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The 3-D Ionization Structure of the Planetary Nebula NGC 6565

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The 3-D Ionization Structure of the Planetary Nebula NGC 6565 A&A 384, 1062–1085 (2002) Astronomy DOI: 10.1051/0004-6361:20020109 & c ESO 2002 Astrophysics The 3-D ionization structure of the planetary nebula NGC 6565 M. Turatto1, E. Cappellaro1, R. Ragazzoni1, S. Benetti1, and F. Sabbadin1 Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy Received 26 November 2001 / Accepted 21 January 2002 Abstract. A detailed study of the planetary nebula NGC 6565 has been carried out on long-slit echellograms (λ/∆λ = 60 000, spectral range = λλ3900–7750 A)˚ at six, equally spaced position angles. The expansion velocity field, the c(Hβ) distribution and the radial profile of the physical conditions (electron temperature and density) are obtained. The distance, radius, mass and filling factor of the nebula and the temperature and luminosity of the central star are derived. The radial ionization structure is analyzed using both the classical method and the photo-ionization code CLOUDY. Moreover, we present the spatial structure in a series of images from different directions, allowing the reader to “see” the nebula in 3-D. NGC 6565 results to be a young (2000–2500 years), patchy, optically thick triaxial ellipsoid (a =10.1arcsec,a/b =1.4, a/c =1.7) projected almost pole-on. The matter close to major axis was swept-up by some accelerating agent (fast wind? ionization? magnetic fields?), forming two faint and asymmetric polar cups. A large cocoon of almost neutral gas completely embeds the ionized nebula. NGC 6565 is in a recombination phase, because of the luminosity drop of the massive powering star, which is reaching the white dwarf domain (log T∗ 5.08 K; log L∗/L 2.0). The stellar decline started about 1000 years ago, but the main nebula remained optically thin for other 600 years before the recombination phase occurred. In the near future the ionization front will re-grow, since the dilution factor due to the expansion will prevail on the slower and slower stellar decline. NGC 6565 is at a distance of 2.0 (±0.5) kpc and can be divided into three radial zones: the “fully ionized” one, extending up to 0.029–0.035 pc at the equator (0.050 pc at the poles), the “transition” one, up to 0.048–0.054 pc (0.080 pc), the “halo”, detectable up to 0.110 pc. The ionized mass (0.03 M) is only a fraction of the total mass (≥0.15 M), which has been ejected by an equatorial −5 −1 3 enhanced superwind of 4 (±2) × 10 M yr lasted for 4 (±2) × 10 years. Key words. planetary nebulae: individual: NGC 6565 – ISM: kinematics and dynamics 1. Introduction three-dimensional structures and physical conditions of the ionized gas was limited by projection effects, leading The late evolution of low and intermediate mass stars to approximate spatial forms and to unrealistic assump- (1.0 M <M∗ < 8.0 M) is characterized by the plan- tions for the main parameters, e.g. electron temperature etary nebula (PN) metamorphosis: an asymptotic giant and electron density constant all over the nebula (Aller branch (AGB) star gently pushes out the surface layers 1984, 1990, 1994). in a florilegium of forms, crosses the HR diagram and A 3-D reconstruction technique for studying at large reaches the white dwarf regime (Aller 1984; Pottasch 1984; and small scales the morphology, physical conditions, ion- Osterbrock 1989). ization, spatial structure and evolutionary status of PNe So far the interpretation of the exuberant mor- has been introduced by Sabbadin et al. (2000a,b) and phologies exhibited by the PNe in terms of detailed Ragazzoni et al. (2001), based on echellograms of mod- erate spectral resolution (R ∼ 22 000−25 000). The key of Send offprint requests to:M.Turatto, the 3-D methodology is simple: the PN is an expanding e-mail: [email protected] Based on observations made with ESO Telescopes at the plasma. Thus the position, thickness and density of each La Silla Observatories, under programme ID 65.I-0524, and elementary volume can be derived from the radial velocity, on observations made with the NASA/ESA Hubble Space width and flux of the corresponding emission. Telescope, obtained from the data archive at the Space The procedure has been applied to: Telescope Institute (observing program GO 7501; P.I. Arsen Hajian). STScI is operated by the association of Universities – NGC 40: an optically thick, very low excitation barrel- for Research in Astronomy, Inc. under the NASA contract NAS shaped nebula with thin arcs emerging at both ends 5-26555. We have applied the photoionization code CLOUDY, of the major axis, powered by a luminous and “cold” developed at the Institute of Astronomy of the Cambridge WC8 star presenting a large mass-loss rate. The fast, University. hydrogen depleted photospheric material ejected by the Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20020109 M. Turatto et al.: The Planetary Nebula NGC 6565 1063 Fig. 1. WFPC2 appearance of NGC 6565 in [OIII] (left) and [NII] (right). The images have been retrieved from the HST archive. North is up and East is to the left. nucleus is gradually modifying the chemical composi- tion of the innermost nebular regions (Sabbadin et al. 2000a); – NGC 1501: an evolved, high excitation, optically thin oblate ellipsoid, denser in the equatorial belt, deformed by several bumps, embedded in a homogeneous, in- wards extended cocoon and ionized by a “hot” and lumi- nous WC4 star exhibiting nonradial g-mode pulsations (Sabbadin et al. 2000b; Ragazzoni et al. 2001). In order to deepen the analysis, we have started a survey at high spectral and spatial resolutions with the ESO NTT. The superb quality of this material allows us to study at unprecedented accuracies objects with different morphol- ogy, e.g. NGC 7009 (the “Saturn” nebula), the tetra-lobed IC 4634, the butterfly HB 5, Mz 3 and NGC 6537, the double-envelope NGC 5882, NGC 6153 and NGC 6818. To simplify the application of the 3-D method we de- cided to begin with an “easy” nebula without FLIERS (fast, low ionization emitting regions), BRETS (bipolar, rotating, episodic jets), ansae, wings, multiple envelopes etc., and naively selected NGC 6565. Fig. 2. [NII]/[OIII] distribution over NGC 6565 (original frames and orientation as in Fig. 1), showing the large stratifi- cation effects and the faint, low ionization regions protruding 2. The nebula from the main nebula in PA 145◦. The bright, compact PN NGC 6565 (PNG 003.5-04.6, Acker et al. 1992) is “a minute oval ring 10 × 8 in PA about 5◦. Considerably fainter along the major axis, and the center is relatively vacant” (Curtis 1918). Its HST ap- NGC 6565 appears peculiar in many respects: pearance is shown in Fig. 1: the [OIII] emission forms a quite homogeneous oval ring, sprinkled by a number of – despite the simple elliptical shape (Stanghellini et al. dark globules and spikes, mainly in the Northern sector. 1993; Gorny et al. 1997), it shows a rich low ionization It is surrounded by a sharp, patchy [NII] layer present- spectrum typical of bipolar PNe (Greig 1972) and is N ing two faint lobes in PA 145◦. Note also the light N-S overabundant (Aller et al. 1988); asymmetry of the nebula with respect to the (barely vis- – the infrared and radio excesses suggest the presence of a ible) central star. The large stratification effects and the large amount of dusty neutral gas causing a detectable external, faint, low ionization regions are better seen in absorption of the optical radiation (Gathier et al. 1986; Fig. 2, presenting the [NII]/[OIII] distribution. Stasinska et al. 1992); 1064 M. Turatto et al.: The Planetary Nebula NGC 6565 – in spite of the high surface brightness of the nebula, in- extracted and treated as single, long-slit spectra. The dis- dicative of an early evolutionary phase, the central star tortion correction and the wavelength calibration were is faint and hot, i.e. it is in, or close to, the white dwarf performed by means of Th–Ar lamp exposures. During the domain. Following Tylenda (1986), NGC 6565 could be frame registration, the various orders were scaled to the in a recombination phase, due to the recent luminosity slit height at Hα, chosen as reference line. Echellograms drop of the powering star. of the spectrophotometric standard star HR5501 (sampled at several positions along each order) were used to obtain Although the nebula is sometimes listed among the dis- the flux calibration. tance calibrators (Pottasch 1984; Sabbadin 1986; Gathier We stress the substantial difference between our obser- 1987; Zhang 1995), its distance is still poorly defined vational procedure and the one generally adopted, which and the distance-extinction law gives contradictory results introduces an interference filter to isolate a single order. (Gathier et al. 1986; Maciel et al. 1986; de Oliveira-Abans On the contrary, we cover the whole λλ3900–7750 A˚ spec- & Faundez-Abans 1991). tral range with a single exposure. This paper is structured as follows: Sect. 3 presents For illustrative purposes, in Fig. 3 we present the main the observational material and the reduction procedure, part of the NTT+EMMI frame of NGC 6565 at PA = Sect. 4 is dedicated to the gas kinematics, Sect. 5 contains 150◦, which includes the external, low ionization regions, the Hα/Hβ spectral maps, Sect. 6 analyses the radial dis- and in Fig. 4 the detailed structure (at the same PA) tribution of the physical conditions (electron temperature of λ4686 AofHeII,˚ λ5007 A˚ of [OIII], λ5876 AofHeI,˚ and electron density), in Sect.
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