NEAT SOUTHERN PLANETARIES : 26
NGC 2792 in Vela

This time we examine two PNe. First is NGC 2792, being located within northern Vela. This followed by the unusual grey PNe, Pb5 or He2-24 [See NSP 26a], that is much fainter and far more challenging. Both objects culminate during summer months. Surrounding objects in this vicinity are numerous and any amateur with moderate apertures could spend an entire night observing just this 15° square degrees region of sky! I have selected only a few, being the prominent star of λ Vel / Lambda Velorum / Al Suhail, the Innes pair, I 11, and an interesting but obscure galaxy, ESO 261-6. [See NSP 26b] Enjoy!

NGC 2792 / He2-20 / h.3149 / Sa2-36 / Wray 16-36 / VV 50 / ESO 314-6 / PK 265+4.1 / PN G265.7+4.1 (09124-4226) [U 397/398] is a moderately bright PNe that lies some 1.3°NE (PA 40°) from the 2.1 magnitude orange star, λ Vel / Lambda Velorum / Al Suhail (09080-4326). (See Surrounding Objects. [See NSP 26b]]) NGC 2792 will be seen with care in 7.5cm. but is certainly much easier in telescopes above 10.5cm.

Fig. 1. NGC 2792 Aladin / He2-20 Wide Field. CDS Aladin Colour
Image about AAO Red (red), J plates (blue), average (green).
Enhanced image with ’silvering.’ Field size: 11½′×11½′ (left) ;

Fig. 2. Close-up of NGC 2792 CDS Aladin Colour Image
Combined AAO Red (red), J plates (blue), average (green).
Enhanced image with ’silvering.’ Image size about 45″×45″ (right)

NGC 2792 has the given brightness of 11.6v or 13.5p magnitude and this difference is quite large compared to many other PNe. It is often said that it subtends the visible diameter of about 9 to 10 arcsec, and 13 arcsec to the inner halo, but telescopically it immediately appears as elliptical. It is also stated in some sources that the outer size of the slightly elliptical halo is a much larger 30″×27.6″, in the ratio that is about the same as the inner region. I.e. 10:8.

This planetary is placed in a starry field among many numerous faint stars that includes the orange 6.1v magnitude star, HIP 45242* / SAO 220962 / PPM 314090 / HD 79524 (09133-4216).

Historical Interlude

Discovered by John Herschel on March 1835 (as HJ 3149), who in his diary almost in a very casual quip, describes NGC 2792 as; “Planetary nebula, observed with Mr. Maclear and another gentleman.” He went on to observe this object three times, and saw it quite differently on each instance. These appeared in his great southern observational tome (Herschel pg.89 (1847), describing NGC 2792 as;

“pF (pretty faint) exactly R (round), equal to a star of 9th m[agnitude], but of a dull light. At first I was inclined to think it double, but with 320[×] it exhibited a uniform round disc; nor did a friend to whom I showed it to see any division. Stars tonight perfectly well defined. In a field with leading stars, or which a diagram was made.”

His second observation was;

“pB (pretty bright), round, 6″ diam[eter], equals in light to a star about 9th m[agnitude]; a very good and careful observation.”

On the last time John Herschel saw NGC 2792, he said;

“Viewed past the meridian. It occurs in a field with about forty stars. Diameter 4 or 5″ at the utmost, 10″ is too large certainly. Very like ∫771 h.3101 [NGC 2452, the planetary in Puppis] But now the night is good it bears magnifying. With 320[×] the disk is dilated into a dim hazy round nebula; yet there is a peculiarity in its appearance which completely separates it from all nebulae of the same size. A very remarkable object.”

After Herschel NGC 2792 was listed in the New General Catalogue (NGC) in 1888, and unusually, has an extended description quite different from most other PNe in this catalogue. Here Dreyer states;

“Remarkable planetary, pretty bright, equal to a star of the 9th magnitude, very small, round, among stars.”

Discussion of John Herschel’s Description

As a broad comment, I find the “remarkable planetary” description in the NGC is just a little odd. This description has been adopted from John Herschel’s written text from his third observation in South Africa of this planetary. Here he concludes by stating “A very remarkable object”, which he bases on; “With 320 the disc is dilated into a dim hazy round nebula, yet there is a peculiarity in its appearance which completely separates it from all other nebula of the same size.” To me this does not match very well against the other more modern observations — and this cannot be blamed just on poor seeing and the like, but perhaps unnoticed dewing on the speculum mirror. Almost contradicting this, John Herschel says before this remark; “But now the night is good and it bears magnification.”

If we were comparing this to the surrounding objects then the number of nebula — like the Vela Supernova Remnant (VSR) or its nearby nebulosity fragment of NGC 2736 / The Pencil (09004-4554) then this would be true. Sure, NGC 2792 is both bright and suitably located to find. However, the overall appearance seems to me far from being truly remarkable at all, especially if you compared it to say Vela’s NGC 2899 or NGC 3132. (Note: Both appear on the next Uranometria 2000.0; Map 399.)

John Herschel said it to be both “Very remarkable planetary…”, then NGC 2792 certainly is a bit more disappointing than it ought to be. Compared to the other PNe appearing in our Neat Southern Planetary Series, it is a bit more thrilling so to speak, but how it rates with all other PNe, I would think it nearer the bottom of the Top 40 than at the top.

Using the Table in the back pages of Kent Wallace’s detailed “Planetary Pages”, NGC 2792 ranks as 70th in brightness and 96th by size. It is also only one of some twenty PNe discovered by John Herschel himself, and in my opinion ranks as 12th or 13th among his discoveries. The NGC contains ninety-six PNe across the entire sky, of which NGC 2792 is one of some twenty-five odd PNe below the declination −40°S.

To me, the NGC description here also slightly overrates this object. Such opinions have disappointingly have spilled into some modern texts, like Mike Inglis’ “Astronomy of the Milky Way : Observer’s Guide to the Southern Sky” (2004), who describes;

“Several planetary nebulae can also be found in Vela, including one that is thought to be amongst the finest in the entire sky. One of these is NGC 2792.”

Observational Descriptions

Telescopically, NGC 2792 seems fairly colourless and featureless to me, or perhaps even slightly greyish. I suspect, though large apertures might also see just the tinge of blueness. It would be interesting to read observations from the larger telescopes to see if the disk shows any features at all. Using either the 10.5cm. f/15 refractor, 20cm.. and 30cm. Newtonians showed nothing different in the disk at all. However, using the O-III filter increases the apparent brightness considerably, as Kent Wallace’s quote below confirms. To me, NGC 2792 catalogue data the object seems larger than against some other PNe I have seen. This PNe for me appears as the similar and smaller version of NGC 3195 in Chameleon (NSP05).

Both AOST1 & 2 describes it as;

“In a field of scattered stars… it is round, greyish and about 10″ across, with fairly uniform light and well defined 10.5cm will show it and the singular image with care, but 15cm makes it easy.”

Auke Slotegraaf, in his “Deepsky Observer’s Companion” and using 15cm (6-inch) f/8.6 Newtonian says;

“…In a low-power sweeping eyepiece, the planetary is readily seen as an 11th magnitude star in a rich field of mixed magnitudes… Examined the disk at various powers, none of which show anything but an even, round disk.”

Also according to Auke Slotegraaf, in the same reference, R.T.A. Innes in South Africa (Union Observatory Circular., 144, p.345 (16th March 1911) says of NGC 2792;

“An 11th magnitude planetary nebula about 20″ in diameter is np. [north-preceding] a pair of 10.5 magnitude stars. No stars within 3′.”

In Mike Inglis’ “Astronomy of the Milky Way : Observer’s Guide to the Southern Sky” (2004) says;

“…a small gray-colored planetary nebula some 10 arcseconds in diameter. Its edges are well defined and show a plain smooth appearance. A small telescope of about 10 cm will show it, about as a tiny disk, but larger apertures will show it with ease.”

Kent Wallace on the 27th March 1992 using a 20cm. C8 describes NGC 2792 as follows;

“At 62.5×, can see directly as a faint star, requiring the O-III filter to identify it as a PNe. Strong response to the O-III filter. Good response to the UHC filter. No response to the Hβ filter. At 100×, a small disk is visible. At 200×, the disk shows up better. No central star is visible.”

Technical Data on NGC 2792

NGC 2792 lies some 8.4 kpc. from the galactic centre and 135 pc. above the galactic plane. It is classed in the V-V classification as Type IV or 4 — ring structure; though apertures below about 40cm. to 50cm. will not see this. It is also classed as Peimbert Type II.

Its internal appearance can be seen in the HST image produced by Howard Bond in 2007. (Fig.3) The brightest region is more diamond-shaped, with the diametrically opposed sides being of near equal intensity. The inner region shows some general mottling with some linear structures shaping the inner region like a cross or an “X.” Beyond this inner region is a slightly larger elliptical halo, which has two very small asymmetrical knots (akin to ansae) on the right and left hand side and bordering its edge. Other images reveal there are two other fainter elliptical structures that can just be seen on the right-hand side of Fig. 3, extending just beyond the inner halo. (See images in the linked Corradi, et al. (2003) paper and the given links.)

True nebula size of the inner portion is about 1500AU or (0.0237 ly.), with the outer regions extending out as far as 15,000 AU (0.07pc. or 0.237 ly.)

Images of the main wavelength shows much diversity within the main central nebulosity. Hα, [NII] and [OIII] show two main areas of concentration, though the HeII bandwidth displays much more complexity. The Hα, [NII] and [OIII] can be seen in Fig. 3 as the two brilliant white regions. (See the German paper on NGC 2792 at; Institut für Astronomie und Astrophysik site.)

Corradi, et al. (2003) first discovered a large [O-III] and Hα+[NII] spherical halo with very low surface brightness that surrounds the main nebulosity and extending out to about 37 to 40 arcsec. This was revealed using CCD imaging with the 3.5-metre ESO / NTT telescope and an occultation bar.

[See these discussed images at;

http://www.iac.es/galeria/rcorradi/HALOES/n2792ha.jpg,
http://www.iac.es/galeria/rcorradi/HALOES/n2792oiii.jpg]

[Other Southern PNe with Halos, see page; http://www.iac.es/galeria/rcorradi/HALOES/].

Fig. 3. HST Image of NGC 2792. Filter image at 555nm. Image size about 25.8″×:24.4″.

One of the most recent dedicated papers on NGC 2792 is by Pottasch et al. (2009), which discusses the abundances of elements in the nebulosity. The paper examines the mid-infrared data from the spectrograph of the Spitzer Space Telescope and the earlier ultraviolet spectrum, and deduce the early evolution of the planetary. They find these observed chemical elements are atypical of many PNe. Also found were small dust grains of silicates about 1μm across. Discussion in what they find is too detailed for our overview here, but the data appears in Table 1, 2 and 3 in linked referenced paper. For amateur observers, the [OIII] to Hβ ratio is about 9.6:1 (960:100), suggesting the OIII filter should be effective when observing for inner structures.

The final chemical abundance found in the nebula model, they find the following results in Table 1;

Also found was the mean [OIII] temperature of the 8 arcsec inner nebulosity, being between 12,000K and 14,200K. HeII lines suggest the nebula temperature is 15,000K, whose electron density (Ne^-) (the source of the energy from the PNN illuminating the visible nebula) being about 10³ cm.^-3. This study finds an electron density of 2500±500 cm.^-3, which is used in estimating the abundance in the table above.

Currently, the distance is estimated by Stanghellini, et al. (2008) to be about 3.05kpc, though many older references tend to give higher values. I.e. AOST1 gives 4.5kpc while the later AOST2 gives 2.5kpc. Lowest value in the literature is 1.89kpc. by Zhang (1995). As Pottasch (2009) says; “The distance of the nebula is very uncertain.” The principle cause seems to be related to the uncertain high extinction in the region, estimated as E(B−V) of 0.55. Future studies of the extinction may improve our knowledge of NGC 2792. Radial velocity of the entire nebula is moving away from us at +14.0 km.s^-1.

Central Star of NGC 2792

The PNN is estimated at 17.04V (earlier stated as 15.74v:) and 17.2p magnitude, being invisible to most amateur telescopes. According to Pottasch et al. (2009) the Teff is 160,000K whose luminosity is 600 times that of the Sun. Earlier estimates in 1988 give the Zanstra temperature as 82,000K, and luminosities about 1200 times the Sun. Pottasch et al. (2008) find the PNN diameter is 0.068R⊙, equivalent to 94,000 km. — somewhere in between the sizes of Jupiter and Neptune. Analysis of the abundances suggest that the original main sequence star was about 1.0M⊙.

This nice PNe is pleasantly interesting when panning across the northeastern portion of Vela.

References to NGC 2792

1. Corradi, R., et al., “Ionized haloes in Planetary Nebulae: new Discoveries, Literature Compilation and Basic Statistical Properties”, MNRAS., 340, 417 (2003) [Paper can be download at the same site which is linked on the top of the page. Size is 17.5Mb., though it is an interesting read.]
2. Herschel, J.F.W.; “Results of Astronomical Observations Made During the Years 1834,5,6,7,8, at the Cape of Good Hope.”; Published by Smith, EIder and Co. (1847)
3. Pottasch, S.R., et al. “Abundances in planetary nebulae: NGC 2372.”, A&A., 502, 189 (2009)
   [Downloadable from the ADS.]
4. Stanghellini, L., Shaw, R.A., Villaver, E., “The Magellanic Cloud Calibration of the Galactic Planetary Nebula Distance Scale.”, AJ., 689, 194 (2008)
5. 5. Zhang, C.Y., “A statistical distance scale for Galactic planetary nebulae”, ApJ.Sup.Ser., 98, 659 (1995)

Last Update : 24th February 2012

Southern Astronomical Delights © (2012)

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