| Abstract
| - 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 ( $\rm distance\simeq2.0$ kpc, $\rm age\simeq 1400$ yr, ionized mass $M_{\rm ion}\simeq 0.06~M_odot$) is a dense (electron density up to 12 000 cm -3), high-excitation, almost-prolate ellipsoid ( $\rm 0.036~pc \times 0.020~pc \times 0.018$ pc, major, intermediate and minor semi-axes, respectively), surrounded by a sharp low-excitation skin (the ionization front), and embedded in a spherical ( $\rm radius\simeq 0.080$ pc), almost-neutral, high-density ( $n{\rm (H\,I)}\simeq 7 \times10^3$ atoms cm -3) halo containing a large fraction of the nebular mass ( $M_{\rm halo}\ge 0.20~M_odot$). 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 ( $M_*=0.66{-}0.68~M_odot$), hot (log $T_* \simeq 5.23$) and faint (log $L_*/L_odot \simeq 2.75$) 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, Vexp(km s $^{-1})=13 \times R\arcsec$, 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 ( λ4686 $\rm\AA $ of He II, i.e. a thirteen fine-structure components recombination line). Some general implications for the shaping mechanisms of Planetary Nebulae are discussed.
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