2005A&A...436..549S

We infer the gas kinematics, diagnostics and ionic radial profiles, distance and central star parameters, nebular photo-ionization model, spatial structure and evolutionary phase of the Planetary Nebula NGC 6741 by means of long-slit ESO NTT+EMMI high-resolution spectra at nine position angles, reduced and analysed according to the tomographic and 3-D methodologies developed at the Astronomical Observatory of Padua (Italy). NGC 6741 (distance≃2.0kpc, age≃1400yr, ionized mass M_ion≃0.06M_☉) is a dense (electron density up to 12000cm^–3), high-excitation, almost-prolate ellipsoid (0.036pcx0.020pcx0.018pc, major, intermediate and minor semi-axes, respectively), surrounded by a sharp low-excitation skin (the ionization front), and embedded in a spherical (radius≃0.080pc), almost-neutral, high-density (n(HI)≃7x10³atoms/cm³) halo containing a large fraction of the nebular mass (M_halo≥0.20M_☉). The kinematics, physical conditions and ionic structure indicate that NGC 6741 is in a deep recombination phase, started about 200 years ago, and caused by the rapid luminosity drop of the massive (M_*=0.66-0.68M_☉), hot (logT_*≃5.23) and faint (logL_*/L_☉≃2.75) post-AGB star, which has exhausted the hydrogen-shell nuclear burning and is moving along the white dwarf cooling sequence. The general expansion law of the ionized gas in NGC 6741, V_exp(km/s)=13xR'', fails in the innermost, highest-excitation layers, which move slower than expected. The observed deceleration is ascribable to the luminosity drop of the central star (the decreasing pressure of the hot-bubble no longer balances the pressure of the ionized gas), and appears in striking contrast to recent reports inferring that acceleration is a common property of the Planetary Nebulae innermost layers. A detailed comparative analysis proves that the ``U''-shaped expansion velocity field is a spurious, incorrect result due to a combination of: (a) simplistic assumptions (spherical shell hypothesis for the nebula); (b) unfit reduction method (emission profiles integrated along the slit); and (c) inappropriate diagnostic choice (λ4686Å of He II, i.e. a thirteen fine-structure components recombination line). Some general implications for the shaping mechanisms of Planetary Nebulae are discussed.