Tomographic and 3-D analyses for extended , emission-line objects are applied to long-slit ESO NTT + EMMI high-resolution spectra of the intriguing planetary nebula NGC 7009 , covered at twelve position angles .
We derive the gas expansion law , the diagnostics and ionic radial profiles , the distance and the central star parameters , the nebular photo-ionization model and the spatial recovery of the plasma structure and evolution .
The Saturn Nebula ( distance \simeq 1.4 kpc , age \simeq 6000 yr , ionized mass \simeq 0.18 M _ { \odot } ) consists of several interconnected components , characterized by different morphology , physical conditions , excitation and kinematics .
We identify four “ large-scale ” , medium-to-high excitation sub–systems ( the internal shell , the main shell , the outer shell and the halo ) , and as many “ small-scale ” ones : the caps ( strings of low-excitation knots within the outer shell ) , the ansae ( polar , low-excitation , likely shocked layers ) , the streams ( high-excitation polar regions connecting the main shell with the ansae ) , and an equatorial , medium-to-low excitation pseudo-ring within the outer shell .
The internal shell , the main shell , the streams and the ansae expand at V { { } _ { exp } } \simeq 4.0 \times R″ km s ^ { -1 } , the outer shell , the caps and the equatorial pseudo-ring at V { { } _ { exp } } \simeq 3.15 \times R″ km s ^ { -1 } , and the halo at V { { } _ { exp } } \simeq 10 km s ^ { -1 } .
We compare the radial distribution of the physical conditions and the line fluxes observed in the eight sub-systems with the theoretical profiles coming from the photo-ionization code CLOUDY , inferring that all the spectral characteristics of NGC 7009 are explainable in terms of photo-ionization by the central star , a hot ( logT _ { * } \simeq 4.95 ) and luminous ( log L _ { * } /L _ { \odot } \simeq 3.70 ) 0.60–0.61 M _ { \odot } post–AGB star in the hydrogen-shell nuclear burning phase .
The 3–D shaping of the Saturn Nebula is discussed within an evolutionary scenario dominated by photo-ionization and supported by the fast stellar wind : it begins with the superwind ejection ( first isotropic , then polar deficient ) , passes through the neutral , transition phase ( lasting \simeq 3000 yr ) , the ionization start ( occurred \simeq 2000 yr ago ) , and the full ionization of the main shell ( \simeq 1000 yr ago ) , at last reaching the present days : the whole nebula is optically thin to the UV stellar flux , except the caps ( mean latitude condensations in the outer shell , shadowed by the main shell ) and the ansae ( supersonic ionization fronts along the major axis ) .