Object of the Week July 7, 2013 - Swings-Struve 1 - An unusual planetary in Sgr

[Steve Gottlieb]

PK 1-6.2 = PN G001.5-06.7 = ESO 457-2 = HD 167362
18 16 12.2 -30 52 08
V = 11.9 (central star in SIMBAD)

The "discovery" name of this nearly stellar planetary is from the 1940 paper by Pol Swings and Otto Struve: "HD 167363 , An object similar to Campbell's Hydrogen Envelope Star" available at at http://www.pnas.org/content/26/7/454.full.pdf

This young, very compact planetary contains a hydrogen-deficient central star displaying a carbon-rich emission-line spectra (WC) very similar to young, massive Wolf-Rayet stars. The central star is surrounded by a small dusty halo showing red [N II] lines. Campbell's Hydrogen Star, with a similar spectra, displays a striking red ring (in larger scopes).

Both Campbell's star and Swings-Struve 1 were actually discovered by Williamina Fleming at Harvard (ApJ, 2, 354) back in 1895! (she found Campbell's Star in 1890). As it was assumed this was a star and not a planetary, Dreyer did not assign it an IC number to either object.

Sw-St 1 was later rediscovered by Merrill at Mount Wilson in 1925, then by Pol Swings and Otto Struve at McDonald Observatory around 1940 and picked up once again at Mount Wilson in 1950 by Karl Henize. What a history! For more information see the 2001 paper "SwSt 1: an O-rich planetary nebula around a C-rich central star" at http://m.mnras.oxfordjournals.org/content/328/2/527.full

Jimi Lowrey mentioned this object to me a couple of nights ago at the Golden State Star Party in northeastern California as he happened to run across the 1940 Swings-Struve paper and wondered if it might display a reddish halo like Campbell's Hydrogen Star. We couldn't confirm a color, though the central star is certainly off-white (Paul Alsing thought it had a reddish tinge) and appears slightly "soft" at high power in my 24" f/3.7. Although situated in a rich Milky Way field, I was able to quickly identify it (without a finder chart) by "blinking" the field with an NPB filter.

The next day I checked my notes database and found I had first observed Sw-St 1 27 years ago (August 1986) with my 13" and commented "blinks very well with an H-Beta filter which dramatically dims all stars. Responds to an OIII filter but to a lesser extent."

Jimi and I, along with several others at the star party, reobserved Sw-St 1 again last night (SQM-reading of 21.6) and found the H-beta response startling -- stars were dimmed nearly 3 magnitudes, but Sw-St 1 shone steadily and dominated the rich field. Campbell's Hydrogen Star will also display this unusual response to an H-beta filter though not as dramatically.

Perhaps more southernly observers can confirm if this planetary displays a red halo, though as the halo is barely non-stellar, so this will probably be a very tough observation. Nevertheless, if you're looking to find another target to use your H-beta filter on besides the Horsehead and the California nebula, check out Swings-Struve 1!

"Give it a go and let us know!"

Good luck and great viewing!

[Jimi Lowrey]

We had a break in the monsoon last night after two weeks of much needed off and on rain showers. I have been wanting to try Swings-Struve 1 with my scope every since Steve, Paul and I viewed it at GSSP.

I found the field easily at 375X the object looked like a carbon star at this power and had a small faint halo. I tried a 7MM @ 697X next and it made a big difference in how it looked there was a very small reddish ring around the star (Paul you were right) and a faint halo beyond the ring. The reddish color ring or shell was very very small like someone had drawn with a sharp red pencil a circle on the edge of the star and beyond the reddish ring was a faint small nebulous shell. Very cool and unusual object I highly recommend you try it and be sure to bump up the power to all the seeing will allow.

[Steve Gottlieb]

Another reddish planetary! Great observation, Jimi. Besides the big jump in aperture, I'm guessing viewing it 11° higher in the sky than GSSP also helped.

Should be a great target for southern hemisphere observers -- Swings-Struve 1 would be practically overhead!

[Bob Douglas]

While observing July 23-25 at Lake Tahoe in California, elevation 6800 feet, I observed SwSt1 using my 28" f/3.6 Starstructure. I only just heard about it from Sue French's article in the August 2014 issue of Sky and Telescope. I used 256x and 366x. Didn't notice any red color. But blinking with a H-beta filter did make it the brightest object in the fov. It's not too often when I get to use this filer. Sue incorrectly gave the size as about 5 arc minutes (yes minutes) in diameter, but it appeared stellar.
During the same nights I observed Pease 1 in M15 and the SNR Sh 2-91 in Cygnus.

It took less than 10 minutes to find Pease 1. Critical to doing this was using the two star patterns indicated in the article "Off the Beaten Path with Steve Gottlieb: Deep Sky Challenges for Early Winter" about Pease 1, with a red dot showing Pease 1. Also very useful were some jpeg images showing the star pattern in the immediate vicinity of Pease 1. In particular there is a small distinct equilateral triangle of equally bright stars just SSW from, and immediately next to, a second small triangle (slightly askew from the first triangle), this second triangle's NE "star" being Pease 1. The second triangle actually is a clump of stars, but two stars and Pease 1 stood out, forming the "second triangle". I used a
3-6mm Televue zoom, switching between the various powers 427x to 854x. Blinking with a UHC filter was also useful, but not necessary.

Sh 2-91 was first shown to me a few years ago by Steve Gottlieb. I used a 21mm Ethos and OIII. It was visible looking directly at it, but using averted vision it really pops out.

[Jimi Lowrey]

Bob Did you try SwSt1 without a filter? I am surprised with 28" that you did not get a hint of the red color.

[Bob Douglas]

Jimi--Yes I did view it without a filter, but alas I didn't see any reddness. And I am very fond of red celestial objects. I was at latitude 39.3 degrees, which may be a factor.

[akarsh]

Jimi showed me this object with his leviathan 48" two nights ago. The object distinctly responded to a filter, had a reddish / pinkish hue, and sported a subtle halo. Seeing wasn't enough to see anything more.

Clear Skies

Akarsh