NEAT SOUTHERN PLANETARIES : 06
IC 5148 — IC 5150 in Grus
Our sixth part of this series reaches out of
the realm of the southern Milky Way. One of the best of the southern
objects here is IC 5148 / IC 5150 in Grus, and is a really
elegant and terrific annular planetary. I have included, for those
that like a tough challenge, and have some aperture, some may like to
try to observe the isolated He2-434 (19338-7433), now placed
in NSP06a] in southern Pavo.
Unfortunately, during the summer months in
the southern hemisphere, roughly between the months of
October and March, there are only handfuls of interesting
objects to occupy our attention. Observers of planetary
nebulae will usually focus most of their attention during
this time of year on the Helix Nebula / NGC 7293
(22297-2112) in Aquarius, but there are several other good
targets, like the Saturn Nebula / NGC 7009
(21042-1121). In this context, really the Helix Nebula is a
southern object, probably much to the chagrin of our
northern observing cousins, as the declination is
−21°S. However, the Saturn Nebula at
−11° declination is equatorial.)
[The link below will bring up a fine sketch of IC 5148
by Rich Jakiel.]
IC 5148 / IC 5150 / ESO 344-5 / PK
002-52.1 / PN G002.7-52.4 (21596-3923) (Grus) was first
pinpointed by Sydney amateur Walter Gale in 1894, near the
Crane’s long ‘neck’. For some reason it
was not marked in the old Sky Atlas 2000.0 but does appear
in the Sky Catalogue 2000.0! The planetary itself is found
in its rather desolate field some 1.3°W of
λ Gru / Lambda
Gruis (22062-3933). It takes at least 15cm. to see its
nebulosity, as the total magnitude is 13.0v / 12.9p. Unlike
the majority of planetaries, this one is large at about 2
arcmin, as most of the available literature states.
To my eyes the O-III filter image is slightly brighter,
but oddly this should not be true, because the observed
spectrum is given as continuous. Usually, for any object
with continuous spectra has no real advantage when using
narrow-band filters.
Observationally, little difference is observed in the
apparent size from the CCD or photographic images. A 20cm.
clearly reveals its colourless annular structure, including
its smallish 25 to 30 arcsec. hole, and consequently, IC
5148 / IC5150 is classified as planetary Type IV or 4. A
30cm. or 40cm. will start to reveal some of the internal
diamond-shaped braided structure, as easily seen in
the available images, where the eastern side appearing
broader and brighter. Contrary to this, AOST1 & AOST2
obscurely states; “…and a considerable area
looks paler (grey), but still luminous.”. This
central hole is much smaller than most of the annular type
planetaries.
In generally accepted PNe theory, the appearance of
central holes are is thought to be due to the strong
radiation pressure pushing the surrounding gas away from
the white dwarf core, leaving the ‘vacant’
space. It is likely that the development of annular
structures are the sign of the last stages of planetary
nebulae formation — just before the nebulous object
transforms into its solitary and long-lived white dwarf
phase. The planetary shell structure when seen in
three-dimensions is probably one spherical shell, if
not, two shells, and that we are probably looking
directly down upon the ‘poles.’
Observations of IC 5148 / 5150
Dennis di Cicco described it as; (S&T., June
1986 p.631);
“IC 5148 is a real show
piece… With 280× and a UHC-filter, the 24[-inch]
in Hawaii showed IC 5148 to be an exquisite giant ring,
irregular with a dark centre and a 15.5 central star. Steve
Gottieb of Berkeley California, observed it with a
13[-inch] reflector and an O-III filter. He could view the
nebula with direct vision and detect the dark centre with
averted vision. This object is the same object as
IC 5150.”
(A nice image taken by him also appears on p.632 in the
same issue of S&T.)
Melbournian Southern Observer and imager Neil Avery on
18th September 2006, wrote me this encouraging e-mail on
his multi-filter imaging attempts of IC 5148/50 (Figure
3.), which I have reproduced here. He says;
“I have imaged one of the PNs that
you suggested with my newly acquired Custom Scientific
4.5nm. Hα and OIII filters.
Compared with the 13.5 nm Astronomiks I get a further three
fold reduction in sky glow. The background sky glow is now
so low that there is not a lot to be gained by a dark site.
This PN has a fairly low surface brightness but the result
is very encouraging. This one is a bit unusual in that the
Hα and [OIII] are of
comparable intensity giving a fairly neutral colour image.
more commonly, H[ydrogen] dominates the outer regions and
O[xygen] the inner. Best wishes.”
Figure 3. Multi-Colour Image of IC
5148/50 by Neil Avery
Summary
Description: IC 5148/ PN G002.7-52.4, North
8 o’clock, Planetary nebula; VV type 4
Optics : Celestron C11 at f/10. Filters, Custom
Scientific 4.5nm. Hα and
[O-III]
Mount : Losmandy G11, autoguided.
Camera : SBIG ST8xe at 20°C Inserts
Hα and OIII, 9×10
mins., binned 2×2
Processing : L=Hα,
G and B= [OIII]
Date: 15 September 2006: Melbourne
Historical Interlude
It is surprising that John Herschel missed this object.
Even some of the more modern references, like
Burnham’s, still do not list this object. As we have
seen throughout this series, the common references for
southern objects used by amateurs are inordinately
incomplete. One of the notable problems with this object is
that the IC catalogue gives it as two objects. Most
references usually selected it as either IC 5148 or the
combined IC 5148/5150. It was given these two different
designations by Dreyer (1908) in the Second Index Catalogue
(IC).
As mention before, Walter Gale discovered this object in
1894. IC 5148 and IC 5150 differs in position by 11″
in RA and 62″ in Dec. IC 5148’s position is
taken from observations made by the veteran American
observer, Lewis Swift, from an observatory on Echo Mountain
near Los Angeles. This planetary is also featured in his
list (Mem. RAS., LIX, p.568 (1899)).
Swifts’ positions were taken from images over very
wide fields — usually having the general notoriety
for having particularly inaccurate positions. IC 5150
refers to the position based on Gale’s observations.
(His catalogue No.3246.) Dreyer’s Index
Catalogue also shows some confusion in the
descriptions. Swift states the object is “vF, L,
lE, *att.”, … very faint, large, little
extended, star attached; while Gale states “pB,
pL, annular” — pretty bright, pretty large
and annular. No confirmation was made between the two
catalogues numbers, so the current odd designations remain.
This discrepancy is slightly unfair on Walter Gale, because
his description is closer to the truth (See AOST1), and it
is for this reason he lists it as IC 5150. Yet the
preferred designation remains as IC 5148.
Technical Data
Professional photography has revealed an outer halo
exists that extends the diameter to about 135 arcsec.
Radial velocity measurements indicate that the nebulosity
is moving towards us by some -23 to -28 km.s-1.
Simbad (2011) currently states -26.2 km.s-1.
Determined by the faint [O-III] emissions, the nebulosity
has an expansion velocity of 53.4km.s-1 —
one of the fastest expansion velocities of known PNe. So
far, the only adopted distance is 900 pc. (2,900ly) first
given by Kingsburgh and English in 1992. Distance by them
is stated as an upper limit and is based on the faint
Hβ (Hydrogen-Beta)
emissions. Due to the faintness of the PNN, determining
distances for this object has been difficult. Since
originally writing this, Stanghellini, et al. (2008), now
state the distance is 1068pc. (Rounded 1070 pc.).
Central Star of IC 5148/50
Visually, the central PNN is at 16.5 magnitude and can
possibly be seen in 40cm. or 50cm. telescope, though I have
to admit that I have never seen it nor read reports of
others. seeing it. It is clearly obvious being central
object in images.
According to Weidmann & Gamen (2011), the central
star seems to be of an O-type (just like He2-434 [NSP 06a]
being found as O-type in the same paper.)
REOSC spectrograph attached to the 2.15-m telescope at CASLEO, Argentina
Subtracting the nebulae emission, called normalised flux
Normalized spectra of O-type CSPN. Absorption lines of HeII
are observed in the spectra. The interstellar line of NaI is indicated.
“O?. We were able to minimize the
nebula contamination and to identify the absorption lines
of the Balmer series and HeII 4686Å (the later
confirming an O-type spectrum). Méndez 1991 classified
this star as hgO(H). Nevertheless IC 5148 does not seem to
be an evolved object.”
Weidmann & Gamen (2011) also importantly point out
in their introduction to this paper, which adds some
perspective and poignancy on the developments in the last
decade on central stars (CSPN) since NSP 06 was written.
“…there are about 3000
catalogued PNe in our galaxy, but only 13% of their
progenitors have been spectroscopically identified.…
The identification of the ionizing star of a PN is not
always an easy work; most of them are optically faint
objects (low luminosity) and sometimes they are not at the
geometric center, because the nebula interacts with the
interstellar medium. Also, PNe are concentrated toward the
plane and bulge of the Galaxy, where crowding and
interstellar dust make difficult to observe and identify
they progenitors. The fact that the PNe are easily confused
with other types of objects (Frew & Parker 2010)
complicates this picture even more, and introduces
confusion in statistical works. At present, many non-PNe
remain hidden in the catalogs of PNe.“
In my view, this planetary is easy to find and is
worthy to glance.
References
- Dreyer, J.L.E., “Second Index Catalogue of
Nebula and Star Clusters, Containing Objects found in the
Years 1995 and 1907.” Memoirs RAS.,
LIX Part II (1908)
- Stanghellini, L., Shaw, R.A., Villaver, E.,
“The Magellanic cloud calibration of the Galactic
planetary nebula distance scale.”,
AJ., 689, 1942 (2008)
- Weidmann, W.A., Gamen, R., “Central Stars of
Galactic Planetary Nebulae II : New OB-type and
emission-line stars”, A&A., 526, 6 (2010)
[Section: 3.3. ‘Notes of some individual
CSPN’]
Last Update : 30th November 2011
Southern Astronomical Delights ©
(2011)
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