NEAT SOUTHERN PLANETARIES : 20
IC 4191 in Musca
Again we now return to the far southern
constellation of Musca, however, our selected object is now
on Musca’s eastern wing. Smaller in size, IC
4191 appears as the bright but minute planetary
nebulae. This object is quite interesting, though during
this time I did use an O-III filter for the first time.
Even so, this PNe was difficult to find because of the
profusely starry field in which it lies. It takes different
observational techniques to observe compared to the much
more larger and flamboyant nebulae we wish to search for at
the telescope.
Again when this was written back in 1999.
little was known of this object. Now in 2011 little has
changed. It still remains a poorly studied object.
Pity!
IC 4191 / He2-89 / Sa2-89 / VV67 / Wray 16-124 /
PK 304-4.1 / PN G3045-4.8 (13088-6739) is a tiny,
moderately bright stellar-like planetary, which was
first found by Williamina Fleming in 1907. Brian
Skiff’s sky positions are 13h 08m 47.52s
−67° 38′ 32.2″, using the GSC
catalogue and using the location of the nearby star GSC
9241-0477 as a guide. IC 4191 is found some 40′ENE
from 4.7 magnitude double star
η Mus / Eta Muscae
(13152-6754), while this PNe field contains the bright 6.4
magnitude reddish star L 5393, some 9.6′SWW
(PA 191°.). In AOST2, David Frew comments that this same
star is useful finding IC 4191.
Observations of IC 4191
Among Musca’s small PNe collection, are three main
objects. I rate The Spiral Planetary
NGC 5189, followed second by IC 4191, with
NGC 4071 lying a close third — making, in some ways,
Musca’s “nebulae triumvirate”) IC 4191
makes quite a contrast to the much larger NGC 4071, as it
appears stellar, except when using very high powers and
large apertures. Visual magnitude is given as 10.6v, while
the photographic magnitude is 12.0p. With care it can be
found in 10.5cm. telescopes, though it is claimed in AOST2
that it can be glimpsed even in 7.5cm. IC 4191 is classed
as a “Type 2” — smooth disk in the
Vorontsov-Velyaminov or VV PNe classification, and for me,
it appears a strikingly blue or aqua-blue
“star-like” nebula. In 20cm, this colour was
still obvious, and once found, it became instantly
noticeable using any wide, low-magnification field. Using
30cm, I thought the colour was much bluer than the planet
Neptune, and incidentally, larger as well! I think the
presence of the red star so close to the planetary
certainly influences the perception of the small
disk’s colouration. This is probably similar to the
problems faced with colour contrasting double stars —
like the famous northern pair
β Cyg / Beta Cygni /
Albireo (19307+2758) or the wider
γ Cru / Gamma
Crucis / Gacrux (12312-5707). I attempted using an
occultation bar to obscure the star, and thought that the
colour did slightly fade. Using the highest power possible
for the seeing conditions, I also once tried moving the
star in and out of the field of the eyepiece. This was only
partly successful. It would be worthwhile to do a blind
test on the object at a Star Party, and compare the
colour with or without the star’s influence.
Positive identification of the PNe is particularly easy
with an O-III filter, by just flickering the filter across
the field of view. (See the Position in Figure 2.) Here the
stars almost vanish while the planetary stays constant.
(Comment: When doing this with the telescope, for some
reason, I always think of Clyde Tombaugh, the discoverer of
Pluto. Call me crazy, but the appearance of the PNe
jumping in and out of visibility seems exactly like the
observational methods of using old blink comparator.) In
the case of IC 4191, I thought the usefulness of the O-III
filter was not as effective compared with other planetaries
of similar brightness and size. Using the O-III with 30cm.
and higher magnification, the disk appeared quite smooth
and uniform, with perhaps slight increases in brightness
towards the centre. I also suspected that the south edge of
the disk was also slightly fainter and fades more
rapidly.
Few modern amateur descriptions exist. One example was
written by Queenslander Greg Thompson, who describes IC
4191 as having a starlike centre, surrounding by a small
but bright and slightly bluish circular glow, which fades
rapidly towards the edges. (“Southern
Astronomy”; Sept./ Oct. p.39 and Field
Sketch.)
Astronomical Objects for Southern Telescopes (AOST2)
(p.293) lists IC 4191 (#576), and says of its nature and
colour;
”…this small planetary nebula
can be picked out by its bright bluish disk from a profuse
star field. The nebula has a bright elliptical prism image
which 7.5cm shows as a tiny point, and is an easy stellar
object &##8230;at the north-west apex of a narrow triangle
with two fainter stars.”
Jenny Kay of the Canberra Astronomical Society Inc.
(“Southern Cross”, June 1988) says of
IC4191;
”12.5″ [f-5] —
50×: The planetary requires some care to pick out in
the busy star field, being almost stellar in appearance.
Once located, it can be noticed that the planetary is only
slightly larger than a true star, and equal in brightness
to a nearby 11 mag field star. It also appears to display a
very deep, blue colour. There is a very strong response to
the UHC filter. 83/151X: Some care still required not to
mistake this very small planetary for a star. The planetary
is 5&″ in size at the most, relatively bright, with a
round disc of uniform brightness …151× offers
the best view where the planetary is a little more
pronounced.”
In a personal communication, Californian amateur Kent
Wallace, observed IC 4191 from Western Australia on 21st
January 1999, states that;
”[Using the C-8 SCT]
at 62.5× and 100× [I] can see a very faint star, but needed
the O-III and UHC filter to identify it as a PNe. Good response to
the O-III and UHC filters. No response to the Hβ. At 200×, I could see a very small
disk when using the UHC filter and averted vision.”
Detailed Analysis of IC 4191
The truth about the PNe nature of this object was finally tied
down as recently as 1948 by the Russian astronomer
Vorontsov-Velyaminov (VV), who listed IC 4191 as VV67. Next
observation of any significance, was one made by Karl K. Henize
between 1961 and 1962 using the 74-inch Reynolds Telescope at Mt.
Stromlo Observatory near the Australian capital, Canberra. His
observational results were a long time coming, and two significant
papers by Henize were published. These were “Observation of Southern Planetary
Nebulae.”; AJ.Sup.Ser.,
14, 25 (1967) April (1967), and a second paper authored by
Westerlund, Beng and Henize; “Dimensions of Southern Planetary
Nebulae.”; AJ.Sup.Ser.,
14, 154 April (1967).The latter paper produced images and
observations that were made between November 1961 and June 1963, when
Henize was visiting Mt. Stromlo (1961-1962) in exchange with the
Dearborn Observatory of the American Northwestern University.
In regards IC 4191, Henize classed the PNe’s structure, by placing it second in
nine-categories of PNe types — “Ellipses gradually brighter in centre in
order of core brightness.” Henize also
found the core’s diameter as 6 arcsec,
about the size as seen in most amateur ’scopes. Observations from Mt. Stromlo found
the outer halo dispersed its light to about 10 arcsec. Henize thought
he had seen some elongation, and using the twenty minute photographic
exposures, made an estimate of the position angle as 50.0°. He
also commented that he thought the nebula appeared as “peculiar triangular shaped.” This is particularly odd PNe description,
but his impression is seen clearly in the image produced in the
original paper. In the professional telescopes appears a very faint
outer envelope can be seen to extend by some 18×11 arcsec.
However, this is invisible in all amateur telescopes. Here IC 4191
remains about 5 arcsec across. Morano (1990) measured the Hβ diameter as 11 arcsec., finding strong
emissions in O-III and Hα. This is
also especially prevalent within the inner parts of the nebulosity.
Like most “average” planetaries, the expansion velocity of the
gaseous shell is 12.0 km.s-1 (1976), while the radial
velocity, measured from the O-III lines, is
-12.7±6.5km.s-1 (1983) in approach.
Images in Ha are particularly interesting, as IC 4191
appears the irregular second disk abut 21 arcsec across,
and possibly surrounded by even faint nebulosity. IRAS
observed in 1982 a moderate flux at 25±6.0 µm, and
also confirming Henize’s “triangular”
description. The O-III image is also irregular, but
slightly smaller than the corresponding
Hα image. No IUE satellite
observations were ever made, but the radio observations
confirm the moderate PNe radio source whose flux is
152m.Jy. (2cm./14.7GHz.) and also the flux of 170m.Jy.
(6cm./5.0GHz.)
Line intensities are measured as
Hβ=100; He(II)=12; OIII=8;
OIII=567; He(I)=19;
Hα=380; N(II)=139; S(II)=5
S(II)=11 against the standard
Hα flux of about
10-11 mW.m-2 (1985). These lines
are not very intense, but because of the
“stellar’ nature of the PNe, the use amateur
filters still prove quiet effective. The
Hβ/[O-III] ratio or
“N”, is the measure of the brightness of the
Hα and [OIII] lines is
5.8 or 6, which is on the small side compared to many
planetaries. (See Hydrogen Beta
(Hβ) and the Line Emissions
from Planetary Nebulae.) Also the spectral lines and
brightness between NGC 4071 (NSP19) and IC 4191 are about
the same. As IC 4191 is much smaller in area, therefore it
appears much brighter in the telescope.
Examination of the intensities of the emission lines
also can be used to determine the so-called electron
densities (using measures of the deep-red [SII] lines) and
the nebula temperatures. First appreciated by Menzel, Aller
and Hebbin in 1941, and further elaborated by M.Peimbert in
1967, these two quantities have become an important means
of examining the nature of gaseous nebulosity. It allows
some explanation of the mechanism of ionisation within the
nebula’s confines, but also allows calculation of the
relative abundances of the various photoionised elements.
Typical values tend to produce electron temperatures (Te)
between 5,000K and 20,000K, and electron densities between
104 and 106
e−.cm-3 (number of electrons
per cubic centimetre) are typical. As a method, this
relates strongly to the atomic structure of matter and the
general conditions within the nebulosity. Such
galactoastrochemistry tells us much about the
evolution of the PNe and PNN combination, including the
element abundances of the original star and the PNN’s
age. For IC 4191, the electron temperatures are taken from
McKenna et al. “[NII] and [OIII] Mean Electron
Temperatures.”; PASP., 108, 610
(July 1996), finding values as [OIII] Te = 9,940K and [NII]
Te = 10,350K, place this PNe in the mid-range of the
106-odd PNe examined in this paper.
Little is known of the central Planetary Nebula Nucleus
(PNN), though the ESO-Catalogue (AG92) states a visual
magnitude of 16.4v, while the other measured B-magnitude is
16.8B [Hence, the B-V=0.4] Earlier texts normally state
“16.6v:: mag”, however, it remains totally
invisible in all amateur telescopes. The spectral signature
of the PNN is uncertain, and the detail of the surrounding
nebulosity is only inferred by its emissions. Zanstra
temperatures for this object were first determined by
Gleize et al. (A&A., 222, 237 (1989)) as
99,000K and 116,000K for HeII, giving the mean PNN Zanstra
temperature of 108,000K. In 1978, the uncertain distance
was determined to be about 2.1 kpc. All values since this
date have done little to improve this distance
estimate.
IC 4071 has proven to be unloved, and some
of its nature will likely be revealed sometime soon. As of
October 2011, no HST image exists or is presently
planned.
Last Update : 27th September 2011
Southern Astronomical Delights ©
(2011)
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