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 ( M _ { ion } \simeq 0.13 M _ { \odot } ) at a distance of 1.7 kpc , seen almost equatorial on : a tenuous and patchy spherical envelope ( r \simeq 0.090 pc ) encircles a dense and inhomogeneous tri-axial ellipsoid ( a/2 \simeq 0.077 pc , a/b \simeq 1.25 , b/c \simeq 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 M _ { \odot } central star , which has recently exhausted the hydrogen shell nuclear burning and is rapidly moving toward the white dwarf domain ( log T _ { * } \simeq 5.22 K ; log L _ { * } /L _ { \odot } \simeq 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 ( m _ { V } \simeq 17.06 ) is a visual binary : a faint , red companion ( m _ { V } \simeq 17.73 ) appears at 0.09 arcsec in PA=190 \degr , corresponding to a separation \geq 150 AU and to an orbital period \geq 1500 years .