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The 3-D Ionization Structure of the Planetary Nebula NGC6565

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The 3-D Ionization Structure of the Planetary Nebula NGC6565 1 Abstract. A detailed study of the planetary nebula NGC 6565 has been carried out on long-slit echellograms (λ/∆λ=60000, spectral range=λλ3900–7750A)˚ 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 cen- tral star are derived. The radial ionization structure is analyzed using both the classical method and the photo- ionization code CLOUDY. Moreover, we present the spa- tial 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=10.1 arcsec, 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? ion- ization? magnetic fields?), forming two faint and asym- metric 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 (logT∗ ≃5.08 K; logL∗/L⊙ ≃2.0). The stel- lar 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 stel- lar decline. NGC 6565 is at a distance of 2.0(±0.5) Kpc and can be divided into three radial zones: the “fully ion- ized” 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 −5 −1 equatorial enhanced superwind of 4(±2)×10 M⊙ yr 3 arXiv:astro-ph/0201413v1 24 Jan 2002 lasted for 4(±2)×10 years. Key words: planetary nebulae: individual: NGC 6565– ISM: kinematics and dynamics A&A manuscript no. ASTRONOMY (will be inserted by hand later) AND Your thesaurus codes are: missing; you have not inserted them 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, I-35122 Padova, Italy 1. Introduction Fig. 2. [NII]/[OIII] distribution over NGC 6565 (original The late evolution of low and intermediate mass stars (1.0 frames and orientation as in Fig. 1), showing the large M⊙ <M∗ <8.0 M⊙) is characterized by the planetary stratification effects and the faint, low ionization regions ◦ nebula (PN) metamorphosis: an asymptotic giant branch protruding from the main nebula in PA≃145 . (AGB) star gently pushes out the surface layers in a flori- legium of forms, crosses the HR diagram and reaches the hydrogen depleted photospheric material ejected by white dwarf regime (Aller 1984, Pottasch 1984, Osterbrock the nucleus is gradually modifying the chemical com- 1989). position of the innermost nebular regions (Sabbadin et So far the interpretation of the exuberant morpholo- al. 2000a); gies exhibited by the PNe in terms of detailed three- - NGC 1501: an evolved, high excitation, optically thin dimensional structures and physical conditions of the ion- oblate ellipsoid, denser in the equatorial belt, deformed ized gas was limited by projection effects, leading to ap- by several bumps, embedded in a homogeneous, in- proximate spatial forms and to unrealistic assumptions for wards extended cocoon and ionized by a “ hot” and the main parameters, e.g. electron temperature and elec- luminous WC4 star exhibiting nonradial g-mode pul- tron density constant all over the nebula (Aller 1984, 1990, sations (Sabbadin et al. 2000b; Ragazzoni et al. 2001). 1994). A 3-D reconstruction technique for studying at large In order to deepen the analysis, we have started a and small scales the morphology, physical conditions, ion- survey at high spectral and spatial resolutions with the ization, spatial structure and evolutionary status of PNe ESO NTT. The superb quality of this material allows us has been introduced by Sabbadin et al. (2000a, b) and to study at unprecedented accuracies objects with differ- Ragazzoni et al. (2001), based on echellograms of moder- ent morphology, e.g. NGC 7009 (the ”Saturn” nebula), ate spectral resolution (R∼ 22000 − 25000). The key of the tetra-lobed IC 4634, the butterfly HB 5, Mz 3 and the 3-D methodology is simple: the PN is an expanding NGC 6537, the double-envelope NGC 5882, NGC 6153 plasma. Thus the position, thickness and density of each and NGC 6818. elementary volume can be derived from the radial velocity, To simplify the application of the 3-D method we de- width and flux of the corresponding emission. cided to begin with an ”easy” nebula without FLIERS The procedure has been applied to: (fast, low ionization emitting regions), BRETS (bipolar, rotating, episodic jets), ansae, wings, multiple envelopes - NGC 40: an optically thick, very low excitation barrel- etc., and naively selected NGC 6565. shaped nebula with thin arcs emerging at both ends of the major axis, powered by a luminous and “cold” 2. The nebula WC8 star presenting a large mass-loss rate. The fast, The bright, compact PN NGC 6565 (PNG 003.5-04.6, Send offprint requests to: M. Turatto, [email protected] ′′ ′′ ⋆ Acker et al. 1992) is “a minute oval ring 10 x8 in p.a. Based on observations made with ESO Telescopes at the La about 5◦. Considerably fainter along the major axis, and Silla Observatories, under programme ID 65.I-0524, and on ob- the center is relatively vacant“ (Curtis 1918). Its HST ap- servations made with the NASA/ESA Hubble Space Telescope, pearance is shown in Fig. 1: the [OIII] emission forms a obtained from the data archive at the Space Telescope Insti- tute (observing program GO 7501; P.I. Arsen Hajian). STScI quite homogeneous oval ring, sprinkled by a number of is operated by the association of Universities for Research in dark globules and spikes, mainly in the Northern sector. Astronomy, Inc. under the NASA contract NAS 5-26555. We It is surrounded by a sharp, patchy [NII] layer present- ◦ have applied the photoionization code CLOUDY, developed at ing two faint lobes in PA≃145 . Note also the light N-S the Institute of Astronomy of the Cambridge University. asymmetry of the nebula with respect to the (barely vis- Turatto et al.: The Planetary Nebula NGC 6565 3 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. ible) central star. The large stratification effects and the selected PA, ranging from 0◦ to 150◦ with a constant step external, faint, low ionization regions are better seen in of 30◦. Fig. 2, presenting the [NII]/[OIII] distribution. The reduction method follows conceptually the stan- NGC 6565 appears peculiar in many respects: dard procedure, including bias, flat field, distortion cor- rection and wavelength and flux calibration. - despite the simple elliptical shape (Stanghellini et al. The slit length, much longer than the order separation, 1993; Gorny et al. 1997), it shows a rich low ionization leads to a considerable superposition of the echelle orders. spectrum typical of bipolar PNe (Greig 1972) and is N This, on the one hand does not affect the emission line overabundant (Aller et al. 1988); spectrum of NGC 6565, on the other hand it does the - the infrared and radio excesses suggest the presence of a flat field exposures. Thus, we were forced to apply a zero- large amount of dusty neutral gas causing a detectable order correction using a white-light flat field. Thanks to absorption of the optical radiation (Gathier et al. 1986; the excellent CCD uniformity, the pixel to pixel sensitivity Stasinska et al. 1992); fluctuation left by this procedure is below 5% (verified - in spite of the high surface brightness of the nebula, on the night sky and the comparison spectrum lines). All indicative of an early evolutionary phase, the central things considered, it is a modest price to pay for keeping star is faint and hot, i.e. it is in, or close to, the white the whole spatial information of the PN emissions in the dwarf domain. Following Tylenda (1986), NGC 6565 λλ3900-7750A˚ spectral range. could be in a recombination phase, due to the recent After bias and flat field correction, the orders con- luminosity drop of the powering star. taining one or more emissions of physical interest were extracted and treated as single, long- slit spectra. The Although the nebula is sometimes listed among the distortion correction and the wavelength calibration were distance calibrators (Pottasch 1984; Sabbadin 1986; Gath- performed by means of Th–Ar lamp exposures. During the ier 1987; Zhang 1995), its distance is still poorly defined frame registration, the various orders were scaled to the and the distance-extinction law gives contradictory results slit height at Hα, chosen as reference line. Echellograms (Gathier et al. 1986, Maciel et al. 1986 and de Oliveira- of the spectrophotometric standard star HR5501 (sampled Abans & Faundez-Abans 1991). at several positions along each order) were used to obtain This paper is structured as follows: Sect.
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