IC 2220 - The Toby Jug Nebula
IC 2220
RA = 7h 56m 48sec Size: 2.5' Distance ~ 90 ±30 pc
DEC = -59' 7' 12", m=+13
Astronomers both professional and amateur have a sense of humor when it comes to naming some of the objects in the universe. There is a nebula shaped like the North American continent, it is called the North American Nebula (no such nebula is named for South America,... yet), another nebula is shaped like the State of California, and it is call the California Nebula, and there are hundreds of nebulas and asterisms that bear the name of the object/shape they resemble. (Horsehead Nebula, Hourglass Nebula, etc.) The most popular asterism is the Big Dipper in the northern constellation Ursa Major.
In examining the nebula IC 2220 shown in Figure 1, Astronomers Paul Murdin, David Allen and astrophotographer David Malin coined the name "Toby Jug Nebula" for the object because of its shape as tankard of a type called Toby Jug. Another name given to this object is the Butterfly Nebula, owing to the similarity in appearance as a butterfly in flight.
IC 2220 is a reflection nebula. Astronomers recognize three types of reflection nebula:
1) Nebula that shine by the hot O and B type stars that were formed in that region of the galaxy. The Pleiades and the central Orion stars are good examples of this.
2) Nebula that shine due to a random encounter by a passing star. Example: California Nebula
3) Nebula that are the result of mass loss from the parent star shining by the reflected 1ight of this parent star. IC 2220 is this third type of reflection nebula.
Hidden at the center of the nebula is 4 diffraction spikes and an overexposed image of the 7th magnitude star HD 65750, an M3 III giant star. Naturally, investigators assumed that the star and nebula were physically connected which turned out to be true as opposed to an optical alignment. Early studies of IC 2220 in the 1970's showed that the distance of HD 65750 and the nebula were about 375 pc, the same distance as NGC 2516 (see below). The "handle" of the Jug in the left side of the nebula was even thought to be a prominence of the illuminating star, expelled from the star in some gigantic mass loss event. The nebula itself was produced during an earlier mass loss phase or possibly by mass ejection of an unseen companion.
The closeness of IC 2220 and the galactic cluster NGC 2516 (1.5' south) prompted Dachs, Isserstedt and Rahe in their 1978 paper to assume that the star was a former member of the cluster. They postulated that
1)the absolute magnitude of the star and cluster are similar, indicating similar distances,
2) the radial velocity of HD 65750 is +20 km/sec, is in excellent agreement with the average radial velocity of the brightest stars in NGC 2516, +22 km/sec, and 3) The proper motion of HD 65750 is similar to that of the average proper motions of NGC 2516's brightest members. They even went as far as estimating the mass of HD 65750 as 5 M¤, assuming it was a typical (former) star of the cluster.
HD 65750 was also found to be an irregular variable with a 0.9m variability over a time scale of at least 11 - 15 years. This irregular brightness is due to the clouds of dust and gas being ejected from the star and moving in front of the star as seen from Earth. Magnitude variations of 0.1 m were even observed on a single night.
In 1997, along comes the HIPPARCOS satellite data and its accurate trigonometric parallaxes. The direct measurement of the distance to HD 65750 was substantially less than the previous estimate of 375 pc, down to 90 ±30 pc by HIPPARCOS data. Thus the theory of it being associated with the cluster NGC 2516 was debunked. Brightness variations in the nebula have been reported and even rotational motion of the one of the nebulas filaments, however these are extremely hard to see even on the best plates taken of the object.
A finder chart for locating the IC 2220, the Toby Jug Nebula is shown as Figure 2.
Figure 1. IC 2220, Toby Jug Nebula, DSS photo taken March 21, 1977, UK 48” Schmidt reflector, 60 min exposure.
Figure 2. Finder chart for IC 2220, Toby Jug Nebula and NGC 2516.
REFERENCES
Dachs, J., Isserstedt, I., 1973, The Dipole Nebula IC 2220, a Southern Reflection Nebula Around the Variable Red Giant HD 65750, Astronomy & Astrophysics, 23, p. 241-245.
Dachs, J., Isserstedt, I., and Rahe, J., 1978, On the Photometric Variations of the Red Giant HD 65750 and of the Surrounding Relection Nebula IC 2220, Astronomy & Astrophysics, 63, p. 353-362.
Murdin, P. Allen, D., Malin, D., 1979, Catalogue of the Universe, Book Club Associates, Cambridge University Press.
Perkins, H., King, D., Scarrot, S., 1981, The Toby Jug Nebula (IC 2220): A Bipolar and Biconical Nebula, Monthly Notices of the Royal Astronomical Society, 196, p.403-408.
Witt, A., Rodgers, C., 1991, CCD Surface Photometry of IC 2220, Publications of the Astronomical Society of the Pacific, 103, p. 415-420.