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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|IC2448: negative image retreived
from NASA's Skyview (DSS).
|IC2448 From Kitt Peak Obs.; OIII
image courtesy Dr. Bruce Balick
This area of southern Carina and into eastern Volans is a wonderful place to explore during the autumn months. The selected planetary this month is IC 2448 - certainly one of the southern gems for the smaller telescopes. I also include a globular star cluster, galaxies, two asterisms, a variables and number of pairs - all within a 7O field of view.
IC 2448/ He 2-19/ SA2-34/WRAY 16-34/ PK 285.7-14.1 (09072-6957) (Carina) lies close to the border of Volans, near the bright star Beta Carinae (Miaplacidus), first discovered by Fleming in 1898. In my mind, it is one of the easiest planetaries to find. In the C8 using a 26mm. Plossel, both Miaplacidus and IC 2448 are seen at the edges of the same field, respectfully NE and SW. [My observing logbook of 1982, however, has an unforgiving 'curse' about this object, as my body was contorted to braking point attempting to use the direct finder close to the fork of the ye' old C-8!] For specific directions it is 34.7'min.arc. S.E.E, but it is best to centre the telescope on Miaplacidus, and move in an easterly direction by about 32'min.arc., then south by 13'min.arc. The field view of IC 2448 can be seen in Figure 1. Visually the observed field contains a number of 12th and 13th magnitude stars, with the 10.2 magnitude (Mph=11.5) planetary being the brightest 'stellar' object in the field. (Burnham's 'Celestial Handbook' and NGC 2000.0 incorrectly says the magnitude is 12th, and similarly, the photographic magnitudes are also underestimated by these same sources as 11.5.)
Medium magnification reveals a tiny featureless greyish disk. My eyes suggested a hint of blue colouration, but this is by no means prominent. The planetary is classified as Type 2b - smooth disk with uniform brightness. Using a 20cm., I could not distinguish any internal structure, and I assume even large apertures will do no better. A description by Burnham's gives a better extension than Fleming's original NGC one - 'vS (very small), R (Round), appearance nearly stellar.' An O-III filter produced a prominent jump in brightness, yet no visible structure. Some debate exists about the actual diameter, with most saying the visual disk is between 8.0" and 10.0"sec.arc. For example, AOST1, Sky Catalogue 2000.0 and Burnham's state 8"sec.arc. Megastar states the diameter is 9"sec.arc., while AOST2 is 10"sec.arc. and Perek/ Kohoutek in 1967 states that the optical diameter is greater than 10"sec.arc. For my eyes, the 10"sec.arc. seems closer to the truth. Photographic images display a slightly larger 12.0"sec.arc. IRAS (1982) shows an infrared diameter of 33"sec.arc., whose emissions are strong at 100µm. This for all planetaries is typical as the circumstellar envelope from the AGB phase discards most of the stellar atmosphere in the surrounding space. For IC 2488 this corresponds to a distance more than one light year from the PNN.
Listed as object #274 in AOST2, the planetary is quoted as; "...about 10" across, slightly oval and the brightest object in a field of scattered stars. Even a 7.5cm. shows the single prism image plainly; it is bright through an O-III filter."
Steve Crouch (Southern Sky 1,4 pg.55) also states;" ...,IC 2448, about one low-power field away to the south west. This is a very prominent object in a 28cm., but a fair amount of magnification is needed for its circular, basically colourless, 8" disk to stand out clearly."
Explanation of the cause of the O-III brightness can be seen in Figure 2. Line intensities are based on the Hydrogen Beta wavelength, measured in milliwatts per square metre (mW.m-2), being set at an arbitrary value of 100. All other wavelengths are then based on this value. The individual line intensities for IC 2448 used in the profile are; H / 100, HeII/ 46, OIII/ 16, OIII/ 1253, He I/ 8, H / 348, NII/ 2.5, SII/ 1 and SII/ 1. Technically, magnitudes of planetaries are set by measuring the major contributors of energy output - Hydrogen-Beta and Oxygen III. In all it these two wavelengths that reflect the integrated magnitude calculated on the cumulative strengths of the all other wavelengths. As O-III (and H ) outputs are close to the visual acuity of the eye, other wavelength play little role in the final magnitude, and are therefore can be ignored. Thus, these two wavelengths are mainly used in the calculation of a visual magnitude. Much professional work has been done on this 'elliptical' nebula since the early 1980's. The PNN (HD 78991) of spectral type 'O' is invisible to most amateur telescopes. It has a visual magnitude of 14.32 (B mag. 13.97) - one magnitude less than stated in Sky Cat. 2000.0. Detail on the central star's temperature in the literature (1982-1989) varies considerably. A mean value is c.76 500OK± 17 000O and the Zanstra temperature is set at 83 000OK. Later observations according to the more accurate 'PN Morphologies Central Star Mass and Nebular Properties.' by Gorney et.al. A&A 318,256 (1997) show a lower Zanstra Temperature of 45 700OK± 800O (Stated 3.5%). Absolute magnitude of the PNN is 0.48, and is said to be 1 800 times more luminous than the Sun. The central mass (1997) is calculated at 0.579 solar masses. For the nebulosity the mass totals some 0.019 (1.9x10-2) solar masses, with an electron density of 40 000 electrons per cubic centimetre. (4x104 cm-3) Electron density tells the observer of the amount of energy available to enable ionisation of the surrounding gas. (However, this parameter is difficult to determine, and is only stated for the brightest of planetaries.) The large this value - the brighter the visual appearance of the nebula. Electron density is studied because it happens to be closely related to both temperature and the mass of the PNN. Morphologies of visible structures for the early stages of PN formation are also likely linked to this parameter. One or two papers on this particular issue has appeared in the professional literature as recently as late 1997.
According to M.Perinotto, IC 2448's PNN has shown some aspects of the P Cygni phenomenon - characteristic of an expanding envelope. The phenomenon observes particular emission lines with a brighter absorption line next to, but always towards the bluer part of the spectrum. This assumption is based on the narrow spectral profiles of the lines of H (656.3nm.) and HeII (468.6nm), which are used to determine the radial velocity -24.0kms-1±4.0 (1983) that indicates it is travelling towards us. From the broadening of the spectral lines the expansion velocity of the inner nebulae is found to be between 11.5 and 13.5km.sec-1 depending on the spectral line in question, though the outer portions of the nebula are travelling at a slower 7kms-1. In size, the true diameter is calculated to be some 0.34 light years or 32 billion kilometres! The evolutionary age of the bright nebulosity is predicted to be a mere 5 500 years.
For distance, the values quoted in the literature vary widely. AOST1 quotes a distance of 1.8 kpc. and this is about half the present estimates. 'Strasbourg-ESO Catalogue of Galactic Planetary Nebulae' by Acker et.al. (1984 &1992); and Cahn and Kaler; (AJ. Sup. Series 22, 319-368 (1987)) both state 3.5kpc. Gorky et.al. (1997) quotes 3.6kpc. This slightly larger value is based on both the diameter and luminosity calculations given above.
Some sources use calculations based on so-called statistical distances, which is an overall average from all 'reputable' sources. Earlier observations are notorious for being underestimates of true distances. In most planetaries, and this tends to make such values lower than the latter day values. For IC 2448, a number of statistical distances quoted in the Strasbourg-ESO Catalogue are; 2.81 (1978) (Source in Sky Cat. 2000.0), 2.54 (1982) and 1.90 (1984).
Since Fleming's discovery 100 years ago, this planetary has become a classical example for the amateurs' eye.
Surrounding Field of IC 2448 (Click on designation for Field Image).
Beta Carinae/ SAO 250495/ Miaplacidus (09132-6943) is one of the brighter stars of the southern skies, and can be found about halfway between the Southern Cross and Canopus. From Sydney it is a circumpolar star. Although today it is in the modern day constellation of Carina, it was once, prior to the mid-17th Century, to lie within the super-constellation of Argo Navis. Later, Argo gained a new sub-constellation called Robur Carolinum. Created by Edmond Halley and officially published in 1679, Robur was made to commemorate the place known as the Royal Oak. This location protected the defeated Charles II, after his army was routed by Oliver Cromwell at the Battle of Worcester in September 1651. La Caille complained bitterly about Halley's addition because it destroyed most of the prominent stars in the Ship. (Halley, however, didn't complain. It ensured that he received his Masters Degrees by royal proclamation!) From about 1720, popularity for the constellation of 'The Oak' was slowly reclaimed back into Argo Navis. Its common usage probably ended in 1800 with Johann Elert Bode's star atlas 'Uranographica'. From then, like many others, Robur simply fell into antiquity as an obscure constellation. American astronomer Benjamin Apthorp Gould in 1879 subdivided this part of Argo Navis into smaller constellations, and Miaplacidus became part of Carina. This change was permanently adopted by the IAU in 1930.
Once known as Alpha Roburis, Miaplacidus was supposed to have been derived from the Arabic Mi'ak or 'waters', while the Latin 'placidus' was added much later. I assume that from Arabia, the Chaldeans would have seen this bright star in Summer glimmering close to the southern horizon of the Indian Ocean. Today, however, Miaplacidus is fully translated to mean 'placid waters' - appropriate because the star lies in the bilge of the ship!
This white star culminates annually at 9 pm. on the 26th March. In order of brightness, the visual magnitude is 1.674, making it 28th among the naked-eye stars. Distance is given as 26.1 parsecs (85 light-years), though the latest information of the Hipparcos astrometric satellite gives a much further distance of 34.09 parsecs (111.2 light-years). The spectrum of this giant star was for many years given as A0 III, though it has recently been downgraded to an A2 IV, indicating a surface temperature of 8 700OK. Measurements suggest an absolute magnitude of -0.4, so the true luminosity is about sixty times brighter than the Sun. Miaplacidus has been shown from the radial velocity that it is approaching us at a pedestrian 5km.sec-1. This is similar to the first measure in 1927, from the famous southern station of Harvard University in Chile. By 1970, careful inspection of the broadening of the spectral lines showed that the rotational (V.sin i.) velocity is c.145km.sec-1. Using general stellar evolution theory,
the rotation of the star would have a 'stellar day' possibly as long as c.60 hours (or 2.5 days) for its nine million
kilometre girth. Proper motion measures is 36.7"sec.arc. per century towards the constellation of Volans, which it will crawl into about the year 9 800 AD. Also in the future it will become the southern pole star in 6 200 AD, approaching by some 5O 42'. It is interesting to note that about the same time, Alpha and Beta Centauri will also be making their closest approach, at a mere 23'min.arc. Today, it stands as a lonely sentinel on the edges of the magnificent southern Milky Way.
Asterism - 'Southern Ursa Major.' or 'Delphinus Australis.' (09079-6442) is a pretty little asterism of nine stars between the magnitudes 8.4 and 11.4, and orientated in an east-west direction. Finding is easy as it lies some 20'min.arc. north of planetary IC 2448. I found it during a broad sweep of the area while moving the telescope towards the charming globular star cluster NGC 2808 - some 2.3O directly north of Miaplacidus. In size, it is about 35'min.arc. in length, with the trapezium shaped 'head' to the west. It is easily visible in a 7.5cm., and using a wide-field with a low magnification is best.
NGC 2822/ ESO 61-4 (09138-6938) is a difficult galaxy to see because it lays a mere 10.2'min.arc. due north of Miaplacidus and hides in the glare of the star's brilliant radiance. (Figure 4) In the RNGC by Sulentic & Tufft, the object is stated as nonexistent - and it is consequently that NGC 2822 does not feature in many modern atlases used by amateurs. ie. Sky Atlas 2000.0. The basis of the deletion seems to have been from a simple comment in the mid-1950's by the famous galaxy observer and cosmologist; Gerald de Vaucoulers. Photographs produce images that cover about 3.3'x2.2', though the visual appearance is closer to about 1.4'x0.9'min.arc. (Note: NGC 2000.0 lists the size as 2.1'min.arc.) The magnitude is given as 11.7p, though visually I thought it to be nearer 12.1 to 12.4. (SIMBAD database gives V=12.01.) In the C-8, I thought I glimpsed a faint east-west smudge, by using an occultation bar placed at the focal plane in both a 12.5mm. Orthoscopic and a 20mm Erfle. Another trick was to place the galaxy just outside the visible field, though I thought the occultation bar did better. If you have a Newtonian, it is advised to make sure that the optics are perfectly aligned, as the artificial spikes will destroy your ability to see it. (This problem is similar to the faint galaxy near Leo's first magnitude, Regulus.) Doubt exists on the galaxy's morphology, but some think it maybe an elliptical. As yet, no redshift measures have been made on this object, so the distance is unknown.
NGC 2836/ ESO 61-3 (09137-6920) lays 18'min.arc further north of NGC 2822 (28'min.arc. from Car), and can be seen in the same field, if Miaplacidus is placed towards the field's southern edge. (Figure 5.) The photographic magnitude of the galaxy is a fairly bright 12.6, and I estimated that the visual magnitude was about the same. Again it is not listed in the RNGC, but for what reason - I don't know! In size, it is about 1.4'x0.18', and consequently appears like a tiny, fuzzy needle. (Photographically, it is some 50% larger.) Although fainter than NGC 2822, this galaxy was easy in the C-8. I suspect that a 15cm. maybe able to see it with care. My second view of this object was with a 30cm., and I could see using averted vision several field stars over the surface of the galaxy. Their magnitudes were around 14 to 14.5, and a larger Dobsonian at Ilford should see these easily.
Images suggest this is a SA-type spiral galaxy with some inner ring structure, which I could not confirm optically, through lack of aperture. Like NGC 2822, no redshifts have been found for this object.
Some 7'min.arc. due east is a faint galactic RR Lyrae variable known as LW Car (09157-6921). I only mention it, as the photographers or CCDer's could easily see it. The magnitude varies between 14.5 and 15.5, in a poorly estimated period.
RMK 10 (09179-6948) is a white and yellowish pair of magnitude 7.8 and 8.1 magnitude, first discovered by Rumker at Parramatta. The pair lies 24'min.arc. east of Beta Carinae. Of the nine measures, last in 1958, the positions remain 10.5"sec.arc. position angle 18O. No significant change has been observed with this pair. It is possible that it is a true binary, though its period is probably very long. In the same field as RMK 10, is the pair R110 (09182-6941), that H.C.Russell first found using the 29cm.(11.5") at Sydney Observatory on 21st May 1881. Russell's measures revealed a P.A. of 26.6O and a separation of 11.1"sec.arc. The magnitude of the two stars is 10 and 11. Little has changed with the pair, and is likely to be just an optical system.
E Carinae/ V345 Carinae / SAO 256583 (09057-7032) is a Gamma Cassiopeia (GCAS) type variable. (See Part 11 of this Series.) This magnitude 4.67 blue-white star lays 24.6'min.arc. (0.97o) southwest of Miaplacidus, and is one of the brightest of its class. Most of these types of stars often vary by up to 1.5 magnitudes - normally without any known periodicity. They are peculiar to the spectral class of sub-giants B-type (B0 III-B3 V), and are exhibited in about 0.4% of all known variable stars. Known as shell stars, they are subject to temporary fades of
one to three magnitudes. The variation of this one is only 0.11 magnitudes between 4.67 and 4.78. V348 Carinaehas a spectral class of B2 Vne.
HJ 4171 (09000-6444) (Vol). Lying due west of Miaplacidus by 1.1O degrees is another faint pair. The quoted magnitudes are 10.1 and 11.1 as first measured by Innes in 1918. Contained in a starry field, the pair is separation is 14.6"sec.arc. at PA of 237O. and is visible in a 7.5cm. Although separation of the pair is easy for small apertures, the magnitude of the stars may require a 10.5cm. to see it clearly in less than perfect skies.
Asterism - 'The Starry Curtain' it a group of 6th, 7th and 8th magnitude stars that winds between Miaplacidus and Alpha Volantis. Sitting in a car in 1993, I found this fascinating area while scanning the southern Milky Way with binoculars. The curtain divides into two parts. Moving NNE from Beta Carinae, the stars appear to weave in a snakelike pattern for about 4O. Although it maybe just visible to the naked eye in country skies, binoculars are really required for the southern part. The northern 4O portion of the curtain surrounds the white 4th magnitude Alpha Volantis. Observations with a C-8 reveal a strong north-south discontinuity of each side of the 'curtain'. Using a 7.5cm. or 15cm., the observer will see the eastern field from Alpha ( ) Volantis containing several 10th to 12th magnitude in a series of intersecting lines. Moving west of Alpha Volantis the number of field stars suddenly reduces. Moving north and south by about 3O of this centre, the starry 'curtain' appears to me as another wavy line joining the brighter southern section.
Although this elegant area is of little astronomical significance, at least finding it is easy!
NGC 2808 (GSC0911-646) (09120-6452) (Carina) is the tenth brightest Globular Star Cluster (GSC) in the sky first discovered by Dunlop (as 256), and was extensively observed by John Herschel in 1836. Between magnitude 6.1 and 6.3, it should be just visible to the naked eye, though some optical aid is required to reveal it as non-stellar. Some debate still exists on the naked-eye visibility Greg Bryant in (Universe 43,12 & 44,1 (1996-97) in his article entitled 'Naked-Eye Globular Clusters' states in Table 2 that NGC2808 is a possible naked-eye cluster. In the number of observations of this object, I have to admit my eyes cannot claim to have seen NGC 2808 visually, with the last time being April 1997 from Mt. Banks. In size it covers some 7'min.arc. with the outliers being up to 13.8'min.arc. from the core's centre. The NGC classification by Dreyer in 1888 is given as 'L,vB, eRi, eCM' - large, very bright, excessively rich, extremely compressed in middle. (See. Figure 6.) In the same field, some 9.5'min.arc. ESE is the John Herschel pair HJ4164 (09135-6455) first measured by Russell in 1879. The stars are magnitude 10.0 and 10.6, separated by 16.4"sec.arc. along position angle 64O.The 'southern' superlatives given to this magnificent cluster are numerous. For example, the general 1920's text of Webb's 'Celestial Objects for Common Telescopes', has on pg.299 a quote by R.T.A.Innes;
"Superb cluster (with) 13th to 15th magnitude stars....like the finest dust, centre blaze."
David Frew 'March Deep Sky Objects' (Universe 30,3 1986);
"This beautiful mag. 6.3 globular star cluster is a much underrated object and is even visible to the naked eye from dark inland skies. A 15cm. telescope will begin the resolution, as the brightest giants are of Mag.13. The overall diameter is about 4' to 5', though the condensed centre is less than half of this. The distance is about
30 000 [light years]."
Andrew Murell (Universe 42,3 1995) in his article entitled 'Carina';
"NGC 2808 is one of the best globular star clusters in the sky, appearing like a cross between Omega Centauri and 47 Tucanae. The cluster is about 14' in diameter and can be fully resolved in a 12" (30cm.), with the outer regions resolved in an 8". It is quite bright, having an integrated magnitude of 6.8."
J Graham Little in 'Ten O'Clock High' (Southern Sky 1,4 pg.53 March/April 1994) states; "The only Globular Cluster of Carina... [and] is visible as a small, fuzzy spot in binoculars. A 15cm. 'scope is required, however, for stellar resolution, as the brightest stars are around magnitude 13. This cluster looks good through a telescope. A sprinkling of tiny stars surrounds a very dense, bright centre. I have noticed on nights of good viewing what appears to be dark lanes towards the extremities of the cluster."
Burnham's 'Celestial Handbook', amazingly does not describe NGC 2808, though it does appear in the object summary on pg.464. It is extraordinary the number of 'standard' texts that do not add this wonderful cluster. This could be expected, as the cluster is in the shadow against the more memorable GSC's of Omega Centauri and 47 Tucanae. It is also in brightness only a few tenths of a magnitude below the vastly overrated northern 'Great Cluster' in Hercules (M13). Observationally, the centre of the cluster is both very condensed and very bright, when compared with the cluster as a whole. To my eyes it appears to have features similar to 47 Tucanae, with the star counts rising exponentially towards the centre. To quote from AOST1;
"[This] beautiful object is a splendid example of the symmetrical strongly compressed type of globular cluster, composed of innumerable faint stars....to small apertures it is a bright nebulous haze with strong central condensation."
My first observation was past members' Beryl van Heast 10.5cm. (4") Newtonian at Bowen Mt. 13.35 UT; 30th December 1978. My observation log states;
"Very bright with a very dense core of unresolved stars, especially [towards] the core. Wonderful!"
It is one of only three known Class I's, and it is the closest of all these by a factor of four! GSC Classes are based on the concentration of stars as seen in the telescope, which was first proposed by Shapley and Sawyer in 1926; where 'I' is highly concentrated, while 'XII' is loose and scattered. Class 'I' label is rather deceptive for NGC 2808. The outer portions of the cluster are easily separated even in a 15cm. Separation of the 'core' stars is a bit more difficult. A 30cm. may just see some of the centre stars, and a 40cm. perhaps will start to break apart the core. Sulentic and Tufft's 1973 revision of the NGC (RNGC) and Roger Sinnott's NGC 2000.0 classification correctly and aptly gives the description; "!Globular Star Cluster, very large, extremely rich, very gradual, extremely compact in centre, 45'min.arc. diameter (core), stars 13....15."
This cluster has proved difficult to determine distance because of the sheer concentration of stars, and as such, information sources vary considerably. Harlow Shapley in 1930 estimated the first distance at 16.2kpc. Lack of variables, especially RR Lyraes, also make distance estimates nearly impossible. Variables are the keys to cluster distance determinations. The first variable was discovered in the early 1930's. By 1962, Sawyer and Hogg had increased this number to four, and yet all remained without known periods until the 1970's. Due to the lack of observations and of star counts, another observational programme was again seriously undertaken by King in 1968 - yet he found no new variables. By 1973, the number of variables increased from four to nine from observations at Mt.Stromlo. The distance of the cluster is now estimated to be about 11.2 kpc. (36 000 lty.), that closely approximates the estimate in 1975 (11.5 kpc.) AOST2 quotes c.7.5 kpc., which is likely a bit of an underestimate of its distance. I assume David Frew has taken this value from Hogg's (1956) distance of 7.6kpc., as I could not find his actual source. Sky Atlas 2000.0 quotes a distance of 9.2 kpc.
In 1963, the first energy distribution was measured and published by Aller (AJ, 68, 69). The globular star cluster was determined to be G2 III, later revised to an F8 type in 1967. These observations also gave an accurate radial velocity of 104 km.sec.-1 in recession.
Age is determined either by H-R Diagrams or by the Iron to Hydrogen ratio or [Fe/H]. The value for NGC 2808 is -1.37, poorer than the Sun, while the Metals to Hydrogen [m/H] ratio is -1.47. Like most globulars, the age is estimated to be between 8 and 12 billion years.
At the end of 1990, the Australia Telescope looked in a number of selected globulars for Hydrogen emissions at the 21cm. line. NGC 2808 was one of the only ones to show an indication of Hydrogen. The discovery was questioned with some vigour, because globulars were suspected to be old objects that used their Hydrogen in the first few billon years when the galaxy was formed. Long ago most of the Hydrogen should have been used up, yet this globular seems to mock the 'standard' theory.
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Douglas Snyder, Oct. 1998