The 3-D shaping of NGC 6741: A massive, fast-evolving Planetary Nebula at the recombination-reionization edge

¹ INAF – Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: sabbadin@pd.astro.it
² INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 11, 80131 Napoli, Italy
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Italy

Received: 29 November 2004
Accepted: 17 January 2005

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 ( kpc,  yr, ionized mass ) is a dense (electron density up to 12 000 cm^-3), high-excitation, almost-prolate ellipsoid ( pc, major, intermediate and minor semi-axes, respectively), surrounded by a sharp low-excitation skin (the ionization front), and embedded in a spherical ( pc), almost-neutral, high-density ( atoms cm^-3) halo containing a large fraction of the nebular mass (). 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 (), hot (log ) and faint (log ) 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, 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  of He II, i.e. a thirteen fine-structure components recombination line). Some general implications for the shaping mechanisms of Planetary Nebulae are discussed.