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these conference proceedings are the condensed result of the 2nd meeting on asymmetrical pne , held at mit during august of 1999 . being the last such meeting of this millennium , and a successor of the successful 1st asymmetrical meeting held at oranim in israel during august 1994 , and published as anns.isr.phys.soc.,11 in 1995 , it was especially good to see that the subject has made significant progress over the last lustrum . the main impact , as you can see immediately by just a quick skim through this volume , has come from the imaging done by the hubble space telescope . the wealth of complex detail revealed by these unprecedented images has left the theorists reeling , the observers gasping , and both groups with plenty ground  based work ahead of them . a special highlight of the meeting was the free hand  out to participants of a cd with superb wfpc2 images by hajian & terzian ! note that references to papers in this volume are from here on indicated by an asterisk next to the authors name . my overall impression from this meeting is that the observed morphologies of pne are shown to be more and more complex , mainly due to the images from hst . the circle with a dot in the middle of 15 years ago is no more ! the impact of the increased spatial resolution data available from the space observations is extraordinary ! archival research with hst data is now becoming a important tool for pne research , especially in combination with ground  based high resolution spectroscopy . to date there are about 125 hst images of pne available . building on the basis created by the work of josef solf in the early eighties ( e.g. solf 1983 , 1984 ) , the spatial resolution and spectroscopic separation at the sub pixel level of small features is now coming of age , and is teaching us a lot about the formation of pne . all this detail , however , is clearly posing a problem for the theorists : the complexity is enormous and at present theory understandably lags the observations . + having said this , theory has also made progress ; several models are getting better at producing the observed gross shapes of nebulae ( frank , garca segura , livio , soker and others ( * ) ) , and attempts at explaining small scale structure ( dwarkadas ( * ) ) are being made . the overall theoretical picture , however , is still quite elusive . mass loading obviously plays an important role in some objects ( e.g. globules in the ring nebula ( dyson ( * ) ) , and perhaps also in ngc6369 , and ngc6751 just from their complex morphology , and the theory for this phenomenon is well developed . symmetry has become very popular and it seems that there are hardly any asymmetrical pne left that do not have some kind of point  symmetry . i recall that at the meeting in la serena ( mass loss on the agb and beyond ) held in 1992 , the concept was hardly known . + the use of movies to show time dependent behaviour of data is also on the increase with several presentations both of observational and theoretical material at the meeting , giving some interesting new ways of visualising data . the movie of m29 s inner nebula ( balick ( * ) ) especially struck me , since it showed the nebula s rotation , which would have got lost had the data been presented any other way . to `` see '' the ism penetrating a pn ( villaver ( * ) ) and thus creating an asymmetry was also spectacular . internet access to this kind of movie is also an excellent way of showing the community what is happening . as a result of the increasing complexity of the observed morphologies , the terminology has recently also proliferated . table  1 lists the terms i heard at this meeting to describe features in pne . they range from the practical to the sublime , and even beyond ... +    c|c|c|c|c|c|c|c    jets & fliers & rings & cocoons & toroidal cocoons + disks & ansae & shells & trails & parallelograms + haloes & real haloes & blobs & lobes & bullets + tori & structures & knots & filaments & bubbles + forks & waists & hoops & loops & double bubbles + tips & cylinders & envelopes & funnels & tuning forks + stripes & dark lanes & arcs & caps & capped bicones + tails & remnants & globules & cometary ... & nipples + rays & corkscrews & rolls & ellipsoids & shadow columns + lis & lattices & petals & slobs & smoking guns +    `` paradigm '' is definitely in , the previously much over - used `` scenario '' is out ; `` weather '' ( which you ignore ... ) is in ; `` physics '' ( ... which you do nt ) is out . other much used key words were : precession , point symmetry , brets , episodic or periodic outflows , irregular , mass loading . finally , a nice piece of nomenclature was reportedly found in the address of a famous observatory , the `` navel absorbatory '' . + it is clear that astronomers are also influenced by `` fashions '' or `` trends '' . please start a trend and use the word `` polarigenic '' ( coined by yours truly about 15 yrs ago ) , as in `` polarigenic mechanism '' for something that produces polarisation . figure  1 shows the possible appearances of bipolar and point symmetric nebulae as projected onto the plane of the sky . under certain circumstances blue and red shifted components can be located on the same side of the object , even though this is , at first sight , counter intuitive . for objects with a precession cone cut by the line of sight or plane of the sky this is observed ( e.g. ic4634 ) and such objects are point  symmetric . m29 is a bipolar that shows plane symmetry in it s inner nebula , and point  symmetry in it s outer lobes which are also both red shifted since they are reflecting light from the central object . so far this is unique among pne . nebulae are put into the following classes :    bipolars : no ( near , or not too far from plane of sky ) , eo ( nearly end  on ) , rr ( both lobes red shifted )    point symmetricals : p1 ( 0 @xmath0 i @xmath1 0.5@xmath2 ) , p2 ( 0.5@xmath2 @xmath0 i @xmath1 90 - 0.5@xmath2 ) , p3 ( 90 - 0.5@xmath2 @xmath0 i @xmath1 90 ) , where @xmath2 the precession cone opening angle , and i is the inclination angle to the plane of the sky of the nebula . some lines of research are promising and should be continued in the near future . here i mention a few , based on what i heard and saw at the meeting , that i find of particular potential interest . the whole issue of what kind of mechanism drives the production of the most asymmetrical and physically different class of pne , the bipolar nebulae ( corradi & schwarz , 1995 ) , depends critically on finding the binary central stars . there is strong circumstantial evidence for the presence of binaries in several bipolars , but it has so far been very difficult to obtain direct determinations of the numbers of bipolars and other pne that contain binary central stars . a promising method is that of radial velocity determinations , especially in the infra  red , where obscuration is less of a problem . bond ( * ) has done pioneering imaging work with hst , which is rightly being continued . + from the theorist s point of view the single versus binary star model for bipolar nebulae is split into two parts : the making of a bipolar nebula starting with an asymmetrical density distribution , and the mechanism to produce that initial density distribution in the first place . to make a bipolar nebula with some outflow in an asymmetrical density distribution , is relatively easy and many models can do this . particularly successful are the models of garca  segura ( * ) , a two wind interaction combined with magnetic field and near break  up rotation , and the work by frank et al . ( * ) based originally on work by icke ( 1988 ) using the kompaneets theory . the nebulae with the highest aspect ratios , such as m29 , pose more of a problem as the required density gradient and other parameters become un  physically high . here the binaries form a more natural system to generate a strong pole to equator density gradient , and their accretion disks can blow disk winds that are highly collimated and fast , explaining the high observed velocities , the strong collimation , and the large wings of the observed hydrogen line profiles . excretion disks , as proposed by morris ( 1987 ) , can shape the slow wind from the primary star into two lobes , through which the fast wind then blows out hh  like objects as in he2 - 104 . in single stars there is no identified mechanism to produce the necessary high underlying density gradient that shapes the wind , and a particular combination of strong magnetic fields , rotation , and equatorial mass loss has to be invoked to give asymmetrical nebulae at all . my overall impression is that binaries are needed for the most extreme bipolars and perhaps for all bipolars . symmetry is most easily explained by precession ; again something that occurs naturally in binaries . also the combination in m29 of both point and plane symmetry is difficult to explain without a binary central object , as is the presence of [ oiii ] lines and a low luminosity central object . i think that an object like he31475 ( see the hst image on the cd handed out at the meeting ) can not be explained by any single star model ! we must go and find those binaries ! there were several papers on the interactions of pne with the ism ( knill  degani ; theory , kerber ; observations , villaver ; modeling , with movie ( * ) ) . this is clearly a very interesting asymmetry producing mechanism , especially affecting the outer parts of the pne , and in some cases even resulting in stars being outside their own nebula . the ratio of density between a typical pn and the ism is about 50 in the galactic plane and 300 in the halo . under certain circumstances the ism can penetrate the pn , giving spectacular interactions , as shown in the presented movie . the morphologies derived from hst images of a sample of pne taken and to be taken in the magellanic clouds , can yield important relationships between morphology and the central star evolutionary stage . this work was started by using galactic pne , extracting statistical information about a sample of which narrow band images had been taken , and making a morphological classification of the nebulae . the result was a plot of their central stars in the hr diagram as a function of morphological class , yielding some intriguing correlations , especially for the bipolars ( stanghellini et al.1993 ) . the large uncertainty in the individual distances to the pne in this sample make this result only statistically interesting . by going to the clouds , the distance uncertainty is gone and much better results can be obtained , again using hst . stanghellini is undertaking this important work . polarimetry was meagrely represented at this conference , as at most meetings that do not specifically deal with the subject . only one out of 55 talks and 2 out of 38 posters contained some item about polarimetry . since polarimetry often can provide a missing and vital piece of information , this is a plea for all observers to use and be more aware of polarimetry . the fact that the title of this meeting contains the word `` asymmetrical '' should already make people think about polarimetry , as it is uniquely suitable to detect asymmetries . + two examples of the importance of polarimetry are : trammell et al . ( 1994 ) found that 77% of 31 agb objects are intrinsically polarised , while johnson and jones ( 1991 ) found 74% of their sample to be polarised . there clearly is a high fraction of agb stars with asymmetries . if there are no features in the polarisation spectrum , there has to be a global asymmetry in the object , and probably dust scattering ; if there are spectral features , local asymmetries such as convection cells or other atmospheric features must be present . the whole critical question of the onset of asymmetry during late stellar evolution , whereby essentially spherical stars produce pne of which many are asymmetrical , can be uniquely studied with polarimetry . the mechanism of dust scattering in the faint loops in m2 - 9 ( schwarz et al . 1997 ) was a supposition until the loops were found to be 60% polarised at right angles to the plane formed by the source , scatterer and observer . such high degree of polarisation is not only easy to observe it also firms up the proposed dust scattering mechanism to a certainty . this then allowed the distance to m29 to be determined accurately from it s expansion parallax . polarimetry provided the key piece of information that allowed the physical parameters of this object to be found . + continued and increased work using polarisation measurements on pne and agb stars  such as that in the press ( apj ) by weintraub , kastner ( who showed the data at this meeting ) , sahai , & hines , who used nicmos to image afgl2688will surely yield important results ! + in summary , this was a meeting clearly dominated by the observers using hst imagery making theorist s lives difficult . the old chinese curse `` may you live in interesting times ! '' is applicable here , and times will become even more interesting in the near future . thanks for inviting me , joel and noam , and see you at the next asymmetrical meeting ! corradi , r.l.m . & schwarz , h.e . 1995 , aa , 293 , 871 icke , v. 1988 , aa , 202 , 1771 johnson , j.j . & jones , t.j . 1991 , aj , 101 , 1735 morris , m. 1987 , pasp , 95 , 1115 schwarz , h.e . 1997 , aa , 319 , 267 solf , j. 1983 , apjl , 266 , l113 solf , j. 1984 , aa , 139 , 296 stanghellini , l. , corradi , r.l.m . , schwarz , h.e . 1993 , aa , 279 , 521 trammell , s.r . , dinerstein , h.l . , goodrich , r.w . 1994 aj , 108 , 984
this paper gives a personal impression of the conference `` asymmetrical pne ii : from origins to microstructures '' , flags some of the highlights , gathers together some facts and terminology , and indicates some promising future lines of work in this field . 
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