The regions in Argo Navis are rich in planetary nebulae. In all, Pyxis contains four; Puppis thirteen; Carina twenty-two, while Vela contains the most, with thirty-three. However, of the seventy-two Argo planetaries, most are below c.5"sec.arc. About half of these are no only small but faint, and give decent challenges for amateurs to find, even for those with larger apertures. The problem that remains is identification because of the star-packed fields. Not having a UHC or O-III filter, makes finding them almost unsurmountable - unless we take due care.

Sometimes I think looking for faint deep-sky objects and 'pushing the photon envelope' is perhaps a bit like fox hunting. It takes a bit more cunning and ingenuity to 'out smart' the object, who hides among all the field stars, almost wishing to avoid capture! Overall, observers seem to have more appreciation for an object that took half-an-hour to find, than the one that is obvious at first glance. Personally, I have seen people get so frustrated - that they would never find their target just because of their attitude. My advice is not to worry if you do not find an object. You can have more fun finding a few 'surprise' objects on your way to your goal!

Selected this month is a planetary 'pair'; NGC 3211, IC 2553, followed by the challenging Sanduleuk pentad; SA2-61; SA2-64; SA2-65; SA2-66; and SA2-68. The last five all lie within a 2.2^O field, which I have termed 'The Five Planetaries'. For the observer this region has a wealth of objects for amateur eyes, and I have selected the most interesting. Included for the surrounding objects are; two field variables, a challenging cluster or two, and some brilliant pairs. (Note: If you do not like the selected pairs this month - then I will know such observers will never become connoisseurs of double stars.)

NGC 3211 / He2-46/ SA-59/ WRAY 16-70/ HJ 1837/ PK 286-4.1(10178-6240) (Car.) is a bright planetary placed some 3.3^O northeast of the Southern Pleiades, and 1.2^O south of the orange star 3.4 magnitude, q Velorum. (See Figure 1 for the STScI image of NGC3211.)Within the field, some 20'min.arc of the planetary is two 6.9 and 8.4 magnitude stars, some 6' apart. Discovered by John Herschel in 1837, the object is easily found in a 10cm. or 15cm. An O-III is a godsend, helping to isolate the planetary from the surrounding starry field. (Figure 2.) The NGC description is the same as given by John Herschel - 'F, S, B -faint, small, bright', and is type 2b - a smooth disk with uniform brightness. Visual magnitude is about 10.0 to 10.2 (brighter than some reference sources), while the photographic magnitude is 10.7.

A few amateur quotations are available, they include;

'Astronomical Objects for Southern Telescopes'; Version 2 (AOST2) on pg. 189 states; "In a beautiful starry field is a small even pale blue disk... fairly bright with no visible central star and a single prism image.... An [OIII] filter helps with small apertures."

Andrew Murrell (Universe 42,3 1995) in his article entitled "Carina";

"NGC 3211 is a planetary nebula about 12" in diameter and a surface brightness of about magnitude 10.5. This nebula could be a challenge for a 4" scope. The surface of the nebulae is smooth. The central star is not visible in Hector (Andrew's 20".)"

J.Graham Little (Southern Sky 1,4 pg.54) says NGC 3211 is;

"A smallish, faintish planetary, 3211 has a diameter of 12 and a mag. of 11.8. It appears, very simply, as a fairly evenly illuminated disk with a grey-blue colouring. A 15cm. 'scope will find this a bit of a challenge."

Apparent visual diameter is some 12"sec.arc., that seems to increase with aperture. Like last month's IC2448, some debate exists on the true diameter. AOST1 states 10"sec.arc., while AOST2 gives it as 12" and Burnham's gives it as 14"sec.arc. Using the "Strasbourg - ESO Catalogue of Galactic Planetary Nebulae"; Acker et.al. (1992) the diameter is 16"sec.arc. It seems the reason is the gradual fading of brightness near the outer edges of the planetary. Therefore, the bigger the aperture, the larger the planetary. As a multiple shell planetary, this is an expected property. In the largest telescopes the diameter extends to 23"sec.arc. (1971), while the infrared image is 33"sec.arc, with a strong flux at 100µm. (microns).

Radial velocities suggest NGC 3211 is moving towards the Sun at -22.0kms^-1±2.0 (1983), while the spectrum reveals a mean expansion velocity of 29kms^-1. The entire nebulosity is 0.1 solar masses, with an electron density is 1 000 e^-.cm^-3.

Visual magnitude of the PNN (HD 89516) is 14.22, but some sources, like Sky Cat. 2000.0 and Megastar, incorrectly quote the older 'B' Mag. of 18.0. Although it is a 'B' spectral type - though this does not easily reconcile with the higher 130 000^OK Zanstra temperature. Early estimates said the temperature was 66 200^OK,

though another estimate by the 'Stoy Method' or 'Energy Balance Method'; suggests 135 000^OK., which is based on H measurements and the amount of ionization. In 1993, another independent result gave the same temperature. These huge variations are typical, as all 'B' stars are notorious for having a wide range of surface temperature. If the Zanstra temperature is true, then the relative luminosity must be 870 times brighter than Sol and the absolute visual magnitude is 4.67. NGC 3211, like last month object IC 2442, also shows weak indication of the P Cygni Phenomena. Once, we estimated that this PNN was c.0.4 Solar Masses. Later estimates are closer to 0.67 ±0.09. This suggests the progenitor was once between 1.0 and 1.3 solar masses.

First, we gave NGC 3211's distance as 1.91 kiloparsecs, but later studies today favour around 3.3kpc. AOST2 is the closest of all the amateur reference, giving the distance as 3.0kpc. Based on the extinction magnitudes, Pottasch derives a distance of 2.5kpc. The mean value, based on the statistical distances, (1971-1994) is about 2.7kpc. Based on the 3.3kpc. distance, the true size of the planetary is 0.42 ltys. or 40 billion kilometres across - eight times the diameter of the entire Solar System! Some estimates place the evolutionary age of the inner bright luminous shell as young as 4 500 years. On a human scale, this dates to the earliest literary writings - like the 'Epic of Gilgamish'.

In all, this is another good example of an easy bluish-grey southern planetary.

IC 2553 / He2-42/ SA2-55/ WRAY 16-67/ PK 285-5.1 (10093-6237) (Car.) is a brighter than NGC 3211, but is a third the size. IC 2553 is an easy target, as it lays 57'min.arc. due west of NGC 3211, in another profusely starry field. Brian Skiff gives its precise position as 10^h09'20.93" -62^O 36' 48", yet we still list it as a 'star' in the Guide Star Catalogue as GSC 8947-1602. Discovered by Fleming in 1893, this Uranian sized PN is visible even in a 7.5cm., though it requires a high magnification in a telescope above 10.5cm. to discern the round featureless disk. (See Figure 3.) AOST2 agrees with this view, which succinctly states; "...the prism gives a single bright image which even a 7.5cm. will show as a minute point of light."

Most of the sources of magnitude, like Sky Catalogue 2000.0 and Burnham's, state 13.0p. If an observer were to attempt to find this object, one would suspect that it might be especially difficult in a 20cm. However, the visual brightness is 10.3, some 2.7 magnitudes brighter! (Megastar wrongly states 10.3p.) Although this is a larger than normal difference seen in planetaries, it is not an uncommon variation.

One obvious problem is that a discrepancy exists with its diameter. Most visual observers state that the size is 4" to 5"sec.arc. in diameter, though some common references state the size to be about 8" to 9"sec.arc. Glancing through the available data, I assume that the larger planetary size is the photographic diameter. Ie. The STScI image is about this size, while the IRAS satellite in 1983 showed also shows a diameter of 8"sec.arc.

I thought the O-III filter produced a dramatic improvement in visibility. Observations reveal strong H and OIII emissions, which seems to account for the difference between the photographic and visual magnitudes. Of all the optical light, some 95% emitted from the nebulosity, is bond into these two wavelengths! Spectral data suggest that IC 2553 is moving towards the observer at 37.3kms^-1± 6.3. Spectra also shows a rough elemental abundance, which was first deduced by the Mexican observers; Getiérrez-Moreno and Moreno. (PASP 12, 1497-1507 (1968)). Comparison, via the spectral line intensities, between NGC 3211 and IC 2553 shows the effectiveness of an O-III filter and the direct-vision prism is about the same. Observers, once they have found both of them, may like to do a quick comparison between the two, and judge for your own eyes.

Distance estimates vary between 2.0kpc. and 4.1kpc., though the average is perhaps closer to 3.0kpc., Sky Cat. 2000.0 is closest at 2.8kpc. AOST1 wrongly gives the distance as 360pc., reflecting the vast improvements in observational techniques used by the professional astronomers since 1968.

Detail throughout the literature on the central star PNN is very confusing. For example, Sky Cat. 2000 gives the visual magnitude of the central star as 12.89, though the Strasbourg-ESO Planetary Cat. states 15.51 (B=15.4). According to Shaw and Kaler (1989), there is some doubt about the actual PNN, and it states that the 15.5 magnitude star maybe a mere field star. Observations of the HI line gives a temperature of 78 000^OK., and the HeII temperature is 100 000^OK. equal to the calculated Zanstra Temperature (A&A 222, 237-247 (1987))

Surrounding Field of NGC 3211 and IC 2553.

RU 88 C1017-628 (10188-6308) is an Open Star Cluster that J.Ruprecht discovered. No one has listed RU88 in any star atlas. It can be found some 28' SSE of NGC 3211. By moving the telescope south, you will see a brighter 9.4 mag star. 5' farther south is the 12th mag RU88. I saw about twenty, 11.5 and 12th magnitude stars in a loose collection some 5' across. Two wide pairs mark the southern boundary. Proper motion studies found the existence of the cluster. Trumpler classification is '3 2 m' - Detached with no central concentration, medium rich.

S Car (10094-6133) is a red Mira-type variable star that is likely one of the first variables stars that southern amateurs select, when embarking on observations of variables. It lies 1.1^O directly north of IC 2553, and conversely, using S Carinae also finds the planetary. Variations follow a regular pulsation of about 149.49 days, between magnitudes 4.5 and 9.9. At maximum, the spectra displayed is M4, decreasing to M6 at minimum. Throughout the cycle, emission lines are prominent in the spectra.

RST 3692 (10203-6240) is a faint pair that is in the same field some 12'min.arc. due east of NGC 3211. Although it is in a very starry field, RST 3692 is the northern object of two 10th mag. stars, some 80"sec.arc. apart. Discovered by Rossiter in 1936, the pair is a close 0.8"sec.arc. at PA 65^O, that incidently points directly to NGC 3211. Both have stated magnitudes 9.8 and 11.8, respectfully. In a 20cm., I could clearly separate the pair at 310X, and suspect that a well-aligned 15cm. could possibly see both stars.