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Astron. Astrophys. 322, 975-981 (1997)

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2. IC 4406

IC 4406 (PN G319.6+15.7) is a large ([FORMULA] long) southern bipolar PN. Optical CCD images of the nebula are presented by Sahai et al. (1991), Schwarz et al. (1992), and Corradi & Schwarz (1993). In these images, IC 4406 appears to be composed by two elongated lobes, mostly prominent in the light of low ionization species such as [NII] [FORMULA], extending from a bright central region showing evidence for the presence of a large torus of ionized gas. CO observations (Sahai et al. 1991) indicate that the ionized gas is located inside a cylindrical molecular cavity; the CO expansion velocity increases from [FORMULA]  km s-1 in the equatorial plane to 60-70 km s-1 along the polar directions of the cylinder. An optical velocity field of the nebula has been obtained by Corradi & Schwarz (1993), but velocities are not resolved there. In the central region, roughly corresponding to the equatorial ionized torus, shocked [FORMULA] emission has been detected (Storey 1984), which is supposed to be produced at the interface between ionized and molecular gas (Sahai et al. 1991). The morphological and kinematical properties of IC 4406 indicate that the mass outflow has been collimated by an equatorial density enhancement in the circumstellar envelope ejected by the progenitor, in agreement with the current view of formation of bipolar PNe (Balick 1987).

The major axis of the nebula lies very close to the plane of the sky. Sahai et al. (1991) derived a nebular inclination (angle i between the symmetry axis of the nebula and the line of sight) of [FORMULA]. [FORMULA]. This inclination makes IC 4406 an ideal target to look for chemical gradients, since every line of sight intersects regions of the nebula which are all approximately at the same distance from the centre. Mixing of emission coming from regions at different distances from the central star is therefore minimized.

Previous studies of the chemical abundances of IC 4406 have been presented by Kaler (1978, 1983), de Freitas Pacheco et al. (1992), and Perinotto et al. (1994), but they lack spatial resolution or are limited to the bright central part of the nebula.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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