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).
* This particular K2III star has the B−V
magnitude of 1.25. Distance is 164.7±9.8pc. or about 540ly.,
from the HIP2 parallax is 6.07±0.36 mas. (2007), making the
absolute magnitude of +0.0 or 88 times more luminous than the Sun.
Surface temperature is 4480K. There is negligible proper
motion.
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.”
As a general comment, dewing is also a significant problem
for today’s observers. I am quite sure most amateurs could
testify to, at least once, viewing some hazy and washed out deep-sky
astronomical object before realising the corrector or mirror has been
subjected to moisture. As similar problem could be the likely
explanation of the many missing James Dunlop deep-sky objects where
none are known to exist. Here many of the objects were described as
“very small round nebulae” which were sometimes
associated with central stars.
Problems with speculum mirrors and dew is not a common
experience for the modern observer. You would suspect that metal
mirrors would tend to hold heat far more than glass or Pyrex mirrors,
and therefore would ward off dew a bit longer. Yet on the heaviest of
moist nights, even metal mirrors would become hazy and make celestial
objects unobservable. Little discussion appears in the literature on
this topic regarding dewing of metal mirrors nor of the best means of
eliminating the problem whilst observing. My assumed guess would be
that the stored latent heat of metal mirrors would prevent dewing but
would have a negative effect be making poorer quality images for
observers.
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;
Table 1. Chemical Composition of NGC
2792
Element | % |
Hydrogen | 12.0 |
Helium | 11.0 |
Carbon | 07.900 |
Nitrogen | 07.641 |
Oxygen | 08.321 |
Neon | 07.799 |
Magnesium | 07.031 |
Silicon | 06.455 |
Sulphur | 06.648 |
Chlorine | 05.049 |
Argon | 06.213 |
Total | 86.057 |
Other Elements | 13.943 |
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
- 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.]
- 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)
- Pottasch, S.R., et al. “Abundances in planetary nebulae:
NGC 2372.”, A&A., 502, 189 (2009)
[Downloadable
from the ADS.]
- Stanghellini, L., Shaw, R.A., Villaver, E., “The
Magellanic Cloud Calibration of the Galactic Planetary Nebula
Distance Scale.”, AJ., 689, 194 (2008)
- 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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