Object Of The Week July 3, 2022 - Abell 41 and the case of MT Serpentis

[wvreeven]

Abell 41, He 2-236, PK 009+10.1

Planetary Nebula

Serpens

RA: 17 29 02.0
DEC: -15 13 04.5

Mag: 15.6

Diam: 0.3' x 0.1'


This week's OOTW is about the relation ship between the shape of planetary nebulae and their central stars. As most of us probably know, planetary nebulae not always are nice, round, spherically symmetrical bubbles of gas. Instead up to 80% of them (!) show complex structures. As a matter of fact:

"Planetary nebulae are classified by different authors into: stellar, disk, ring, irregular, helical, bipolar, quadrupolar, and other types, although the majority of them belong to just three types: spherical, elliptical and bipolar. [cut]

The huge variety of the shapes is partially the projection effect - the same nebula when viewed under different angles will appear different. Nevertheless, the reason for the huge variety of physical shapes is not fully understood. Gravitational interactions with companion stars if the central stars are binary stars may be one cause. Another possibility is that planets disrupt the flow of material away from the star as the nebula forms. It has been determined that the more massive stars produce more irregularly shaped nebulae. In January 2005, astronomers announced the first detection of magnetic fields around the central stars of two planetary nebulae, and hypothesized that the fields might be partly or wholly responsible for their remarkable shapes."

(The above two paragraphs almost literally were copied and pasted from Wikipedia (https://en.wikipedia.org/wiki/Planetary_nebula).)

Back to our OOTW. About 2º west of Xi Serpentis lies the planetary nebula Abell 41. It's central star is a short period variable known as MT Serpentis. MT Ser has a very short period of 0.11322653 days or about 2.7 hours. At first it was thought that MT Ser is a cataclysmic variable. This means a white dwarf and a "normal" star orbiting each other. It was long thought that MT Ser was the shortest period cataclysmic variable known. However, more recent research seems to indicate that this in fact is a system of two white dwarfs orbiting each other in 0.22645306 days or about 5.4 hours.

(Source https://www.aanda.org/articles/aa/pdf/2001/39/aah2736.pdf)

In 2010 a observations were published showing evidence that the "symmetry axis of the model nebula is within 5º of perpendicular to the orbital plane of the central binary system. This provides strong evidence that the close-binary system, MT Ser, has directly affected the shaping of its nebula, Abell 41."

(Source https://arxiv.org/abs/1006.5873)

Here are images from respectively SDSS, ESO and POSS for Abell 41. I have not been able to find higher resolution images (not sure if the one on Flickr actually is of this nebula!).

4777 4778 4779

I observed this nebula in August 2016 using my 20" dob. My notes read

"At 256x visible without a filter as a tiny disk. UHC gives a beautiful view of a disk in a rich star field. It forms a triangle with a faint double star and a single star of about m=12. OIII dims the stars too much. H-Beta makes the disk disappear."


As always,

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

[Ivan Maly]

It so happened that I observed this nebula just last week. My observation is similar to yours.

Abell 41, Ser (PK 9+10.1). 2022-06-29, Spruce Knob (summit, 4800 ft). Clear, calm, dry. Transp. 8/10, seeing 1", SQM 21.58 mag/arcsec^2. Telescope aperture 508 mm, F/4. Ocular 5.5 mm. Small nebula in a rich star field. It stands out with OIII filter. Sketched.

4785

[Gastronaut]

Dear Ivan,

Nice report of I just returned from Calhoun County Park, Grantsville, WV; observed there last Wednesday and Thursday nights (6/29 - 7/1/2022). Ive been to the "Knob" many times but only in the fall of the year. Curious as to the "breeze" as Spruce Knob can be windy in the extreme (I've only ever observed from the East Parking Lot, tucked up along the tree line to minimize exposure to the wind in the Fall.) Unfortunately not likely to get there at all this year as I will be at Stellafane in the July dark window, Mid Coastal Maine in August and at Okie-Tex in late Sept. Feel free to message me privately.

RW

[Ivan Maly]

Nice to see you here, Robert. No wind to speak of that night, how about that?

[Steve Gottlieb]

A few observations going back to 1990. Curiously, I didn't find much of a response to filters.

24" (8/13/15): I picked up Abell 41 unfiltered at 375x as a faint, small, round disc in a rich Milky Way field. 500x provided a better view with a dark background and more contrast. At this high magnification the planetary was easily held continuously and noted as slightly elongated but at 15" diameter, was surprisingly small for an Abell. The mag 16.5 central star was seen at 500x, though I didn't check to see if it was visible at any lower magnification. No response to an OIII filter.

18" (8/1/08): barely picked up unfiltered at 175x as an extremely faint, small, round glow. There was a very weak, if any, contrast gain using the NPB filter and the planetary was still difficult. It was perhaps slightly easier to view at 280x.

17.5" (7/22/00): picked up unfiltered at 220x as a very faint, small disc, ~15" diameter. Easier to view at 280x and could almost hold continuously. Appears to brighten towards the center and increase in size with averted vision. This low excitation PN gave no response to a UHC filter at 140x and 280x or with a H-beta filter at 140x.

17.5" (7/21/90): at 220x without filter appears as a faint, small, round disc of low even surface brightness. Very unusual filtration response as dims with OIII and UHC filters! At 280x without filter, the disc is very evident.

[Uwe Glahn]

In contrast to Steve I noted a positive reaction to filters, even if the reaction is not strong. Maybe the central star is the troublemaker which is at the border of visibility in larger instruments and disturb the very faint glow of the nebula itself?

My notes are as follows:

PN: Abell 41 (PNG 009.6+10.5, PK 009+10.1, Hen 2-236, Sa 2-205)

16", 225x, UHC, NELM 6m4
faint Abell PN contrariness to the bright appearance on the blue and red POSS photoplates; the PN is not visible without a filter; with [OIII] filter and from 129x up a small, faint and diffuse glow is popping in and out of view; the best view is with 225x and UHC filter; with this combination a small, round and not sharply defined glow is difficult but steadily visible with averted vision

sketch:
4788
home (http://www.deepsky-visuell.de/Zeichnungen/Abell41.htm)



27", 419x, [OIII], NELM 6m5+
with higher magnification the central star is steadily visible with averted vision; a faint glow around the central star is also visible even without a filter; with [OIII] filter the central star disappears and the nebula of the PN pops out clearly; this glow is visible with direct vision as a small, round nebula with good defined edges but without showing the characterizing shell structure