Object of the Week July 28, 2013 - Sharpless 174 - a moving planetary?

[reiner]

Sh2-174 The planetary nebula abandoned by its central star?

Cepheus

23 46 49
+80 56 20

Type: Planetary nebula (?), 10'x15'


Sharpless 174 has a size of about 10'x15' and belongs hence to the very large planetary nebula. Its central star has a relatively large proper motion. During its movement through the interstellar matter, the PN looses part of its shell which is left behind as a trail visible in HII light. The central star is therefore no longer in the center of the HII-emitting part of the PN.

Image by Stephane Zoll in HII and OIII
Sharpless174_Zoll.jpg

Due to the apparent lack of a central star (there was no suitable start in the HII part of the nebula), Sh2-174 was first classified as an HII region. The part of the nebula, that shines in OIII light is, however, concentric around a White Dwarf, which gave evidence for Sharpless 174 being a PN. The parts of this object that are visible in HII and OIII light appear therefore slightly displaced (see animation below).

RW Tweedy, R Napiwotzki, The planetary nebula abandoned by its central star

OIII and HII frames of Sh2-174



My observations of this object
This displacement is visible at the eyepiece. The PN is quite faint, but still bright enough to allow for a detection of the slight displacement when switching from H beta to OIII filter. The disk of the PN appears diffuse without clear edges. OIII emission is displaced to the NW, while H beta and hence HII emission is more extended and displaced towards SE, in agreement with the narrow band images. Using an UHC filter, both components appear simultaneously, yielding a round appearance of the PN.


New interpretation of the nature of Sharpless 174

Entering David Frew, Dep. of Physiscs at Macquarie University in Sydney. During his PhD time, the made a survey of the nearby PN and PN candidates, gathering new data and re-examining existing data. One of his aims was to set up a new distance scale for PN and to prepare a sample of all known *bona fide* PN nearer than 1000 pc. Such a sample would allow examination of PN also with statistical tools and, of course, be to the delight of us amateuer astronomers, providing us a new observing project :-).

Thesis of David Frew

While there were not that many new PN to add to the sample of true PN, there were several, formerly acknowledged PN to be rejected from the sample. One of them was Sharpless 174. So, what's the point of rejecting Sharpless 174 from being a true PN after the reason for its peculiar appearance and for the apparent lack of a CS seemed to be solved?

The first point is the White Dwarf star. It is cooler than other WD of evolved PN of similar age and did not fit into the typical post-AGB scenario of a PN central star. The WD is simply by a factor of 100x too old for being a PN central star.

The nebula itself shows several features that are not in line with it being a PN.

a) The line shifts of the nebula indicate no co-movement of the gas with the White Dwarf

b) There is no obvious limb brightening of the nebula which would be expected for a shell ejected in the PN formation phase (as compared with a solid sphere of gas)

c) There is no bowshock of the nebula in direction of the movement of the White Dwarf (going along with a). Also a low ionization NII rim preceeding the nebula is missing. Such a bow shock would be expected if the highly diluted material of the evolved PN would interact with the interstellar medium (as it does for other evolved PN of similar age).

David Frew ends up with the notion: Sharpless 174 is not a PN shell (i.e. material ejected by the CS at the end of its AGB phase) but a Strömgren sphere, i.e. unrelated interstellar medium excited by the moving White Dwarf. The "shift" of the OIII and HII zones and the lag of the HII zone results from the higher recombination time of HII as compared with OIII (explaining also the tail in the similar nebula Sharpless 68).

This makes entirely sense and even it this were not the last word on Sharpless 174, it does not diminish the fun of hunting this obscure object. The statement "I have observed the shifted Strömgren spheres of a White Dwarf" is at least as cool as " I have observed an old PN", isn't it?

There are a few more "famous" PN that could be mere Strömgren spheres, such as Sharpless 68, DeHt 5, TK 2 or even Abell 35.

“Give it a go and let us know!”

Good luck and great viewing!

[hajuem]

Hallo Reiner!!

Interesting Object!! Cool Observation!! Do you see a chance for 16" und UHC (Astronomic)? I hope for good Conditions in August on the top of the EWS!!

Lg Hajü
www.astromerk.de

[reiner]

Hi Hajü,

I have observed Sharpless 174 only with my 22", but I guess it should not be a problem with a 16", in particular under good conditions. Don't forget to try the different filters to see the "movement" of the emission!

[Jimi Lowrey]

Hi Reiner,

Very cool object.

Do you get much of a response to the H beta filter?

[Steve Gottlieb]

Reiner, it seems to me that the RA is roughly 2 min too large. I realize at this declination that doesn't translate into a huge angular difference, but perhaps you can check the coordinates?

[reiner]

Hi Jimi and Steve,

yes, there is a good response with the H beta filter. It is about the same as for the OIII filter, maybe slightly less.

And yes, the coordinates were off. I corrected that with the data from Messier45. The Simbad data point to the WD, which is not the center of the emission.

[Uwe Glahn]

Reiner,

I observed this PN with 27" + [OIII] one year ago from excellent conditions in the Austrian Alps and got very good response from the [OIII] with the typical morpology you showed in the [OIII] frame.

Sh2-174.jpg
27", 113x, [OIII], NELM 7m+, Seeing V

One month later I tried the same PN with Hß and got only a bad reaction with the filter, no PN switch or noticeable moving. "Brightest" part was found around a fainter star group 8' SSW from GSC 4614878 with a fainter arm to the GSC. Another very faint plob was around the brighter star group 6' SSE of the GSC. It fits good into the Hß frame but was visible much fainter than the [OIII]. Perhaps the conditions were not as good as the observation one month before. I have to revisit the PN especially with Hß.

[reiner]

Hi Uwe,

the visibility of the displacement of the HII and OIII zones certainly depends on the conditions. The displacement was very obvious on a very transparent night with Matthias in the Southern Black Forest (his estimate was 7m0). The W limit of the emission was about the same with OIII and H beta filters. With OIII, the brightest part was actually the western rim (as in your sketch). With H beta, the object extended substantially more towards E and, if I recall it correctly, the W rim became much less obvious.

[Atlas]

Hi Reiner,

last night I had a chance to visit Sh 2-174. I could see some nebulosity at 90x and 150x even without filter. The narrow band filters worked precisely as you described. The OIII filter enhanced the northwestern rim (the blue part) while everything else disappeared from view. The H beta filter revealed the large hooklike structure across the center and towards the southeast (the red part), while suppressing the northwestern rim. The UHC filter showed all parts of the object making it appear like as large disk. Very interesting object!

Greetings
Johannes

[Uwe Glahn]

Hi Reiner,

again very little to no response with the Hß filter. I tried the PN last Thursday under very good transparency (parallel to you at the Silvretta) with my 27", 113x (6,1mm AP). The nebula was visible even without filter; strong reaction with [OIII] and NPB filter, especially NW rim, bright and easy direct vision object; with Hß I suspected only a very faint glow around the star chain 8' S of the 9mag GSC. But it was not clear if it was the filter blurred star chain or the nebula itself. The filament to the GSC was again not visible. perhaps I have to check my Hß?

[Marko]

It took me a while to write up my Aug observation of this curious object. What I can say is the OIII is all but gone in an 18" but using the NPB filter I pick up most of the Ha/Hb glow. My observation does see the OIII glow best although quite dim a little better to the west. This was at 8000 feet in close to excellent skies so I am not sure a 16" would see the OIII but maybe with better eyes.

Here is what I reported at 8000 feet with 21.6+ skies and fantastic transparency and very good seeing
(I suspect seeing is not too critical for this object).
SH2 -174 LBN 598
BrtN 10.0' E SH2 23 46 49.1 +80 56 20
12:20am 8mmDe 211x NPB filter frames the detectable glow to be about 1/2 FOV [8.9']. I see a bright FStar [GSC 4614:878] as 'up' and that is North. The glow loops to the right and down and approaches but is gone before getting to a bright but dimmer FStar 1/3fov away [6.8' to GSC 4614:901]. The glow continues to loop down with 3 faint stars about 1/2fov [10.2'] far down and slightly right where these 3 are in the middle of the glow. If we look from N star off to left and down there seems to be some glow that with movement extends down through and around several bright stars brighter than the line of 3 on other side [GSC4614:1286 is in middle of this group]. Now switch to OIII filter 8mmDe I don't feel confident enough to say I see any glow to the left around the group of 4 stars as I had seen with the NPB filter. I do detect the right side with movement and glow now is all the way and strongest just on the east side of the field star to the right [GSC 4614:901] is the strongest part of the OIII glow and is vertical [NS] elongated. I do not detect the glow on the left using OIII This object was reported to have the OIII shifted from the H2.