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NEAT SOUTHERN PLANETARIES : 08
IC 2448 in Carina

This area of southern Carina and into eastern Volans is a wonderful place to explore during the autumn months. Our selected planetary this time is IC 2448, being certainly one of the southern gems for the smaller telescopes.

For our surrounding objects I have also included one globular star cluster (NGC 2808), several galaxies, two asterisms, some variables and number of pairs — all within the approximate field of view of 7° around this planetary.

IC 2448 / He2-19 / Sa2-34 / Wray 16-34 / PK 285.7-14.1 / PN G285.7-14.9 (09072-6957), in Carina, lies close to the border of Volans, near the bright star β Car / Beta Carinae / Miaplacidus / HIP 45238 (09132-6943), IC 2448 was first discovered by Fleming in 1898. In my mind, it is one of the easiest southern planetaries to find. In 20cm. with the 26mm Plössel eyepiece, both Miaplacidus and IC 2448 are seen at the edges of the same field, respectively NE and SW. [My observing 1982 logbook, however, has an unforgiving personal 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 20cm C8!] For specific directions it is 34.7′SEE, but it is best to centre the telescope on Miaplacidus, and move in an easterly direction by about 32′E, then 13′S. 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 PNe being the brightest stellar-like 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 11.5p.

Medium magnification reveals a tiny featureless greyish disk. My eyes suggested a hint of blue colouration, but this is by no means prominent. IC 2448 is classified as Type 2b — smooth disk with uniform brightness. Using 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 arcsec. For example, AOST1, Sky Catalogue 2000.0 and Burnham's state 8″. Megastar states the diameter is 9 arcsec, while AOST2 is 10 arcsec and Perek/ Kohoutek in 1967 states that the optical diameter is greater than 10 arcsec. For my eyes, the 10 arcsec seems closer to the truth. Photographic images display the slightly larger size of 12.0". IRAS (1982) shows an infrared diameter of 33 arcsec, 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.

Observational Descriptions

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, p.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.”

Scott Mellish, (ASNSW) observed IC 2448 from the ASNSWI site at Wiruna. He sketched the PNe at 11pm on the 2nd February 2000, writing in his column “Sky Sketcher Post Mortem”, “Universe”, 48, 4, May (2000);

“This is a nice little planetary nebula, and fairly bright in the 16-inch [f/4.7 Dobsonian] at 191× mag. The Oxy III filter makes the object more distinct in appearance, and there is the hint of some mottling present. Perhaps higher power and more aperture would not leave this in question. However, this object is quite small at some 8″ in diameter. With a magnitude of 10.4, it should not prove too difficult in scopes around the [20cm] 8-inch mark.”

Technical Data

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 100.0. All other wavelengths are then based against this value. Individual line intensities for IC 2448 used in the profile are; Hβ=100, HeII=46, O-III=16, O-III=1253, He I=8, Hα=348, NII=2.5, SII=1 and SII=1. Technically, magnitudes of planetaries are set by measuring the major contributors of energy output — Hβ and O-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 Hβ) 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 1980s.

Central Star of IC 2448

IC 2448 PNN has been identified as star HD 78991, being of O-type spectral class. Invisible to most amateur telescopes. its shines at 14.32v / 13.97B magnitudes — 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 76,500&plusmhn;17,000K and the Zanstra temperature is set at 83,000K. Later observations according to Gorney et al. (1997) showed the lower Zanstra Temperature of 45 700±800K (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 M⊙. For the nebulosity the mass totals some 0.019 M⊙ (1.9×10-2 M⊙), with an electron density of 4×104 electrons per cubic centimetre. 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 Hα (656.3nm.) and He II (468.6nm), which are used to determine the radial velocity -24.1±4.0 km.s-1 (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 13.5 km.s-1 depending on the spectral line in question, though the outer portions of the nebula are travelling at a slower 7 km.s-1. In size, the true diameter is calculated to be some 0.34ly or 32 billion kilometres! The evolutionary age of the bright nebulosity is predicted to be about 5 500 years.

For distance, the values quoted in the literature vary widely. AOST1 quotes a distance of 1.8kpc — about half the present estimates. Both Acker et al. (1984 &1992) and Cahn, Kaler (1987) state the distance is 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 there are several of statistical distances quoted in the Strasbourg-ESO Catalogue, whose mean is 2.81 (1978). Source in Sky Cat. 2000.0), 2.54 (1982) and 1.90 (1984).

Since Fleming’s discovery one-hundred years ago, this planetary has become the classical example for the amateurs’ eye.

References

  1. Acker et al.; “Strasbourg-ESO Catalogue of Galactic Planetary Nebulae”; 1984 ≈ 1992
  2. Cahn, Kaler; AJ. Suppl. Series, 22, 319-368 (1987)
  3. Gorney, et al. “PN Morphologies Central Star Mass and Nebular Properties.”; A&A, 318, 256 (1997)
  4. Stanghellini, L., Shaw, R.A., Villaver, E., “The Magellanic cloud calibration of the Galactic planetary nebula distance scale.”, AJ., 689, 1942 (2008)

References

  1. Stanghellini, L., Shaw, R.A., Villaver, E., “The Magellanic cloud calibration of the Galactic planetary nebula distance scale.”, AJ., 689, 1942 (2008)

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Last Update : 06th November 2011

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