---
abstract: 'The nebula Mz3 has arguably the most complex bipolar morphology, consisting of three nested pairs of bipolar lobes and an equatorial ellipse. Its three pairs of bipolar lobes share the same axis of symmetry, but have very different opening angles and morphologies: the innermost pair of bipolar lobes shows closed lobe morphology, while the other two have open lobes with cylindrical and conical shapes, respectively. We have carried out high-dispersion spectroscopic observations of Mz3, and detected distinct kinematic properties among the different morphological components. The expansion characteristics of the two outer pairs of lobes suggest that they originated in an explosive event, whereas the innermost pair of lobes resulted from the interaction of a fast wind with the surrounding material. The equatorial ellipse is associated with a fast equatorial outflow which is unique among bipolar nebulae. The dynamical ages of the different structures in Mz3 suggest episodic bipolar ejections, and the distinct morphologies and kinematics among these different structures reveal fundamental changes in the system between these episodic ejections.'
author:
- 'Martín A. Guerrero, You-Hua Chu, and Luis F. Miranda'
title: 'Mz3, a Multipolar Nebula in the Making'
---

Introduction
============

Mz3, the Ant Nebula, is perhaps one of the most stunning bipolar nebulae. The [*Hubble Space Telescope*]{} (*HST*) color image presented by the Hubble Heritage Program (STScI-PRC01-05, PI: B. Balick, V. Icke, R. Sahai, and J. T. Trauger) reveals a complex system of three nested pairs of bipolar lobes. These bipolar lobes are roughly aligned along the same axis of symmetry, but have vastly different shapes, opening angles and detailed morphologies. In addition, a faint ellipse of emission aligned along the equator of these bipolar lobes is seen. Not only is the morphology of Mz3 complex, but its nature is also uncertain. While usually classified as a young planetary nebula (PN), Mz3 has also been suggested to be a circumstellar nebula of a symbiotic star, based on the high density of its core [@ZL02], its near-IR colors [@SK01], and the spectrum of its central star.

Previous high-dispersion spectroscopic observations of Mz3 have detected several pairs of bipolar lobes, and their kinematic properties led to the suggestion of episodic bipolar ejections [@LM83; @MW85]. More recently, @Retal00 reported the discovery of fast, 500, collimated outflows. Detailed modeling of the structure of Mz3 has been hampered by the limited detector sensitivity or sparse slit coverage of these previous observations. Therefore, we have carried out new long-slit, high-dispersion echelle observations of Mz3, emphasizing particularly the morphological features that have not been observed previously. These echelle observations, combined with the high-resolution *HST* narrow-band images and *Chandra* X-ray observation, allow us to produce a complete spatio-kinematic model of Mz3, adequately representing the three pairs of bipolar lobes and the equatorial ellipse. While our results confirm the previous suggestion that the multipolar structure was produced by episodic bipolar ejections [@LM83; @MW85], we are able to describe the kinematic properties and determine the formation process more precisely. This paper reports our new observations and analysis of the physical structure of Mz3.

Observations
============

Archival HST Images
-------------------

Narrow-band WFPC2 images of Mz3 in the H$\alpha$, H$\beta$, and \[N [ii]{}\] $\lambda$6583 emission lines were retrieved from the *HST* archive (Proposal IDs 6502 and 9050, PI: Balick, and Proposal ID 6856, PI: Trauger). The images we used in this work are listed in Table 1 with their integration times, filters, and the location of Mz3 on the WFPC2 (PC or WFC). These images were calibrated via the pipeline procedure, including the analog-to-digital correction, bias and dark image subtraction, and flat-field correction. We removed the cosmic rays and combined different exposures obtained with the same filter using standard IRAF routines. The H$\alpha$ and \[N [ii]{}\] images of Mz3, displayed in Figure 1, are used to analyze the nebular morphology. The H$\alpha$ to H$\beta$ ratio map of Mz3, shown in Figure 2-[ *left*]{}, is used to investigate the distribution of intranebular extinction.

Archival Chandra X-ray Observations
-----------------------------------

The Advanced CCD Imaging Spectrometer (ACIS) on board the *Chandra X-Ray Observatory* was used on 2002 October 23 to obtain a 40.8 ks exposure of Mz3 (Observation ID: 2546; PI: Kastner). Mz3 was positioned at the nominal aim point of the ACIS-S array on the back-illuminated S3 CCD. We retrieved the level 1 and level 2 processed data from the *Chandra* Data Center and further processed the data using the *Chandra* X-Ray Center software CIAO v3.0.2 and the Calibration Data Base CALDB v2.25. The background count rate is consistent with the quiescent background[^1] for most of the observing time. Only two background “flares” of short duration occurred. After excluding these high-background periods from our analysis, the net exposure time was reduced to 39.6 ks. We used this dataset to extract an image in the 0.5-1.8 keV band at a resolution of $\sim$10, and overplotted the X-ray contours on the *HST* WFPC2 H$\alpha$ image in Figure 2-[*right*]{} to illustrate the relative distribution of X-ray-emitting gas and the ionized nebular material.

Echelle Observations
--------------------

High-dispersion spectroscopic observations of Mz3 were obtained on 2002 June 23 and 24 using the echelle spectrograph on the CTIO 4m telescope. The spectrograph was used in the long-slit mode to obtain single-order observations of the H$\alpha$ and \[N [ii]{}\] $\lambda\lambda$6548,6584 lines for an unvignetted slit length of 3. The 79 line mm$^{-1}$ echelle grating and the long-focus red camera were used, resulting in a reciprocal dispersion of 3.4 Å mm$^{-1}$. The data were recorded with the SITe 2K No. 6 CCD with a pixel size of 24 $\mu$m. This configuration provides a spatial scale of 026 pixel$^{-1}$ and a sampling of 3.7 pixel$^{-1}$ along the dispersion direction. The slit width was set to 09, and the resultant instrumental FWHM was 8. The angular resolution, determined by the seeing, was better than 12.

The echelle observations were made with the slit oriented along different position angles and placed at various offsets from the central star, in order to sample the complex morphological features of Mz3. The slit positions and exposure times of these observations are given in Table 2. Although both H$\alpha$ and \[N [ii]{}\] lines are available, only the \[N [ii]{}\] $\lambda$6583 line is used to analyze the kinematics of Mz3 because of its smaller thermal width. The echellograms of the \[N [ii]{}\] $\lambda$6583 line are presented in Figure 3, where an \[N [ii]{}\] image is also presented with the slit positions overplotted.

Results
=======

The *HST* narrow-band images of Mz3 reveal three pairs of bipolar lobes and one elliptical feature along the equator of these lobes; they are marked in Figure 1 as BL1, BL2, BL3, and EE, respectively. These features are also detected in the echellograms and marked correspondingly in Figure 3. In the following sections, we discuss detailed morphologies and kinematics and propose spatio-kinematical models for each of these structures in Mz3.

BL1: the Hourglass-Shaped Inner Bipolar Lobes
---------------------------------------------

The innermost pair of bipolar lobes (BL1), called Inner Bipolar Lobes (IBL) in @Retal00, have an hourglass morphology, with a narrow waist along the east-west direction (Fig. 1). The position-velocity diagram, i.e., the long-slit echellogram, also shows a tilted hourglass pattern (Fig. 3), which can be produced by two shells expanding oppositely along the polar axis with the south pole tilted toward us. The velocity difference between the walls of each lobe is $\gtrsim$100, much larger than the $FWHM$ of the \[N [ii]{}\] line at the walls, 10–15. The sharp morphology and narrow \[N [ii]{}\] line shape at the walls of the BL1 lobes indicate that the material originally residing in the lobes has been evacuated and compressed into thin shells by a bipolar outflow. The bipolar expansion of the lobes is a direct consequence of the bipolarity of the outflow. The lateral expansion of the lobes, on the other hand, may be driven by the thermal pressure of hot gas shock-heated by the outflow impinging on the circumstellar material. The hot gas in the central cavities of the BL1 lobes has been detected in X-rays [Fig. 2-[*right*]{}; @Kastner03].

The *HST* images of the BL1 lobes reveal protrusions from their polar caps indicative of blowouts. The southern lobe shows a single protruding blister at its polar cap, while the northern lobe shows multiple blister-like structures extending from the polar cap and converging into a single blister at the end. The \[N [ii]{}\] echellogram covering these regions, shown in Figure 4, reveal gas motions reflecting the blowout process. The blister at the cap of the southern lobe shows a spindle-shaped \[N [ii]{}\] line that broadens up to 100  at its leading edge, while the multiple blister-like extensions of the northern lobe show multiple velocity components and the convergent blister at the end shows a bubble-like structure expanding rapidly both laterally and radially. The \[N [ii]{}\] echellograms also detect nebular knots outside the BL1 lobes along the axis of symmetry, as marked in Figure 4. Exterior to the southern lobe, a bright knot at $\sim$19  from the central star is detected at roughly $-$30 km s$^{-1}$ from the systemic velocity ($v_{\rm sys}$). Exterior to the northern lobe, a counterpart of the southern knot is detected at $\sim$19 from the central star with roughly +30 km s$^{-1}$ offset from $v_{\rm sys}$; in addition, a fainter knot is detected at $\sim$25 from the central star with a velocity offset of about +40 km s$^{-1}$.

The different characteristics of the northern and southern lobes of BL1 are probably caused by the detailed interactions between the bipolar outflow and the dense circumstellar material. The circumstellar material has a high concentration in the equatorial plane, as indicated by the higher extinction around the waist of BL1. The sharp band of obscuration over the northern BL1 lobe at $\sim2\farcs5$ north of the central star suggests that dense equatorial material is located in front of this lobe and therefore confirms the orientation of BL1 implied from its kinematics. The variations in the local extinction, as derived from the H$\alpha$/H$\beta$ ratio map shown in Figure 2-[*left*]{}, support this hypothesis: the H$\alpha$/H$\beta$ ratio is higher on the northern lobe than on the southern lobe, and therefore extinction towards the northern lobe is higher, indicating larger amounts of intervening material. In addition to the surrounding material that obscures the northern BL1 lobe, the central star of Mz3 is embedded in a thick, extended shell detected through the mid-infrared emission of dust [@Quinn96].

This H$\alpha$/H$\beta$ ratio map discloses additional clues on the distribution of absorbing material within Mz3. The H$\alpha$/H$\beta$ ratio, i.e., the extinction, is especially enhanced at the projected edge of the lobes and along the bright optical filaments, suggesting that the expanding lobes carry large amounts of dust and suffer from self absorption. In agreement with Smith’s (2003) conclusions based on the different amounts of extinction derived from infrared H [i]{} and \[Fe [ii]{}\] lines, we conclude that a significant fraction of the extinction towards Mz3 is local rather than interstellar. The local nature of the extinction in Mz3 and its nonuniform distribution affects the morphology of the diffuse X-ray emission which is anticorrelated with the amount of extinction (Fig. 2), as typically observed in other PNe [@Kastner02].

To determine the dynamical age and inclination of the polar axis for each of the BL1 lobes, the shell morphology and position-velocity relation need to be analyzed and modeled quantitatively. We have adopted a simple expression to approximate the radial expansion velocity of an hourglass as a function of the latitude angle, $\theta$ [@SU85]: $$v(\theta) = v_{\rm e} + (v_{\rm p} - v_{\rm e}) \times 
\sin (|\theta|)^\gamma,$$ where $v_{\rm e}$ and $v_{\rm p}$ are the expansion velocities at the equator and pole, respectively, and the exponent $\gamma$ sets the lobe geometry. We have also assumed a homologous expansion so that $$r(\theta) = \Delta t \times v(\theta)$$ where $\Delta t$ is the time since the lobe was formed.

Using the model outlined above, we have determined $v_{\rm e}$ and $v_{\rm p}$, the age, the exponent $\gamma$, and the inclination with respect to the sky and PA of the symmetry (polar) axis of each of these bipolar lobes. The best fits for the southern and northern BL1 lobes are shown in Figure 5 and the parameters of these fits are listed in Table 3. As expected from the different morphological and kinematical properties of the southern and northern lobes, the best-fit parameters to each lobe are not exactly the same, though both fits have similar inclination of the symmetry axis with respect to the plane of the sky, 15–20, and kinematical age[^2], (600$\pm$50)${\times}(\frac{D}{\rm kpc})$ yr, where $D$ is the distance in kpc to Mz3.

If the bright knots at the tip of the bipolar lobes share their inclination angle, then the true de-projected velocity of these knots is in the range between 90 km s$^{-1}$ and 150 km s$^{-1}$. For comparison, we have also included in Tab. 3 the parameters of the best fit to the northern lobe considering the extension and kinematics of the converging blister at its polar cap. The shorter kinematical age of the northern lobe when its blister is considered may be suggestive of acceleration of the gas motions caused by a blowout process.

BL2: the Cylindrical Bipolar Lobes
----------------------------------

The bipolar lobes of BL2, the Outer Bipolar Lobes 1 (OBL1) in @Retal00, have an almost rectangular morphology, with the PA’s of the western and eastern edges having a difference as small as $\sim$5 (Fig. 1). The lobes have a width of $\sim$24 and a length up to $\sim$85 for the northern BL2 lobe, i.e., the aspect ratio is 7:1. Their edges are rather straight, bending inwards only at the location where these lobes contact the inner BL1 lobes. The detailed morphology of BL2 shows a complex system of long filaments extending radially outwards. These filaments originate from a collection of knots at the base of the BL2 lobes that form a cavity-like structure just outside the BL1 lobes (Fig. 4).

The \[N [ii]{}\] echellogram of the BL2 lobes along PA 8  (i.e., roughly the BL2 symmetry axis) shows two velocity components with a velocity gradient of $\sim$1.1 arcsec$^{-1}$ (Fig. 2). The difference in velocity between these two components, $\sim$110, does not change significantly with the position along the symmetry axis of these lobes. Along the orthogonal direction, the echellograms at PA 98  and offset 14, 19, and 26 show hollow position-velocity ellipses in BL2 (Fig. 2). Material in these lobes is thus mostly confined in the thin walls of hollow cylinders. This material cannot just flow along the walls of the cylinder, as a cross section of such a cylinder would have exactly the same observed velocity. Instead, the apparently constant velocity-split implies that the section of the cylinder expands with a constant, $\sim$55, expansion velocity. Meanwhile, the velocity along the walls increases with the distance from the central star and must be faster than the transversal velocity; otherwise the lobes will not show the high aspect ratio, $\sim$7:1, that characterizes them.

The Hubble law-like expansion of the BL2 lobes suggests that these lobes were made in a single, explosive event. As the difference in velocity between the blue- and red-shifted components at a given location of BL2 is the same, $\sim$110, the kinematical age of BL2 can easily be derived assuming that the 24 width of BL2 are simply due to expansion along this direction. The kinematics are well reproduced using a cylinder[^3] tilted with respect to the plane of the sky with fixed, 55, expansion velocity across its section and linearly increasing velocity along the walls (Figure 6). The best fit model has an inclination of 20$\pm$5  against the sky plane, in agreement with the previous value reported by @MW85, and a kinematical age of (1,000$\pm$100)${\times}(\frac{D}{\rm kpc})$ yr. At the maximum distance of 85 from the central star of Mz3, the de-projected expansion velocity is $\sim$320.

BL3: the Conical Lobes
----------------------

We have named BL3 the pair of bipolar lobes with conical shape, called the Outer Bipolar Lobes 2 (OBL2) by @Retal00. These lobes have an opening angle of $\sim$50 and their limbs point directly to the central star of Mz3. In the images in Fig. 1, the conical lobes BL3 are composed of multiple knots with long, radial tails stretching out up to 60 from the central star of Mz3. The distribution of these knots and filaments is looser than this of the filaments in BL2. Indeed, the knots and filaments in BL3 look disconnected, more like individual streams of material than as part of a contiguous structure.

Further information on the kinematics and structure of BL3 can be derived from the echellograms at PA 43, 52, and $-$28 through the central star, and at PA 98 and offset 14, 19, and 26 to the south of the central star of Mz3 (Figs. 3 and 7). In the echellograms at PA’s 43, 52, and $-$28, the knots and filaments composing BL3 appear as tilted straight features with different slopes on the position-velocity space. The structure and kinematics of BL3 in these echellograms is somehow confused by that of BL2, but the echellograms at PA 98  resolve unambiguously BL3 from BL2. In these echellograms, the velocities of the knots and filaments of BL3 are mostly distributed, but not completely confined, along ellipses. The radial velocities of these ellipses as well as the velocity differences between their red- and blue-shifted sides increase radially from the central star of Mz3. The distribution in the position-velocity space of these knots and filaments suggests that, unlike BL2, material in BL3 is not completely confined to the walls of the conical lobes. This is illustrated by the feature seen in the echellograms at PA 98 and offset 14 South and 19 South at relative position $\sim-$15 and $(v-v_{\rm sys})\sim-$100 (Figs. 2 and 7). This feature looks like a small velocity ellipse whose spatial size and difference in velocity increase from the echellogram at offset 14 South to that at 19 South, suggesting that this filament is opening into a conical structure.

The kinematics of the knots and filaments of BL3 derived from these echellograms show that their expansion velocity follows a Hubble law. We have modeled the kinematics of the BL3 lobes assuming that they have a conical shape with opening angle $\sim$50, and that the expansion velocity is directed along the walls of the cone and increases linearly outwards from the central star of Mz3. Following this model, we have fit the observed kinematics (Fig. 7) and derived an inclination angle of the symmetry axis with the plane of the sky of 12$\pm$3. The de-projected expansion velocity at 60 from the central star would be 180$\pm$30 and the kinematical age of BL3 is (1,800$\pm$200)${\times}(\frac{D}{\rm kpc})$ yr. The inclination angle and kinematical age derived from this fit have greater uncertainty than these fitting BL1 and BL2, because the discrete nature of BL3 makes difficult to judge the goodness of the fit and to determine the best-fit parameters.

EE: the Equatorial Ellipse
--------------------------

The *HST* images of Mz3 displays an additional feature unnoticed in previous images, a closed ellipse with size 82$\times$32 oriented along PA$\sim$85, i.e., almost along the nebular equator (Fig. 1). This structure, referred to as the Equatorial Ellipse (EE), is delineated by filamentary arcs especially prominent at the northeast and southwest of Mz3.

EE is revealed as dramatic high-velocity arcs in the echelle observations along the slits oriented at PA 98 and offsets 3 North, and 4, 14, and 19 South of the central star, as well as in the slits at PAs 8, 52, 43, and $-$28 (Fig. 3). The measured expansion velocity is close to 200 with respect to the systemic velocity. It is interesting to note that the arcs in the echellograms of the slits passing through the central star are disrupted by radial filaments of BL3. It is also interesting to note that the arcs detected in the echellograms at PA 98 and offsets 3 North and 4 South show marked point-symmetry.

It is clear from these results that Mz3 shows an equatorial outflow moving at high velocity. Its three-dimensional geometry, however, is difficult to envision because the fragmented information revealed by the observations and the likely interaction of EE with BL3. In the following, we will consider four different geometrical models for this outflow: (a) an extended equatorial disk, (b) a ring collimating a bipolar ejection, (c) a pair of wide-opened bipolar lobes, and (d) an oblate ellipsoid-like shell.

Although equatorial disks have been proposed to play an important role in the collimation of bipolar PNe, there is no detection of high velocity equatorial disks in PNe. An example of equatorial disk can be found in the bipolar nebula around $\eta$ Carinae which shows an equatorial structure that seems to be an extended equatorial disk (Smith 2002). If EE in Mz3 were a circular equatorial disk, then the expansion law with radius on the disk can be inferred from any echellogram of a slit passing through its center, simply by applying a scaling factor that depends on the inclination of the disk with respect to the line of sight, because all velocities along such a line share the same inclination angle with the line of sight. For a circular disk, its inclination angle can be derived from the observed minor-to-major axes ratio of the projected ellipse. The size of EE of 82$\times$32 corresponds to an inclination against the plane of the sky of the rotation axis of the circular disk of $\sim$23. Using this value for the inclination of the disk and the information on the expansion velocity law on the slit at PA 52 passing through the center of Mz3, we have determined the expansion law with radius on the disk which is plotted in Figure 8. The velocity in the disk decreases smoothly with radius up to a given radius, when the velocity decreases sharply. Using this velocity law, we have produced the position-velocity plots expected for significative slit positions (Fig. 9). The model deviates significantly from the position-velocity arcs observed along PA 98 with different offsets from the central star (Fig. 9). We conclude that EE cannot be interpreted as an expanding disk.

A detailed study of the spatio-kinematical properties of an expanding ring collimating a pair of bipolar lobes is presented by @SU85 for the bipolar nebula around the symbiotic Mira variable R Aqr. In an expanding ring, the ring itself projects an ellipse onto the sky, long-slit echellograms along the ellipse major axis show two arcs in the position-velocity space, one shifted to the blue and the other to the red, and long-slit echellograms along the ellipse minor axis reveal a characteristic hourglass shaped line. The morphology and kinematics of Mz3 observed in the echellograms of the slits along PA 98 are compatible with this model expectations; however, the slit at PA 98 and offset 26 South of the central stars does not detect emission outside the observed ellipse, nor the slits at PAs 52, 43, 8, and $-$28 show the expected hourglass shape. We conclude that an expanding ring that collimates bipolar lobes is not appropriate for the three-dimensional geometry of EE.

We have also considered the possibility that EE is composed by a pair of wide-opened, champagne-glass-shaped bipolar lobes tilted with the line of sight so that the flow vector points almost directly to us at the location of the equatorial waist. This model would explain the observed kinematics: at locations near the central star, the observed velocity is large because the line of sight is close to the direction of the flow vector, while at increasing distances from the central star, the lobes bend and close so that the direction of the flow vector diverges from the line of sight and the observed velocity decreases. The projection of these lobes onto the plane of the sky, however, would not produce an elliptical shape, but two interwined arcs pointing at opposite directions, as observed, e.g., in the central regions of MyCn18 [@Sahai99]. We thus disregard this model as the three-dimensional geometry of EE.

Finally we consider an oblate shell which expands much faster along the equator than along the poles and whose symmetry axis is close to the plane of the sky. Assuming a homologous expansion for this shell, we have produced synthetic position-velocity plots that can be compared with the observed ones to determine the best fit parameters (Figure 10). The best-fit shell model is a flat ellipsoid-like whose symmetry axis is tilted against the line of sight by 70$\pm$5, and the ellipsoid-like has an equatorial expansion velocity $\simeq$200, a polar velocity $\simeq70\pm$20, and a kinematical age (1,000$\pm$50)${\times}(\frac{D}{\rm kpc})$ yr. This model explains satisfactorily the high velocity arcs observed in the slits at PA 98. It also accounts for the disruption of EE by BL3, which has bored a hole near the polar regions of EE. Finally, this model also explains the point-symmetric distribution of arcs observed in the slits at PA 98 and offsets 3 North and 4 South; at these locations, the detectability of the shell is optimized because the shell is seen tangentially and the optical path is thus larger than at other locations.

The Multipolar Structure of Mz3
===============================

Previous spatio-kinematical studies of Mz3 have revealed an increasing level of complexity in this nebula. @LM83 studied the inner bipolar lobes (BL1) and concluded that these lobes are hourglass in shape with the symmetry axis close to the line of sight. In a later paper, @MW85 determined with greater accuracy a spatio-kinematical model of the inner bipolar lobes. Moreover, they extended the spatio-kinematical study of Mz3 to the outer regions, reporting the presence of different sets of bipolar lobes. The low spatial resolution of the narrow-band images available by then, however, hampered Meaburn & Walsh’s study: the bipolar lobes BL2 and BL3 were not distinguished from each other; the equatorial ellipse was interpreted as an additional bipolar lobe; and the detection of a high-velocity component in the Na[i]{} line, correctly interpreted as related to a high-velocity outflow from Mz3, was not associated to the equatorial ellipse EE. More recently, @Retal00 obtained high-dispersion spectroscopic observations along the major axis of Mz3 that allowed them to describe the kinematics of the bipolar lobes BL2 and to find high-velocity, $\sim$200, components at the location of the blowout at the tips of the inner bipolar lobes BL1. Because of the limited spatial coverage of their study, the association between these high-velocity kinematical components and the equatorial ellipse EE was not as clearly seen as evidenced in our echelle observations obtained at different slit positions (Fig. 3). Finally, in a simultaneous study of Mz3, @SG04 have derived spatio-kinematical models and kinematical ages for the three pairs of bipolar lobes that are in complete agreement with these derived here.

The present study reconciles many of the previously reported kinematical features of Mz3 into a more comprehensive view of its physical structure. Mz3 consists of four distinct structures, an oblate ellipsoid-like shell and three pair of bipolar lobes with almost coincident symmetry axes. The properties of these structures are especially singular among similar structures observed in bipolar PNe. Unlike the slowly expanding rings or tori observed in some bipolar PNe, the oblate ellipsoidal-like shell expands at high velocity along the equator of the bipolar lobes. Similarly, very few multipolar PNe have pairs of lobes exhibiting the notable differences in opening angle, morphologies and detailed small-scale structures as the three pairs of bipolar lobes of Mz3; BL1 has hourglass-shaped expanding bubbles filled with X-ray-emitting hot gas (Kastner et al. 2003), while BL2 and BL3 are composed of knots and filaments following a Hubble flow with cylindrical and conical shapes, respectively.

Multipolarity has become a common feature among bipolar nebulae. A growing number of bipolar nebulae have been noted to have multiple systems of bipolar lobes either sharing the same symmetry axis or having different symmetry axes, e.g., M2-9, M2-46, NGC2440, and Hen2-104 [@HL94; @MGS96; @LMBH98; @Solf00; @Corradi01]. Among these multipolar nebulae, the case of Mz3 is of especial interest because the kinematical ages of the different systems of bipolar lobes in Mz3 are small and of the order of the difference in kinematical ages among them. The inner bipolar lobes BL1 have a kinematical age 500–600${\times}(\frac{D}{\rm kpc})$ yr, the cylindrical lobes BL2 and the equatorial ellipsoid EE are $\sim$1,000${\times}(\frac{D}{\rm kpc})$ yr old, and the outer bipolar lobes BL3 have a somewhat more uncertain kinematical ages of $\sim$1,800${\times}(\frac{D}{\rm kpc})$ yr. Mz3 is thus a multipolar nebula in the making, where BL1 corresponds to the most recent ejection from Mz3 central star, EE and BL2 are older and probably coeval, and BL3 is finally the oldest structure, although its kinematical age is the most uncertain and we cannot rule out a formation closer in time to that of BL2.

The different kinematical properties of the three pairs of bipolar lobes suggest distinct formation scenarios. The ballistic motion of the two outermost bipolar lobes of Mz3, BL2 and BL3, indicates that the gas within these lobes expands freely under its own inertia. Most likely, these lobes are the result of two episodes of explosive mass ejection or outbursts that occurred $\sim$1,800${\times}(\frac{D}{\rm kpc})$ yr and $\sim$1,000${\times}(\frac{D}{\rm kpc})$ yr ago. The last episode of mass ejection responsible of BL2 also resulted in high velocity ejecta along the equatorial plane that formed EE, the equatorial ellipse. On the other hand, the morphology and hot gas content of the innermost pair of lobes, BL1, indicate that they resulted from the interaction of highly pressurized hot gas with the surrounding material. This hot gas may be produced by the onset of a fast stellar wind. An alternative origin has been proposed by @Kastner03 who attribute the X-ray emission to the action of an X-ray jet along the symmetry axis of Mz3. Our observations indeed reveal bipolar collimated outflows along the symmetry axis of Mz3 (the knots further away the leading edges of the BL1 lobes as seen in Fig. 4), but not with the high velocities required to produce the observed X-ray emission. Note, however, that the outflow detected in our observations may trace high density material accelerated by a much higher velocity jet that, being responsible of the X-ray emission, would elude optical detection because its low density.

The oblate shell forming the equatorial ellipse EE of Mz3 is a very singular structural component. Many bipolar PNe show equatorial disks or tori, but all of them have modest expansion velocities, $\sim$30. Bipolar nebulae around symbiotic stars also show equatorial disks or tori, but expansion velocities are modest, too. The only exception among symbiotic stars is the remarkable elliptical shell or ring around Hen2-147 with an expansion velocity $\sim$100 [@Corradi99]. Thus, the $\ga$200 equatorial outflow in Mz3 is the most extraordinary among bipolar PNe and nebulae around symbiotic stars.

The equatorial outflow of Mz3 rivals that of the massive star $\eta$ Car. The equatorial outflow around $\eta$ Car shares many similarities with this found in Mz3: the nebula around $\eta$ Car has several systems of bipolar lobes [@Ishibashi03] and the formation of the equatorial outflow has been timed during or around the moment when the main bipolar lobes in $\eta$ Car, the Homunculus Nebula, were formed. Despite these similarities, the equatorial outflows in both nebulae are notably different. The equatorial outflow in $\eta$ Car has been described as an extended equatorial disk expanding with velocity proportional to the angular distance to center [@Davidson01; @Smith02], while the physical structure of the equatorial outflow in Mz3 is best described by an oblate shell. Furthermore, their detailed morphologies are different and very likely indicate different origins: in $\eta$ Car, the equatorial outflow seems to be composed of multiple jet-like features located along the equatorial plane, while in Mz3 the equatorial outflow shows the limb-brightened morphology characteristic of a thin shell.

The formation of multipolar nebulae can be explained as the result of recurrent outbursts as those observed in massive stars in binary systems during the Luminous Blue Variable (LBV) phase, e.g. $\eta$ Car. In low mass stars, recurrent outbursts can be related to nova-like eruptions on the accreting hot component of a symbiotic star or to structural instabilities in the late evolution of the central star of a PN (e.g., thermal pulses). In symbiotic novae, the timescales of successive outbursts are determined by the mass of the accreting white dwarf, the mass loss rate of the red giant, and the accretion efficiency of the wind capture which is related to the binary interaction [e.g. @PK95]. Recurrence periods of a few hundred years are typical of symbiotic novae [@PK95; @Corradi99]. The formation of multipolar PNe is far more difficult to explain, as it requires the alternation between a dense, slow wind and a fast, tenuous wind. The evolution of the central star of the PN in a binary system provides a natural scenario for recurrent outbursts during the evolution through a common envelope phase or as the result of accretion and nova-like outbursts on the white dwarf component of a symbiotic star. This raises the similarities between Mz3 and other symbiotic stars like R Aqr and Hen104, or other suspected symbiotic stars yet classified as PNe, e.g., M2-9, and casts doubts on the true nature of Mz3 as a PN.

Even if we accept that Mz3 has formed as the result of recurrent nova-like outbursts in a symbiotic star, the physical structure of this bipolar nebula is rather unique. The successive collimated ejections in Mz3 are rather regular in time, but they have very different morphological and kinematical properties, which suggest very distinct conditions and formation mechanisms. In Mz3, we are thus witnessing the formation of a multipolar nebula which evolves dramatically between periodic outburst episodes.

M.A.G. and L.F.M. acknowledge support from the grant AYA 2002-00376 of the Spanish MCyT (cofunded by FEDER funds). We thanks Miguel Santander García for providing us with the results on their spatio-kinematical modeling of Mz3 before publication. We also thank the referee, Dr. Matt Redman, for his valuable comments.

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[lrcl]{}       H$\alpha$ & 350     & PC1 &    9050\
      H$\alpha$ & 900     & WF3 &    6856\
      H$\beta$ & 1300     & WF3 &    6502,6856\
      \[N [ii]{}\] & 1300     & PC1 &    9050\
      \[N [ii]{}\] & 900     & WF3 &    6856\

[rrr]{} 0     & 8          & 600        \
2 W & 43          & 900        \
0     & 52          & 900        \
3 N  & 98          & 1800        \
4 S  & 98          & 1800        \
8 S  & 98          & 1800        \
14 S  & 98          & 1800        \
19 S  & 98          & 1800        \
26 S  & 98          & 1800        \
0     &$-$28          & 900        \

[lrrrrr]{} Southern Lobe & 100     & 15     & 15 & 8 & 600    \
Northern Lobe & 80     & 15     & 20 & 12 & 600    \
Northern Lobe and Blister & 140     & 15     & 20 & 10 & 520    \

[^1]: Reported by M. Markevitch (2001), available at http://cxc.harvard.edu/contrib/maxim/bg/index.html.

[^2]: The distance to Mz3 is highly uncertain. Hereafter we have chosen to show explicitly the dependence of the kinematical age on distance.

[^3]: Actually, these are not cylinders, as they are opening gradually with distance from the central star, but the angle of divergence, $\sim$5, is too small to affect significantly the model fits.
