Neat Southern Planetaries 18

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 XVIII

For Neat Southern Planetaries XVIII, I have selected two faint and difficult planetaries near the border of eastern Centaurus and Crux. As planetaries go, they may not rank as the most elegant, but they do have the advantage of being quite near to one of the very best examples of an Open Star Cluster - the absolutely fantastic and southern observer's delight, the Jewel Box, which I like to term "Gemma Australis Magnifica"

He2-90/ PK 305+1.1/ Sa2-90/ WRAY 16-125/ PNG305.1+01.4 (13097-6120) (Centaurus) is 2^O S.E.E. of k Crucis (NGC 4755) and is not listed in any of the popular catalogues or atlases. First discovered by K.Henize in 1964, it is often listed, like in Megastar 4.0, under the alternative designation Sa2-90. (See Postscript.) It best found by using the double star J Centauri/ D133 (13227-6059), and then move westwards by 1.6^O The PNe's position is not marked in Sky Atlas 2000.0, but if you are using Uranometria 2000.0, the PNe's field is marked as a tiny triangle of stars near the bottom of Map 432 and next to the RA line marked 13h 08m. The 10'x10' field shows the STScI Image (Figure1) of the small planetary, which is west of the triad of stars. This same triad can be also be seen in the attached field-finder chart in Figure 2. I attach this chart, as it is essential to find the PNe. The magnitude limit is about 15.0, and the stars whose mags are above 10th have there visual displayed.

He2-90 is listed as 13.7p mag and subtends 10"arc.sec. I could not see He2-90 directly using 30cm, but applying averted vision, revealed a small 4"arc.sec star-like "blip", that magically and unexpectedly disappeared in the O-III ! I thought the visual mag to be about 14th, once I compared it to the star chart.

I asked Scott Mellish (Observation Officer of the ASNSW and author of the series in UNIVERSE "Sky-Sketcher's Post Mortem") to observe this object with his 40cm f/4.7 Dobsonian. The observation (and sketch which will appear in a future issue) were made under 7/10 seeing and transparency on the 15th February 1999 using a 7mm Nagler, where Scott states;

"Almost gave up on this one. [He2-90 is] impossible to define as a planetary without a finder chart. Stellar in appearance, and remains so even in O-III filter, [and] even 8"arc.sec. in size seems far too large. More like 2"arc.sec."

This is truly a tiny and faint planetary in a profusely starry field. At first glance you might wonder why I even bothered looking for such an inelegant object. Honestly, the reason was that I found an interesting paper specifically on He2-90, and found it has characteristics quite unlike nearly all known planetaries - primarily it is very low O-III emissions. For example, if the relative intensity of the Hb line is 100, then the emissions from the O-III line is O-III=205 (and the Ha=856) normally, values of O-III would be ten times this value. It is no wonder the O-III doesn't work as well as it should !

Studies into the Unusual Planetary He2-90.

The ESO-Strasbourg Catalogue states that the diameter is less than 10"arc.sec, while the PASP, 103, 275-293 (1991) states that the Ha image is a circular 12"sec.arc., surrounded by a shell of faint nebulosity.

One difficulty with observations of this PNe is that it lies a mere 0.4^O NE from the northeast edge of the Coal Sack, (euphemistically called "The Black Magellanic Cloud") and the effects of extinction on the PNe in this region of the sky is uncertain.

The most significant (and only) paper He2-90 was produced by Costa, de Freitas Pacheco and Maciel "He2-90: a southern planetary nebulae with low metal abundances." (A&A, 276,184-186 (1993))

The central star is visual magnitude 16.5 (B.mag=15.6), whose error somewhere between ±0.25 and ±0.50 magnitudes. It is believed the PNN nucleus is not very luminous, so that the UV energies that are required to illuminate the nebulosity are below par and are not very energetic - hence the poorness of the viewed PNe. Spectral measures also find a low carbon to nitrogen ratio of [C/O]=0.3, suggesting the original PNN's progenitor's mass is fairly small, perhaps around 0.6-0.7 M¤. Again the spectra since the mid-1980's has very low abundances of elements such as Oxygen, Nitrogen, Sulphur, Neon and Argon, suggesting a low mass and/or an under-luminous star.

Not many of these objects like this are known, and include the PNe's SwSt-1/ PK 001-06.2 / He2-377 (18162-3052) (Sgr) and Hu2-1/ PK051+09.1/ ARO 100 (18498+2051) (Her).

Observations were obtained by Costa et.al. (1993) on 11th May 1991 and 4th April 1992, using the 1.6-metre Cassegrain at the National Astrophysical Observatory, at Brasópolis, Brazil. The resolution obtained by the spectrograph is 0.7nm., with the spectra surrounding the planetary was taken elsewhere by A.Damineli on the 6th June 1992 surrounding the red Ha region was accurate to about ~0.04nm.

Graph 2 shows the line intensities of the major emissions between 372.7 and 733.0nm, using the data from Table 2 of Costa et.al (pg.187). A comparison spectra of a "standard" PNe is also given. This clearly shows the deficiency in both O-III and the other lightweight noble gases. (Compare this with the information on He2-111 (NSP 16), which has this effect reversed and an excess of O-III.)

Measuring the H-Beta flux and the diameter of ~6"sec.arc, enabling a calculation of distance using the Shklovsky Method, gives a distance about 1.5kpc. (This is presently one of the only estimations for this object!) Measuring the slight blue-shift of the sharp spectral lines of SII and NII, finds that the radial velocity of He2-90 is -31km.sec^-1 - so it is moving towards us. Also from the spectra finds the expansion gas velocity is presently at a pedestrian 3kms^-1.

Gleise et.al. (A&A, 222,237-246 (1989)) finds a Zanstra Temperature 50 000K, and later by Kaler and Jacoby (Ap.J.,372, 215(1991)), who determined a more accurate T_z=51 000K. The latest measures of He2-90's PNN find these values maybe about 15% too high.

It appears the that the atmosphere surrounding the PNN is fairly compact and dense, with the luminosity being some one thousand times more luminous than the Sun. [log(L*/L ¤_3.0] This confirmed by observations by the IRAS satellite and in the Near Infra-Red (NIR). IRAS observations in 1982 at 100 mm makes He2-90 is a strong flux PNe at this wavelength, and supports the NIR observations of a "D" classification in 1987 - where the major of visual brightness in these wavelengths come from circumstellar dust - an uncommon property with PNe's. (See also the forthcoming NSP XXII on NGC 6445 in Sagittarius that also shows significant quantities of "dustiness".)

Hydrogen-Alpha profiles produced by Damineli (1992) shows a PNN superwind velocity of 1 050 kms^-1.

(Note: A discussed in earlier parts of the NSP series, the velocity of this outflow emanates from the PNN itself. As the gas radial travels away from the PNN, its mass collides with the much slower 10-20kms^-1 atmosphere loss that once occurred during the Asymptotic Giant Branch (AGB) stage, and causes the illumination of gaseous structures that we see in the telescope.)

From Damindi's (1992) data, Costa et.al. infer a PNN solar radii of c.0.38 R¤ (500 000km.), and a "speed of sound" near the top of the photosphere of c.15kms^-1. For comparison, the speed of sound at sea level on Earth is about 0.3kms^-1 and across the Sun's photosphere c.1.2kms^-1.

Overall, the past evolution of object and the cause of the very low abundances remains particularly uncertain, though it is likely produced by a low mass progenitor or a property of under luminous star.

He2-90 Update: 23rd June 1999

Attached is an image (Figure 3 - immediately below) by Mike Kerr which shows the weird nature of this Planetary. There is few images that I know of that look like this! The image shows two unusual features;

1) The stunning red colouration.

2) The polar "spikes".

There is obviously unusual characteristics with this object, which reflects the low O-III abundance and the low PNN temperature.

From the a private communication by e-mail on the 21st June 1999 he says;

"I've got an image for you of He2-90. The image is 100 arc-seconds square with north up and east to the left.

I produced this 'true-colour' image using the narrow band image process I described at the last (ASNSW) Technical Meeting. The images are from the Innsbruck University database and are Ha and [OIII] images taken with ESO's 3.5 m New Technology Telescope in Chile. I have applied a logarithmic scale to the images to compress the dynamic range and better show both the bright and faint parts of the object. I assume the two spikes are not part of the object but are diffraction spikes from the telescope, which have been enhanced because of my processing.

At the H-a wavelength(s), He2-90 appears to have a bright, circular central disk about 6 arc-seconds in diameter surrounded by a fainter circular halo about 14 arc-seconds in diameter. [OIII] emission is limited to a central, stellar-sized disk about 2.5 arc-seconds in diameter. This would seem to match pretty well with Scott's observation. All in all, quite a strange planetary!"

**Figure 3** - He2-90 Imaged by Mike Kerr

Surrounding Objects of He2-90.

J Centauri/ D133/ D133 AB-C/ HIP 65271 (13227-6059) is a distinctly blue double star/ triple was discovered by Dunlop in 1836. Respective visual magnitudes are 4.5 and 6.2, with J Cen A being SAO 252284/ HD 116087 and J Cen B / SAO 252283/ HD116072. The pair is widely separated by 61.7"sec.arc along position angle of 343^O. (Latest data gives c. 60.0"±0.3" and 345.5^O) Russell in 1879 made the last serious measure of the pair, when it became obvious that this is just a pair of chance alignment. Little has changed in there positions since then, and looking at the proper motions of the components only confirms today's view that this is just another pretty optical duo. The two individual spectra are B3 and B2.5V/ B3.

Contained in a dense starry field, this stunningly blue duo is simple wonderful - even in the smallest of telescopes. I though it was very similar in many respects to Eta (h) Muscae / D131 (NSP 19) (near IC 4191 in central northern Musca) which I observed on the same night.

The secondary component is again double. Known as FIN 208AB, this was revealed by Finsen in 1930. This is certainly a binary, and both stars are equally 5.2 magnitude. However, this is not an easy pair, and it is unlikely that amateur telescopes could see them, as the separation is a small 0.2"sec.arc, and this may have increased between 1930 and 1990, but this is hard to tell with only six measures to date. Also the position angle certainly has decreased from c.168^O to 137^O. The Washington Double Star Catalogue (WDS96) states in the Notes; "Too close, measures uncertain." As yet no interferometry observations have been made.

One of these components is a suspect Beta Cepheid type variable star V790 Cen, whose visual variations change roughly between 6.16 and 6.27 in an unknown period. Estimates for the visible AB system range between 6.2 and 6.4, so it could possible that variability is lower than 6.27. Which component is the actual variable is not known, but the combined spectral class of this star is typically B2.5 VN - B3V.

Several "mini-asterisms", centred about 1^O west, 1^O northwest, and 1.2^O southwest, finds three obvious SW to NE winding lines of stars across the surrounding fields between 8th to 10th magnitude. All were easily be seen in dark skies in binoculars, each extending about 3^O to 3.5^O.

NGC 5168/ Cr 273 (13311-6056), is a moderately faint open cluster, due west of J Centauri by 1.1^O. Looking for NGC 5168 in a 20cm, and after glancing at available information, I thought the cluster would be brighter and far more interesting - but this wasn't true. Catalogued as 9.1 mag and 4' across with about 50 stars, the cluster is classed as "1 2 m -", but all I could was an equally bright white and yellowish pair surrounded by a faint collection of a dozen or so 13th mag stars. This uncatalogued pair is separated by 5.6"sec.arc at position angle 46^O, and is only identified in the Tycho star catalogue. The field is positively identified by the 7th magnitude star 23' SWW of the pair. Placing this star on the western edge of the field centres in on the cluster. Looking at the photographic image from the Mt. Stromlo's "Southern Open Cluster South of -45^O " shows an estimated visual mag of 11.5. The NGC says "Cl ,vF ,S ,vRi, st..15 - Cluster, very faint, small, very rich, star down to 15th.", and this is the same description as adopted by the RNGC.

I suspect that a 30cm or larger should start to reveal the quantity of faint stars surrounding the central pair.

Hogg 16 (13292-6112) and Cr 272 (13305-6116) are due west of NGC 5168 by 50' and 60' along PA 108^O, are the respective open clusters. In a 20cm or greater, Hogg 16 looks like a "K" shaped asterism while Cr 272, slightly fainter of the two, looks like a backwards "S".

Frankly, I think both Cr 272 and Hogg 16 are brighter than NGC 5168.

At the bottom southern edge 28' from J Cen in a wide-field eyepiece is a 10.5 mag orange-red star. This is the suspected red LB variable NSV 6200 Cen (13217-6134), approximately varying between 10.5p and 11.3p mag in an unknown period. I thought this star quite prominent, and slightly brighter than given 10.5p mag - more likely 10.0 mag.

Overall a very pretty and dense field of stars, worthwhile even if you only look for the pair J Centauri !

Postscript :

Problems With Designations for Southern Planetaries.
Designations for southern deep sky objects usually observed by amateurs and professionals exclusively followed the NGC and IC catalogued objects, and little else, at least until the mid-1960's. For most observers this remained adequate, due to the general lack of aperture and the true number of objects known. Since the revolution of discovery in the 19th Century, most deep-sky objects observed by amateurs has remained fairly static. Yet by the 1950's and 60's many new objects became under the scrutiny of professionals, that eventually filtered down to the amateur ranks. Three types of objects significantly increased in numbers - open star clusters, galaxies and planetaries. In terms of their "newness", both open cluster and galaxies were always, and their numbers increase simply by measuring the astronomical photographic plates, and cataloguing the results. However among the planetaries this proved more difficult because many were stellar in nature, and the photography could not separate them from the background stars. Using the emission spectra of the PNe to advantage, deeper and better quality plates taken near the bright Ha or O-III lines, and then comparing these to a "standard" full wavelength plates, meant that new PNe objects could be found rapidly. In the 1960's this proved to be the case. Many of the new objects were certainly bright enough for amateur telescopes, but without adequate finder charts they remain invisible. In the 1980's the general availability of narrow-band O-III filters (and large Dobsonian Telescopes) meant that disclosing a PNe position for a majority of these planetaries became easier. The number of PNe's visible for amateurs has increased five or six-fold. One legacy, however, is the use of designations with these objects, which can lead to some confusion.

The literature after about 1965 is rather ambiguous when the designations used. Most serious amateurs, and all the professionals, use the PK number, which was introduced in 1967 by the PNe specialists Perek and Kohoutek, and originally contained 1050 objects, but has been added to since. The numeral after the "PK" abbreviation is given in galactic coordinates of the specified object. However, amateur texts often favours designations based on the discoverer. The southern "He2-" survey was conducted between 1964 and 1967 by the American observer Karl Heinze, and lists some 468 objects. Centred mainly on the red plates near the Ha wavelength, it reveals many more objects. This catalogue was published in 1967, (Ap.J. Supp.,14,414 (1967)) and remains the first serious southern catalogue of many new planetaries that were beyond the non-NGC or non-IC objects.(Note: Amateurs can observe about 60% of these.) Overall, the majority of this catalogue are 10"sec.arc. or less in size.

To confuse thing further Heinze's third catalogue is given the designation of "HEN", which comes from a more through Ha survey, that was published in 1976 (Ap.J.Supp.,30,491,(1976)). Some modern papers sometimes use the designation "He3-" instead of the HEN designation. Ie. The "Stingray Nebula" He3-1357.

For some reason, some amateurs and a few professional tend to favour the "Sa2-" designations over the "He2-" designations, even though Karl Heinze published these PNe's some nine years earlier! It seems to me that favouritism is assessed by the author, and that designation stays "fixed' in the astro's mind. It seems to me that the preference for either designation is dictated not by time of the discoverer finds the object, but by the convention of the individual observer. It is recommended that the designation used should follow the convention used in the ESO-Strasbourg Catalogue (Versions; 1986, 1992, 1994 and the just released 1996), and choose the discoverer.

The second important southern catalogue is the "Sa2-" and to a lesser extent the "Sa3-". These were planetaries that were discovered by N.Sanduleak (Publ. Warner & Swansey Obs.,2,1 (1975) and 2,57 (1976)

Later catalogue designations seem not to have these problems, except perhaps if only one or two were found by a particular observer. For southern amateur observers, the "Longmore" (1977) or "Longmore-Tritton" (1980), "Abell" (1966) objects are likely more commonly listed. These are different than the "He2-" and "Sa2-" planetary because they are usually much larger ring-like structures with low surface brightness.

For amateurs, it is preferred that you follow this convention. In most cases, the "He2-" designation has precedence over "Sa2-" , though in the literature, it is common (and confusingly) used exclusively by one catalogue or researcher.

The IAU Designations recommend using the PK number, but for amateurs, these are hard to use or talk about (especially in conversation) to other amateurs. Some professional favour the ESO-Strasbourg Catalogue "PNG" number designations, and these tend to be similar to the PK numbers. PNG's have the same problems as mentioned above, with the PK numbers. The worst problem is certainly having to use both, as they don't always correspond in the same order! Amateurs using the PNG Catalogue like have cursed as much as I have, especially having to use the "Designation Conversion Table" to find information on the object in question - most irritating! (This problem is similar to the translations that have to be done in Stephen Hynes' book "Planetary Nebulae - A Practical Guide and Handbook for Amateur Astronomers" )

Next in NSP 19: The Jewel Box and then to Musca's best planetary treat - the wonderful NGC 4071 !

My Favourite Northern Planetaries - I (By Andrew James)

NGC 2371-72/ PK189+19.1/ PNG189.1+19.8 (07256+2929) is a bipolar nebula in the constellation of Gemini, some 3.1O SW (@PA 221degrees) from Alpha Geminorum, and forming a flat triangle with Alpha and Beta Gem. The photographic magnitude is 13.0, while the overall size subtends about 44.0", but telescopically the central bright luminosity is about a quarter of this. It is classed as 3a+2. The central star is fairly bright when compared to the nebulosity's brightness, and a 25cm should have little trouble in see in it.

Finding this planetary is really very quick. Simply centre your telescope on 3.8 magnitude Iota (i) Geminorum (60 Gem) (07257+2748), and move the telescope 1.6^O directly north. This is easy to do, as visually Iota Gem forms a near-perfect 4.5^O equatorial triangle with Alpha and Beta Gem. Centred in the field should be NGC2371-72, which is easily seen in a 20cm, and possibly even a 15cm. could glimpse an "uneven glow". (I also read a claim by one northern observer who could not see it in a 30cm.!)

Telescopically this planetary is quite interesting, as the two lobes are obvious, and therefore the separation as two objects in the NGC. The field in a 20cm. (78X) is quite spartan, containing two 9th magnitude stars and a sprinkling of 11th to 12th mag stars. Although the planetary is obvious, there is a 14.3 magnitude star (GSC1922:1155) to the southwest of the PNe by 15"sec.arc. and just outside the boundary of the PNe's disk.

An image was taken by H.D.Curtis - the combatant with Harlow Shapley in "The Great Debate" on the scale of the Universe and the truth of the existence of extragalactic galaxies. Here he obtained, using the 36" Crossley reflector, a detail image of the planetary structure. (See S&T February pg.219 (1986)) At first, you might think that the V&V classification of "3a+2" refers to the two visible lobes, but it seems, after looking at Curtis's image that the "3a" refers to the inner portion of the nebula, while the "2" - refers to the faint outer extended envelope.

The late Walter Scott Houston said of this planetary;
"It looks similar to M76 in Perseus and like a miniature version of the Dumbbell, M27 is Vulpecula."

There is an interesting comparison with this object and those south of -50^O declination and those in the equatorial and northern hemisphere regions of the sky. I thought NGC 2371/72 compare well with the bipolar PNe (BPNe) He2-111 next to Alpha Centauri, as I though these both appeared similar. (See Postscript 1)

NGC 6210/ is a bright planetary in the constellation of Hercules that I have often found once or twice using the globular cluster M13 some 12.7degrees due south . This bright pale blue planetary nebula was discovered by Wilhelm Struve in 1842, who listed it as deep-sky nebula object "S5 N". Its listed magnitude is 9.3p, while the nebulosity subtends some 16.2"arc.sec. in a round and well defined disk. Classified as "2+3b", this planetary is easily visible in a 7.5cm, which shows little detail or structure in apertures below 30cm. I personally like this object, as it is a good test for O-III and UHC filters. Unlike most planetaries. the central star is easily visible in apertures in 20cm or greater. The description in AOST2 is unusual, as the text says that the prism image is said to produce a bright violet line. I tried this, but though that the line is produced by the central star (which is not mentioned in the AOST2 text.)

A recent written description, that it "right-on-the-money" is by Susan.C. French appears in Sky and Telescope, Vol. 96, 1, July 1998 pg.99-100, and here she says;

"Through my 15cm. telescope it's recognizable even at 40X as a tiny disk of uniform brightness tinted robin's-egg blue. The perceived colour seems to vary with aperture, or perhaps with the observer; reports range from sky blue to distinctly greenish.

The nebula is about 16" in diameter. On a good night I can glimpse its central star at high power, but even in a large scope the star is nearly overwhelmed by the brightness of the nebula. High powers show NGC 6210 as oval, with the long axisoriented east-west."

One really neat bright northern planetary.

Postscript 1:
There is somevigour debate with the status of bipolars being classed as planetaries, as they are quite evolutionary different. If a new term is ever applied, I think that the term "neons" should be used, because of the strong bright colours and their dualistic lampshade shapes.

Postscript 2:
The apparent strong disparity with planetaries "visible" and those deemed "suitable" for amateur 'scopes between these two regions is in reality an illusion. Amateur astro's should note a few points;

* The objects in the northern hemisphere were discovered some twenty to forty years before most of the faint and far southern objects.

* Just because the object doesn't appear in the NGC and IC catalogues doesn't mean that the objects is not visible in medium to modern telescopes. This applies to all types of astronomical deep-sky objects, yet is more prevalent to planetaries, a few globulars and a fistful of galaxies.

* The advent of the large dobsonians has meant that fainter objects are available, but at a cost to smaller and harder to resolve "stellar" objects - which applies particularly to planetaries.

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Douglas Snyder, August. 1999