KNOTS IN NEARBY PLANETARY NEBULAE 1,2 C. R. O Dell. B. Balick. A. R. Hajian. W. J. Henney. and A. Burkert

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1 The Astronomical Journal, 123: , 2002 June # The American Astronomical Society. All rights reserved. Printed in U.S.A. KNOTS IN NEARBY PLANETARY NEBULAE 1,2 C. R. O Dell Department of Physics and Astronomy, Vanderbilt University, Box 1807-B, Nashville, TN 37235; cr.odell@vanderbilt.edu B. Balick Department of Astronomy, University of Washington, Box , Seattle, WA A. R. Hajian US Naval Observatory, 3450 Massachusetts Avenue, NW, Washington, DC W. J. Henney Instituto de Astronomía, Universidad Nacional Autónoma, Apdo. Postal 3-72 (Xangari), Morelia, Michoacan, Mexico and A. Burkert Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117, Heidelberg, Germany Received 2002 February 18; accepted 2002 March 15 ABSTRACT HST emission-line images of five of the arguably closest planetary nebulae have shown that there is a progression of characteristics of their knots. This progression begins with dark tangential structures showing no alignment with the central star and location near the main ionization front. At the end of the progression in the largest nebulae, the knots are located throughout much of the ionized zone, where they are photoionized on the side facing the central star and accompanied by long tails well aligned radially. This modification of characteristics is what would be expected if the knots were formed near or outside the main ionization front, obtaining densities high enough to lead to their being only partially ionized as they are fully illuminated by the Lyman continuum (Lyc) radiation field. Their expansion velocities must be lower than that of the main body of the nebular shell. Their forms are altered by exposure to the radiation field from the star, although it is not clear as to the relative role of radiation pressure acting on the dust component vis-à-vis ionization shadowing. The one object that does not fit into this sequence is NGC 2392, which is the most complex nebula in our sample. In this case the inner part of the nebula is composed of a series of loops of material, some being ionization bounded, which cover only a small fraction of the area illuminated by the star. This complex structure may be what gives rise to the large variations in electron temperature inferred from low spatial resolution observations. Cometary-form knots are seen in the outer part of this object, with these objects closely resembling those found in the largest nebula in our sample, NGC Key words: planetary nebulae: individual (NGC 2392, NGC 6720, NGC 6853, NGC 7293, IC 4406) 1. INTRODUCTION The planetary nebulae (PNe) continue to produce surprises as we have entered the phase of subarcsecond images (e.g., Sahai & Trauger 1998; Hajian & Terzian 1996). Some of the newly discovered detailed features must represent interesting but second-order effects, while others, such as those discussed in this paper, represent processes fundamental to understanding the structure and evolution of this class of objects. Since the PNe are an important source of processed stellar material of the most common metals in the interstellar medium, it is important that we understand the basic processes occurring in this class of objects. Aspects of the PNe can have broad repercussions in areas such as the evolution of abundances in galaxies and star formation. 1 Based in part on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS Based in part on observations obtained at the Cerro Tololo Inter- American Observatory and the Kitt Peak National Observatory, which are operated by AURA, under a cooperative agreement with the National Science Foundation The basic forms of the PNe and how they may be generated have been recently reviewed by Balick & Frank (2002) and will not be reprised here. Suffice it to say that the forms are a result of multiple physical processes that peak up with the expulsion of material as the precursor star reaches the top of the asymptotic giant branch and continues with shaping through interactions with the parent star s stellar wind and radiation field. In this paper we address a single important problem, the nature of the knots found in the PNe, concentrating on the characteristics of the nearest objects and using the imaging ability of the Hubble Space Telescope (HST). In the previously studied exemplar of these objects, NGC 7293, it was shown (O Dell & Handron 1996) that these knots contain a large fraction of the entire ejecta from the star, trapping that material into shielded molecular zones, thus raising the possibility that this material undergoes a very different processing as it returns to the interstellar medium. In x 2 we present a brief summary of the known characteristics of the objects studied. Section 3 describes the new imaging and spectroscopic observations. Section 4 gives the results from this investigation, and x 5 discusses these results and their implications.

2 3330 O DELL ET AL. Vol PROPERTIES OF THE NEBULAE IN OUR SAMPLE It is important to understand the basic properties and models for the five planetary nebulae investigated in this program. In some cases there is a substantial body of data that has led to accurate three-dimensional models for the nebula, while in other cases there have been surprisingly few useful observations. All of the nebulae possess an approximate axial symmetry. Four of them (NGC 6720, 6853, 7293, and IC 4406) appear to be prolate spheroids with strong concentrations of material in their equators, such structures being natural products of a bipolar model. Their long axes correspond to the plumes seen in orthogonally oriented bipolar nebulae. NGC 6720, 6853, 7293, and IC 4406 are almost certainly ionization bounded in their equators, but not all are ionization bounded along their long axes. NGC 2392 is the most complex object in our sample, since it possesses significant complexity in both its inner and outer structures NGC 2392 NGC 2392 (the Eskimo Nebula) is composed of a bright inner disk of about full width. This inner disk shows up in a wide variety of ionization states, being obvious in everything from He ii through [S ii]. Surrounding this inner ring is a lower surface brightness outer disk of a nearly circular diameter of radius. The amorphous portion of the outer disk is clear in high ionization state emission, but there are numerous low-ionization discrete features within it. The object is complex kinematically but well studied. This has been done most thoroughly in [O iii] 5007 emission at a variety of position angles (P.A. s) (O Dell, Weiner, & Chu 1990, henceforth OWC) and in [N ii] 6583 emission in slits stepping across the full nebula (Balick, Preston, & Icke 1987). The inner disk produces a pattern of emission spots distributed as if they lie on an incomplete spheroid (the tips are vacant) of ratio of diameters of 2 : 1 pointed about 20 to the line of sight of the observer, with the near side pointed toward P.A. = 202. There is a significant density enhancement toward the equator of the spheroid. The maximum expansion velocity is 60 km s 1. These basic numbers and their interpretation were first presented by Weedman (1968) using a smaller data set. The kinematics of the amorphous outer disk in [O iii] indicates that it is an oblate spheroid oriented as if it is on the equator of the inner prolate spheroid and has a maximum expansion velocity of 16 km s 1.Aswe will see in x 4.1, there are numerous other features that are both well imaged and well mapped kinematically that are less certain as to where they fit into the three-dimensional picture. The distance derived from the kinematic model and the observed expansion motions (6 mas yr 1 ) in the plane of the sky (Liller & Liller 1968) is 880 pc, which means the inner disk has a minor radius of pc and a kinematic age (the present size divided by the present expansion rate) of 1055 yr. We note, however, that Hajian, Terzian, & Bignell (1995) obtain an upper limit (5 mas yr 1 ) to the tangential velocity that is less than the value of Liller & Liller (1968), and, if this is correct, then the distance, size, and age are all larger than adopted here NGC 6720 NGC 6720 (M57, the Ring Nebula) presents the appearance of an elliptical figure with a well-defined boundary along the minor axis, where it has its highest surface brightness. If one does not include images in the He ii 4686 line, the object looks like a ring, but He ii images have shown for many years that there is material in the middle of the nebula and that the ring appearance is due to looking only at the lower states of ionization (although there may be a density diminution toward the central star). Masson (1990) first derived the current model of this nebula, arguing from monochromatic images that it is a prolate spheroid with an equatorial density enhancement, a model confirmed and refined by Guerrero, Manchado, & Chu (1997, henceforth GMC97), who argue that the major axis is inclined 30 from the observer s line of sight, and the near tip of the figure is pointed toward P.A. = 60. H and radio continuum images show clearly that the density rises steadily from the central parts to the boundary of the nebula. The object is ionization bounded along the equatorial plane, as indicated by the progression and scale of the various ionization boundaries, the semiminor axis (in [S ii]) being 31>4. Although the GMC97 images reach fainter, a more complete set of images in terms of spectral coverage is supplied by Lame & Pogge (1994). GMC97 argue that the nebula is optically thin to Lyman continuum radiation along its major axis, providing the source of ionization for the outer halos that are observed, the innermost being a series of arcs and loops, the outermost of diameter about being more nearly smooth and circular. This ionizing source can be either or both Ly continuum photons arriving directly in the cones along the major axis or scattered photons. The best angular resolution ground-based images are those made with the Subaru Telescope (Komiyama et al. 2000), although their selection of filters only contained one dedicated emissionline filter (H). Harris et al. (1997) have determined the parallax of the central star to be mas, which corresponds to a distance of 700 þ pc. A semiminor angular extent of 3000 would then correspond to :03 þ0:07 pc. From their analysis of the observed velocity ellipse in [O iii], GMC97 derive an expansion velocity of 45 km s 1. They do not state whether this is the equatorial expansion velocity or the maximum (semimajor axis) velocity. However, since the line splitting of their spectra in the direction of the central star is 80 km s 1 and that value must be greater than twice the equatorial expansion velocity, then 45 km s 1 must be the maximum value, which would mean that the equatorial expansion velocity is about 30 km s 1. If this is the case, then the kinematic age of the nebula would be 3400 þ yr. In x we find from HST images an age of yr. This latter, shorter age is to be preferred as it is derived directly from observations and is free of the modeling assumptions necessary when working from the radial velocities NGC 6853 NGC 6853 (M27, the Dumbbell Nebula) presents a thick dumbbell form lying along P.A. 201 with a half width of , showing the ionization stratification of an ionization bounded nebula with a high-ionization central core (Barker 1984). Observations made as part of an imaging photometric study of NGC 7293 (O Dell 1998a, henceforth CRO98b) show the half width in He ii emission to be about Perpendicular to the long axis of the dumbbell form is a skewed, bright-rimmed elliptical form possessing several internal structures, which show up in the full range of ion-

3 No. 6, 2002 KNOTS IN NEARBY PLANETARY NEBULAE 3331 ization states. This geometry suggests a prolate spheroid with a broad equatorial concentration of material that is viewed nearly in the plane of the equator. Like NGC 6720 and NGC 7293, the He ii emission indicates that the equator of the spheroid is effectively a disk. A striking feature is a wedge of low ionization with a center line pointed toward P.A. = 244 and aligned toward the central star. The divergence of this wedge is such that it appears to arise about half way out from the star, thus excluding this as being a radiation-shadowed region. Like NGC 6720 (GMC97) and NGC 7293 (Walsh & Meaburn 1987; CRO98b), there are a series of faint outer features (Moreno-Corral, López-Molina, & Vázquez 1992; Papamastorakis, Xilouris, & Paleologou 1993) forming a halo around the bright inner region forming the dumbbell feature. Harris et al. (1997) derive a parallax of mas, corresponding to a distance of 380 þ70 50 pc. This means that the semiminor axis corresponds to a size of :04 þ0:06 pc. Bohuski, Smith, & Weedman (1970) find an expansion velocity of 31 km s 1, which would mean that the kinematic age of the nebula is 10,000 þ yr. Moreno-Corral et al. (1992) determined that the rate of expansion in the plane of the sky was no more than 2>3 century 1, which would give an upper limit to the age of 14,600 yr. The surface brightness of this object is about half that of NGC 6720 and 5 times that of NGC 7293, which argues that it is actually smaller than indicated by the parallax, so that in our discussion we will adopt the 1 value of the radius of 0.27 pc and an age of 12,700 yr NGC 7293 NGC 7293 (the Helix Nebula) is remarkably similar to NGC 6720 in appearance, showing similar ratios of axes, bright lobes along the minor axis, long arcs within the bright ring, and a central core of He ii emission (CRO98b). The most striking difference is in scale, with the semiminor axis in H being (CRO98b). Again, the object shows welldefined ionization structure along its minor axis, indicating that it is ionization bounded in that direction. Remarkably similar models for NGC 7293 have been derived by Meaburn et al. (1998, henceforth M98) and in CRO98b, using both the emission-line images in multiple ions and kinematic data. This model is one of a prolate spheroid with a strong density concentration toward the equatorial plane, which is a filled disk. The major axis of the spheroid is inclined about from the line of sight and the near side pointed toward P.A. = The region perpendicular to the equatorial plane is likely to be optically thin to Lyman continuum radiation, which allows the photoionization of material in the outer shell (Walsh & Meaburn 1987; CRO98b). This independently derived model is very similar to that for both NGC 6720 and NGC 6853, with NGC 6853 and NGC 7293 being similar in size and age, while NGC 6720 is significantly smaller and younger. A quite different model is proposed by Speck et al. (2002). They argue that the nebula is nearly spherical in form and is density (rather than ionization) bounded in all directions, using as their primary argument the presence of a low surface brightness halo beyond a distance of The reality of that halo is very sensitive to the far wing characteristics of the point-spread function of the low-resolution (FWHM = 1<3) imaging system employed. A fully densitybounded model would require the density in the material concentrated near the equator to begin to drop dramatically just at the point where the ionization front would have been formed, i.e., that, as one enters the H + +He 0 zone, the density begins to drop sharply. CRO98b includes the results of calculations by G. Ferland with the CLOUDY program that show the semiminor axis agrees very closely with the size expected for photoionization of the nebula. This means that to argue for the equator being density bounded requires the density to begin to drop very exactly where the ionizing photons are being depleted, an unlikely occurrence. In addition, the photometry of the surface brightness of the heads of the cometary knots of the nebula (O Dell & Handron 1996), when compared with the predictions based on the total flux of the nebula in H, indicates that the nebula is primarily radiation bounded; i.e., it is optically thick to Lyc radiation in most directions. In that case the observed surface brightness was less than expected, indicating that the photoevaporation flows from the knot surfaces are not in the recombination-dominated regime (López-Martín et al. 2001; Henney 2001). Harris et al. (1997) derive a parallax of mas, corresponding to a distance of 207 þ46 24 pc. This means that the semimajor axis of corresponds to a size of 0.33 þ0:07 0:04 pc, and if the equatorial expansion velocity is 31 km s 1 (M98) the dynamical age of the nebula is 10,600 þ yr. This indicates that NGC 6853 and NGC 7293 are similar in size, age, and physical characteristics, with the significant difference in appearance being due to the relative proximity and more nearly equatorial viewing angle of NGC IC 4406 IC 4406 is unlike the other nebulae in our program since it does not have an accepted common name that communicates well a major feature of the appearance of the object. However, R. Coker (University of Leeds) points out that the Retina Nebula is appropriate in light of the numerous dark narrow features that are obvious on the HST images presented here. The object is one of the prototype planetary nebulae designated as bipolar because of their appearance. However, the appearance depends closely upon the spatial orientation of the object, with the prolate spheroids used to describe three (NGC 6720, 6853, 7293) of the four nebulae listed earlier in this study being similar to a bipolar object because they are all argued to have density concentrations near the equatorial plane. Those objects are like IC 4406 in having evidence for being ionization bounded in the equatorial plane and are probably optically thin (density bounded) along their long axis. IC 4406 has been imaged well in multiple investigations, although the contour plots shown in the Sahai et al. (1991, henceforth S91) study are probably the most useful for comparison with the HST images produced here. Confirmation of the object being ionization bounded in the equator is provided by the presence of H 2 lobes (Story 1984) outside of the minor axis of the ionized portion. The axis along the two plumes is pointed toward P.A. = 93, and the velocity gradient is observed in CO (S91) and [N ii] (Corradi & Schwarz 1993), indicating that the closer end is in that direction. S91 also determine that the long axis is inclined at an angle of 78=5 3 with respect to the line of sight, which makes it similar to NGC The half width to the ionization front (as measured on our [S ii]) image is

4 3332 O DELL ET AL. Vol. 123 The central star is faint and seen against the high surface brightness nebula, so no trigonometric parallax is available. Various indirect methods have determined values around 1.6 kpc for the distance, which is adopted here (1.5 kpc, Acker 1978; 1.65 kpc, Maciel & Pottasch 1980; 1.8 kpc, Cudworth 1974). This means that the distance from the central star to the equatorial ionization front is 0.11 pc. S91 indicate that the CO emission in the surrounding molecular neutral zone shows an expansion velocity of 23 km s 1, while the [N ii] study of Corradi & Schwarz (1993) had insufficient spectral resolution to resolve that line. Adopting the CO expansion rate of 11.5 km s 1 gives a dynamic age of 9000 yr, within the range of the other nebulae in our sample. However, we will see in x that the expansion velocity of the optical portion of the nebula is only about 4 km s 1, which would increase the dynamic age to 27,000 yr, making it the oldest nebula in our sample if the distance is 1.6 kpc. We will see in x that there are several arguments indicating that the true distance is much less. 3. OBSERVATIONS AND DATA REDUCTION The observational material in this study is composed of imaging data obtained with the WFPC2 (Holtzman et al. 1995) camera of the HST and spectra obtained with several different ground-based telescopes. The HST images were usually examined at the scale of 0>0996 pixel 1, characteristic of CCD detectors 2 4, so that the resolution is primarily determined by undersampling of the HST image. A few images are examined with CCD-1 s scale of 0>0455 pixel 1, which samples the image at about two pixels across the full width at half maximum (FWHM) intensity. The spectra are all of resolutions smaller than the thermal widths of hydrogen emission and can provide information about the internal kinematics of the nebulae Imaging Observations We made new imaging observations with HST for the nebulae NGC 2392, 6720, 6853, 7293, and IC These were combined with previous HST observations of the same objects. In some cases the two epochs of observations were used to determine changes in the position of features within the nebulae and in others to increase the signal-to-noise ratio (S/N) or to extend the range of emission lines imaged. Multiple observations were made in order to allow correction for cosmic-ray events on the images. The goal of the imaging observations was to record each nebula in the widest relevant range of ionization states, using existing spectra as a guide. This meant that we worked with the narrowband filters F673N ([S ii]), F658N ([N ii]), F502N ([O iii], F469N (He ii), and the H and H isolating filters F656N and F487N. In the simplest form of photoionization theory (CRO98b) these filters trace the primary ionization front ([S ii]), the H + +He 0 ([N ii]), the H + +He + ([O iii), and the H + +He +2 (He ii) zones respectively, while the H and H emission arises from all of regions of ionized material. Observations were made in the medium-bandwidth filter F547M for determination of the nebular continuum for purposes of calibrating the emission-line images into absolute energy surface brightness units. The new (program GO 8726) and archival observations used in this program are listed in Table 1. All of these observations were cosmic-ray cleaned and combined using tasks developed for use with IRAF 3 and whenever relevant were converted into absolute energy surface brightness units using the method and calibration coefficients of O Dell & Doi (1999). 3 IRAF is distributed by the National Optical Astronomy Observatory, which is operated by AURA under cooperative agreement with the NSF. TABLE 1 HST WFPC2 and NICMOS Observations of Program Objects a Observations NGC 2392 NGC 6720 NGC 6853 NGC 7293 IC 4406 New WFPC2 Observations: Program... GO-8726 GO-8726 GO-8726 GO-8726 GO-8726 GO-9314 Date Jan Sep Nov Aug Jun Jan 19 F469N F487N F502N F547M F656N F658N F673N Archival WFPC2 Observations: Program... GO-8499 GO-7632 GO-6280 GO-5086 GO-7286 GO-6119 Date Jan Oct Nov Aug Oct Jul 30 F469N F502N F555W b F656N F658N Archival NICMOS Observations: c F212N F215N a Total exposure times in seconds for each filter. b Not used in detailed analysis. c Images out of focus.

5 No. 6, 2002 KNOTS IN NEARBY PLANETARY NEBULAE Spectral Observations High velocity resolution observations were made with the two nearly identical echelle spectrographs mounted on the 4 m telescopes at Kitt Peak National Observatory (KPNO) and Cerro Tololo Inter-American Observatory (CTIO). In both cases the instrument was used without cross dispersion. A single spectral order containing both H at 6563 Å and the adjacent [N ii] lines at 6548 and 6583 Å were isolated with a narrow bandpass interference filter. The instrumental FWHM of the spectra was about 8 km s 1, as determined from the helium+argon comparison lamp. Spectra of NGC 7293 were obtained from both KPNO and CTIO using slits projecting angles of about on the detectors, with the pointing determined to include one or more of the cometary knots that characterize this nebula (O Dell & Handron 1996; Meaburn et al. 1998). In both cases the slit orientation was controlled to lie along a radial line from the central star and hence along the tail found to the outside of the bright cusp of the targeted cometary knot. The KPNO target was , using the position based system of nomenclature proposed by O Dell & Burkert (1997). A pair of 900 s exposures was made on 2000 October 7. The CTIO target was , and again a pair of 900 s exposures was made, this time on 2001 August 30. In addition to the spectra of the cometary knot, CTIO spectra were made at position angles of 0,45,90, and 135 in overlapping settings chosen to cover from the central star to the outer edge of main ring of emission. Spectra of NGC 2392 were again made from KPNO in the long-slit mode on 2000 January 21 with the central star at the middle of the slit and at position angles of 7 and 128 in order to lie along interesting linear features on the outer boundary of the nebula. Two 600 s exposures were made at P.A. = 7 and three exposures of 600 s at P.A. = 128. IRAF tasks were used to correct these long-slit exposures for curvature and to produce the wavelength calibration, which has an uncertainty of less than 1 km s 1. Long-slit spectra of NGC 2392 were also obtained at KPNO on 2001 April 18 in [O iii] (11 slit positions of 180 s each) and 2001 April 19 in [N ii] (five slit positions of 300 s each). The slit was oriented at P.A. s of 0 and 90 and was stepped to several locations perpendicular to the slit. In this way, the kinematics of the nebular structure were mapped at a variety of nebular positions covering the majority of the nebula. Long-slit spectroscopic observations of IC 4406 were conducted at CTIO on 2001 April 13 in [N ii]. Two integrations each lasting 600 s were obtained at P.A. = 85 (i.e., along the major axis), and two integrations each lasting 300 s were obtained at P.A. = 5 (i.e., along the minor axis). 4. RESULTS FROM THIS INVESTIGATION 4.1. NGC 2392 This object is probably the most inherently complex of our sample, as demonstrated in Figure 1, a series of monochromatic images. The basic features of the appearance have already been described, but the nomenclature we adopt here is in some cases different from that in earlier papers. Our system of designation is illustrated in Figure 2. The H,[Nii], [O iii], and [N ii] images are the uncalibrated images, which is acceptable since those lines dominate the HeII 4686 [OIII] 5007 F547M Hα 6563 [NII] 6583 [SII] Fig. 1. Monochromatic images of NGC 2392 shown in array. In all cases the vertical axis is toward P.A. = 296. The intensity scales are logarithmic compressions in all three NGC 2392 figures. The horizontal field is and the vertical field is throughout.

6 3334 O DELL ET AL. Vol. 123 Fig. 2. Nomenclature adopted in the text illustrated on lower ([N ii] and higher ([O iii]) ionization HST images signal from their filters. This is not the case for the F469N (He ii) and F673N ([S ii]) image, where the lines are sometimes weak in comparison with the background continuum. Normally one would correct for this continuum by scaling the strong line free F547M image, but this is not acceptable here, especially for the He ii image, because the two-photon component will be stronger where the emitting region is less dense (i.e., the color of the continuum can change). We have therefore corrected for the atomic continuum by simply scaling and subtracting the H image. This was done for both [S ii] and He ii, using regions of high and low ionization, respectively, to establish the correct subtraction of the continuum component What the Images Tell Us about the Structure of the Nebula The wide variation in appearance is best depicted in color coded images, as shown in Figure 3. Both of these color images cover a range of ionizations. The [S ii]-[n ii]-[o iii] sequence depicts the lower ionization range, while the [N ii]- [O iii]-[he ii] image gives the higher ionization range. Both of these show that the inner disk (Minkowski s face ) is not a continuous disk; rather, it is composed of a series loops and filaments. There appear to be two continuous loops of low ionization, with centers displaced north and south of the central star. This is very similar to the double loop forms seen in SN 1987A (Burrows et al. 1995), MyCn18 (Sahai et al. 1999), and He2-104 (Corradi et al. Fig. 3. Color images of the difference in appearance in NGC 2392 in high- and low-ionization states. In each case the lowest ionization is red and the highest blue, with the middle ion depicted as green.

7 No. 6, 2002 KNOTS IN NEARBY PLANETARY NEBULAE ) The surface brightness contrast between the loops and intraloop regions is high, about 20. This can be explained either as a very narrow shell seen in projection or as the loops being rings. The loop features are very sharp (0>2) and do not trail out toward the center, as would be expected for a thin shell. These two characteristics together argue that the loops represent curved filaments (rings) rather than shells. Examining the loops near P.A. = 35 and P.A. = 215, where the confusion is least, shows that the loops have [S ii], [N ii], and [O iii] in close progression, with [O iii] displaced no more than 0>1 from the [S ii] peak. The ratio of [O iii]toh is about 8 in these loops, values typical of photoionization from a hot star. However, there is no He ii emission on the inner edges of these features. The big differences in ionization state are very difficult to understand if the filaments are as physically close to the hot nucleus as they appear. NGC 2392 has one of the largest values of t 2, the measure of fluctuations in the electron temperature (Peimbert, Torres-Peimbert, & Luridiana 1995), where the electron temperature derived from the [O iii] lines is 15, K, and the ratio of the Balmer continuum to H yields These values were derived from ground-based spectroscopy, where it was unavoidable that one averages over areas of several square arcseconds. The presence of multiple loops of highly different ionization structures is probably a guide to the solution of the t 2 problem, at least in this object. The different loops probably have different sources of formation and conditions of photoionization, and a detailed study would probably reveal that they also have quite different electron temperatures. The He ii emission is strongest in a rosette pattern in the inner nebula, having a diameter of This is well inside the characteristic of the low-ionization bright disk. In addition, there are three well-defined high-ionization features (called here the bulges) at P.A. s of 3, 134, and 233. The last two are in less confused fields and show no [S ii] emission and weak [N ii], confirming they are high ionization. It is as if the inner nebula is optically thin to Lyc radiation in these directions. The high-ionization disk surrounds the inner disk and varies little in surface brightness (OWC) except where it shows some enhancement in the vicinity of the low-ionization features in this area (the three fuzz features and knots+tails). Its elliptical border is ill defined, but we measure it to be , which is larger than the values of derived from ground-based images (OWC). The direction of the long axis is difficult to determine, but is about P.A. = 30, which is different from OWC s value of 45. There are three features, which we call fuzz, in the sectors with P.A. values (south fuzz), (west fuzz), and (north fuzz). The closest portions are 9>3 from the central star (southern part of the west fuzz), while the farthest parts of the north and south fuzz are at 18>8. The fuzz features all show similar structure, being highly mottled in appearance with some indication of radial linear structure in their inner potions. One sees at least 10 knots in the south fuzz, but no clear evidence of any knots in the other two. All three are low ionization, although some have [O iii] emission. Many of the fine-scale features within the fuzz show the expected photoionization sequence toward the central star for [S ii], [N ii], and [O iii]. However, there is no detected He ii emission in the fuzz nor in the knots and tails (discussed in the next section). There are a few features that appear darker than the background in [O iii], indicating that they are not highly ionized and lie in the foreground to at least a significant fraction of the material producing the high-ionization disk. The fuzz does not appear in He ii Characterization of the Knots and Tails There are approximately 60 features, which we call knots, spread out rather evenly at different P.A. s in a zone from to radius, with a peak population at about radius. These knots are relatively weaker in [S ii], as compared with H, than their accompanying tails, an effect that can be explained by the knots being above the critical density for the [S ii] emission (4300 cm 3 and 14,600 cm 3 for the 6716 and 6731 Å components, respectively). The eight knots for which surface brightness photometry was performed (see below for their precise locations), in order to test the hypothesis of their being local ionization fronts around a neutral core, have characteristic electron densities of for their H emitting layers, under the assumption of their being resolved cusps. The density would be even higher if they are not fully resolved and would be higher still in the region of higher density where the [S ii] emission arises. Unlike NGC 7293, where the tails beyond the knots are nearly linear structures bounded by radial lines from the central star, the tails accompanying NGC 2392 s knots are only sometimes well bounded. They certainly lie close to radial lines passing from the star through the center of the knot, but they often deviate within the tail feature and show orientations up to 8 from the radial direction. Moreover, many of the tails are broad; i.e., they widen faster than the shadow of the bright knot at their head. One can test the hypothesis that the bright heads of the knots are ionization fronts by comparing their average surface brightness with predictions based on a knowledge of the Lyc flux of the central star. This method has been used previously to determine that the knots in NGC 7293 (O Dell & Handron 1996) and the proplyds in the Orion Nebula (O Dell 1998a) are optically thick to the Lyc. If NGC 2392 is optically thick to the Lyc, then its total flux (F H ) in the H recombination line of hydrogen will be a direct measure of the luminosity of the star in the Lyc. This means that the expected relationship between the average surface brightness [hs(h)i 4F(H)/ 2, where F(H) is the photon flux in H from the knot s cusp and is the chord diameter of the knot perpendicular to a line drawn toward the central star], and the distance (h) to the central star will be hsðhþ H eff H i ¼ eff F H ; where eff H and eff H are the hydrogen recombination coefficients defined and given values by Osterbrock (1989). We derived F H from our calibrated images in the H filter (F487N) to be 13.1 photons cm 2 s 1, which is ergs cm 2 s 1. From this flux we expect there to be a relation hs(h)ih 2 = if both the nebula and the knot are optically thick to the Lyc. If the knot is optically thin to the Lyc, then hs(h)ih 2 will be smaller than this value and if the nebula is optically thin to Lyc, then hs(h)ih 2 will be larger. We derived hhs(h)ih 2 i from a sample of eight knots selected to be sufficiently isolated that ð1þ

8 3336 O DELL ET AL. Vol. 123 accurate photometry could be performed. They have P.A., h, and values of 40=0, 14>4, 0>32; 9=1, 14>3, 0>32; 316=5, 16>3, 0>25; 307=6, 15>9, 0>24; 282=0, 16>0, 0>33; 185=4, 15>3, 0>27; 167=1, 13>9, 0>25; 107=7, 16>4, 0>25. This set gave hhs(h)ih 2 i = This means that if the knots are optically thick to the central star s Lyc radiation, the nebula absorbs only 8.2% of that radiation; i.e., the nebula is quite open. If the knots are not optically thick or advection effects are important in their ionization balance, then an even smaller fraction of the total radiation would be absorbed by the star. Since the ionization structure of the knots indicate that they are mini-ionization fronts and the analysis of Henney (2001) establishes them as comfortably in the recombination-dominated regime, the 8.2% value is probably characteristic for the nebula as a whole. This is quite different from the case for NGC 7293, where a similar analysis (O Dell & Handron 1996) indicates that the nebula is optically thick. Several of the tails appear dark in [O iii], indicating that they are not highly ionized and that they lie in front of a significant portion of the material producing the high-ionization disk. They are neutral in He ii, indicating that they do not have He +2 and that there is no He ii emitting region beyond them. Sometimes they appear dark in the continuum-subtracted He ii image, but, this is well within the uncertainties in the continuum subtraction; thus these dark features are artifacts of the data processing, rather than intrinsic to the object Features Extending beyond the High-Ionization Disk We see several low-ionization features outside of the high-ionization disk. Two of these are rather obvious extensions of tails of individual knots (P.A. s of 90 and 188 ), while the broad feature between P.A. = is seen to be composed of multiple tails projecting back to knots. Phillips & Cuesta (1999) confirmed the existence of these features that had previously been seen on OWC s [N ii] image. They constructed a model to explain their existence based on a sporadic, precessing jet. Unless one is to invoke this mechanism to explain all of the knots and tails, these new images render their model untenable What the New Spectra Tell Us about the Nebula Our spectra are similar in nature and quality to those used in earlier studies of the kinematics of the multiple components of the Eskimo Nebula (O Dell & Ball 1985; Gieseking, Becker, & Solf 1985; Balick et al. 1987; OWC). They do, however, provide a more homogeneous coverage of emission lines and P.A. s. As noted previously, the spectra of the inner disk are broken down into multiple components, which taken together approximate elliptical shells from which three dimensional models have been calculated. The HST images presented in this paper show that the inner disk is composed of a series of arcs and probably closed loops, which raises the question of the correctness of the previous velocity-analysis approaches. Traces of the surface brightness across the inner disk show ratios of contrast between the narrow features and the spaces between them of about 15, which would be difficult to obtain if the thin features were formed by limb brightening of thin shells. One can make identification of particular velocity features with features in the inner disk, but a detailed analysis cannot be made because of the 20 times different spatial resolution of the images and the spectra. The contour plots of these and earlier images suggest that the discrete velocity knots blend together to for a nearly continuous velocity ellipsoid. However, this could be the result of the blending of the outer contours of individual knots ordered approximately as a velocity ellipsoid. Even if there is such a global order to their motions and structure, the knotty nature of the spectra supports the interpretation of the images and the photometry of the cometary knots that the inner structure of the Eskimo Nebula which forms the inner disk is composed of many fine features which are ionization bounded, but these cover but a fraction of the total area and most stellar radiation passes through this structure and illuminates the outer parts of the nebula. Our spectra show that both the bright inner regions of the cometary features and their outer tails are always indistinguishably at the same radial velocity as the systemic velocity for the nebula. Our [N ii] spectra at P.A. = 128 clearly show the features that are the extension of the inner broad tail features which lie outside the high-ionization disk are also at the systemic velocity. The pair of [N ii] spectra obtained at P.A. = 187 cross the feature that extends on the low-ionization images from what appears to be the tail of a knot + tail feature past the boundary of the high-ionization disk. In contrast with the broad features near P.A. = 128, the inner part of this feature is redshifted about 8 km s 1 with respect to the systemic velocity, while the outer portion is redshifted an additional 8 km s NGC What the Images Tell Us about the Structure of the Nebula The large-scale ionization structure is well delineated by the very complete coverage of ions in our data set. However, the large-scale structure is well resolved on good ground telescope images (Lame & Pogge 1994; they separate the two components of the [S ii] doublet and include [O i] 6300), so that no basic improvement is possible. However, a few comments are possible. The HST image is shown in Figure 4. The distribution of He ii emission is different from NGC 7293 in an important way, in that NGC 6720 has a dip in the He ii surface brightness as the central star is approached, whereas in NGC 7293 it rises monotonically toward the star. This indicates that in NGC 6720 there is a drop in density toward the middle. There are two bright streaks crossing the central region. These streaks have their greatest enhancement in He ii, indicating that the streaks are formed by material inside of the bright ring. The bright ring of the nebula has numerous arcs or loops, and in this fashion strongly resembles NGC These arcs are darker in flux calibrated plots of the surface brightness ratio of [O iii] and H. This indicates that they lie outward of the He + zone. As we discuss in the next section, it is possible that these arcs are composed of a series of knots Determination of the Expansion Age from the Images [N ii] images of NGC 6720 obtained using WFPC2 on 1998 October 16 (GO 7632) and 2000 September 26 (GO 8726) were aligned and subtracted (Fig. 5, left). The residual shows a pattern indicating nebular expansion. The 1998 image was then magnified by various amounts and subtracted from the later one until the residuals showed no structure (Fig. 5, right). In this way we find that the nebula

9 No. 6, 2002 KNOTS IN NEARBY PLANETARY NEBULAE 3337 Fig. 4. This image of NGC 6720 has a vertical axis pointed toward P.A. = 34. The color coding is [N ii]: red, [O iii]: green, and He ii: blue. The dark orthogonal lines are due to the seams where the WFPC2 individual CCD images join. The double nature of these lines is due to our use of two data sets with slightly different pointings. expanded uniformly and homologously by a factor of in 1.95 yr. The resulting expansion age is 1500 yr with a 15% uncertainty. The expansion velocity along the minor axis, 45 km s 1 (GMC97), and the angular radius of its minor axis, , imply an expansion distance of 450 pc Characterization of the Knots and Tails The knots are numerous but appear almost exclusively in extinction. The only exception is a detectable bright cusp in [N ii] in a few objects on the side facing the central star. Tails are common but lack the tight radial alignment seen in the cometary knot tails in NGC 2392 and NGC These tails are prominent in extinction as compared with the head, being intermediate in this property between NGC 6853 (where the dark tails are relatively much stronger) and NGC 7293 (where the dark tails are much weaker). The knots are present in all of the ions observed, being most visible in [O iii] and least visible in [N ii]. A few are invisible when viewed, with only a few being neutral when [N ii]. This combination of facts indicates that the knots lie in the outermost ionized zone, at least as far out as the He 0 zone, which gives rise to the [N ii] emission. The [S ii] images are underexposed for these purposes but certainly show numerous dark features. The fact that there are no dark objects seen only in [S ii] argues that the features do no lie in the main ionization front, although the low S/N ratio of these images does not allow this conclusion to be definitive. In a recent paper (Speck et al. 2001) show that H 2 emission follows along the arcs within the bright ring. In addition, they show a marginally resolved but ubiquitous

10 3338 O DELL ET AL. Vol. 123 Fig. 5. Differences of [N ii] images using WFPC2 from the dates shown. The left panel shows the residual after image alignment, whereas the right image was magnified by a factor of prior to subtraction. The residual of the right panel shows no systematic pattern, implying uniform nebular expansion. structure of H 2 knots. They conclude that the H 2 emission comes from the neutral cores of knots, a conclusion we share based on the good correlation of dark knots with bright H 2 emission NGC 6853 Since the field of view of the WFPC2 is much smaller than the angular size of the nebula, these new images provide little additional information about the ionization structure of the object. The exception to this statement is the low-ionization wedge that points toward P.A. = 244, which these HST images show to be composed of multiple clumps of knots. In contrast with the lack of information about the global structure, these images provide a wealth of information about the knots within the nebula. The first arguments for the presence of knots were presented by Meaburn & Lopez (1993, henceforth ML) from their 1>1 resolution images in [O iii], although a detailed comparison with the WFPC2 [O iii] images shows that each of the dark knots reported there are actually clumps of multiple dark knots. An image of quality intermediate between that of ML and the WFPC2 was obtained as one of the first images with the European Southern Observatory s Very Large Telescope (VLT). 4 The VLT image shows clear indication of dark knots oriented radially with respect to the central star. Our HST image is shown in Figure Characterization of the Knots and Tails The knots in NGC 6853 are found with a wide variety of forms. As first indicated by ML, they are most visible in extinction in [O iii] against the bright background of the 4 See phot html. nebula. They vary enormously in form, illumination, and size. The smallest are about 0>3 in diameter and the largest about 1>0 diameter at the central core. Some appear as nearly circular disks, while others show a tail of material pointing away from the central star. Very few knots have the clear cusp+tail structure of the prototype cometary knots found initially in the Helix Nebula. Radial tails become more common the farther away from the central star one looks, indicating that we are seeing intrinsically radial structures but under a variety of conditions of orientation, those farther out selectively being viewed from a line of sight more nearly perpendicular to a radius vector from the central star. None of the knots show a noticeably bright rim in [O iii], which indicates that they are all located well out in the ionization structure of the nebula, beyond the He + zone. A significant fraction, but not most, of the knots have a bright boundary cusp on the side facing the central star. As would be expected for distribution of knots both on the nearer and further sides of the nebula, there are some bright edged knots that do not show up in the [O iii] image, indicating that they are on the far side of the nebula. The fact that the bright cusps represent local ionization fronts on the starward side of a neutral core is shown by the presence of [S ii] emission on that side. Like the cometary knots in the Helix Nebula (O Dell, Henney, & Burkert 2000), the bright cusps show the surprising result that they are more diffuse in [N ii] emission than in H Photometry of the Bright Cusps As discussed in x 4.1.2, valuable information can be obtained by comparison of the observed surface brightness of the bright cusps with values predicted from the integrated flux from the nebula. We have done a similar analysis for six bright knots. Adopting a total observed flux in H (CRO98b) of F(H) = ergs cm 2 s 1 leads one

11 No. 6, 2002 KNOTS IN NEARBY PLANETARY NEBULAE 3339 Fig. 6. This 99>5 75>5 section of the HST image has a vertical axis of P.A. = 15. The upper left corner is 9 00 at an angle of P.A. = 166 from the central star. The pattern of dark and bright cusp knots and their associated dark tails are discussed in the text. The color coding is [N ii]: red, H: green, [O iii]: blue. to expect hs(h)ih 2 = if the nebula is optically thick to Lyc radiation and h is characteristic of the radial distance, i.e., that the knot lies near the plane of the sky. For the two bright knots with hhi =16 00, the hhs(h)ih 2 i is 1.5% of the predicted value; for the two knots with hhi =30 00 it is 4.9%; and for the two with hhi =43 00 it is 15%. This trend exactly agrees with what is expected for our model of NGC 6853, where the disk of the nebula is seen nearly edge-on, and the fact that only one of the objects is dark in [O iii] and that one is in the farthest pair. This means that the smaller angle objects selectively are on the far side of the nebula, and their angular distance considerably underestimates their true radial distance NGC What the Images and Spectra Tell Us about the Cometary Knots and the Structure of the Nebula The regions imaged are illustrated in Figure 7. The northern quadrant field of our program GO 8726 observations is the same as that of program GO The primary purpose of program GO 7286 was STIS spectroscopy of the cometary knot , but simultaneous parallel observations were made with WFPC2 and NICMOS. These fields are also illustrated in Figure 7. Enlargements from sections of the parallel observations are shown in Figure 8. The WFPC2 portions of Figure 8 are of a section of the nebula that appears as an arc on ground-based images. It is but one of several arcs whose arrangement leads to the common name of the Helix Nebula. Examination of this image reveals that this arc is actually composed of a host of cometary knots. This result had been anticipated in the CO study of Young et al. (1999), where they saw multiple velocity components within the resolution of their radio telescope. These same images show that the cometary knots are more diffuse in [N ii], which is contrary to the usual appearance of ionized nebulae, but is the same result shown by the slitless spectra of H and [N ii] of the cometary knot (O Dell et al. 2000), where it is argued that the puzzling reversal is due to not all of the ionized gas having reached its equilibrium temperature, being cooler near the local ionization front. The NIC3 image shown in Figure 8 is out of focus, due to thermal problems with NICMOS. However, the resolution is still quite high as compared with large ground-based and small satellite telescope images. This H 2 image was created by subtracting the continuum-dominated F215N filter image from the H lm plus continuum F212N filter image. This image shows that this outer H 2 arc is actually composed of a series of compact features, which are probably the cores of cometary knots. This argument supports the same conclusion drawn by Cox et al. (1998) from their H 2 observations. The fact that the arcs are seen to break down into cometary knots at sufficient angular resolution does not mean that the primary emission from the nebula is composed of unresolved knots. Where the knots are few in number, on the inner edge of the low-ionization ring, they are found with a narrow range in sizes and give no suggestion of images overlapping one another. One also sees in the ground-based

12 3340 O DELL ET AL. Vol. 123 GTO-5086+GO-5311 GO-5977 GO-7286 GO-7286 STIS SLIT Enlarged FOV NIC3 Fig. 7. Fields covered by the several images and spectra of NGC 7293 shown on this ground-based image in H. The program GO-7286 images with the WFPC2 optical and NIC3 infrared cameras were made in parallel with spectroscopy with the STIS. A section of the GO-7286 WFPC2 image and all of NIC3 image are shown enlarged in Fig. 8. long-slit spectra that the knots appear as discrete objects with specific characteristic velocities superposed on an amorphous emission at slowly changing velocities. The local velocity of the nebula s emission was determined at multiple positions from both the cometary knot centered spectra and the CTIO spectra at steps of 45 in P.A. The results are summarized in Figure 9. This figure indicates that the [N ii] velocity of expansion decays from about 24.5 km s 1 at the center, whereas the H expansion velocity maximum is again at the center, but only 16 km s 1. The expansion velocity for [N ii] is in excellent agreement with that found by M98. Since the [N ii] emission must arise from a relatively thin low-ionization region outside of where the bulk of the H emission occurs, this pattern is the same as in most PNe. The fact that there is [N ii] emission even in the direction of the central star means that there is sufficient material near the pole of the nebula to form at least He +2 +H +,He + +H +, and He 0 +H + ionization zones, which makes it likely that the nebula is radiation bounded even in that direction Spatial Motions of the Cometary Knots The spatial motion of the cometary knots in the Helix Nebula will probably contain useful information about the method of formation of these objects, especially when compared with that of the nebula itself. This requires the accurate determination of both the radial and tangential velocities with respect to the nebula. Radial velocities have been determined for 50 cometary knots by M98 with an accuracy of a few kilometers per second using the [N ii] line at 6583 Å. We have measured velocities for two additional cometary knots ( and ) on our (2000 October) CTIO spectrum centered on the former object as shown in Figure 10. Throughout this paper we use the position-dependent system of designation of Helix Nebula cometary knots (O Dell & Burkert 1997) rather than the serial numbers (shown as M1, M2, and M3 in Fig. 10) employed by M98. The radial velocities are indicated next to the object designation label in Figure 10. A single radial velocity was determined on our 2001 August spectra for the cometary knot , with the result of V = 24 1kms 1 from both the H and [N ii] lines. The systemic velocity of the Helix Nebula from optical lines is V = 27 km s 1 (M98) and V = 26 km s 1 from CO emission (Healy & Huggins 1990). Comparison of these radial velocities with the CO emission can be useful. M98 has compared (in their Fig. 6) the cometary knot radial velocities with that of CO emission as determined by Healy & Huggins (1990). In a plot of CO radial velocity versus P.A., Healy & Huggins indicate there is a sinusoidal variation with an amplitude of 19 km s 1. The M98 radial velocities for their sample of cometary knots show a smaller range of radial velocities (14 km s 1 ). The low spatial resolution CO velocities were determined along the arcs that form the helical shape of the nebula, but we have seen in the current HST images of the outer regions that these arcs are broken down into cometary knots. This makes it puzzling as to why there should be a

13 No. 6, 2002 KNOTS IN NEARBY PLANETARY NEBULAE 3341 Hα 35x30" [NII] 35x30" H 2 F215N 51x48" 51x48" Fig. 8. Sections of the WFPC2 parallel images outlined in Fig. 7 shown at the top, where one sees how the outer arc of NGC 7293 breaks down into individual bright cusp knots. This is visible in the H image but not the [N ii] image because of the photoionization effects discussed in O Dell et al. (2000). The bottom frame shows the NIC3 image outlined in Fig. 7 and shows how the lower arc in NGC 7293 is broken down into H 2 knots. North is up. Expansion Velocity (km/s) " 200" 300" Distance from Central Star Fig. 9. Splitting of the two components of emission from the nonknot emission from NGC 7293 shown as a function of distance from the central star (open squares, H; filled circles,[n ii]). systematic difference in the velocities. Another way of stating this is that, if the CO emission arose from a photondominated region outside of the main body of ionized gas, one would expect that there could be significant differences in the motions; but if the CO also arises from the neutral cores of the cometary knots, then the difference is puzzling. This difference is corroborated if one adds our radial velocity for to their data set, since it falls in the middle of the largest gap in the PA range of their data and is at a location where the CO data would predict a radial velocity 17 km s 1 more positive than is observed. The difference in the velocity patterns is likely to be due to the fact that the optically studied cometary knots are located at smaller distances from the central star. This could mean that the cometary knots have formed from portions of the nebula having different radial velocities (recall that the [N ii] expansion velocity is significantly greater than the H expansion velocity). However, when one considers only the observed radial velocities, this trend is different than for the expansion velocities shown in Figure 9, where one sees that the expansion velocity decreases with increasing distance from the central star. Since the well defined optical cometary knots fall inside of a distance of , their velocities certainly fall within that of the main body of emission as measured by H. An alternative reconciliation of the CO and optical line velocities that still accepts that the CO emission arises from inside the knots is that the difference is due to expansion at

14 3342 O DELL ET AL. Vol V(sys)=-27.2+/-3 km/s V(exp)=18.8+/-3 km/s Proper Motions 4.5+/-9 km/s /-5 km/s km/s M km/s M1 M km/s km/s km/s Fig. 10. North-northwest section of NGC 7293 imaged by the HST shown in this H image, with north at the top. Bright cusp knots included in the measurement of proper motions are circled and labeled with their position-based names (e.g., ) and sometimes their serial names (e.g., M2) as described in the text. The location of the ground-based spectrograph slit is shown. Where available, radial velocities of the knots are given as are the summary data on the motions in the main body of the nebula. The nebular velocities are given at the upper left, and the knot group proper motion relative to the central star is shown at the top center. the knots ionization fronts. This motion will selectively be toward the central star and will produce a shifting of the optical emission-line velocities. However, the magnitude of this effect is quite model dependent and has not been calculated. This means the role of this effect is promising, but remains uncertain until accurate hydrodynamic modeling has been done. The time base between two sets of F656N observations of the northwest portion of the Helix Nebula is 5.99 yr, which means that one WFPC2 coarse pixel (0>0996) corresponds to a tangential velocity of 16.3 km s 1 if the distance to the nebula is 207 pc, as indicated by the parallax of the central star (Harris et al. 1997). Since Figure 9 indicates that this is less than or equal to the characteristic expansion velocity of the nebula, it was thought worthwhile to measure the proper motion of the cometary knots in the field of view. The features measured are indicated by circles in Figure 10, these being cometary knots sufficiently isolated to allow optimum measurement. The one exception to this guideline was the pair and that was sufficiently close that they could be measured together. The method of measurement was developed by P. Hartigan (Hartigan et al. 2001) for determination of the motion of features in Herbig-Haro objects. The method involves shifting a small sample centered on the cometary knot with respect to a similar sample of the reference image until a minimum value of the difference of the images is obtained. The result was an average tangential angular velocity of mas yr 1 in the direction P.A. = 52. Since the stars within the field of view were shifted into alignment in order to correct for differences in the absolute pointing and shifts between the four CCDs used in WFPC2, this is a motion with respect to these stars. This method gives the total tangential motion of the measured cometary knots, whereas the motion relative to the rest of the nebula is the most relevant number. Under the assumption that the central star s motion is characteristic of

15 No. 6, 2002 KNOTS IN NEARBY PLANETARY NEBULAE 3343 the motion of the center of mass of the nebula, one can make the necessary correction once the central star s motion is known. Cudworth (1974) determined this using photographic material and 20 reference stars (V 15) to be 40 mas yr 1 toward 73. A more recent determination using a CCD detector, six reference stars (V 16) and a seven year time base (H. Harris 2001, private communication) gave 33 mas yr 1 toward 87. We have adopted a value of 36.5 mas yr 1 toward 89 for the central star. This gives for the corrected motion perpendicular to a line from the cometary knots to the central star of 13 5kms 1, and a corrected motion along a line toward the central star of 5 9kms 1. The large uncertainty in the tangential motion of the central star, together with the difficulties of measuring these extended sources, means that the final results are so uncertain as to provide only the most limited constraints on models of the cometary knot system. The situation is exacerbated by a lack of knowledge of the expansion motion of the nebula in the plane of the sky. Adopting a polar axis inclination of 29 from the line of sight and an equatorial expansion velocity of 16 km s 1 gives an expected expansion motion of 14 km s 1. Given the uncertainty in the derivation of the motion of the cometary knots away from the central star and the uncertainty in the prediction of the similar motion of the nebula, one cannot say that any useful conclusion can be drawn from a comparison of 4.5 9kms 1 (the cometary knots) with 14 km s 1 (the nebula) IC 4406 We have the advantage of both new superior images of this object in addition to spectra of unprecedented spectral resolution. These will be discussed in that order in this section What the Images Tell Us about the Dark Features The new images confirm the basic bipolar nature of the nebula as reported previously (S91; Gruenwald, Vieges, & Broguiére 1997). In the equatorial plane we see the normal progression of ionization states, with the otherwise underexposed [S ii] images confirming the presence of an ionization boundary at the sharp north and south edges, beyond which H 2 lobes have been seen (Story 1984). The overall structure is represented well in a logarithmic depiction, as shown in Figure 11. It is, however in the high-resolution CCD1 images that unique aspects of the dark lanes are revealed (Fig. 12). We can make some general comments on the visibility of the dark lanes in the images in different calibrated filters (F502N, F656N, and F658N being adequately exposed for this purpose), and then we present the detailed results of an analysis of a well-defined dark lane. We see the dark lanes in all of the emission-line filters. They are more visible in calibrated [O iii]/h images, which indicates that they originate in a region outside of the [O iii] (H + +He + ) zone. They almost disappear in [N ii]/h images, which indicates that they are seen in front of most of the ionized material producing both ionic emissions, i.e., they lie very near the ionization front. A detailed analysis was made of a single well-defined dark lane. This lane is located 1>3atP.A.=47 from the central star. A nearly linear portion was sampled in profile using a sample width of 1>1 (24 CCD1 pixels). For this purpose the local regions of the calibrated images were normalized to one another, so that the depth of the dips when crossing the dark lane is a direct measure of the extinction. The results are shown in Figure 13. We see that there is a progression of maximum depths in decreasing order [O iii], H, and [N ii]. If this progression were entirely due to the shape of the interstellar reddening curve, the depth of H and [N ii] should be the same, since their emission is at very similar wavelengths. If the dark lane was completely optically thick at all of the wavelengths, then variations in the apparent extinction would be due to the relative amounts by which the background brightness of the nebula fills in the dark lane, owing to the finite point-spread function (PSF) of the imaging system. However, this cannot be the explanation because the PSF should be essentially the same at H and [N ii] and sharper at [O iii], which would make the latter two of the same depth and [O iii] less dark. Having eliminated these two families of possibilities, one can attribute the differences as being due to the location of the material causing the dark line with respect to the emitting volumes for each ion. The progression of depths indicates that the feature lies on the near side of the [N ii] emitting zone (H + +He 0 ). The absence of a brightening in either [N ii]or[sii] at the edge of the dark lane argues that it actually lies outside of the ionization front. However, such a brightening is hard to detect and could produce the lower apparent extinction in [N ii] What the Spectra Tell Us about the Structure of the Nebula The results of the analysis of our spectra of IC 4406 are summarized in Figure 14. We see that the splitting indicated by the [N ii] line is 8 km s 1, indicating an expansion velocity in the [N ii] zone of only 4 km s 1. This is comparable to the value of 6 km s 1 for the expansion velocity in [O iii] found by deconvolution of 3 km s 1 resolution Fabry-Perot spectra by Robinson, Reay, & Atherton (1982). These values are in marked contrast with the CO expansion velocity of 23 km s 1 found by Sahai et al. (1991). Since the spatial FWHM of the CO study ( ) was larger than bright core of the nebula, it is hard to draw conclusions from the CO velocities. Our results are quite different from those in the earlier study by Corradi & Schwarz (1993). Their study was made with a resolution of about 40 km s 1, whereas ours is 7 km s 1. At that resolution they could not resolve the [N ii] emission into its two velocity components and could only present a systematic variation of the average velocity along a slit aligned along the long axis, passing through the plume. However, their variation in this composite velocity over the central is 21 km s 1, while our spectra show that the two components vary by only 6 km s 1 in this same range. There are multiple indications that the adopted distance of 1600 pc and corresponding radius of 0.11 pc are too large, the latter placing this object intermediate in size between the compact core of NGC 2392 and the large objects NGC 6853 and NGC At the adopted distance its radius is comparable to NGC 6720 s value of 0.10 pc. However, the form is one only found in the compact PNe. Furthermore, this size, combined with the expansion velocity determined here would make it the oldest PN in our sample and one of the oldest known. Finally, its H surface brightness is half that of NGC 2392 and 3 times that of NGC All of these comparisons argue that IC 4406 s distance has been significantly overestimated and its intrinsic size is about 0.04 pc.

16 3344 O DELL ET AL. Vol. 123 Fig. 11. HST WFPC2 GO 8726 and GO 9314 pointings combined to produce this image of IC The color coding is as in Fig. 6, and the image is , with the vertical axis at P.A. = DISCUSSION 5 STScI release PR98-39 available at pr/1998/39/index.html Other PNe Showing Similar Features There are several additional nearby PNe that show structures similar to our program object. NGC 3132 was imaged as part of the Hubble Heritage program. 5 This image shows that this PN s appearance is dominated by a thin bright ring of low ionization within which there is a quite homogenous disk of [O iii] emission. There are numerous dark knots, many with short radial tails appearing just at and outside of the [N ii] ring. Across the middle of the [O iii] disk is a lane of knots, which appear best in extinction in [O iii] and these are accompanied by [N ii] emission on the side facing the central star. In most ways this object s knots and their location appear similar to NGC Knots are found in different forms in roughly half of PNe (Gonçalves, Carradi, & Mampaso 2001). Generally they are best seen in lines of low ionization and are called low-ionization structures. However, with the possible exception of NGC 2392, the knots under consideration here are generally connected together in filaments or give the appearance of having once been connected in that way, until photoionization and ablation left only the densest, most massive knots behind. In addition, the knots in our targets are generally distinct for their numbers and large-scale distribution, implying that their formation was primordial or at least the result of a process that acts globally. There are several other PNe in which strings of knots appear to lie along or near the outer surface: NGC 6818, NGC 3918, portions of NGC In all cases it is credible that the surface containing the knots was, until recently, associated with an ionization front. We shall return to mention the formation of knots under these conditions. In other cases the knots are embedded both near the surface and throughout the nebular interior, such as NGC The Progression of Characteristics of the Knots Since all of the PNe in our study are of the same basic structure and are simply seen at a progression of phases in

17 No. 6, 2002 KNOTS IN NEARBY PLANETARY NEBULAE 3345 Fig. 12. Central region of IC 4406 shown in the same fashion as in Fig. 11, except that the field of view is now 33>6 square and only the higher resolution CCD1 (0>0455 pixel 1 ) images were used. the evolution of the nebular shell, it is reasonable to look for any progression of characteristics of the associated knots. There appear to be progressions that fit four of the objects quite well, in the size order IC 4406 (R = 0.04 pc), NGC 6720 (R = 0.10), NGC 6853 (R = 0.27), NGC 7293 (R = 0.33). The first progression is in the appearance of the knots. In IC 4406 we see the knots as dark lanes with a hierarchy of lengths and orientations and no bright edges. In NGC 6720 one again sees almost exclusively only dark features, but now they are usually well-defined knots, and there is frequent indication of dark tails that are close to radial, but deviate significantly from the exact radial orientation. In NGC 6853 we see well-defined knots, some with photoionized cusps facing the central star and others entirely dark. Radial dark tails are present around a minority of the features. In NGC 7293 the knots all have photoionized cusps and bright tails exactly oriented along radial lines from the central star. There is some extinction in the tails, but this is generally less than in NGC 6720 and NGC The next progression is that of location. In IC 4406 the knots are almost certainly very close to the main ionization front, while in NGC 6720 we see knots in the H + +He 0 zone that produces the [N ii] emission. In NGC 6853 the knots are certainly in at least as close as the H + +He 0 zone and some are likely to be in the H + +He + zone that produces [O iii]. In NGC 7293 the knots appear to extend from the outermost zones well into the H + +He + zone, but not into the H + +He +2 zone. Since there is a steady advance of the ionization front as the PN shell expands, the above progression can be explained as the knots forming in an early phase of the expansion and near the main ionization front. As the shell expands, some of the knots are left behind in the inner ionization zones, where they are subjected to sculpting by the radiation field of the central star. This can be done through radiation pressure acting on the dust component of the