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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⊙
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. 3 presents the the flux calibration. observational material and the reduction procedure, Sect. We stress the substantial difference between our obser- 4 is dedicated to the gas kinematics, Sect. 5 contains the vational procedure and the one generally adopted, which Hα/Hβ spectral maps, Sect. 6 analyses the radial distribu- introduces an interference filter to isolate a single order. tion of the physical conditions (electron temperature and On the contrary, we cover the whole λλ3900-7750A˚ spec- electron density), in Sect. 7 we derive the distance, the tral range with a single exposure. nebular mass and the central star parameters, in Sect. 8 For illustrative purposes, in Fig. 3 we present the main the radial ionization structure and the distribution of the part of the NTT+EMMI frame of NGC 6565 at PA=150◦, gas (ionized and neutral) are presented, Sect. 9 concerns which includes the external, low ionization regions, and in the application of the photo-ionization model (CLOUDY), Fig. 4 the detailed structure (at the same PA) of λ4686A˚ Sect. 10 describes the 3-D structure of the nebula in dif- of HeII, λ5007A˚ of [OIII], λ5876A˚ of HeI, λ6300A˚ of ferent ions, a short discussion and the conclusions are pre- [OI], λ6563A˚ of HI, λ6584A˚ of [NII], λ6731A˚ of [SII] and sented in Sect. 11. λ7135A˚ of [ArIII]. They suggest that the faint, low ion- ization regions visible in Fig. 2 are the projection of ex- tended cups protruding from the main nebula. Note also 3. The observational material and reduction the large stratification effects, the [OI] rises in the out- technique ermost parts and, in spite of the blurred Hα appearance Six long-slit echellograms of NGC 6565 (exposure time due to thermal motions plus expansion velocity gradient 900s; spectral resolution 60000 with a slit 1.0 arcsec wide) plus fine structure, the similarity of the HI and [OIII] dis- have been obtained on July 28, 2000 at the ESO NTT tributions. Following Williams (1973), these features are equipped with EMMI in REMD mode using grating #14, indicative of an optically thick nebula powered by a high grism #3 and CCD #36 (spectral range λλ3900-7750A,˚ temperature star. mean spatial scale along the slit 0.27 arcsec pix−1). During A first, qualitative picture of the NGC 6565 struc- the observations the sky transparency was excellent and ture can already be inferred from Figs. 1 to 4: the ob- the seeing fluctuated between 0.53 and 0.89 arcsec. The ject is an ellipsoid with extended polar cups, projected 30 arcsec long slit was centered on the nebula for all the almost pole-on. The possible optical appearance of the 4 Turatto et al.: The Planetary Nebula NGC 6565
Fig. 3. Main part of the NTT+EMMI frame of NGC 6565 at PA=150◦ (in logarithmic scale); the principal nebular emissions are identified and the strongest night sky lines (N.S.) indicated. The spatial scale is given by the N.S. extension, corresponding to the slit length of 30′′.
Fig. 4. Detailed structure (in logarithmic scale) of some representative emissions observed in NGC 6565 at PA=150◦. The orientation is as in Fig. 3. The fluxes are multiplied for the factor given in parenthesis to render each emission comparable with λ5007A˚ of [OIII]. The λ6300.304A˚ night sky line was removed from the [OI] nebular emission. Part of the [OI] λ6363A˚ emission (echelle order 96) appears as a moustache in the lower-left corner of the λ6300A˚ image (echelle order 97). The fast, blue-shifted wisp in the lower-left corner of the [NII] and [SII] frames is real. A hot pixel column grazes the Hα emission. The zero-velocity pixel column (zvpc) and the central star pixel line (cspl), defined in Sect. 4, are shown in the lower–right panel. nebula seen equatorial-on is illustrated in Fig. 5, show- spatial properties of the equatorial matter. The intensity ing the PN NGC 6886 in [OIII] and [NII]. NGC 6565 and peak separations, 2rzvpc, in the different emissions are pre- NGC 6886 are almost twins in many respects: nebular sented in columns 6 to 11 of Table 1. spectrum, physical conditions, ionization structure, evolu- Typical errors for 2Vexp(main) and 2rzvpc are 1.0 km tionary phase, luminosity and temperature of the central s−1 and 0.15 arcsec for the strongest forbidden emissions star. Likely, they derive from similar progenitors. ([OI], [SII], [NII] and [OIII]) to 3.0 km s−1 and 0.3 arcsec for the faintest ones ([NI], [OII], [ClIII] and [ArV]). The 4. The gas kinematics corresponding inaccuracies for the recombination lines are: 2.0 km s−1 and 0.2 arcsec for HI, 1.5 km s−1 and 0.2 arcsec We introduce two important definitions relative to each for HeI and 3.0 km s−1 and 0.3 arcsec for HeII. The error emission line, which will be extensively used in the paper in Vexp(cup) (column 5) is 3.0 km s−1 in all cases. (they are shown in the lower-right panel of Fig. 4): The correlation between cspl and zvpc, evident in Ta- 1 ) the zero-velocity pixel column (zvpc, see Sabbadin et ble 1, is summarized in Fig. 6, showing both Vexp and al. 2000a, 2000b, and Ragazzoni et al. 2001), corre- rzvpc vs. I.P.. For reasons of homogeneity, the peak sepa- sponding to the recession velocity of the whole nebula, rations at different PA are normalized to the [OI] value. collects the photons of the ionized gas expanding per- NGC 6565 follows the Wilson’s law (“the high- pendicularly to the line of sight; excitation particles show smaller separations than the low 2 ) the central star pixel line (cspl), parallel to the disper- excitation particles”, Wilson 1950) with some caveats: sion, represents the nebular material projected at the apparent position of the star, whose motion is purely - the “high ionization” zone (I.P.>50 eV) is poorly de- ˚ radial. fined, containing only two emissions: λ7005A of [ArV] is exceedingly faint and λ4686A˚ of HeII consists of thir- In a certain sense the zvpc and the cspl are comple- teen fine structure components whose spectral distri- mentary: the first is independent on the expansion velocity bution presents a noticeable blue-shifted tail lowering and gives only spatial information, the second measures the measurement reliability (in fact, 2Vexp(HeII) from the velocity field and contains only kinematical informa- λ6560A˚ is 38±3km s−1); tion. By combining the zvpc and the cspl we will derive - the “medium ionization” region (20 eV Fig. 5. HST imaging of the PN NGC6886 in [OIII] and [NII], showing the possible appearance of NGC 6565 seen almost equatorial-on. Table 1. Expansion velocity in the cspl and peak separation in the zvpc for NGC 6565. ′′ Ion λ I.P. 2Vexp(main) Vexp(cup) peak separation( ) (A)˚ (eV) (km/s) (km/s) PA=0◦ PA=30◦ PA=60◦ PA=90◦ PA=120◦ PA=150◦ [NI] 5198 0.0 70.9: – 8.7: 8.4: 8.0: 7.6: 8.0: 8.3: [OI] 6300 0.0 67.7 55 8.7 8.4 8.0 7.6 8.0 8.3 [SII] 6731 10.4 65.0 54 8.5 7.8 7.5 7.3 7.5 7.9 [OII] 7319 13.6 59.4: – 7.8: 7.7: 7.2 7.0 7.4 7.8: HI 6563 13.6 55.8 48: 7.1 6.7 6.2 6.0 6.2 6.5 [NII] 6584 14.5 60.6 54 8.2 7.6 7.4 7.1 7.4 7.8 [SIII] 6312 23.4 57.5 – 7.6 6.9 6.7 6.3 6.5 6.7 [ClIII] 5537 23.8 57.9: – 7.0: 6.9: 6.5: 6.1: 6.3: 6.7: HeI 5876 24.6 58.7 52: 7.5 6.8 6.5 6.2 6.5 6.8 [ArIII] 7135 27.6 57.2 50 7.3 7.0 6.8 6.3 6.5 7.0 [OIII] 5007 35.1 56.7 49 6.9 6.4 6.3 6.0 6.3 6.4 [ArIV] 4740 40.7 32.4 – 3.9 3.6 3.5 3.5 3.6 3.9 [NeIII] 3967 41.0 55.2 – 7.2 6.6 6.5 6.3 6.6 6.6 HeII 4686 54.4 44.2: 42: 4.5 3.5: 3.8: 3.7 4.0 4.1 [ArV] 7005 59.8 17.0: – – 3.4: 3.2: 2.7: 3.0: 2.7: portional to the distance from the central star, in quali- very faint (untilted?) emission having different kinemati- tative agreement with the results of the classical papers cal properties with respect to the main nebula. This “halo” of Wilson (1950) and Weedman (1968), performed on a (barely visible in Figs. 2, 3 and 7) is more evident in [NII] consistent sample of objects. than in Hα and [OIII]. It appears double-peaked in [NII] −1 The exact value of the proportionality constant, (2Vexp=25±3 km s ), with the blue-shifted component A=Vexp/r′′, remains unknown, since at the moment no stronger than the red-shifted one in the southern half of point of the nebula possesses both the expansion velocity the nebula, and vice-versa in the northern half. In both and the distance from the star. Although a rigorous treat- Hα and [OIII], a single, broad emission (FWHM=25±5 −1 ment of the problem is presented in Sect. 7, an intuitive km s ) is visible. solution is evident in Fig. 4: by chance the combination of The existence of a faint, low ionization envelope around spatial scale and spectral dispersion in the NTT+EMMI NGC 6565 supports the Tylenda’s (1983, 1986) suggestion echellograms gives emission line structures of NGC 6565 that the nebula is in a recombination phase. reproducing the tomographic maps in the slice covered by The only kinematical study of NGC 6565 reported in the slit. the literature is by Meatheringham et al. (1988), who de- −1 −1 Let’s come back to the gas kinematics. Further infor- rived 2Vexp=41.8 km s in [OII], 28.8 km s in [OIII] −1 mation is in the position–velocity maps, i.e. in the com- and 16.6 km s in HeII from the peak separation, and −1 plete radial velocity field. The maps, shown in Fig. 7 for 2Vexp[OIII]=54.4 km s from the width at 10% maxi- the six PA, are in the nebula rest frame. The systemic mum intensity (following Dopita et al. 1985, this gives to −1 heliocentric velocity is Vr⊙ = −5.2 ± 0.5 km s . We a good approximation the highest expansion velocity). have selected HeII, [OIII] and [NII] as representative of The disagreement with the results of Table 1 can be high, medium and low ionization regions, respectively, and ascribed to the different spatial and spectral accuracies ′′ Hα, which gives the overall distribution of the ionized gas (Meatheringham et al. 1988 used a slit width of 3.9 and a (Flux(Hα) ∝ Ne2∝N(H+)2, where Ne and N(H+) are the spectral resolution ≃26000), and to the different reduction electron and proton densities, respectively). techniques. To be noticed that the width at 10% maximum intensity in our echellograms gives 2Vexp[OIII]=90(±4) A number of interesting features can be identified from km s−1. Further comments will be contained in subsection these maps: the complex expansion velocity field charac- 10.2. terized by the polar cups, the fast low excitation blue– shifted wisp in PA=0◦ and 150◦, the large stratification effects, the blurred appearance of Hα due to a combina- 5. The Hα/Hβ spectral maps tion of thermal motions, fine-structure and expansion gra- dient, and the blue-shifted tail in HeII (asymmetric distri- In general (see Aller 1984, Pottasch 1984, and Osterbrock bution of the thirteen fine-structure components). In ad- 1989), the PN extinction is obtained by comparing the dition, our spectra point out the presence of an external, observed Balmer decrement (in particular Hα/Hβ) to the 6 Turatto et al.: The Planetary Nebula NGC 6565 Fig. 6. Polar expansion velocity (left) and equatorial peak separation (right) vs. I.P. in NGC 6565, showing the linear relation between Vexp and distance from the central star. The intervals of the peak separations at different PA, normalized to the [OI] values, are reported. The ions deviating from the normal (cfr. text) behavior are marked. Fig. 7. Position-velocity maps for representative ions of NGC 6565 at the six PA observed. The orientation of each PA is indicated in the HeII panel. The contour step is ∆logF=0.30. intrinsic value given by Brocklehurst (1971) and Hummer to 4.4–4.6 just beyond the Hα peaks), then gradually de- & Storey (1987). The estimates of (Hα/Hβ)obs reported creases to 3.7–4.1 (the last statement is uncertain, because in the literature for NGC 6565 span the range 3.79 (de of the low signal–to–noise in the outer regions). Freitas Pacheco et al. 1992) to 4.65 (Aller et al. 1988). So far, Hα/Hβ variations were reported for a few PNe They all represent mean values over the slice of nebula by comparing direct, narrow-band imagery, and ascribed covered by the slit. to the presence of absorbing dust within (or around) the The spatial and spectral accuracies achieved by the objects (for example, in NGC 6302 by Bohigas 1994, in superposition technique used (±0.15 arcsec and ±1.0 km NGC 6445 by Cuesta & Phillips 1999, in NGC 2440 by s−1, respectively) allow us to extend the Hα/Hβ analysis Cuesta & Phillips 2000, and in NGC 6781 by Mavro- to the whole spectral image. Note that the fine structure matakis et al. 2001). of the lines (the spectral distribution and the relative in- The advantage of a “spectral map” is evident: it tensities of the seven components of Hα differ from the shows the de-projected slice of nebula covered by the slit, corresponding quantities of the as many components of whereas the “direct images superposition” refers to the Hβ, Clegg et al. 1999) is uninfluent for an expanding gas PN projection on the sky. at Te≃104 K. In general, two main factors affect the Hα/Hβ ratio: Fig. 8 shows the Hα/Hβ isophotal contours su- a ) the amount of dust along the line of sight. We can perimposed to the Hα spectral emission (recall that write: Flux(Hα)∝N(H+)2) for some representative PA of c(Hβ) = c(Hβ) + c(Hβ) (1) NGC 6565. The large scale appearance of the Balmer ra- tot interstellar local tio is the same in all the six spectra: there is a minimum where c(Hβ)local is null for optically thin, density (3.6–3.8) in the central regions, it increases outwards (up bounded PNe. But NGC 6565 is decidedly optically Turatto et al.: The Planetary Nebula NGC 6565 7 Fig. 8. Isophotal contours of the Hα/Hβ ratio superimposed to the Hα spectral image for some representative PA of NGC 6565. The orientation is as in Fig. 7. The isophotal contours range between 3.6 (the outermost) and 4.6, with a constant step of 0.2. thick to the UV stellar radiation, mainly in the dense, The other diagnostics present in the spectra (like equatorial regions. Moreover, Gathier et al. (1986) and λ4711A/˚ λ4740A˚ of [ArIV], λ5198A/˚ λ5200A˚ of [NI], Stasinska et al. (1992) suggest that the infrared and λ5517A/˚ λ5537A˚ of [ClIII] and the [OII] red quartet radio excesses of the nebula could be due to a large around λ7325A)˚ are too weak for an accurate spatial anal- amount of local absorbing material. Our Hα/Hβ spec- ysis. tral maps indicate that the neutral gas, if present, This procedure, performed at all the six PA of mainly surrounds the equatorial regions; NGC 6565, gives very similar results, as expected for an b ) the Te distribution in the ionized gas: the higher the ellipsoid seen almost pole-on. Thus, only two representa- electron temperature, the smaller the intrinsic Hα/Hβ tive PA are illustrated here: PA=0◦ (along the apparent ratio. For example, we have Hα/Hβ=2.74, 2.85 and major axis) and PA=90◦ (along the apparent minor axis). 3.02 for Te=20000 K, 10000 K and 5000 K, respectively The results are summarized in Fig. 9. (assuming the case B of Baker & Menzel 1938, and log Ne=3.00; Brocklehurst 1971, Aller 1984, Hummer 6.1. Te[OIII] & Storey 1987). A radial variation of Te within NGC 6565 is expected, In order to get the intrinsic line ratio I(λ5007A)/˚ due to the complexity of the ionization structure. I(λ4363A),˚ the observed line intensities must be de- reddened. We have just found that Hα/Hβ systematically Note that a large amount of dust in the equato- changes over NGC 6565 and, at the moment, we cannot rial regions of the nebula should produce spatial and identify the main cause for the variation (Te or absorp- wavelength-asymmetric Hα/Hβ spectral maps, whereas a tion). Thus, we will consider two extreme cases: radial Te variation should not. Unfortunately, our resolu- tions are inadequate (by a factor of almost 2) to test such A) the Hα/Hβ ratio only depends on the amount of ab- a possibility. In the present case, the discrimination be- sorption along the line of sight. In practice this corre- tween a) and b) requires a detailed analysis of the nebular sponds to assume a constant Te=10000 K; physical conditions. B) the absorption is constant over the nebula and the Hα/Hβ variation is due to Te. We discuss the case of 6. The physical conditions c(Hβ)=0.45; the choice of other, reasonable values of c(Hβ) doesn’t modify the conclusions here obtained. The radial profiles of Te and Ne are obtained from the classical diagnostic line ratios (ions in p3 configuration The resulting Te[OIII] profiles, shown in Fig. 9, middle for Ne and in p2 or p4 configurations for Te; Aller 1984, panel, appear very similar in the two cases: the electron Osterbrock 1989) and from the absolute Hα flux distri- temperature is large (14000–15000 K) in the innermost re- bution (Sabbadin et al. 2000a, b; Ragazzoni et al. 2001). gions, it rapidly decreases outwards down to about 10000 A compilation of the relevant references for the transition K, and then it slightly increases (the last statement is probabilities and collision strengths used in this paper to weakened by the faintness of the [OIII] auroral emission). derive Te, Ne and the ionic and chemical composition is Previous Te[OIII] determinations in NGC 6565 are mean given by Hyung & Aller (1996) and Liu et al. (2000). values and range from 9700 K (Acker et al. 1991) to 10300 In all cases we make use of the zvpc, which is indepen- K (Aller et al. 1988; de Freitas Pacheco et al. 1992). dent on the expansion velocity field, and corresponds to The knowledge of the detailed Te[OIII] radial profile the equatorial, densest nebular regions of NGC 6565. The allows us to eliminate the dependence of the Hα/Hβ ratio logical steps of our analysis are the following: on Te, thus obtaining c(Hβ). Our results indicate that: - first we obtain Te[OIII] from λ5007A/˚ λ4363A˚ (the line - c(Hβ)interstellar=0.45(±0.02); 4 −3 ratio is almost independent on Ne for Ne<10 cm ); - in the innermost regions c(Hβ)local ≃0. Once corrected - Ne[SII] is then derived from λ6717A/˚ λ6731A˚ (using for the interstellar component, the ratio Hα/Hβ is low, Te[OIII] to take into account the weak dependence of due to the large Te of the ionized gas. Te decreases the ratio on Te); outwards and, accordingly, Hα/Hβ increases; - Te[NII] comes from λ6584A/˚ λ5755A˚ (adopting Ne[SII] - the Hα/Hβ rise beyond the Hα intensity peak is for its weak dependence on Ne); mainly caused by the presence of neutral absorb- - last, Ne(Hα) is given by the Hα flux distribution in the ing material around the ionized nebula. The max- zvcp (both Te[OIII] and Te[NII] are considered for the imum of c(Hβ)local, 0.12(±0.03), occurs at about Te dependence of the Hα emissivity). 1.2(±0.1)×rH+ (rH+ being the radius of the Hα peak). 8 Turatto et al.: The Planetary Nebula NGC 6565 Fig. 9. Radial physical conditions and other parameters in NGC 6565 at PA=0 ◦ (left) and 90◦ (right). Top panel: the observed Hα/Hβ profile in the zvpc (left ordinate scale; continuous line) and the adopted c(Hβ)tot profile (right ordinate scale; dashed line). Middle panel: Te[OIII] for cases A (i.e. c(Hβ) variable across the nebula; continuous line) and B (c(Hβ)=constant=0.45; long–dashed line) and Te[NII] (dotted line). Bottom panel: Ne[SII] (thick continuous line) and Ne from the Hα flux for some representative values of ǫl×rcspl×D (arcsec Kpc). The three radial zones discussed in the text are indicated in the lower right panel. The adopted, final c(Hβ)tot profile for the two selected moderately the de-reddened line fluxes, on the other hand PA of NGC 6565 is shown in Fig. 9, top panel. It will be it implies that H+ ≫H◦ (in other words: if the electron used in the next Sections to correct the observed intensi- temperature of the almost neutral gas around NGC 6565 ties through the relation: is ≃5000 K, there is no neutral gas!). We will deep the I(λ)corr question in the next Sections. log = fλ c(Hβ) (2) I(λ)obs where fλ is the interstellar extinction coefficient given by Seaton (1979). 6.2. Ne[SII] Notice that the adopted c(Hβ)tot profile is obtained under the assumption Te(H+)≃Te[OIII]≃Te[NII]; this is The intensity ratio λ6717A/˚ λ6731A˚ of [SII] depends questionable in the outermost (knotty) regions, where the strongly on Ne and weakly on Te (Aller 1984; Osterbrock ionization drops and H+ and H0 co-exist. 1989). The accurate knowledge of the c(Hβ) profile is Lowering Te(almost neutral gas), c(Hβ)local decreases unimportant, because the two lines are very close. The and goes to zero for Te(almost neutral gas)≃5000 K. In resulting Ne[SII] distribution for the two selected PA of this case the overall Hα/Hβ spectral distribution of Sect. 5 NGC 6565 is included in Fig. 9, bottom panel (thick con- is due to Te variations. On the one hand this modifies only tinuous line). Our analysis suggests that: Turatto et al.: The Planetary Nebula NGC 6565 9 - the Ne[SII] profile is aligned to the Hα one, and both - jHα is the emission coefficient (case B of Baker & Menzel peak internally to the λ6717A˚ and λ6731A˚ maxima 1938), interpolated from Brocklehurst (1971) values; (which, in their turn, are lightly misaligned). This in- - Vl is the “local volume”, i.e. the nebular volume iden- + dicates that the S emission occurs in a recombining tified by a single pixel of the zvpc; Vl is given region; by l×b×s×(D/206265)3, where l=slit width (1.0′′), - the Ne[SII] distribution shows peaks up to 2800±200 b=pixel spatial scale (0.27′′) and s is the radial depth −3 cm ; of the zvpc, s=rcspl∆V/Vexp, with ∆V=pixel spectral −1 −1 - the [SII] density peak is anticorrelated to rzvpc; resolution (1.79 km s ), Vexp=28 km s (see Table + + - the S emitting zone, being co-spatial to the N one 1) and rcspl is the (still unknown) angular radius of the (see Fig. 1), has a patchy and inhomogeneous struc- cspl (i.e. the Hα nebular size in the radial direction); ture favoring the detection of the densest, i.e. bright- - ǫl, the “local filling factor”, corresponds to the fraction est, parts and indicating that the local filling factor, of the local volume Vl which is actually filled by matter ǫl, in the external, low ionization layers of NGC 6565 with density Ne. is ǫ <1. l We can make the reasonable assumptions that Ne= + Previous Ne[SII] determinations in the nebula (mean 1.15×N(H ) and ǫl=constant along the zvpc, thus obtain- values) date from Aller et al. (1988, Ne=1600 cm−3), ing: −3 1.07 × 109 F (Hα) Acker et al. (1991, Ne=1460 cm ) and de Freitas Pacheco × 1/2 −3 Ne = −0.47 ( ) (4) et al. (1992, Ne=1000 cm ). Moreover, Meatheringham Te ǫl × rcspl × D et al. (1988) obtain Ne=1550 cm−3 from λ3726A/˚ λ3729A˚ Eq. (4), combined with Ne≃Ne[SII], represents an im- of [OII]. portant link between the spatial and the dynamical prop- erties of the ionized gas (i.e. between the zvpc and the cspl). It can be used to derive the distance if independent 6.3. Te[NII] information on the radial radius, rcspl, is available (for ex- The line intensity ratio λ6584A/˚ λ5755A˚ of [NII] depends ample, ǫl=1 and rcspl=rzvpc in a homogeneous PN present- strongly on Te and weakly on Ne (Aller 1984; Osterbrock ing a large degree of symmetry). In the case of NGC 6565 1989). Adopting the Ne[SII] distribution we have derived we have the opposite situation: the distance will be ob- the Te[NII] one shown in Fig. 9 (middle panel, dotted tained in the next Section by means of the extinction- line). Te[OIII] and Te[NII] are only partially superim- distance correlation; thus, Eq. (4) will be used to derive ǫl posed: [OIII] is present all over NGC 6565, whereas [NII] and the radial size of the cspl. is peaked in the outermost, low ionization regions. Their Figure 9, bottom panel, shows the Ne distribution for values are similar; the electron temperature discrepancy, three representative values of ǫl×rcspl×D (1.25, 2.5 and normally observed in PNe, can be ascribed to inaccuracy 5.0 arcsec Kpc). Note the Ne radial asymmetry (the de- in the atomic parameters, small scale ionization fluctua- cline outward the maximum is steeper than the internal tion and, mainly, weakness of the auroral lines (Aller 1990; rise), and the similarity with the [SII] density profile, the Gruenwald & Viegas 1995; Mathis et al. 1998). Previous alignment of the peaks and the close overlap for ǫl×rcspl Te[NII] determinations in NGC 6565 are mean values and ×D=2.5 arcsec Kpc. span in the range 8900–9500 K (Aller et al. 1988; Acker et al. 1991; de Freitas Pacheco et al. 1992). 6.5. Physical conditions: general considerations Our analysis shows that the equatorial regions of 6.4. Ne from the Hα flux NGC 6565 consist of three, distinct radial zones, indicated Following Sabbadin et al. (2000a, b) and Ragazzoni et in the right-bottom panel of Fig. 9: al. (2001), the accurate Ne radial distribution can be ob- - the “fully ionized” nebula is the innermost one (roughly tained from the absolute Hα flux in the zvpc. The de- extending up to the Hα peak). Ne monotonically in- reddened Hα profile must be further corrected for the in- creases outwards, while Te first decreases then remains strumental resolution, thermal motions and fine structure almost constant; (for details, see Sabbadin et al. 2000a). These corrections - the “transition” zone located just beyond the electron were superfluous for the diagnostics just analyzed, i.e. density peak. Here Ne rapidly decreases to a few hun- λ5007A/˚ λ4363A,˚ λ6717A/˚ λ6731A˚ and λ6584A/˚ λ5755A,˚ dreds cm−3 and Te softly increases outwards; because they are forbidden lines and each ratio refers to - the outermost nebula, the “halo”, presenting a gentle the same ion and is emitted in the same nebular region. gradient of Ne; nothing can be said on Te . For a pixel of the Hα zvpc we have: halo 2 + 4πD F (Hα)=4πjHαN(H )NeVlǫl (3) Note that: where: - the overall Ne profile can be obtained from the Hα flux - D is the nebular distance; in the zvpc, whereas Te is limited to the main nebula, 10 Turatto et al.: The Planetary Nebula NGC 6565 due to the weakness of the [OIII] and [NII] auroral Equally improbable are the solutions for 1≫ ǫl; for ǫl=0.1 lines. Much deeper echellograms are necessary (and we have rcspl=12.5 arcsec, that is a nebula very stretched highly advisable) to determine Te up to the faintest along the polar axis; r(equatorial)/r(polar) ≃0.26. regions; A more stringent upper limit to ǫl, ǫl≤0.35, results - the Ne of the halo shown in Fig. 9 (bottom panel) must from the intuitive assumption r(polar)≥r(equatorial), i.e. be considered as an upper limit since we have adopted rcspl ≥rzvpc. Moreover, by extrapolating to the polar re- the same kinematical properties for the main nebula gions the anticorrelation between Ne[SII](peak) and rzvpc and the halo. A better approximation is given by: found for the equatorial zone (cfr. Sect. 6.2), we derive rcspl≃5.0 arcsec (and ǫl≃0.25). This is confirmed by the Nehalo(Fig.9) Nehalo(Fig.9) Nehalo = ≃ (5) morphological analysis of the “NGC 6565-like” PNe pro- [ rhalo × V expmain ]1/2 2 rmain V exphalo jected more or less equatorial-on, contained in the cata- logues of Acker et al. (1992), Schwarz et al. (1992), Man- - Te refers to the ionized gas, but there are growing chado et al. (1996), and Gorny et al. (1999), e.g. NGC 6886 evidences indicating the presence of a considerable (PNG 060.1-07.7; see Fig. 5), M 2-40 (PNG 024.1+ 03.8), amount of almost neutral, patchy gas (at unknown Te) K 3-92 (PNG 130.4+03.1), M 2-53 (PNG 104.4-01.6) and in both the “transition” zone and the “halo”; M 1-7 (PNG 189.8+07.7). Up to now we have obtained the physical conditions in Thus, in the following we will adopt rcspl=5.0±0.5 the equatorial regions of NGC 6565 using the zvpc. The arcsec (corresponding to 0.048±0.005 pc), and ǫl=0.25, + linear relation between expansion velocity and nebular ra- constant across the S emitting region (consistent with dius, found in Sect. 4, suggests that the same analysis can the patchy structure of the low ionization layers). The be performed also in the polar regions. The main problem position-velocity proportionality constant, A=Vexp/rcspl, −1 −1 is represented by the line fluxes, which are considerably results to be 5.6±0.6 km s arcsec ; this agrees with lower in the cspl than in the zvpc. the impression that the NTT+EMMI emission line struc- Only a rough Ne profile can be derived for the (almost) ture of NGC 6565 nicely reproduces (to within 20%) the polar regions, showing density peaks of 1000(±200) cm−3 tomographic map of the nebular slice covered by the slit; in [SII] and 500(±100) cm−3 from the Hα flux (adopting in fact, in Fig. 3, 4, 7 and 8 one arcsec corresponds to 3.11 4 pixels along the dispersion (cspl) and to 3.70 pixels along Te=10 K and ǫl=1; see subsect. 7.2). the slit (zvpc). −1 The kinematical age of the nebula, tkin∝A ≃1750 7. The nebular distance, radius and mass and the yr, represents a lower limit to the actual age, tN6565, central star parameters since the dynamical history of the gas is unknown (Aller 7.1. Distance 1984; Schmidt-Voigt & K¨oppen 1987, and Dopita et al. 1996). An estimate of tN6565 can be obtained by assum- In Sect. 5 we have obtained c(Hβ)interstellar=0.45±0.02, ing a nebular ejection at Vexp(superwind) gas located in the equatorial regions mainly affects the M∗>0.70 M⊙ (Sch¨onberner 1981, 1983, Iben 1984), northern portion of the image. M∗>0.90 M⊙ (Wood & Faulkner 1986, Vassiliadis & All this, added to the different general morphology of Wood 1994), ≃ the nebula in [OIII] and [NII] (quite homogeneous and M∗ 0.65 M⊙ (Bl¨ocker & Sch¨onberner 1990, Bl¨ocker diffuse in [OIII], inhomogeneous and patchy in [NII]; Fig. 1995), 1) indicates the presence of noticeable density fluctuations These discrepancies reflect the different choices by the ≃ at small spatial scale (below the seeing, 0.7 arcsec) in the authors of: outermost, recombining nebular regions, which are rich of knots, globules and condensations. - the semi-empirical AGB mass-loss law, controlling both the evolution along the upper AGB and the transition 7.4. Central star parameters into the PNe region, - the residual, hydrogen rich envelope (and its correlation The accurate value of mV∗ has been obtained from with the thermal-pulse cycle phase), and the fast post- WFPC2 frames of NGC 6565 taken through the broad- AGB mass-loss, determining the horizontal crossing of band filter F555W (λc=5407A,˚ bandwidth=1236 A).˚ The the HR diagram, resulting magnitude is mV∗=18.88(±0.05), where the un- - the treatment of the gravo-thermal energy release and known star color is the main source of inaccuracy. Previous neutrino energy losses, defining the fading to the white mV∗ estimates reported in the literature span the range dwarf regime. 15.9 (Shaw & Kaler 1989) to 20.3 (Gathier & Pottasch 1988). Although a deep analysis of the central star evolution The HI and HeII Zanstra temperatures, TZHI and is beyond the aim of this paper, our observational data TZHeII respectively, are derived in the classical way (Aller suggest the following, qualitative picture: the massive pro- 1984; Pottasch 1984; Osterbrock 1989) by comparing the genitor of NGC 6565 underwent a long AGB phase (fa- de-reddened Hβ and HeII λ4686A˚ nebular fluxes with voring the degeneration of the core), a strong superwind (mV∗)o. The integrated Hβ and λ4686A˚ fluxes were ex- leaving a small, hydrogen rich envelope (causing a rapid trapolated from the overall line profile at each PA, as- horizontal crossing), and a quite fast final fading towards suming a circular symmetry of the nebular image. We the white dwarf domain (highly degenerated core and neu- −2 obtain log F(Hβ)obs = −11.20(±0.03) mW×m and trino losses). I(λ4686A/H˚ β)obs=0.12(±0.02). The corresponding val- According to the critical analysis by Bl¨ocker & ues reported in the literature are in the range -11.27 Sch¨onberner (1990), Tylenda & Stasinska (1994) and (photoelectric photometry; O’Dell 1962) to -11.1 (slit Bl¨ocker (1995), we assume M∗(NGC 6565)≃0.65 M⊙. 12 Turatto et al.: The Planetary Nebula NGC 6565 Fig. 10. Absolute emission line radial profiles (erg cm−3 s−1; logarithmic scale) in the zvpc of NGC 6565 at PA=0◦ and 90◦. The orientation is as in Fig. 9. The nebula has been considered at a distance D=2.0 Kpc. 8. The radial ionization structure O+/H+ abundance is obtained only at, or close to, the line peaks. Also, the unfavorable position of λ3967A,˚ at The detailed knowledge of the plasma diagnostics in the the extreme blue edge of the frame, makes the quantitative (fully ionized + transition) zone of NGC 6565 allows us Ne++/H+ analysis quite uncertain. to solve the equations of statistical equilibrium, thus ob- At this point some caveats are in order: taining the point to point ionic concentrations from the line intensities (Peimbert & Torres-Peimbert 1971, Barker 1) because of the luminosity drop of the fast evolving 1978, Aller 1984, Osterbrock 1989). central star, the ionization and thermal structure of −3 −1 The absolute emission profiles (erg cm s ) in the NGC 6565 are out of equilibrium: with time the Hα ◦ ◦ zvpc at PA=0 and PA=90 are shown in Fig. 10. peak recedes (the ionized nebula zooms out) and the All the main ionic species are well represented in the recombination processes prevail. They are faster in echellograms, with the exception of O+, whose principal the high ionized species; this explains, at least qual- emissions (λ3726A˚ and λ3729A)˚ fall outside our spectral itatively, the reversion of the Zanstra discrepancy re- + + range. Thus, we are forced to derive the O /H abun- ported in Sect. 7.4: TZHI is larger than TZHeII because dance from the λ7319.92A/H˚ α ratio, where λ7319.92A˚ is Hβ maintains a “longer memory” than λ4686A˚ of the the strongest line of the red O+ quartet, whose relative stellar luminosity. intensities are practically constant in the density range Actually, the post-AGB evolution slows down and here considered (De Robertis et al. 1985). Since the ra- the luminosity gradient rapidly decreases while ap- tio λ7319.92A/(˚ λ3726A+˚ λ3729A)˚ is in the range 0.015 to proaching the white dwarf domain (Sch¨onberner 1979, 0.075, mainly depending on Ne (Keenan et al. 1999), the 1981, 1983; Wood & Faulkner 1986; Vassiliadis & Turatto et al.: The Planetary Nebula NGC 6565 13 Fig. 11. The radial ionization structure of NGC 6565 at PA=0◦ (top) and PA=90◦ (bottom). Three solutions are shown for O0/H+, corresponding to Te(neutral)=5000 K, 7500 K and ≃ Te(ionized). The orientation is as in Fig. 9. Wood 1993, 1994; Bl¨ocker 1995). Because of the high we are forced to consider different scenarios. Three gas density we can consider NGC 6565 in quasi- possible solutions are given for O0/H+, correspond- equilibrium: the H+ recombination rate, dH+/dt∝- ing to Te(neutral)=7500 K, Te(neutral)=5000 K and Ne2, is large enough to ensure a short “relax- Te(neutral)≃Te(ionized). ation” of the main nebula (the recombination time, 5 Fig. 11 shows the resulting radial ionization structure trec(yr)=1/αBNe≃1.2×10 /Ne, is a few dozen years; of NGC 6565 at PA=0◦ (top) and PA=90◦ (bottom). Only αB=effective recombination coefficient of hydrogen). 2) No correction for seeing is applied to the observed emis- two elements (i.e. helium and oxygen) present an almost sion line profile; although this causes some inaccuracy complete ionization structure. Note that hydrogen is well for the sharpest radial distributions (like [OI], [NII] represented in the main nebula, but the lack of the neutral and [SII]), it doesn’t modify the general results here term appears problematic in the outermost regions, where H◦ abounds (see Sects. 6.1 and 7.3). obtained. + + ++ + 3) No precise information is yet available for the elec- This is confirmed by the (He /H + He /H ) ra- ± tron temperature of the patchy, almost neutral gas in dial trend: it is approximately constant (0.105 0.005) up the external part of the “transition” zone, where the to the “transition” zone and later it increases outwards. Such a behavior is typical of an optically thick PN ionized recombination and cooling processes dominate. This + ˚ by a high temperature central star (the He region ex- mainly concerns the [OI] λ6300A emission, which is + much more sensitive to the physical conditions than tending further than the H one; Hummer & Seaton 1964; are the [NII] and [OII] lines (Williams 1973). Thus, Alexander & Balick 1997). Previous He/H abundances re- ported in the literature for NGC 6565 (mean values over 14 Turatto et al.: The Planetary Nebula NGC 6565 the whole nebula) span in the range 0.095 to 0.108 (Aller et al. 1988; K¨oppen et al. 1991; de Freitas Pacheco et al. 1992). 8.1. Ionization structure for Te(neutral)≃Te(ionized) This corresponds to the common assumption of using a single temperature (Te[OIII] and/or Te[NII]) all over the nebula, even in the regions where recombination starts to be significant. In this case the value (O0/H+ +O+/H+ +O++/H+) is quite constant across the nebula (but in the innermost regions, where O+3 and even higher ionization stages dom- inate). O/H results 6.0(±1.0)×10−4, to be compared with the values of 5.9×10−4, 8.7×10−4 and 10.9×10−4 reported by Aller et al. (1988), K¨oppen et al. (1991) and de Freitas Pacheco et al. (1992), respectively. Fig. 12. Reconstruction of the (ionized+neutral) radial 0 + The most striking feature of Fig. 11 is the low O /H density profile in NGC 6565 at PA=90◦ East sector. The abundance in the outer regions of NGC 6565. This is orientation is as in Fig. 9. Thin continuous line: the ob- a direct consequence of the “large” value adopted for served Ne≃N(H+) profile (from the Hα flux in the zvpc), Te(neutral), favoring the emissivity of the forbidden line for ǫl=1.0. Thick continuous lines: the N(Htot) distribution 0 + λ6300A˚ with respect to Hα. TheO /H under-abundance in the “transition” zone (from Eq. (8)) for three values of indicates a scarce efficiency of the charge-exchange reac- Te(neutral). Dashed and dotted thin lines: the N(Htot) + 0← 0 + tion O + H →O + H (Williams 1973; Aller 1984; Os- profile in the “transition zone” and in the “halo” for some + 0 terbrock 1989), implying that H ≫H (in contradiction representative values of time elapsed from the start of the with the result just obtained from He). recombination (from Eq. (10)). All this is likely due to the unrealistic assumption Te(neutral)≃Te(ionized), for the following reasons: In the general form, the total abundance (relative to - the low value of O0/H+ in the outer regions of NGC 6565 hydrogen) of an element X with atomic number n is given disagrees with a number of observational evidences by: X Pn Xi mentioned before, indicating the presence of a large = i=0 (6) H 1 j amount of neutral, dusty and patchy gas around the Pj=0 H ionized nebula; For oxygen, neglecting the higher ionization terms, this - H+≫H0 means that the nebula is almost optically can be written as: ◦ thin to the UV flux of the star, i.e. the total O O + O+ + O++ = (7) nebular mass is close to the ionized nebular mass H H◦ + H+ (Mtot≃Mion≃0.03M⊙). Thus, NGC 6565 should be an Assuming O/H=constant across the nebula, we obtain under-massive nebula ejected and excited by a massive for each radial position: ◦ + ++ ◦ + O O O central star, contradicting the theoretical and observa- H + H + + + + + ≃ H H H (8) tional evidences of a direct relation between Mneb and + O H H M∗ (Sch¨onberner 1981; Pottasch 1983; Vassiliadis & which provides the H0 distribution in the “transition” Wood 1994; Boffi & Stanghellini 1994; Bl¨ocker 1995; zone of NGC 6565, since O/H is derived from the “fully Buckley & Schneider 1995; Dopita et al. 1996). ionized” nebula. The impasse caused by the uncertainties on O◦/H+, due to Te(neutral), can be overcame by linking the “tran- 8.2. Ionization structure for Te(neutral) This must be considered as an upper limit, since the thin– Table 2. Total chemical abundances in NGC 6565 thick transition did not occur at the beginning of the stellar drop, but during the decline (the exact moment He/H=0.105(±0.003) depending on the nebular mass; for M ≃0.15 M⊙ the −4 neb O/H=6.0(±1.0)×10 ≃ − thin–thick transition started at log L∗/L⊙ 2.8, i.e. about N/H=1.5(±0.3)×10 4 400 years ago). Ne/H=1.6(±0.4)×10−4 More precise information comes from the observed S/H=6.3(±1.0)×10−6 density profile in the “transition zone” and in the “halo”. Ar/H=1.8(±0.4)×10−6 The Ne depletion rate for recombination is given by: + dNe/dt = −αBNeN(H ) (9) Assuming Ne≃N(H+) and Te=10000 K, and neglecting the recombination delay due to expansion, we obtain: Tylenda’s (1983, 1986) models: only the innermost nebula −6 N(Htot)= Ne(t)/[1 − 8.2 × 10 tNe(t)] (10) is ionized by the fading UV luminosity of the star, whereas which provides N(Htot), the density in the “transition the outermost part, unattainable by the direct radiation, zone” and in the “halo” at the start of the recombina- simply “ remembers” the UV flux received just before the tion phase, once is known the present Ne(t), the electron luminosity drop. In this scenario, the three-zones division density at time t (in years) elapsed from the beginning of scheme proposed in Sect. 6.5 marks two distinct evolution- the recombination process. ary times of the star: the “fully ionized” nebula refers to This is summarized in Fig. 12 for PA=90◦ East sector, the present low luminosity phase, the “halo” to its past i.e. along the apparent minor axis of NGC 6565. It shows: glory. The “transition” zone is a special case; on the whole - the whole Ne≃N(H+) profile in the zvpc (from the it is recombining, but the process has been gradual (the Hα flux, for ǫ =1.0); according to the discussion in l outermost regions are the first to recombine), and decel- Sect. 6.5, we adopt Ne =Ne (Fig. 9) / 2, halo halo erated (the luminosity gradient of the star decreases in - the N(H ) distribution in the “transition” zone for Te tot time). Moreover, following Tylenda (1986) and Marten (neutral)≃Te(ionized), 5000 K and 7500 K (from Eq. & Szczerba (1997) the high ionization species recombine (8)), faster than the low ones, and for a given ion, the shorter - the density profile, N(H ), in the “transition zone” and tot the wavelength of the transition, the faster is the decline in the “halo” for some representative values of t (from of the respective emission. Eq. (10)). The electron temperature behavior in the “transi- In spite of the heavy assumptions (in particular: αB is tion” zone reflects the rate of change in kinetic energy of a function of Te and ǫl<1.0 in the real nebula), Fig. 12 (ions+atoms+electrons) and is the net result of two con- suggests that: trasting processes: the recombinations favor an increase of Te since they tend to remove slow electrons, whereas the - the recombination of the main nebula indicatively losses by forbidden lines lower Te (Aller, 1984). All this, started about 400 years ago; added to the inhomogeneous structure of the outermost - the nebular mass is at least 0.10 M⊙, hence most of the layers, makes the situation quite complex. Following the gas is presently neutral (M =0.03 ± 0.01, Sect. 7.3); ion time dependent ionization calculations by Harrington & - the electron temperature in the external, almost neutral Marionni (1976), Tylenda (1986) and Marten & Szczerba regions is lower than in the “ionized” nebula (5000 (1997), the electron temperature gradually decreases in K Table 3. Input parameters of the model nebula axis) as representative of the whole equatorial regions of NGC 6565 identified by the zvpc. The input parameters of the model nebula are given in Radial density profile N(Htot) in top panel of Fig. 13 Table 3. The adopted N(Htot) profile corresponds to the (cfr. Sect. 8.2) “fully ionized” part of N(H+) at PA=90◦ East sector, for Filling factor 1 Chemical abundances: ǫl=1 (see Fig. 9). It is arbitrarily extended to the “tran- C, Si, Cl Aller et al. (1988) sition” and the “halo” in order to obtain a nebular mass He, N, O, Ne, S, Ar this paper of about 0.30 M⊙. The choice of different N(H) profiles in other elements PN (CLOUDY default) the outermost regions does not modify the resulting ion- Dust PN (CLOUDY default) ization structure, since these layers cannot be reached by Star blackbody with T∗=120000 K and the UV stellar flux. Concerning the ionizing source, we use logL∗/L⊙= 2.0 a blackbody distribution having the Zanstra HeII temper- ature and luminosity of the central star of NGC 6565 (see Sect. 7.4). Once it is convolved for a seeing of 0.65 arcsec, the line emissivity (per unit filling factor) of the model & Wood (1994) and Bl¨ocker (1995) give contrasting re- nebula at 2.0 Kpc can be compared with the emission per sults. unit volume presented in Fig. 10. The results are shown Although a clear answer needs first and second epoch in Fig. 13. HST imagery in the main nebular emissions, our feeling is The steady-state model nebula convolved for seeing that NGC 6565 is still contracting, because of the observed (third panel in Fig. 13) closely reproduces the “fully ion- reversion of the Zanstra discrepancy (or, at least, it seems ized” part of NGC 6565 (bottom panel) up to R≃1.0×1017 to be still contracting, due to the time delay in the nebular cm. An even better fit might be obtained through a mod- adjustement to the changing UV luminosity of the star). est adjustment of the chemical abundances. The discrep- We conclude this Section with the total chemical abun- ancies begin in the “transition” zone and increase outward dances (Table 2). They are derived in the usual way, (“halo”). i.e. multiplying the observed ionic abundances for the The comparison of the model vs. observed nebula of- corresponding ICFs, the correcting factors for the un- fers an encouraging support to the results obtained in the observed ionic stages (obtained both empirically, Barker previous Sections. In particular, Fig. 13 confirms the pe- 1983, 1986, and from interpolation of theoretical nebular culiar properties of [ArIV] at λ4740A,˚ as pointed out in models, Shields et al. 1981, Aller & Czyzak 1983, Aller Sect. 4: the structure of Ar+3 (I.P.=40.7 eV) mimics that 1984, Osterbrock 1989). Following the critical analysis by of higher ionization species since it is present in the in- Alexander & Balick (1997), we have considered the to- terval (40.7–59.8) eV, the upper limit being the ionization tal line intensities (i.e. integrated over the whole spatial potential of Ar+4, i.e. it overlaps the He++ zone. profile and the expansion velocity field). Neon shows the opposite behaviour of Argon: Ne++ In general, the chemical abundances of Table 2 are in is expected to within the (40.9–63.4) eV range, whereas satisfactory agreement with the values reported in the lit- in Fig. 13 the [NeIII] emission at λ3967A˚ extends to the erature (Aller et al. 1988; K¨oppen et al. 1991; de Freitas medium-low ionization region (see also Sect. 4 and Figs. 5 Pacheco et al. 1992), but for the N overabundance sug- and 10). We have verified on a number of model nebulae: gested by Aller et al. (1988) which is not confirmed here. the abnormal extension of Ne++ only occurs in the pres- ence of a hot central star. According to Williams (1973), 9. The photo-ionization model Butler et al. (1980), Shields et al. (1983) and Clegg et al. (1986), a large number of stellar photons with ener- In order to test the results obtained for the physical condi- gies well above the HI ionization threshold maintains a tions and the ionization structure, and to disentangle the high degree of ionization in the nebula right up to the controversial points of the foregoing Sections, we have ap- point where Ho prevails, and the charge-exchange reac- plied the photo-ionization code CLOUDY (Ferland et al. tion Ne+3+Ho →←Ne+++H+ populates the 5P, 3Do and 3P 1998) to a mass of gas having the same density distribu- levels of Ne++. The ground state is then reached via ra- tion and chemical composition of our PN, and a powering diative decay, and the collisionally excited lines at λ3868A˚ source similar to the central star of NGC 6565. and λ3967A˚ are emitted. The usual caveat: CLOUDY is a “steady-state” model. Concerning the electron temperature, the radial be- It works in the “fully ionized” part of NGC 6565, which is havior of the model nebula (Fig. 13, top panel) closely in quasi-equilibrium (see Sect. 8), but it cannot depict the reproduces the observed one (Fig. 9, middle panel) up external, recombining regions originated by the luminosity to the “transition” zone. The external, fast Te drop of drop of the central star. the model nebula clearly refers to a steady-state situa- We consider only a single sector of a single PA tion, which does not take into account the presence of the (East sector of PA=90◦, i.e. along the apparent minor broad recombining region in NGC 6565. Turatto et al.: The Planetary Nebula NGC 6565 17 Fig. 13. Model nebula vs. NGC 6565 at PA=90◦, East sector. Top panel: radial profiles of Te (left ordinate scale) and + ◦ Ne, N(H ), N(H ) and of the input density N(Htot) (right ordinate scale) for the model nebula in the case ǫl=1.0. Second panel: absolute radial flux distribution (erg s−1 cm−3) of the model nebula in the main emissions. Third panel: same as in the second panel, after convolution for a seeing of 0.65 arcsec, reproducing the observational conditions of the true nebula. Bottom panel: absolute flux distribution in the main emissions of NGC 6565 at PA=90◦, East sector (cfr. Fig. 10). Notice the quantitative agreement between E(Hα) of the N(Htot) profile under the assumption ǫl=1, whereas in the model nebula, convolved for seeing, and the observed + 1/2 the real nebula N(H )= ǫl ×Ne[SII]= 0.5×Ne[SII]. one. This is a consequence of the clumpiness found in In Fig. 13 the most striking discrepancy concerns the ≃ Sect. 7.2 (i.e. ǫl 0.25). In fact, the former is obtained from radial O++ profile: in NGC 6565 (bottom panel) it closely reproduces the H+ one, as expected of an optically thick 18 Turatto et al.: The Planetary Nebula NGC 6565 nebula powered by a high temperature central star (Flower showing the nebula from different directions, separated by 1969, Williams 1973), while in both the model nebula and 15◦. Each couple of images constitutes a stereoscopic pair, the convolved model nebula (second and third panel, re- illuding the reader to see NGC 6565 in 3-D. spectively) H+ extends further than O++ and the outer- The opaque reconstruction in Hα is shown in Fig. 14 most ionized regions are O++-depleted. for two absolute flux cuts: log E(Hα)=-19.55 erg s−1 cm−3 We stress that the application of the photo-ionization (upper part of each panel) and -18.50 erg s−1 cm−3 (lower eff + code CLOUDY to the other PA of NGC 6565 gives results part). Since E(Hα)=h ν α Ne N(H ) ǫl, the adopted ◦ Hα coinciding with the ones just presented for PA=90 , East cuts correspond to Ne≃300 cm−3 (upper) and Ne≃1000 −3 4 sector. cm (lower) (for Te=10 K and ǫl=1). We conclude that the nebular modeling is a powerful The complexity of the nebular structure in the faint, instrument for interpreting the observed properties of each low ionization polar cups is more evident in Fig. 15, refer- PN. In the specific case of NGC 6565, the nebula is so ring to λ6584A˚ of [NII] at the two cuts: log E(λ6584A)=˚ peculiar that it cannot be completely reproduced even by a -19.40 erg s−1 cm−3 (upper part of each panel) and -18.40 detailed and sophisticate code like CLOUDY. An evolving erg s−1 cm−3 (lower part). In this case E(λ6584A)˚ ∝Ne model is required, which takes into account the nebular N(N+), thus a precise Ne cut cannot be assigned: Fig. 15 reaction to the rapidly changing UV luminosity of the star. simply represents the “faintest” and the “brightest” low NGC 6565 is not a unique case. The recombination ionization regions, respectively. phase must be quite common in PNe, almost ineluctable NGC 6565 in the high ionization HeII emission at in the presence of a massive, i.e. fast evolving, central star. λ4686A˚ (not shown here for reasons of space) consists of Tylenda (1986) lists a dozen well-studied objects, includ- a compact, almost tubular structure recalling the Hα one ing the famous NGC 7027, NGC 7293, NGC 6853 and (high cut) presented in Fig. 14. Moreover, the spatial form NGC 6720, and Corradi et al. (2000) suggest that a re- in [OIII] at λ5007A˚ closely reproduces the Hα one (Fig. combining halo is present in NGC 2438, but many other 14), as expected of the similarity of the spectral images candidates are contained in the Acker et al. (1992) and and the radial profiles (see Sects. 2, 4 and 8). Finally, the Kohoutek (2000) catalogues on the basis of their optical distribution of the neutral gas in the transition zone of appearance, spectroscopic characteristics and star faint- NGC 6565 (obtained from Eq. (8) using the approximation + ◦ ++ ness. Notice that the combination of large optical exten- Htot/H ≃O /O and assuming Te(neutral gas)=6000 K sion and high surface brightness makes the Ring nebula and Te([OIII])=11000 K) mimics the N+ structure shown the ideal laboratory for analyzing in detail the recombi- in Fig. 15, although peaked a bit further away. nation process in PNe. In short, NGC 6565 is an inhomogeneous triaxial el- lipsoid seen almost pole-on (major axis=10.1 arcsec pro- 10. The spatial structure jected in (5,7), intermediate axis=7.1 arcsec in (−85,7), minor axis=6.0 arcsec in (5,−83)). The matter along and 10.1. Tomography close to the major axis was swept-up by some accelerating The reconstruction of the emission line structure in the agent (high velocity wind? ionization? magnetic fields?), nebular slices covered by the slit was introduced by Sab- forming two fast, asymmetric and faint polar cups pro- badin et al. (2000a, b) and Ragazzoni et al. (2001). In jected in (18,7; i.e. they are lightly misaligned with re- the case of NGC 6565 we have selected λ4686A˚ of HeII, spect to the major axis) and extending up to about 10.0 λ5007A˚ of [OIII] and λ6584A˚ of [NII] as representative of arcsec from the star. The optical nebula is embedded in a the high, mean and low ionization regions, respectively, large cocoon of almost neutral gas, which is the result of and Hα as a marker of the whole ionized gas distribution. the recombining processes occurred during the luminosity The spectral images of the forbidden lines were decon- decline of the powering star. volved for seeing, spectral resolution and thermal motions, The [OIII] and [NII] appearance of NGC 6565 seen while also fine structure was considered for the recombi- from different directions is illustrated in Figs. 16. It gives nation lines. They were de-projected using the expansion a representative sample of the possible morphologies as- law derived in subsect. 7.2, i.e. adopting Vexp/r=5.6 km sumed by our nebula when changing the line of view, and s−1 arcsec −1. The resulting tomographic maps (quite sim- confirms its structural similarity with NGC 6886 (sug- ilar to the spectral images shown in Figs. 4 and 7) were gested in Sect. 3), as well with K 3-92, M 1-7, M 1-59, assembled with the procedure described by Ragazzoni et M 1-66, M 2-40 and M 2-53 (see the imagery catalogues al. (2001), thus obtaining the spatial structure presented by Schwarz et al. 1992, Manchado et al. 1996, and Gorny in the next Section. et al. 1999). Also the observed kinematics depends on the line of view, as shown in Table 4 (upper panel). NGC 6565 can 10.2. 3-D morpho–kinematical structure be either a compact, roundish, fast expanding object or In order to render the 3-D structure, we adopt the method a bipolar, elongated, slow one with all the intermediate introduced by Ragazzoni et al. (2001): a series of images cases. The morpho–kinematical range further spreads out Turatto et al.: The Planetary Nebula NGC 6565 19 −3 Fig. 14. Opaque reconstruction of NGC 6565 in Hα at Ne≃300 cm (for ǫl=1, upper part of each panel) and Ne≃ 1000 cm−3 (bottom part), as seen from 13 directions separated by 15◦. The line of view is given by (θ, ψ), where θ is the zenith angle and ψ the azimuthal angle; the upper-right image is the rebuilt-nebula seen from the Earth, i.e. from (0,0). Each horizontal couple represents a “direct” stereoscopic pair, allowing the reader to have 12 3-D views of the nebula in as many directions, all together covering a straight angle. We repeat here the instructions given by Ragazzoni et al. (2001): to obtain the three-dimensional vision, look at a distant object and slowly insert the figure in the field of view, always maintaining your eyes parallel. Alternatively, you can use the two small dots in the upper part of the figure as follows: approach the page till the two dots merge (they appear out of focus); then recede very slowly, always maintaining the two dots superimposed, till the image appears in focus. Fig. 15. Same as Fig. 14, but for [NII], showing the faintest and the brightest (upper and lower part of each panel, respectively) low ionization regions of NGC 6565. Fig. 16. [OIII] and [NII] appearance of NGC 6565 from 6 directions separated by 30◦, showing the different mor- phologies when changing the line of view. The upper-right panel (θ, ψ=0,0) corresponds to the rebuilt-nebula seen from the Earth (to be compared with Fig. 1). The scale is the same of Figs. 14 and 15. Remember that projection(θ,ψ)=projection(θ±180◦,ψ±180◦). in the case of a “butterfly” PN, like Hb 5, Mz 3 and was at high excitation, a bit larger and much brighter than NGC 6537. at present). In the near future the ionization front will re- Decreasing of the spatial resolution exacerbates the grow, since the dilution factor due to the expansion will problem. For example, by putting NGC 6565 at the dis- prevail on the slower and slower luminosity decline. tance of the Large Magellanic Cloud (i.e. 50 Kpc; Ko- The nebula, at a distance of 2.0 Kpc, can be divided vacs 2000 and references therein) we obtain the results into three radial zones: of Table 4, bottom panel. In this case the echellograms - the “fully ionized”, extending up to 0.029–0.035 pc completely lose the spatial information due to the neb- at the equator (≃0.050 pc at the poles), ula compactness. 2Vexp[OIII](peak separation) cannot be - the “transition”, up to 0.048–0.054 pc (≃0.080 pc at measured any longer (in Table 4 it has been replaced by the poles), 2Vexp[OIII](FWHM)) and the morphological classifica- - the recombining “halo”, detectable up to 0.110 pc. tion is quite uncertain. The ionized mass (≃0.03 M⊙) is only a fraction of the All this evidenziates the difficulties implied in the anal- total mass (Mtot≥0.15 M⊙), which has been ejected by an −5 −1 ysis of the high dispersion spectra and questions the re- equatorial enhanced superwind of 4(±2)×10 M⊙ yr liability of the interpretations, evolutionary correlations lasted for 4(±2)×103 years. and spatio-kinematical models based on echellograms at From all the points of view NGC 6565 results to be inadequate spatial and spectral resolutions. an exciting laboratory which deserves further attention. Amongst the many and important aspects left unresolved by this paper we mention: 11. Discussion and Conclusions - the neutral gas distribution and kinematics. Detailed The long travel dedicated to the study of NGC 6565 is radio and infrared observations are highly advisable; over. We started with the kinematical properties, passed for instance, the spectroscopically resolved structure through the physical conditions and arrived to the dis- in the H2 emission at 2.122 µm, which is a signpost of tance and radius of the nebula and to the temperature and the bipolar structure (Kastner et al. 1996); luminosity of the star. The radial ionization structure was - the filling factor. We obtain ǫl=0.25 in the external, low later explored, using both the “classical” method and the ionization regions by comparing Ne(Hα) and Ne[SII]. photo-ionization code CLOUDY, and the spatial structure The same analysis should be performed in the internal, investigated by assembling the tomographic maps. high ionization parts using [ArIV] (λ4711A/˚ λ4740A)˚ In nuce: NGC 6565 is a young (2000–2500 yr), patchy, and [ClIII] (λ5517A/˚ λ5537A).˚ Deeper echellograms optically thick ellipsoid with extended polar cups, seen are required; almost pole-on. It is in a recombination phase, because of - the faint and fast (shocked?) polar cups, overlooked in the luminosity drop of the massive powering star, which our study mainly focused on the equatorial regions; is reaching the white dwarf domain. The stellar decline - the mechanisms and physical processes forming and started about 1000 years ago, but the nebula remained shaping a PN like NGC 6565. This point appears prob- optically thin for other 600 years before the recombination lematic, mainly because of the huge number of models phase occurred (thus, at Galileo Galilei times NGC 6565 proposed for bipolar PNe. 20 Turatto et al.: The Planetary Nebula NGC 6565 Table 4. Morpho-kinematical characteristics NGC 6565 at 2.0 Kpc Projection 2Vexp[OIII] 2Vexp[NII] 2Vexp[OIII] 2Vexp[NII] morphology apparent size (peak separation) (10% maximum intensity) [km s−1] [km s−1] [km s−1] [km s−1] [arcsec] along the major axis 57(±2) 61(±2) 90(±4) 105(±4) elliptical 10.0×8.5 along the intermediate axis 42(±2) 46(±2) 50(±4) 77(±4) bipolar 20.0×8.5 along the minor axis 34(±2) 39(±2) 40(±4) 62(±4) bipolar 20.0×10.0 NGC 6565 at the distance of the Large Magellanic Cloud Projection 2Vexp[OIII] 2Vexp[NII] 2Vexp[OIII] 2Vexp[NII] morphology apparent size (FWHM) (peak separation) (10% maximum intensity) [km s−1] [km s−1] [km s−1] [km s−1] [arcsec] along the major axis 57(±4) 47(±4) 80(±4) 86(±4) - 0.40×0.34 along the intermediate axis 44(±4) 36(±4) 60(±4) 66(±4) - 0.80×0.34 along the minor axis 36(±4) 31(±4) 48(±4) 55(±4) - 0.80×0.40 They include: single AGB progenitors (+ fast wind + et al. 2001). The first is given by RR=∆V/Vexp, ∆V being magnetic fields + photoionization), a planetary sys- the spectral resolution. RR mainly depends on the intru- tem, a binary system undergoing common envelope mentation, since the expansion velocity range of the PNe evolution, a close companion presenting a dense ac- is quite sharp (indicatively: 20 km s−1 Alexander, J. & Balick, B. 1997, AJ, 114, 713 Iben, I. Jr. 1984, ApJ, 277, 333 Aller, L. H. 1984, Physics of Gaseous Nebulae, Reidel, Dor- Jacoby, G. H. & Kaler, J. B. 1989, AJ, 98, 1662 drecht Kastner, J. H., Weintraub, D. A., Gatley, I., Merrill, K. M. & Aller, L. H. 1990, PASP, 102, 1097 Probst, R.G. 1996, ApJ, 462, 777 Aller, L. H. 1994, ApJ, 432, 427 Keenan, F. P., Aller, L. H., Bell, K. L., et al. 1999, MNRAS, Aller, L. H. & Czyzak, S. J. 1983, ApJS, 51, 211 304, 27 Aller, L. H., Keyes, C. D. & Feibelman, W. 1988, PASP, 100, Kingsburgh, R. L. & Barlow, M. J. 1992, MNRAS, 257, 317 192 Kohoutek, L. 2000, Abhandlungen aus der Hamburger Stern- Baker, J. G. & Menzel, D. H. 1938, ApJ, 88, 52 warte, XII Barker, T. 1978, ApJ, 193, 209 Kohoutek, L. & Martin, W. 1981, A&AS, 44, 325 Barker, T. 1983, ApJ, 267, 630 K¨oppen, J., Acker, A. & Stenholm, B. 1991, A&A, 248, 197 Barker, T. 1986, ApJ, 308, 314 Kovacs, G. 2000, A&A, 363, L1 Bl¨ocker, T. 1995, ApJ, 371, 217 Liu, X. -W., Storey, P. J., Barlow, M. J., et al. 2000, MNRAS, Bl¨ocker, T. & Sch¨onberner, D. 1990, A&A, 240, L11 312, 58 Boffi, F. R. & Stanghellini, L. 1994, A&A, 284, 248 Maciel, W. J., Faundez-Abans, M. & de Oliveira-Abans, M. Bohigas, J. 1994, A&A, 288, 617 1986, RMxAA, 12, 233 Brocklehurst, M. 1971, MNRAS, 153, 471 Manchado, A., Guerrero, M. A., Stanghellini, L. & Serra- Buckley, D. & Schneider, S. E. 1995, ApJ, 46, 279 Ricart, M. 1996, The IAC Morphological Catalog of North- Butler, S. E., Heil, T. G. & Dalgarno, A. 1980, ApJ, 241, 442 ern Planetary Nebulae, IAC Chengalur, J. N., Lewis, B. M., Eder, J. & Terzian, Y. 1993, Marten, H. & Szczerba, R. 1997, A&A, 325, 1132 ApJS, 89, 189 Martin, W. 1981, A&A, 98, 328 Clegg, R. E. S., Harrington, J. P. & Storey, P. J. 1986, MNRAS, Mathis, J. S., Torres-Peimbert, S. & Peimbert, M. 1998, ApJ, 221, 61P 495, 328 Clegg, R. E. S., Miller, S., Storey, P. J. & Kisielius, R. 1999, Mavromatakis, F., Papamastorakis, J. & Paleologou, E. V. A&AS, 135, 359 2001, A&A, 374, 280 Corradi, R. L. M., Goncalves, D. R., Villaver, E., Mampaso, Meatheringham, S. J., Wood, P. R. & Faulkner, D. J. 1988, A. & Perinotto, M. 2000, ApJ, 542, 861 ApJ, 334, 862 Cuesta, J. P. & Phillips, J. P. 1999, AJ, 117, 974 O’Dell, C. R. 1962, ApJ, 135, 371 Cuesta, J. P. & Phillips, J. P. 2000, ApJ, 543, 754 Osterbrock, D. E. 1989, Astrophysics of Gaseous Nebulae and Curtis, H. D. 1918, Publ. Lick Obs. 13, 55 Active Galactic Nuclei, Mill Valley, CA Univ. Sci. David, P., Le Squeren, A. M. & Sivagnanam, P. 1993, A&A, Peimbert, M. & Torres-Peimbert, S. 1971, ApJ, 168, 413 277, 474 Pottasch, S. R. 1981, A&A, 94, L13 de Freitas Pacheco, J. A., Maciel, W. J. & Costa, R. D. D. Pottasch, S. R. 1983, in IAU Symp. N. 103, Planetary Nebulae, 1992, A&A, 261, 579 ed. D. R. Flower, p. 391 de Oliveira-Abans, M. & Faundez-Abans, M. 1991, RMxAA, Pottasch, S. R. 1984, Planetary Nebulae, a Study of Late 22, 3 Stages of Stellar Evolution, Reidel, Dordrecht De Robertis, M. M., Osterbrock, D. E. & McKee, C. F. 1985, Ragazzoni, R., Cappellaro, E., Benetti, S., Turatto, M. & Sab- ApJ, 293, 459 badin, F. 2001, A&A, 369, 1088 Dopita, M. A., Lawrence, C. J., Ford, H. C. & Webster, B. L. Reay, N. K., Pottasch, S. R., Atherton, P. D. & Taylor, K. 1985, ApJ, 296, 390 1984, A&A, 137, 113 Dopita, M. A., Vassiliadis, E., Meatheringham, S. J., et al. Reay, N. K. & Worswick, S. P. 1977, MNRAS, 179, 317 1996, ApJ, 460, 320 Sabbadin, F. 1986, A&A, 160, 31 Ferland, G. J., Korista, K. T., Verner, D. A., et al. 1998, PASP, Sabbadin, F., Benetti, S., Cappellaro, E. & Turatto, M. 2000b, 110, 761 A&A, 361, 1112 Flower, D. R. 1969, MNRAS, 146, 171 Sabbadin, F., Cappellaro, E., Benetti, S., Turatto, M. & Zanin, Frank, A. 1999, NewA Rev., 43, 31 C. 2000a, A&A, 355, 688 Gathier, R. 1987, A&AS, 71, 245 Sahai, R. & Trauger, J. T. 1998, AJ, 116, 1357 Gathier, R. & Pottasch, S. R. 1988, A&A, 197, 266 Schmidt-Voigt, M. & K¨oppen, J. 1987, A&A, 174, 211 Gathier, R. & Pottasch, S. R. 1989, A&A, 209, 369 Sch¨onberner, D. 1979, A&A, 79, 108 Gathier, R., Pottasch, S. R. & Pel, J. W. 1986, A&A, 157, 171 Sch¨onberner, D. 1981, A&A, 103, 119 Gorny, S. K., Schwarz, H. E., Corradi, R. L. M. & Van Winckel, Sch¨onberner, D. 1983, ApJ, 272, 708 H. 1999, A&AS, 136, 145 Schwarz, H. E., Corradi, R. L. M. & Melnick, J. 1992, A&AS, Gorny, S. K., Stasinska, G. & Tylenda, R. 1997, A&A, 318, 96, 23 256 Seaton, M. J. 1979, MNRAS, 187, 73P Greig, W. E. 1972, A&A, 18, 70 Shaw, R. A. & Kaler, J. B. 1989, ApJS, 69, 495 Gruenwald, R. & Viegas, S. M. 1995, A&A, 303, 535 Shields, G. A., Aller, L. H., Keyes, C. D. & Czyzak, S. J. 1981, Habing, H. J. 1996, A&ARv, 7, 97 ApJ, 248, 569 Harrington, J. P. & Marionni, P. A. 1976, ApJ, 206, 458 Shields, G. A., Dalgarno, A. & Sternberg, A. 1983, PhRva, 28, Hummer, D. G. & Seaton, M. J. 1964, MNRAS, 127, 217 2137 Hummer, D. G. & Storey, P. J. 1987, MNRAS, 224, 801 Sjouwerman, L. O., van Langevelde, H. J., Winnberg, A. & Hyung, S. & Aller, L. H. 1996, MNRAS, 278, 551 Habing, H. J. 1998, A&AS, 128, 35 22 Turatto et al.: The Planetary Nebula NGC 6565 Soker, L. & Rappaport, S. 2001, ApJ, 557, 256 Spitzer, L. Jr. 1978, Physical Processes in the Interstellar Medium, John Wiley & Sons, New York Stanghellini, L., Corradi, R. L. M. & Schwarz, H. E. 1993, A&A, 276, 463 Stasinska, G., Tylenda, R., Acker, A. & Stenholm, B. 1992, A&A, 266, 486 Tylenda, R. 1983, A&A, 126, 299 Tylenda, R. 1986, A&A, 156, 217 Tylenda, R. & Stasinska, G. 1994, A&A 288, 897 Vassiliadis, E. & Wood, P. R. 1993, ApJ, 413, 641 Vassiliadis, E. & Wood, P. R. 1994, ApJS, 92, 125 Weedman, D. W. 1968, ApJ, 153, 49 Williams, R. E. 1973, MNRAS, 164, 111 Wilson, O. C. 1950, ApJ, 111, 279 Wood, P. R. & Faulkner, D. J. 1986, ApJ, 307, 659 Zhang, C. Y. 1995, ApJS, 98, 659 This figure "h3325f1.jpg" is available in "jpg" format from: http://arxiv.org/ps/astro-ph/0201413v1 This figure "h3325f2.jpg" is available in "jpg" format from: http://arxiv.org/ps/astro-ph/0201413v1 This figure "h3325f3.jpg" is available in "jpg" format from: http://arxiv.org/ps/astro-ph/0201413v1 This figure "h3325f4.jpg" is available in "jpg" format from: http://arxiv.org/ps/astro-ph/0201413v1 This figure "h3325f5.jpg" is available in "jpg" format from: http://arxiv.org/ps/astro-ph/0201413v1 This figure "h3325f7.jpg" is available in "jpg" format from: http://arxiv.org/ps/astro-ph/0201413v1 This figure "h3325f8.jpg" is available in "jpg" format from: http://arxiv.org/ps/astro-ph/0201413v1 This figure "h3325f14.jpg" is available in "jpg" format from: http://arxiv.org/ps/astro-ph/0201413v1 This figure "h3325f15.jpg" is available in "jpg" format from: http://arxiv.org/ps/astro-ph/0201413v1 This figure "h3325f16.jpg" is available in "jpg" format from: http://arxiv.org/ps/astro-ph/0201413v1