By Andrew James, Astronomical Society of New South Wales, Inc.
(This is a special series appearing on   Doug Snyder's  Planetary Nebulae Observer's Home Page)


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Neat Southern Planetaries XIX

 We now return to the far southern constellation of Musca, which contains a few planetaries and large varieties of interesting objects, including the first “really neat” object in this series the “Spiral Planetary” NGC 5189 (NSP 1). The next best planetary is arguably NGC 4071, which we will discuss in some detail. Little in the amateur literature appears about the object - a trait that persistently dogs many southern observers.

NGC 4071 (Figure 1)/ He2-75/ Sa2-82/ WRAY 16-104/ PK298-4.1/ PNG 298-4.8. 0 (12043-6719) (U 450) was discovered by John Herschel in 1838, some 1.9O NWW (PA 294.4O ) from 4th mag red variable Epsilon (e) Muscae (12176-6758). Originally, he thought the object was a bright nebula, and had no inkling that it was a PNe. It was Karl Henize in his Southern Ha survey in 1961 who became first to discovered its nature. Although the PNe has not been given any  proper name, to me it appears either like a thumbprint or a three dimensional like the shape of one huge intergalactic eye. As the “Thumbprint Nebula” already exists in Draco (12429+7816), so perhaps it is best to call this the “Eyeball Nebula.”   

NGC 4071 appears is a profusely starry field of star lying along Musca's western wing. Although from Sydney’s latitude it is circumpolar, the planetary culminates at 9pm on the 6th May and at midnight on 22nd March . The telescopic field is marked some 14' to the NNE of NGC 4071, with a line of four stars in a flat arc, some 13' long, and another 9.4 star 20' further east. The four stars in a line each have respective magnitudes, north to south, of 9.3, 10.0, 9.3 and 9.2. Closest of these stars, the brightest and last star in this list, is given known as HD104995/ PPM 778794/ CPD-66 1701 (12054-6714). Later values state visual and “B”magnitudes of 9.4V and 10.8B, while the spectral class lies between B8 and K0 III. (Investigating the surrounding field to 30' also finds eighteen infra-red sources) The visually reddish 10th mag star is also the pair DON 1095.  (See Surrounding Field and “Finder Chart” Figure 3.)

Although few amateurs, until recently knew of NGC 4071's existence, mainly as it was classed as a nebula. This wonderful 12.9ph mag elliptical planetary nebula is surprisingly brighter than  expected, but unlike most PNe’s, its surface brightness is quite low, and this may account for some of the difficulty in seeing it using high magnification. This is similar to the problems with small apertures seeing the Helix Nebula in the constellation of Aquarius. Those who are fortunate to own an Hß filter, it is likely that it will be of some use with NGC 4071. As the PNe is classed as a low-excitation object, considerably gains in contrast certainly will enhance the faint detail inside the nebulosity. I don’t have one of these filter, so an observation by one or two observers would be greatly appreciated!

 The inner nebulosity is stated as 63"arc seconds across, but this coincides to its small visual appearance of about 40". Photographically NGC 4071 displays an unusual dark bar, centrally bisects the inner parts of the nebulosity. However, this curio is likely invisible to all amateur telescopes. In 25cm, the entire nebulosity is just visible to the naked-eye without a filter. Possibly even 20cm could see the PNe, if the skies were very dark. I have personally glimpsed NGC 4071 using an O-III in 20cm in moderately poor seeing conditions, though I admit is was quite a faint smudge. Using the O-III filter, the nebulosity literally jumps at you, and this becomes far more obvious in apertures above 20cm. Telescopes exceeding 25cm begin to reveal significant structure, though these features unfortunately remain colourless and faint. Using the direct-vision prism was almost useless in 20cm, as most of the nebulosity was extinguished with significant light loss from the three prisms.

 Observational Descriptions. 

John Herschel in 1837 simply described NGC 4071 as;

"vF,R, 40"arcseconds, has a vS star in centre; in a field of at least 80 or 90 stars."

Herschel’s observation is a bit odd, as he describes a bright star in the centre to the nebulosity, when clearly there is no star, but more towards the southwestern edge. I could not find any mention of this star in any of the bright catalogues, including the latest “USNO-A2.0", that contains over 526 280 881 sources and just 3.6 Gbytes in size! All this catalogue gives is twenty-five between 16.0 to 19.5 stars all within 1', but only encompassing the nebulosity. None is given within it.                                            

Also, and paradoxically, few amateur observational descriptions exist for this object. The main reason seems to be just because some common star atlases like Sky Atlas 2000.0 (Edition 1) and Norton’s still do not list it. It also doesn’t appear listed at all in Burnham’s Celestial Handbook. One exception, however, is Uranometria 2000.0. Oddly, NGC 4071 appears in the Sky Catalogue 2000.0, though the stated diameter of 75"arc seconds seems a bit too large. This situation is similar to the presumed “non-existent” NGC 5844 (15107-6141) in Triangulum Australe (NSP 4).

 I asked Scott Mellish to kindly make an observation for me, which he so gratefully provided, with  a description and field sketch. (Figure 2) Observed on the 18th February 1999 from a dark-sky site in the Warrumbungle National Park, and using fellow ASNSW member Mick McCullagh’s 18" f/4.5 Dobsonian, he says of NGC 4071;

“Use of an O-III filter makes a big difference. NGC 4071 has a couple of stars superimposed across it surface. Using averted vision I thought it does appear slightly mottled. Overall, this is a nice soft nebulous smudge.” 

The object doesn’t appear in AOST1. It is likely Hartung did not include the object at the time, as it was still considered by most amateurs to be a diffuse nebula. Incidentally, like its big brother in Musca, NGC 5189 (13335-6559) (NSP 1). David Frew in AOST2 (pg.292) says of NGC 4071;

 

“...It is a fairly difficult object in 30cm but an O-III filter makes it comparatively easy. It is about 1' across, and of fairly even surface brightness [,] though the preceding edge seems brighter; a faint field star is superposed on the south edge. D [distance] is unknown.”

 

Jenny Kay of the Canberra Astronomical Society Inc (“Southern Cross”, June 1998) says of the PNe;

“12.5" [f-5 Newtonian]-- 50X: Too faint. 105X: There is uncertainty at this power, barely suspecting that there may be a hint of a glow at the edge of a very faint 14 mag field star. This star lies in the east of a small triangle with two others, the star pattern being a guidepost to locate the planetary. 151X: With much care and patience, the planetary can be detected as an extremely faint, relatively large, round glow, with a diameter of 40". ...[in a] 8" -- This planetary is too faint for me to detect.”.  

                         

Kent Wallace from Atascadero, California while observing from Western Australia (20.01.98) using a Celestron 8 SCT (Personal Communication) says;

 “At 62.5X, [it is] a good sized diffuse disk requiring the O-III filter and averted vision. Good response to the O-III and UHC filters. No response to the Hß filter. At 100X can see the disk with averted vision alone, very faint. At 200X, [I] can see the disk, but the vision is very poor.”

Looking at the photographic image I think finds some similarity to NGC 6445 (17493-2001) in Sagittarius (NSP 21). Here the symmetrical structure appears as two bright illuminated “bookends” along the north and south parts of nebulosity. More specifically, both NGC 4071 and NGC 6445 share the “new” and rarer form of PNe known as bipolar planetary nebulae or abbreviated BPNe. About forty-five BPNe's are now listed as such.

Like many southern objects, the absolute position is partly uncertain. Brian Skiff states that the true position is 12h 04m 15.3s -67O 18' 35", which Acker and Milne determined in 1982. This position also appears in the Strasbourg-ESO PNe Catalogue. I have attached Figure 3, which is a finder chart for NGC 4071, and includes several objects already mentioned in the text.

 

Catalogue Data.

The Strasbourg-ESO Catalogue data (1992) finds the expansion velocity of the visual envelope as +14.1kms-1 (1984), though later data suggests a slightly 15.2kms-1, and the radial velocity is +11.0 ± 3kms-1 (1983) away from the Sun. In 1971, the largest diameter was measured as 63"arc.seconds. Electron densities of the visible nebulosity shows weak densities of Ne- (OIII)=620±60 and Ne- (SII)=410±50, which also explains the elusiveness of the PNe visually in the telescope.  Until the mid-1990's, no observations by were obtained in the NIR, IRAS, RADIO or IUE for this object. Most significantly, when it was alluding that this PNe had strange structures, the ESO took a monochrome image (2.2m.) by Baessgen and Bremer in 1988, later appearing in paper by Meatheringham, Wood and Faulkner (1989)

The line intensities of the PNe are as follows; Hß=100, He(II)=63, OIII=786, Ha=294, N(II)=259, S(II)=30 and  S(II)=24. The data here shows the strength of he OIII is quite high compared to Hß ([OIII/Hß]. (See Postscript) This indicates ratios almost equivalent in “standard”  PNe, except with the unusual excesses of Nitrogen and Sulphur - the primary precursor to BPNe. Over all the intensity is below par, and this account for the difficulties in seeing this in the telescope.

 

The most significant observations in the 1990's were the  multi-wavelength imaging taken at the ESO Observatory in Chile. The individual wavelengths (Ha, HeI, OIII, NII.. These images (Figure 4) have been adopted from the Innsbruck PNe Database, which was taken from the observations of C.Y.Zhang on 1st July 1996. (The colourised versions of these images can be found at This may also be found alternatively at the site of the Innsbruck PNe Database. Looking at the previously mentioned line intensities, the distribution becomes light in the main emission wavelengths becomes quite apparent. For example, one of the most interesting things is the dark bar of NGC 4071. Composition of this feature is stronger in Ha and NII, while the contribution from the HeI and OIII is minor. Another is the distribution of the HeI image, which concentrates the energies to the inner part of the expanding nebulosity. Compare this image with the Figure 5, showing the combined light image. Also, compare the individual wavelength images to theoretical one.     

Overall, the amount information about the PNN in Strasbourg-ESO Cat and other references is particularly poor. For example, the stated 19.2v mag and 18.20 B.mag - and these are likely both uncertain by more than half a magnitude. SIMBAD suggests that the central star is 16.11 magnitude, but the spectral class is still remains uncertain. Studies by Gorny, Stasinska and Tylenda “Planetary Nebulae Morphologies,Central Star Masses and Nebulae Properties.” (A&A.,318,256 (1997)) suggest the mass of the PNN may be between 0.698? and 0.839?, while the Zanstra temperature is 151 000K, and by calculating the size of the PN Nucleus, the energy is some  95 times more luminous that the Sun. This particular reference also states the diameter of NGC 4071 is 0.56pc. (1.7ltys.), making it among the largest of known PNe. Presently, the distance is estimated as 1.3kpc., though earlier observations once suggest to be a further away at 1.8kpc. (The value is same as today's Shklovskii distance.) Unreliable statistical distances calculated, and against all known PNe's, by using various means, now range anywhere between 0.8 and 1.4kpc. Age is currently estimated to be 15 900 years.

 

The General Nature of NGC 4071.

Much has been investigated about this object in recent decades.

The first of the significant papers in the 1990's is “Morphology of bipolar planetary nebula - I Two dimensional spectrophotometry” by G. Pascoli (Astron&Astrophys. Suppl. Series, 83, 27-39 (1990)) He concluded that NGC 4071 is a possible bipolar planetary (BPNe), and ranks among the Helix, Ring Nebula (M57) and NGC 2440. However, its characteristics seemed to match the Mz 1/ Menzel 1/ He2-130 (15302-5859) in Norma, discovered by Donald Menzel  in 1922. Pascoli comments on his observations of NGC 4071, whose statements can be verified by looking at Figure 5. His words gives  some factuality to the nature of the BPNe. He says;

“NGC 4071 presents a very contorted aspect with ansae which seems to be pair related to two symmetric bright condensations. The two central ansae are nearly parallel and straight, the outer ones are curvilinear and seem delineated as a faint polar nebulosity. One finds a similar morphology in [Mz 1/] He2-130 ...Comparison with other bipolar nebulae such as NGC 7293 [and] NGC 6720 is interesting.”       

 

Figure 5 clearly and easily shows the two bright condensations, that are nearly equal in brightness. (Figure 10 in the Pascoli’s paper). The two condensations are joined by dark bar intersecting the nebulosity. Pascoli inspection of the Ha images that were available to him clearly show these features.

 Pascoli followed his first paper with an intricate analysis that includes computer modelling. (“Morphology of bipolar planetary nebulae - II The three-dimensional structures”; Astron.& Astrophys, 232, 184-194 (1990)). Although the mathematical analysis and theoretical internal structures are quite complex, the resultant map confirms much of NGC 4071, and other BPNe/ PNe structures. Here he describes NGC 4071 as a “late elliptical”, whose expected orientation, between the main central axis and the line of sight, to be tilted by about 65O. The mean structural radius is given as 0.291 pc (0.95lty) with an expansion velocity of 15.2kms-1.

 Without getting into too much complexity, the structures drawn in Figure 5 does look similar to Figure 2. Three images are displayed, the plan and end elevation, showing the generalised structure of the observed nature of NGC 4071. The latter manipulation uses the general formulae from this paper, accounting for its general spatial orientated.

NGC 4071 has proven to be almost unique, and some of its  nature will likely be revealed in the next century. As of October 1999, no HST image exists or is presently planned.

 Surrounding Field of NGC 4071.

 DON 1095 (12047-6703), is a very faint pair discovered in 1929 by H.F. Donner. Visually the primary is around 10th magnitude, and lies 14.7' almost due north of NGC 4071. By magnitude, this reddish pair is 10.7 and 13.8, with a separation of 4.5"arc sec. along position angle 339O. Little is known about this pair, and no one else has bothered to contribute much about it at all. The primary is listed in the Tycho catalogue as T89862081:1 or GSC 35994:1, with the B-V of 0.988 confirm its reddish colour in the telescope. Surprisingly, the parallax on the star is large 141.7± 47.4 milliarc seconds (mas), corresponding to a close distance of 7.06±  1.56 pc (23.0 ±5.1 lty.) A 20cm easily resolves the pair, though 25cm or greater would improve this.

 Epsilon (e)  Muscae/ HIP59929/ SAO251830/ HD106849 (12176-6758) lays 2.2O NW from Alpha (a) Muscae, along Musca's western wing. The star   e Mus mean magnitude is 4.06 while the spectral class is M5 III. Considered as a SRb red variable, the period varies over about 40 days, fluctuating between about 3.99 and 4.31 magnitudes. Like all SRb variables, the variations in brightness are unpredictable. Some astronomers tend to think their light-curves are sometimes caused by layering of several multiple periods. Paradoxically SRb's can also remain fixed at constant brightness over long periods, before again resuming their fluctuations. Little information exists on this particular variable, and both period and amplitude remains imprecise.The Hipparchos satellite finds the distance as 92 ±28 pc. (300  ±90 ltys.) from the measured 0.01080 ±0.0048 m.arc seconds. Also Epsilon Muscae has the high proper motion of pmRA=-231.26 pmDE=-26.37.

 [Comment: These motions are similar to the reddish orange Epsilon (e) Crucis/ HIP60260/ SAO251862/ HD107442  (12213-6024) some 7.5O due north of  e Mus in both magnitude, parallax 0.01430±0.0056 and proper motion; (pmRA=-171.06 pmDE=91.82). This is also the suspected variable NSV 5568 and is listed in the Espin-Birmingham Catalogue as EsB 344. The K3/K4 spectral class and is likely a Type III luminosity class sub-giant, with the B-V mag quoted values given between 1.389 and 1.603.]

 The following stars are pairs near NGC 4071 listed below, but no one has listed them as pairs in many common star atlases like Uranometria 2000.0 and Sky Atlas 2000.0.

 

h 4471/ Lambda ( l ) Muscae HIP 57363/ SAO251575/ HD102249 (11457-6644) is bright 3.63 mag star that is also a wide pair 3.3O NWW (PA288O) of Epsilon Muscae or 5.3O NWW of Alpha Muscae. Alternatively, h4471 can be also found 1.9O ESE of NGC 4071 along PA 285.9O.

This pair is 3.6 and 12.8 mag, separated by 40.6"sec.arc along position angle 275O. I could easily see the companion in the C8, and I estimate that even a 10.5cm should have no problems. It is a slight possibility in 7.5cm scopes, though this might prove difficult because of the difference in magnitude  (Dm) is 8.8. Spectral class of the primary is A0III, and the colour seemed to me white and yellowish.

Looking at the Hipparchos data, the distance of the bright central star is 39.29±3.84pc or 128.1  12.6 lty, taken from the parallax of 0.02545  ± 0.0054"sec.arc. Also the proper motion of the star is quite high in RA. (pmRA=-100.42"sec.arc. per century and pmDE=33.21"sec.arc. per century.)  This motion is roughly shared with both  e Mus and  e Cru, and it is interesting to speculate this may not be coincidence! (See comment on  e Mus above.)

h.4471 is very likely just a chance alignment, although this white pair has changed little since discovery.

 

HDO 215/ Zeta 2 , z Muscae/ HIP 60320/ SAO251866/ HD107566 (12221-6731) is a 5.15 mag star 35' NE (@PA46O) of  e Muscae or 1.7O E and 14' S of NGC 4071. Again, this is a A5V bright star with a faint companion, separated by 32.4"sec.arc. at position angle 130O. The secondary is 10.7 magnitude, and so this white/ yellowish pair can easily be seen in 7.5cm. Placed in an attractive background of stars, little has changed with this pair since discovered in 1900.

 

Next Installment: Another Musca planetary IC 4191.

 Postscript:

H-Beta and the Line Emissions from Planetary Nebulae.

 It has long been known that PNe can be calibrated by measuring the single line strength of Hß. This value is measured photoelectrically, and is the amount of energy (flux) liberated from the PNe structure (or even the PNN itself) measured in mW.m-2 and abbreviated F (Hß - the absolute Hß  flux. It is not easy to imagine this quantity. For example, the Sun produces energy that can be felt by exposure of skin to the warm Sun. Energies here are ~1 368 Watts per square metre or 1.368mW.m-2 ,and useful, for example, in charging electric cells to generate electrical power. Such energies are quite intense and enough to permanently damage the retina of the eye in less than 10 000th of a second! This energy is for all wavelengths, but is much smaller for the Hß output alone.

 For planetaries (and stars), these quantities are very very tiny. For example, NGC 4071's measured flux is 2.239x10-12 mW.-2 , some 1018 times fainter than Sun’s output! To avoid having to write down so many small numbers, values for the absolute fluxes are given as the log of the measured flux. Ie. log -11.66 ± 0.02 mW.m-2. Typical values for these indices compared to all PNe may range anywhere from -9 to -16, with the midrange of the distribution being about ~ -11.5

Among the southern planetary, NGC 3242 has the highest Hß flux of -9.789 or 1.626x10-10 mW.m-2 - 72 times more intense than NGC 4071's surface area. A low Hß flux PNe is He2-77 / Sa3-16/ PK298-0.1/ PNG298.1-07 (12091-6307) at log -13.28mW.m -2. [This PNe appears in south western Crux, and being 17.6 magnitude and 26"arc seconds across, this PNe remains impossible to see in any amateur telescope.]  

Calibration of the absolute Hß fluxes are made by examination of several “standard stars”, Ie. 
p3 Orionis and a Lyrae (Vega), or the bright PNe. For southern planetaries, NGC 3242 and IC 418 are used as Hß  “standard” candles.

In other terms, the relative Hß fluxes are used to compare the strengths of the other emission lines seen in the PNe, and allow differences between other PNe to be quickly examined. This is made by giving the absolute Hß flux the value of exactly 100.0. From this other prominent PNe lines can be assessed. For example, the values for NGC 4071 (From the Strasbourg-ESO Planetary Nebulae Catalogue)are;

Hß=100,  He(II)=63, OIII= -, OIII=786, He I=-, Ha =294, NII=259, SII=30 and  SII=24.  

For the bright PNe NGC 3918, the values (From the Strasbourg-ESO Planetary Nebulae Catalogue) are; Hß=100, He(II)=16, OIII= 15, OIII=1667, HeI=15, Ha=374, N(II)=259, S(II)=6 and  S(II)=10. 

Amateurs should be concerned with the important ratios of [OIII/Hß], [OIII/Ha] and , as they tell much about of the telescope appearance of the PNe and the effectiveness of the OIII filter, both photographically and visually.

The experience French amateur PNe observer Yann Pothier ( ) uses the ratio between the Hydrogen Beta (Hß). By combining the two lines from the two OIII emissions, gives some indication of the effectiveness of the OIII filter on the object. This is calculated by;

 

n =  [OIII, 496nm+501nm] /  [Hß, 486nm]

Where ;  N = PNe  Effectiveness Ratio.

Example 1; NGC 4071 OIII is (786 + 0), Hß is 100. Therefore, N= 7.86, rounded to 8.

(Note: OIII at 486nm was not measured accurately. However, it is small in value.)

 Example 2: NGC 3918 OIII is [1667+15]/100= 1682/100 = 16.8 or 17

(Typical of a moderately high excitation PNe.) 

 Correlation with known PNe finds that the smaller this value the lower the excitation of the PNe’s nebulosity. Values can range anywhere from 0.1 to 180, with the mean being about 13.0

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