The central star is very bright but the nebulosity pops out with the filter. An even haze all the way around. Photos show a brighter part on one end but I could not see it."

[Bertrand Laville]

Hi All,

Here are my drawing, with a 25'' Obsession, and a detailed report at*:
http://www.deepsky-drawings.com/abell-36/dsdlang/en

the resume of which beeing*:
Abell 36 is a large, pale NP, L1 to L3 in a scale from 1 to 10, with an easy seen annularity. The irregularities of structure and the sape of the ring need a long and study observation.
It is not an easy Abell PN.

Clear skies
Bertrand
http://www.deepsky-drawings.com/dsdlang/en/


Abell 36 PK 318+41.1 T635 BL 2008 04 03.jpg

[Uwe Glahn]

In my experience Abell 36 belongs to the more easy Pn of the Abell catalog and is a great chance to get started in the Abell world - so nice choice Owen.
Problem could be the southern Declination of around -20°DEC, especially for the people in GB or Northern Europe. I observe from around 48°N, so the high is no problem at all.

I copy my descriptions of my Abell observing project below.

8", 47x-80x, [OIII], NELM 6m5+
nothing visible without filter; with [OIII] a 1:2 N-S elongated glow could be seen; can hold glow steadily with averted vision; bright CS; with 80x the S part looks a little bit brighter

16", 51x-100x, [OIII], NELM 6m0+
easy Abell PN, can hold it with direct vision, with max. AP 2:3 N-S elongated disk around 11,8mag CS, with AP 4mm southern part much brighter

16", 75x, [OIII], NELM 6m5+
easy, can already see the PN without filter, 2:3 E-W elongated PN, disk is shifted to the S side, flatly N side, S of the CS faint dark hole, the brightest part at the W side, very faint outer parts at the NW
Abell36.jpg