IC 4191 in Musca

Again we now return to the far southern constellation of Musca, however, our selected object is now on Muscas 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 Skiffs 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.

NGC 4071

Observations of IC 4191

Among Muscas 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, Muscas 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 disks 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 stars 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 PNes 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 cores 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 Ám, and also confirming Henizes 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 nebulas 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

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