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

Abstract

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 M ion 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), almostneutral, high-density (n(H I) 7 × 10 3 atoms cm −3 ) halo containing a large fraction of the nebular mass (M halo ≥ 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, V exp (km s −1 ) = 13 × R , 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 finestructure components recombination line). Some general implications for the shaping mechanisms of Planetary Nebulae are discussed.

The H surface brightness-radius relation: a robust statistical distance indicator for planetary nebulae

“…In the meantime, PHR J1315-6555 was shown by Parker et al (2011) to be a physical member of the intermediate-age open cluster ESO 96-SC04. The compact object NGC 6741 has been suggested to be a possible member of Berkeley 81 (Sabbadin et al 2005), but while the distances are comparable, the radial velocity of the cluster is 8kms −1 greater (Sabbadin et al 2005;Magrini et al 2015), suggesting non-membership of the PN. It should be noted that the recent increases in numbers of both Galactic PNe and open clusters (Dias et al 2002) have increased the probability of positional coincidences between these two classes of object.…”

The structure of planetary nebulae: theory vs. practice

“…Unfortunately, a detailed comparative analysis (Sabbadin et al 2005) proves that the "U"-shaped expansion profile is a spurious, incorrect result caused by a combination of unsuited assumptions, spatial resolution, and diagnostic choice.…”

“…Besides the -in fieri -detailed spatio-kinematical study of individual PNe (Ragazzoni et al 2001;Turatto et al 2002;Benetti et al 2003;Sabbadin et al 2004Sabbadin et al , 2005, we decided to investigate a general, long-lasting open problem: inferring the radial density profile in expanding nebulae.…”

Expansion Velocity Investigation of the Elliptical Planetary Nebula NGC 6803

“…For other permitted lines, we assume V f s = 5 km s −1 , while for the forbidden lines, we ignore, i.e. 0 km s −1 (see Sabbadin et al 2004Sabbadin et al , 2005Sabbadin et al , 2006. For the turbulence, we applied V trb = 5 km s −1 only to the forbidden lines (Sabbadin et al 2006).…”