The 3--D ionization structure of NGC 6818: a Planetary Nebula threatened by recombination

Abstract

Long-slit NTT+EMMI echellograms of NGC 6818 (the Little Gem) at nine equally spaced position angles, reduced according to the 3-D methodology introduced by Sabbadin et al. (2000a,b), allowed us to derive: the expansion law, the diagnostics and ionic radial profiles, the distance and the central star parameters, the nebular photo-ionization model, the 3-D reconstruction in He II, [O III] and [N II], the multicolor projection and a series of movies. The Little Gem results to be a young (3500 years), optically thin (quasi-thin in some directions) double shell (Mion~0.13 Msun) at a distance of 1.7 kpc, seen almost equatorial on: a tenuous and patchy spherical envelope (r~0.090 pc) encircles a dense and inhomogeneous tri-axial ellipsoid (a/2~0.077 pc, a/b~1.25, b/c~1.15) characterized by a hole along the major axis and a pair of equatorial, thick moustaches. NGC 6818 is at the start of the recombination phase following the luminosity decline of the 0.625 Msun central star, which has recently exhausted the hydrogen shell nuclear burning and is rapidly moving toward the white dwarf domain (log T*~5.22 K; log(L*/Lsun)~3.1). The nebula is destined to become thicker and thicker, with an increasing fraction of neutral, dusty gas in the outermost layers. Only over some hundreds of years the plasma rarefaction due to the expansion will prevail against the slower and slower stellar decline, leading to a gradual re-growing of the ionization front. The exciting star of NGC 6818 (mV~17.06) is a visual binary: a faint, red companion (mV~17.73) appears at 0.09 arcsec in PA=190deg, corresponding to a separation >=150 AU and to an orbital period >=1500 years