A VISIR Mid-infrared Imaging Survey of Post-AGB Stars

A VISIR Mid-infrared Imaging Survey of Post-AGB Stars
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  21 The Messenger 144 – June 2011  Astronomical Science  A VISIR Mid-infrared Imaging Survey of Post-AGB Stars cumstellar material in either a dusty torus or a disc. Our team has discovered somediscs/tori in the heart of PNe (Lagadecet al., 2006; Chesneau et al., 2006;Matsuura et al., 2006; Chesneau et al.,2007) using adaptive optics on ESO’s Very Large Telescope (VLT) and mid-infrared (MIR) interferometry at the VeryLarge Telescope Interferometer (VLTI).But the role of these discs/tori in theshaping of the nebulae is still unclear, aswe know neither the fraction of the totaldust mass that is present in these cen-   tral cores, nor the fraction of objectsexhibiting such a disc/torus structure.In order to observe the inner region of post-AGB stars, we need MIR observa - tions, as the dust optical depth is smallerat longer wavelengths. The MIR is the only wavelength range at which we can ob- serve the inner morphology of stars fromthe AGB to the PPN phase. Furthermore,the main source of radiation for thesesources in the MIR is direct emission from dust, while at shorter wavelengths it is scattered light. Mid-infrared imaging is thus the best way to study the dusty structures inside these evolved stars.Many MIR imaging observations of post- AGB stars have been made in the past.But the only available MIR imaging survey(Meixner et al., 1999) has been madewith 3-metre-class telescopes and suf  - fers from limited angular resolution for themorphological study of the observedobjects; in addition the survey selection isbiased, as known bipolar nebulae wereobserved. The survey consisted of only17 resolved sources. Some work hasbeen done using 8-metre-class tele - scopes, but always focusing on particularindividual bright, well-known objects.Here we present the results from the rstmid-infrared N  -band imaging survey of a large number of post-AGB stars with8-metre-class telescopes. We aim at asystematic survey probing the inner dustyregions of post-AGB stars.  Target selection and observations  The large sample of observations camefrom ve distinct observing runs from VISIR/VLT, Michelle/Gemini North and T-Recs/Gemini South. Eric Lagadec 1 Tijl Verhoelst 2 Djamel Mekarnia 3 Olga Suarez 3, 4  Albert A. Zijlstra 5 Philippe Bendjoya 3 Ryszard Szczerba 6 Olivier Chesneau 3 Hans Van Winckel 2 Michael J. Barlow 7 Mikako Matsuura 7, 8 Janet E. Bowey 7 Silvia Lorenz-Martins 9 Tim Gledhill 101 ESO 2 Instituut voor Sterrenkunde, KatholiekeUniversiteit Leuven, Belgium 3 Fizeau, OCA/UNS/CNRS, Nice, France 4 Instituto de Astrosica de Andalucia,Granada, Spain 5 Jodrell Bank Centre for Astrophysics,University of Manchester, UnitedKingdom 6 N. Copernicus Astronomical Center,  Torun, Poland 7 University College London, UnitedKingdom 8 Mullard Space Science Laboratory,Univerisity College London, UnitedKingdom 9 Universidade Federal do Rio de Janeiro, Brazil 10 University of Hertfordshire, UnitedKingdom Post asymptotic giant branch (AGB)stars are key objects for the studyof the dramatic morphological changesthat low- to intermediate-mass starsundergo during their evolution from the AGB towards the planetary nebulastage. There is growing evidence thatbinary interaction processes may playa determining role in shaping manyobjects, but so far direct evidence forbinarity is still weak. We report on asystematic study of the dust distributionaround a large sample of post-AGBstars that probes the symmetry-break-ing in the nebulae around these systems.  According to our current understanding of stellar evolution, all stars with mainsequence masses in the range 1–8 M  A   evolve via the asymptotic giant branchphase to the planetary nebula (PN) stage. As they ascend the AGB, their mass-lossrate increases from solar-like values of  10 –14   M  A  /yr up to 10 –4   M  A  /yr. The inte - grated mass lost from these stars is anessential component of galactic evolu - tion, as it is the main source of   s -processelements in the Universe and the mainproducer of carbon. AGB stars are alsothe main contributors to the dust phaseof the interstellar medium (ISM). Duringthe last stages of AGB evolution, theremains of the convective hydrogen enve - lope are ejected during nal, quiescentand sporadic mass-loss events. Dustgrains and molecules, predominantly CO,form in their winds, forming large circum - stellar envelopes that can be detectedin the infrared and millimetre domains. A departure from spherically symmetric mass-loss is observed in a substantial fraction of suspected AGB to PN transi - tion objects. In particular, multipolarstructures are often associated withprotoplanetary nebulae (PPNe). Thethree-dimensional morphologies of theseobjects are projected on the sky, thusmaking it difcult to determine theintrinsic morphology of PPNe and PNe.But it is estimated that around 80%of all PNe show aspherical morphologies.Hubble Space Telescope observationsof PNe, for example, show a large rangeof morphologies, including elliptical,bipolar, multipolar or round nebulae.Hydrodynamical models explain manyof the observed structures from a struc - ture-magnication mechanism, wherea fast wind from the central star of the PNploughs into the earlier slow AGB wind,amplifying any density asymmetry alreadypresent: this is the generalised interact-   ing stellar wind model (GISW). Anothermodel has also been proposed to explainthe shaping of PNe. In this model shap-   ing occurs at the end of the AGB phase,when fast collimated jets are triggeredand shape a bipolar nebula. If the direc - tion of the jets changes with time, thenmultipolar nebulae can be formed.Such jets could be formed through inter - action with a companion, e.g., in anaccretion disc (see Balick & Frank [2002]for a review).Much of the theory of the shaping of PNand PPN relies on the presence of cir -  22 The Messenger 144 – June 2011  Astronomical Science Lagadec E. et al., A VISIR Mid-infrared Imaging Survey of Post-AGB Starsclose to the central star; the objects with detached shells are characterised by the presence of a clear double-peakeddistribution to the SED, with a rst peak at a wavelength shorter than 1 μm dueto the central star, and a second peak due to the cool dust in the shell. The ux is much lower in the near-IR due to the absence of dust close to the central star.Figure 3 shows examples of both SEDs.  The very strong correlation between the presence of a dense core and the bipo-   lar morphology is an indication that thedense cores play a role in the shaping of the nebulae. Two main classes of modelshave been proposed to explain the shap - ing of nebulae. The rst class of modelsis based on the GISW models and herea fast wind from the central star of a PPN or PN interacts with a slower wind, a remnant of the AGB phase, assumed tobe toroidal. In the second class of mod - els, the primary shaping agents are high-speed collimated outows or jets that arecreated at the end of the AGB phase orat the beginning of the PPN phase. Theinteraction of these jets with a spherical AGB wind will create lobes that are in factOur sample contains the brightest post- AGB stars observable from Paranal andincludes PPNe, R CrB stars, RV Tauristars and “Water Fountains”. R CrB starsare hydrogen-decient post-AGB starswith known obscuration events. RV Tauristars are pulsating post-AGB stars,located at the high luminosity end of thePopulation II Cepheid instability strip. These RV Tauri stars are likely to harbourcompact dusty discs. Water Fountains are oxygen-rich PPNe characterised by the presence of blue and red-shifted OH and H 2 O masers, hence their curious class name. Some  bona fde PN andevolved massive stars were also observed.Most of the observations were obtainedwith the MIR instrument VISIR on the VLT. The excellent observing conditionsduring our run (0.43 mm of precipitablewater vapour in the atmosphere), com - bined with the burst mode on VISIR,allowed us to obtain high quality diffrac - tion-limited images. The burst modereadout allows every single frame of anexposure to be saved. In this way it ispossible to follow rapidly evolving eventsor to improve the spatial resolution bytaking short enough exposures to freezeatmospheric turbulence, as in luckyimaging (e.g., Law et al., 2006). This modecan be used only for objects that arebright enough to provide a high enoughsignal-to-noise (S/N) in a single elemen - tary frame.We thus observed 93 evolved stars ina quasi-uniform way in the mid-infrared,with a spatial resolution of the order of 0.3 arcseconds (which is the diffractionlimit at this wavelength). Two kinds of objects: resolved cores and detached shells From the set of 93 objects we observed,59 appear as point sources. Among the extended targets, we resolved a wealth of different structures, such as resolved central cores, dark central lanes, de- tached shells, S-shaped outows. If weconsider only the PPNe from our sample,we arrive at a sample of 52 detectedobjects. For the largest objects that are clearlyresolved, we notice that the PPNe can be divided into two categories. On the one hand the objects with a dense cen- tral core, in the form of a bright centralsource or a dark lane, with most of theemission coming from the poles, indicatethe presence of a large amount of dust,making the central regions optically thick even in the MIR. Four examples are shownin Figure 1. On the other hand, some ob-  jects do not have such a dominant cen-tral core, and we can observe either a detached shell or the central star (exam - ples in Figure 2). The objects without acentral core all have an elliptical morphol -ogy, while the objects with a central core are either bipolar or multipolar. Bothtypes of sources are differentiated in theirspectral energy distribution (SED): the objects with a dense central core or an equatorial dark lane have a rather atSED in the near-infrared wavelengthrange, due to the presence of hot dust IRAS 10197IRAS 16594IRAS 17311IRAS 174411  1  1  1  Figure 1. VISIR 10 μm images of a sample of post- AGB stars with resolved central cores. The desig-nation of each target is given and the spatial scaleis shown.  23 The Messenger 144 – June 2011 Most AGB stars have a large-scale circu - larly symmetric morphology, while PNedisplay a variety of morphologies fromelliptical to bipolar or multipolar. Opticalimaging surveys of PNe indicate that80% of the PNe show a clear sign of departure from circular symmetry, andthus that approximately 20% of PNeare spherical. The shaping of the PNe isthought to occur at the very end of the AGB phase or the beginning of the PPNphase. It is thus surprising that in oursample of 25 resolved PPNe, we do notnd any circular ones. The fact that we do not observe anycircular PPNe could be a sample selec - tion effect. We selected our targetsas bright IRAS 12 μm sources. To be a bright emitter at these wavelengths,an object needs to have dust with a tem- perature of approximately 300 K, whichmust therefore be located close to thecentral star. This is the case for the starswith a central core, which are aspheri-   cal. The spherical PPNe are fainter thanthe non-spherical ones in the mid-infrared,due to the lack of a central torus/discemitting in this wavelength range. At theend of the AGB phase, the envelopesof the AGB progenitors of circular PPNeare ejected and rapidly cool down whileexpanding. There are thus very fewspherical PPNe that are bright in the mid-infrared. Furthermore those bright PPNeare compact and thus difcult to spa-   tially resolve. The best way to detect suchspherical envelopes is thus at longer wavelengths, and such detached shells are actually observed in the far infraredwith the Herschel Space Observatory.Formation of point-symmetric structures A few objects that we resolved display apoint-symmetric morphology, with anS-shaped envelope. One of our targets,IRAS 17441 (shown lower right in Fig - ure 1), displays a tilt between the orienta - tion of the resolved central dusty torusand the tips of the observed S-shapedstructure. Such a tilt was observedby Volk et al. (2007), and measured tobe almost 90 degrees. They suggestedthat a precession of the dusty toruscould explain the observed S-shapedstructure of the nebula. They estimatedcavities. If the direction of the jetschanges with time, multipolar nebulaecan be shaped.Both models require the presence of acentral torus/disc in the core of the neb - ula. Our observations clearly indicatethat the bipolar and multipolar nebulae do have such a central structure in their core.Departure from circular symmetry  All the PPNe we resolved in our survey show a clear departure from circular sym - metry. Some circular shells are resolved,but only around massive evolved stars. A dramatic change in the distribution of the circumstellar material is often ob-served when a star evolves from the AGBto the PN phase (Balick & Frank, 2002). Figure 3. Typical spec -tral energy distributions of the two classes of objects observed. Left: an object with a dense equatorial dusty torusand a bipolar/multipolarmorphology. Right: an object with a detached shell and an ellipticalmorphology. IRAS 07134IRAS 17163IRAS 19374IRAS 195001  1  1  1  Figure 2. VISIR 10 μm images of a sample of post- AGB stars with detached shells. Annotations are asin Figure 1. 17441 IRAS 1950010 100Wavelength (m)    F   l  u  x   (   J  y   )   F   l  u  x   (   J  y   ) 111010100100Wavelength (m)  24 The Messenger 144 – June 2011 evolution of low- and intermediate-massstars. This dust is ejected into the ISMduring the PPN phase. Our observationsshow the existence of two paths for this dust ejection, via a detached shell or an expanding torus. It is now becomingclear that the bipolar objects with anequatorial torus have been formed via theinteraction with a binary companion. This opens up a new eld of research tostudy the impact of the presence of abinary companion on the dust formationby evolved stars. In order to better under - stand the importance of PPNe for thelife cycle of dust, it would be interestingto study how these different paths affectthe dust production by these objects.Spatially resolved VISIR mid-infraredspectra of these sources will allow us tostudy the dust composition at different locations in these PPNe, enabling us to better understand the evolution of dustduring the PPNe phase, from its forma - tion to its ejection, and how the presenceof a dense dusty disc affects its compo - sition.Finally, the mass loss from evolved starsis a key ingredient for our understand-   ing in many elds of astrophysics, includ -ing stellar evolution and the enrichment of the ISM via stellar yields. The aim of anongoing ESO large programme (PI: C.Paladini), which will combine MIDI + VISIR+ Herschel observations of a sample of  evolved stars, is to constrain the geome- try of this mass loss at different spatialscales. We will then be able to fully un-derstand the dust evolution from its for - mation in a circumstellar envelope until itsinjection into the ISM, and better under - stand the life cycle of dust. ReferencesBalick, B. & Frank, A. 2002, ARA&A, 40, 439Chesneau, O. et al. 2006, A&A, 455, 1009Chesneau, O. et al. 2007, A&A, 473, L29Corradi, R. L. M. & Schwarz, H. E. 1995, A&A,293, 871Guzman-Ramirez, L. et al. 2011, MNRASLagadec, E. et al. 2006, A&A, 448, 203Lagadec, E. et al. 2011, MNRAS, in pressLaw, N. M. et al. 2006, MNRAS, 368, 1917Matsuura, M. et al. 2006, ApJl, 646, L123Meaburn, J. et al., 2008, MNRAS, 385, 269Meixner, M. et al. 1999, ApJS, 122, 221Stanghellini, L. et al. 2007, ApJ, 671, 1669Soker, N. 1998, ApJ, 496, 833 the dynamical age of the envelope,assuming a distance of 1 kpc and anexpansion velocity of 100 km/s, tobe approximately 100 yr. According tothis model, the torus should thus pre-   cess with a rate of around 1°/yr. As ourobservations were made four years afterthe observations presented by Volk etal. (2007), we should see a tilt of the torusof about 4° between the two observa - tions. The images provided by theseauthors show that the orientation of the torus that they observed is exactly the same as the one we observed. The torusin the core of IRAS 17441 is thus not pre - cessing at such a high rate.It appears that we cannot explain theobserved S-shaped structures with aprecession of the central tori. This couldbe due to the fact that we underesti - mated the dynamical age of the nebula orthat another mechanism is responsiblefor the S-shaped structure. A more plau - sible explanation is that the S-shapedstructure is not due to the precession of the torus itself, but to precessing out - ows inside this torus. The presence of such outows has been observed inthe PN NGC 6302, which has a morphol - ogy very similar to that of IRAS 17441(Meaburn et al., 2008). These outowsare of a Hubble-type, which means thattheir velocity is proportional to the dis - tance from the source. A torus similar tothe ones observed in the core of theseobjects is also seen in the core of NGC 6302. The properties of such out - ows can be theoretically describedby a sudden ejection of material, a “bul - let”. Such bullets naturally account formultipolar ows, which could arise natu - rally from the fragmentation of an ex-plosively driven polar-directed shell. Wepostulate that the S-shaped structureobserved in IRAS 17441 is due to highspeed outows triggered at the end of the AGB phase, or the beginning of thePPN phase, likely during an explosiveevent.Chemistry and morphology  Amongst the PPNe clearly resolved in our survey, 18 have known dust chemistry: either oxygen-rich, carbon-rich or a dualdust chemistry with both carbonaceousand oxygeneous dust grains in their envelopes. For the oxygen-rich sources,we nd that 10 out of 11 are bipolaror multipolar, while the remaining one iselliptical. For the carbon-rich sources,we nd that two are bipolar or multipolarand two elliptical. The three objectswith a dual dust chemistry are multipolaror bipolar. This is in agreement with therecent work by Guzman-Ramirez et al.(2010), which shows a strong correlation between dual dust chemistry and the presence of an equatorial overdensity.  The dual dust chemistry could be due either to the formation of PAHs in an oxy - gen-rich torus after CO photodissocia - tion, or to the presence of a long-livedoxygen-rich disc formed before the star turned carbon-rich due to the third dredge-up.Stanghellini et al. (2007) also studied thecorrelation between dust compositionand morphologies. They determined,from a study of 41 Magellanic Cloud PNe, that all PNe with oxygen-rich dust are bipolar or highly asymmetric. Our studyagrees with this nding, and it seemsthat oxygen-rich PPNe appear to bebipolar or multipolar. The low C/O ratio of these bipolar nebulae could be due tothe interaction with a binary companionduring a common envelope phase, or, inthe case of single star evolution, resultfrom conversion of carbon to nitrogen. The common envelope interaction willlead to the ejection of the envelope earlier than in the single star evolution scenario, leading to a less efcient dredge-up of carbon, and thus a lower C/O ratio. Theconversion of carbon to nitrogen occursfor massive AGB stars in the hot bottom-burning process. It is thus likely that thebipolar PPNe have progenitors with largermasses than the elliptical ones. This isin agreement with the work by Corradi & Schwartz (1995), who showed that bipo - lar PNe tend to have a higher progenitormass. Soker (1998) proposed that thisresult could be explained in the paradigmof binary system progenitors, as prima - ries that undergo a common envelopephase, and thus become bipolar, tend tohave a higher mass. Future directions  A large fraction of the dust in galaxies isproduced during the late stages of the  Astronomical Science Lagadec E. et al., A VISIR Mid-infrared Imaging Survey of Post-AGB Stars
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