The 3-D shaping of NGC 6741: A massive, fast-evolving Planetary Nebula at the recombination-reionization edge
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 (distance≃2.0 kpc, age≃ 1400 yr, ionized mass Mion≃ 0.06 M⊙) is a dense (electron density up to 12 000 cm-3), high-excitation, almost-prolate ellipsoid (0.036 pc × 0.020 pc × 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 (radius≃ 0.080 pc), almost-neutral, high-density (n(H I)≃ 7 ×103 atoms cm-3) halo containing a large fraction of the nebular mass (Mhalo≥ 0.20 M⊙). 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⊙), hot (log T* ≃ 5.23) and faint (log L*/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, Vexp(km s-1)=13 × 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 Å of He II, i.e. a thirteen fine-structure components recombination line). Some general implications for the shaping mechanisms of Planetary Nebulae are discussed.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- June 2005
- DOI:
- 10.1051/0004-6361:20042447
- arXiv:
- arXiv:astro-ph/0502101
- Bibcode:
- 2005A&A...436..549S
- Keywords:
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- planetary nebulae: individual: NGC 6741;
- ISM: kinematics;
- and dynamics;
- Astrophysics
- E-Print:
- 27 pages, 18 figures, accepted for publication in A&