Two Rings but No Fellowship: Lotr 1 and Its Relation to Planetary Nebulae

Home , Abell 70, Helix Nebula, LoTr 5

arXiv:1309.4307v1 [astro-ph.SR] 17 Sep 2013 o.Nt .Ato.Soc. Astron. R. Not. Mon. ..Tyndall to A.A. relation stars. its central and barium 1 possessing LoTr nebulae fellowship: planetary no but rings Two ..LópezJ.A. 02 oe ta.21) n nesodi em feither ⋆ of terms in al. understood et Marco and Tyndall De 2010), e.g. evo- & al. (see (Moe et common-envelope extensively Jones form a studied 2012, of to been has influence PN lution shaping to a thought The for even 2006). resulting essential cases some the almost in shape be and binary to (PN), a believed nebula and is planetary star companion progenitor planetary the or between interaction The INTRODUCTION 1 cetdxx xxxxx.Rcie xxxxxx x nori in xx; xxxxxxxx xxxx Received xx. xxxxxxxx xxxx Accepted © 2 1 4 3 8 7 6 5 uoenSuhr bevtr,Aos eCodv 17 Ca Córdova and 3107, de Physics Alonso of Observatory, School Southern Astrophysics, European for Centre Bank Jodrell otenArcnLreTlsoe OBx9 bevtr 793 Observatory 9, Box Observat PO Telescope. 9, Large Box African PO Southern Observatory, Astronomical African South bevtroAto´mc ainl ard n etode Centro and NSW Madrid, 2109 Astronómico National, Ryde, Observatorio North Observatory, UK Astronomical 7AL, AutónomaAustralian CV4 Nacional Astronom´ıa, Warwick, Universidad de of Instituto University Physics, of Department -al [email protected] E-mail: 03RAS 2013 6 .Martell S. , 1 , 2 ⋆ .Jones D. , 000 –6(03 rne 7Otbr21 M L (MN 2018 October 17 Printed (2013) 1–16 , n otAB–sas iais eea tr:ceial peculia chemically stars: – general binaries: stars: – ind post-AGB and an be words: may Key which episodes. 1, transfer WeBo mass 1 and the of A70 in ages than difference such, with As morphology shells, orientations. elongated nebular different slightly different with two and respectively, in years, consists 000 1 LoTr of nebula ABSTRACT ohv oainpro f64dy,wihi osbesg fms ac mass of sign gian possible III a is K1 H which a double-peaked days, enh broad 6.4 of s-process presents of of consists also period sign that rotation any present a 1 have not LoTr to does VLT-FO of giant with cool star confirm The obtained photometry.We central dwarf. SuperWASP spectra the as of of well combination nature under as a syst NTT-EMMI, being star using and from central 1 UCLES the far LoTr both still of of int analyses nebula process and detailed rare stars formation first represent barium the a systems present of – Such formation A70. systems the and post-AGB into 1 investigation r WeBo the rapidly 5, further LoTr K-type, as a transfer b such of P, post-mass PNe signature accretion-induced period, an orbital the constitutes an shows likely with most system system The a days). is, that 1500 ( system star central 7 .Pollacco D. , or1i lntr euatogtt oti nintermediate-per an contain to thought nebula planetary a is 1 LoTr 2 ...Boffin H.M.J. , lntr eua:idvda:LT ,WB ,A67 tr:AGB stars: – 70 A66 1, WeBo 1, LoTr individual: nebulae: planetary 5 eMeio neaa aaClfri,CP 20,Mexico 22800, C.P. California, Baja México, Ensenada, de n .Santander-Garc´ıa M. and , srbooá SCIT,SpainAstrobiolog´ıa, CSIC-INTA, srnm,Uiest fMnhse,M39L UK 9PL, M13 Manchester, of University Astronomy, il 90,Snig,Chile Santiago, 19001, silla ia omxx xxxxxx xxxxxxxx xxxx form ginal r 95 ot Africa South 7935, ory Australia , ,SuhAfrica South 5, 2 otC ytm Msasie l 09 n iulyre- visually and 2009) al. between et fall (Miszalski binaries ob- systems of period lack post-CE intermediate to due The morphology, their post- and servations. formation including 2009) PNe binaries, on al. (AGB) effect intermediate- et Branch Winckel about van Giant known WestAsymptotic days, ‘Gen- & is (P=100-1500 (Kahn the little period model very by Winds’ However, required 1985). Stellar as Interacting 2006) enhancement eralised density common- Blackman equatorial & ejected an (Nordhaus the forming or (CE) 2000) axisymmetric envelope Rappaport an & out (Soker carving (CFW) nebula wind fast collimated a .Miszalski B. , α msinlns hs rgni tl nla.The unclear. still is origin whose lines, emission A T E tl l v2.2) file style X 3 , 4 .Faedi F. , 8 madtesurrounding the and em ytmsmlrt other to similar system neetbti shown is but ancement or1peetavery a present 1 LoTr ten10ad say, and, 100 etween 5 r ttn in,and giant, otating n o white hot a and t .Lloyd M. , reit period ermediate potnte to opportunities to.Hr,we Here, stood. rto.LT 1 LoTr cretion. 0 n 35 and 000 7 S,AAT- RS2, o binary iod h binary the cto of ication 1 , 2 A.A. Tyndall et al.

(a) (b)

Figure 1. Deep, narrowband images of PN LoTr 1, in (a) [O iii]λ5007 A,˚ and (b) Hα+[N ii]λ6584 A.˚ North is to the top of the image, East is left. The central star is visible at α = 05:55:06.6, δ = −22:54:02.4. Overlaid on image (b) are the slit positions of the spatio-kinematic data presented in section 2.1.2. Slit 1 (E-W) was acquired using NTT-EMMI and is centered on the central star. Slits 2–4 (N-S) and 5–8 (E-W, width of slits 6, 7 and 8 exaggerated to clearly show the positioning due to overlapping with slit 1) were acquired using AAT-UCLES. All slits were taken in [O iii]. solved systems (e.g. Ciardullo et al. 1999). Soker (1997) (Bidelman & Keenan 1951) – population i G/K-type AGB claimed that these interacting systems are the most likely stars that show an over-abundance of carbon and s-process CSPNe (central stars of planetary nebulae) to form the clas- elements, in particular barium (Thevenin & Jasniewicz sical ‘butterfly’, or bipolar, morphologies, but with few sys- 1997). A now-canonical model for the formation of these tems known and limited investigations, this has yet to be Ba ii stars states that they form not through CE evolution1, confirmed. Only by finding and studying CSPNe with inter- as is the case for close binaries found within PNe, but rather mediate periods can we substantiate this claim and relate via a wind-accretion scenario (Boffin & Jorissen 1988). Here, the processes at work in formation of PNe by both post-CE the future Ba ii star is polluted whilst on the main sequence and intermediate-period CSPNe. by the wind of its companion (Luck & Bond 1991; the com- The planetary nebula (PN) LoTr 1 (α = 05:55:06.6, δ panion having dredged up these s-process elements during = −22:54:02.4, J2000) was first discovered by A.J. Long- its thermally pulsing AGB phase), but with the system re- more and S.B. Tritton with the UK 1.2-m Schmidt tele- maining detached. After the envelope is ejected to form the scope (Longmore & Tritton 1980). It is generally noted surrounding nebula, the AGB star evolves into a WD, while that LoTr 1 belongs to the so-called ‘Abell 35-type’ group the contaminated star retains its chemical peculiarities to (Bond et al. 1993) of PNe showing evidence of a binary cen- form the remnant Ba ii star. tral star system consisting of a cool central star (a rapidly ro- One important prediction to come out of the wind- tating subgiant or giant), and an optically faint hot compan- accretion model is that the accreting star, i.e. the fu- ion (a white dwarf with effective temperature, teff ∼ 100 kK) ture Ba ii star, also accretes angular momentum from the since these giant stars are too cool to ionise the surrounding companion to become a rapid rotator (Jeffries & Stevens nebula. Four PNe fell into this category: Abell 35 (here- 1996; Theuns et al. 1996). Indeed, photometric monitor- after, A35), LoTr 5 (Thevenin & Jasniewicz 1997), WeBo 1 ing of LoTr 5 (Thevenin & Jasniewicz 1997) and Webo 1 (Bond et al. 2003) and Abell 70 (Miszalski et al. 2012, here- (Bond et al. 2003) has revealed that their cool components after, A70). However, Frew (2008) determined that A35 is are in fact rapid rotators with a rotation period of a few days, most likely not a true PN, but rather a Strömgren zone in thus providing further evidence for this formation scenario. the ambient interstellar medium (ISM). This claim is sub- This is further evidenced by the fact that Montez et al. stantiated by Ziegler et al. (2012), who find that the central (2010) found that the x-ray emission from the binary cen- star may in fact have evolved directly from the Extended Horizontal Branch to the White Dwarf (WD) phase (a so- called AGB-manquéstar). As such, we choose not to con- 1 We note, however, the existence of some exceptional systems sider Abell 35 in our comparisons among this group. that also experience similar enrichment in close binaries, i.e. the Another common factor amongst this particular group Necklace nebula (Miszalski et al. 2013), and which are most likely of PNe is evidence for the existence of ‘Barium (Ba ii) stars’ linked to dwarf carbon stars.

© 2013 RAS, MNRAS 000, 1–16 The Planetary Nebula LoTr 1 3 tral stars of LoTr 5 is most likely due to the chromospheric posure time, texp, in each filter was 1800s and the binning activity from a spun-up companion. set to 2 × 2 (≡ 0.33′′ pixel−1). The seeing was ∼0.7′′. Most Ba ii stars are not observed to be within PNe, al- The images show LoTr 1 to have an apparent double- most certainly because the lifetime of the PN is very short shell structure, with the central shell having a circular profile with respect to the lifetime of the stellar system. However, with an angular diameter of 47′′±4′′, and the outer shell a this does not completely rule out the possibility that some more irregular, but still roughly circular, appearance, with Ba ii stars may be formed without passing through a PN a diameter of 2′26′′±4′′. Furthermore, in [O iii]λ5007 A˚ the phase. Indeed, there are a few examples of field stars that outer shell appears brighter in the northwest and southeast – have been shown to consist of a rapdidly rotating cool star this, coupled with the slight deviation from circular symme- linked to an optically faint hot component in a similar fash- try, could be considered evidence for an inclined, elongated ion to the Abell-35 group, but without current evidence structure, where the brighter areas result from a projection for a surrounding PN. As mentioned by Bond et al. (2003), effect. HD 128220 (P = 872 days) is one such system, made up Interaction with the ISM is also frequently invoked of an O subdwarf and a G0 giant companion. O subdwarfs to explain deviation from symmetry and brightening of overlap with CSPN in terms of their log g and teff , implying the nebular shell (e.g., Jones et al. 2010), however this that they too are found to be in a post-AGB phase of evolu- would not produce the apparent axisymmetry in the neb- tion (Howarth & Heber 1990). 56 Pegasi is another example, ular brightening (as the nebula would only interact strongly whereby it possess a system consisting of a K0 giant and a with the ISM in the direction of its motion; Wareing et al. hot white dwarf companion with evidence for an overabun- 2007). dance of Ba ii, and has an estimated orbital period of 111 days (Griffin 2006). It is possible that both of these systems 2.1.2 Nebular spectroscopy have gone through a PN phase in the past, but it has since dissipated into the surrounding ISM. This fact, as well as the Longslit echelle spectroscopy was carried out using AAT- knowledge that in Ba ii central stars the chemical pollution UCLES and NTT-EMMI, focusing on [O iii] emission over process happened very recently – either during or immedi- eight different slit positions (see figure 1 (b)) in order to gain ately prior to the formation of the PN – makes it highly velocity profiles across a good sample of the nebula. It is im- important to study such systems as A70 and WeBo 1, as it portant to ascertain the true three-dimensional shape of the will allow us to gain greater insight into both the s-process surrounding nebula in order to fully understand and con- within AGB stars and mass-transfer mechanisms. strain the shaping process, and subsequently give a clearer In this paper, we present photometric and spectroscopic insight into mass-transfer mechanisms. Imaging is insuffi- observations of LoTr 1 and its central star along with com- cient on its own due to a degeneracy between PN inclination plementary data of A70 and WeBo 1, in order to try to relate and morphology – for example, when the symmetry axis of 2 the evolutionary processes of these three systems , and see if a bipolar nebula is aligned perpendicular to the plane of they belong to a common “fellowship” of s-process enriched the sky, it makes the nebula appear spherical (Kwok 2010). cool CSPN inside ring-like nebulae. If the nebular morphology is classified incorrectly then the fraction of aspherical PNe possessing a binary core will also end up being inaccurate, and this information is already exceedingly limited for longer period binaries due to selec- 2 OBSERVATIONS AND ANALYSIS tion biases for those stellar systems possessing wider orbits. 2.1 LoTr1 Longslit spectroscopy can be used to acquire spatially re- solved velocity maps of the constituent parts of the nebula 2.1.1 Imaging in order to recover the ‘missing’ third dimension of the morphology one cannot gain from imagery alone. The resulting The deep [O iii]λ5007 A˚ and Hα+ [N ii]λ6584 A˚ images spectra are then plotted as position-velocity (PV) arrays. shown in figures 1(a) and (b) were acquired on 2005 March On 2005 March 03, a spectrum was acquired from the 03 using the red arm of the European Southern Observatory nebula using EMMI in its single order echelle mode, em- (ESO) Multi-Mode Instrument (EMMI; Dekker et al. 1986), ploying grating #10 and a narrowband [O iii]λ5007 A˚ filter mounted on the 3.6m New Technology Telescope (NTT) of to prevent contamination from overlapping echelle orders. the La Silla Observatory. EMMI was used with the mosaic ′′ ′′ The maximum slit length of 330 and a slit width of 1 was of two MIT/LL CCDs, 2048 × 4096 15 µm pixels. The ex- − used to give a resolution, R ∼ 54,000 (5.5 km s 1). A 1800s exposure was taken at a P.A. of 90◦ crossing the central star (slit position 1 in figure 1(b)), and the data were 2x2 binned 2 PN K 1-6 (Frew et al. 2011) is another potential candidate for to give a spatial scale of 0.33′′ and a velocity scale of 3.9 km this group of objects, as it also shows evidence of possessing both s−1 per pixel. The seeing was ∼0.7′′. a G- or K-type giant (inferred from imagery, looking at both On 2005 January 14, spectra were acquired from the optical and 2MASS near-IR colours) and a very hot sub dwarf or nebula of LoTr 1 using the 79 lines/mm grating on the white dwarf (inferred from GALEX archival images) at its core. UCL CoudéEchelle Spectrograph (UCLES) of the Anglo- However, no stellar spectroscopy is available as yet to look for Australian Telescope (AAT). UCLES was operated in its signs of chemical enrichment (Frew et al. 2011). More recently, ′′ Miszalski et al. (in press) presented evidence for a carbon and longslit mode with a maximum slit length of 56 and a slit ′′ −1 s-process enriched giant at the centre of the planetary nebula width of 1.97 to give a resolution R ∼ 20,000 (15 km s ). Hen 2-39. Due to it being a newly investigated system, Hen 2-39 The EEV2 CCD (2048×4096 13.5 µm pixels) was used with − is not included in this study either. binning of 2×3, resulting in a pixel scale of 3.88 km s 1

(a) (b)

Figure 2. PV arrays showing reduced, longslit [O iii]λ5007 A˚ NTT-EMMI and AAT-UCLES spectra of LoTr 1, both aligned E-W, to show the overall nebular structure. West is to the top of the array. The velocity axis is heliocentric velocity, Vhel. The display scale has been modified to highlight the spatio-kinematic features referred to in the text. Cross-section 0′′ defines where the central star is found. The gap between the two CCDs in figure (a) is visible as a black strip across the frame at cross-section 58′′ (see text for details).

(a) Slit position 2 (b) Slit position 3

Figure 3. PV arrays showing reduced [O iii]λ5007 A˚ AAT-UCLES spectra of LoTr 1. The velocity axis is heliocentric velocity, Vhel. The display scale has been modified to highlight the spatio-kinematic features referred to in the text. Slits 2–4 are N-S (North is to the top of the array), slit 5 is E-W (West is to the top of the array). Cross-section 0′′ defines where the central star is found. The continuum of a field star is visible at cross-section −8′′ in figures (a) and (b), and at cross-section +18′′ in figure (d).

© 2013 RAS, MNRAS 000, 1–16 6 A.A. Tyndall et al. pixel−1 in the spectral direction and 0.48′′ pixel−1 in the spatial direction. 1800s exposures were taken at five different slit positions (shown in figure 1(b)) using a narrowband filter to isolate the 45th echelle order containing [O iii] emission line profile. Slits 2–4 were taken at a position angle (P.A.) of 0◦, and slit 5 was taken at a P.A. of 90◦. The seeing during the observations was ∼2′′. A further three slits (slit positions 6–8 in figure 1(b))were acquired using the same instrument and CCD on 2013 January 3 with a seeing of ∼1.5′′ and a binning of 2×2 (≡ 0.32′′ pixel−1 in the spatial direction). Here, a slit width of 1′′ was employed for slits 6 and 8 (R ∼ 45,000 ≡ 6.7 km s−1) and 1.5′′ for slit 7 (R ∼ 30,000 ≡ 10 km s−1)3. All the spectra were cleaned of cosmic rays and debi- ased appropriately. The UCLES spectra were wavelength calibrated against a ThAr emission-lamp, rescaled to a linear velocity scale appropriate for the [O iii]λ5007 A˚ emission, and corrected to heliocentric velocity, Vhel. Due to the optical set-up of EMMI, it was necessary to perform the wavelength calibrations using a long-exposure (3600s) ThAr emission lamp at the start and end of the night to gain a good number of arc lines, before cross-correlating with shorter-exposure (200s) Ne lamps taken immediately after Figure 4. SuperWASP photometry of LoTr 1. Upper panel: Pe- each observation, to account for any drift due to telescope riodogram showing a clear detection of periodicity at 6.3967 ± and instrument flexure (with small shifts accounted for with 0.0005 days. Middle panel: The data points folded on the period a linear correction). obtained from the periodogram. Lower panel: The data binned The reduced nebular spectra of LoTr 1 are presented in into phase bins, overlaid is a sinusoidal fit with a peak-to-peak figures 2 and 3, as position-velocity (PV) arrays. In each PV amplitude of 0.061 magnitudes. array, cross-section 0′′ defines where the central star is found. In figure 2 (a), the bright lines located around the central star at cross-sections +3′′ and −3′′ are most likely artefacts In slit 1 (figure 2(a)), emission is clearly detected from ∼ ′′ ∼ due to the comparative difference in brightness between the outside of the central shell at cross-sections 40 and − ′′ central star and the nebula (Jones et al. 2010). 40 , associated with the outer shell (see section 2.1.1). The closed velocity ellipses shown in the PV arrays pre- Here, the emission from the eastern side appears blue-shifted sented in figures 2 (a), 3 (c), and the central PV array of 2 with respect to the nebular Vsys, and the west appears red- (b) (i.e. representing slit positions 1, 4 and 7), have a major shifted; this is indicative of an inclined and extended struc- axis which has the same length as the diameter of the in- ture, e.g. an elliptical nebula, where the approaching “lobe” ner shell (see section 2.1.1). This indicates that the nebular is tilted slightly to the east of its receding counterpart. Con- structure is indeed an isolated, closed shell rather than a sideration of the other slits presented in 3 (b), (c) and (d), projection effect related to a bipolar structure being viewed confirms this asymmetry in the velocity profile across the end-on. No significant asymmetries are observed in these ve- nebula, but indicates that the symmetry axis may lie closer locity ellipses, which are consistent with a spherical shell or to the northeast-southwest direction than east-west. How- an elongated ovoid viewed directly along the symmetry axis. ever, any deviation from the line of sight must be rather small given the almost circular appearance of the shell in The expansion velocity, Vexp for the inner shell is measured to be 17±4 km s−1 at the location of the central star, while the images (figure 1). −1 Determination of the exact structure and inclination of the Vexp for the outer shell is measured to be 25±4 km s falling within the typical range for a PN (Weinberger 1989). the nebula would require a more extensive, higher signal- Assuming typical expansion properties for the nebula (i.e. to-noise (given the outer shells faintness) dataset, covering velocity proportional to distance from the central star), the more of the physical extent of nebula, and a detailed spatio- latter velocity is then the maximum expansion velocity (for kinematical model such as that presented in Jones et al. an elliptical shell viewed pole-on) or the uniform expansion 2012. However, it is clear from both the imaging and spec- velocity of the shell (in the case of a sphere). The heliocentric troscopy presented here that LoTr 1 shows a double-shelled structure with evidence for an elliptical and slightly inclined systemic velocity, Vsys of this central shell was determined to be 14±4 km s−1. outer shell, and a morphologically similar inner shell but with a different orientation.

3 The slit width for slit 7 was altered to try to match the seeing conditions during the night. However, as all observations were 2.1.3 Stellar photometry carried out with the same binning in the spectral direction and the same grating (i.e. approximately the same dispersion), they can The ﬁeld of LoTr 1 has been observed by WASP-S be- all be used to qualitatively assess the spatio-kinematic structure tween 2006 May 4 and 2012 February 17 with a total of of the nebula (varying on scales ≫ the slit-widths employed, due 21407 photometric points obtained with the two cameras to the angular size of the nebula also being ≫ the slit-width). DAS 226 and 228. WASP-S is a wide ﬁeld survey camera

140 1000

120 800 ) ) −16

−16 100 600

80 400

Flux (ergs/cm/cm/s/Å * 10 200 Flux (ergs/cm/cm/s/Å * 10 40

0 20

−200 0 1500 2000 2500 3000 4200 4400 4600 4800 5000 5200 5400 Wavelength (Å) Wavelength (Å)

(a) (b)

Figure 5. (a) Flux-calibrated FORS2 spectrum of the central star system of LoTr 1. (b) Flux-calibrated IUE spectra of LoTr 1 (solid line), and of NGC 7293 (dashed line) used to determine the parameters of the white dwarf. Note also the presence of the Mg II emission at 2800 A˚ which is a sign of chromospheric activity from the cool companion (e.g. Jasniewicz et al. 1996; Montez et al. 2010) situated at SAAO, Sutherland, South Africa, and together with its northerly sister, SuperWASP-N, is designed to ob- tain extremely accurate photometry of bright stars in order to search for transits from exoplanets (see Pollacco et al. 2006, for a more detailed description of the facility, the data reduction and archive). Data mining tools and the public archive are discussed by Butters et al. (2010). After reduction through the instrument pipeline (Pollacco et al. 2006), the time series were examined with a simple Lomb-Scargle filter for periodic signal detection. From the light curves re- produced in figure 4 taken from Jones et al. (2011), a period of 6.4 days was derived with a peak-to-peak amplitude of 0.061 magnitude. This is close to the value of 6.6 days with an amplitude of 0.1 magnitude in V, initially derived by Bond et al. (1989). Given the period, this amplitude is probably too high to be considered due to irradiation ef- fects (De Marco 2006), so the most likely explanation is that the periodicity is following spots on a cool star – the signature of a rapid rotator, analogous to the systems of LoTr 5 (5.9±0.3d, Thevenin & Jasniewicz 1997) and WeBo 1 Figure 6. LoTr 1 FORS2 stellar spectrum (solid line) alongside (4.69±0.05, Bond et al. 2003). three synthetic spectra with [Ba/Fe] = 0.1, [Ba/Fe] = 0.2, and [Ba/Fe] = 0.5, smoothed to match the FORS2 resolution and plotted over a wavelength range which includes the Ba ii 4554A˚ 2.1.4 Stellar spectroscopy line. On 2012 February 10, a low-resolution spectrum was acquired of the central star of LoTr 1 using the FO- The central star of LoTr 1 was classified as a K1 III- cal Reducer and low dispersion Spectrograph (FORS2; type giant by comparing the flux-calibrated FORS2 spec- Appenzeller et al. 1998) with grism 1200g, on the Antu unit trum with UVES POP standard stars (Bagnulo et al. 2003) (UT1) of the Very Large Telescope (VLT) based at ESO- that had been rebinned and smoothed to match the resolu- Paranal. FORS2 was operated with a maximum slit length tion of the FORS2 data. To refine the analysis and derive ′ ′′ of 6 and a 0.5 slit width (R ∼ 3000), and at a position the possible s-process overabundance, we used the stellar ◦ angle of 45 . The data were 2×2 binned to give a spatial spectral synthesis code of R. Gray, spectrum version 2.764, ′′ −1 scale of 0.25 pixel , with a dispersion on the spectral axis with models from Castelli & Kurucz (2003). The best fit pa- −1 of 0.3A˚ pixel . A single 600s exposure was taken under rameters were determined to be teff = 4750± 150 K and log ′′ seeing of ∼ 0.7 . g = 2.0 ± 0.5, which is in agreement with the K1 III spectral The stellar data were extracted to 1-D and flux- type. Using the average value of the absolute magnitude, Mv calibrated against standard star Hz 4, and are presented in figure 5 (a). All reductions were carried out using standard starlink routines. 4 See http://www1.appstate.edu/dept/physics/spectrum/

Figure 8. Hα+[N ii]λ6584 A˚ image of PN A70 showing all slit positions. Slits 1–4 were acquired in [O iii]λ5007 A˚ using VLT-UVES, and slit 5 was acquired in Hα and [O iii]λ5007 A˚ using SPM-MES. The central star is visible at α = 20:31:32.2, δ = −07:05:17.0.

To check that the giant and the planetary nebula are indeed linked and not merely a chance superposition, we computed the radial velocity difference between the nebular lines in the FORS2 spectrum and the giant star’s absorption lines, cross-correlating our spectrum (shifted so that the nebular lines are at zero velocity) with our synthetic K1 III spectrum. The resulting stellar velocity of 4±2 km s−1 with respect to the nebula implies that the cool central star and nebula are physically related. The FORS2 spectrum was used to check for signs of Ba ii pollution at 4554A.˚ Figure 6 shows the FORS2 spectrum of LoTr 1 plotted alongside three synthetic spectra with various barium enhancements, from [Ba/Fe] = 0.1 to [Ba/Fe] = 0.5. An over-abundance of 0.5 or greater classifies the system as a definite Ba ii star, with a value of 0.2–0.5 being possibly a ‘mild’ Ba ii star (Pilachowski 1977). Due to the relatively low S/N and resolution of the spectrum, we are unable to determine a definitive value for the barium abundance in LoTr 1, but we can clearly state that it is Figure 7. Hα emission seen in LoTr1. much less than 0.5, and therefore does not show any mea- surable barium enhancement. This is contrary to both A70 and WeBo 1 (see Miszalski et al. 2012 and Bond et al. 2002, = +0.7 for a giant of this spectral type as given in Allen’s respectively), which both possess definite Ba ii stars. ‘Astrophysical Quantities’ (2000), the flux for the spectrum On 2013 January 3, 3×30 minute spectra were acquired shown in figure 5 (a) that allows the derivation of an appar- from the CSPN of LoTr 1 using AAT-UCLES with the 79 ent magnitude, mv = +12.6 (accounting for 20% slit-losses), lines/mm grating, operated in full echelle mode with a slit ′′ ∼ and extinction AV =0.1285 (Schlegel et al. 1998), one can width of 1 to give a resolution R 45,000. All spectra were determine a rough distance to the star of 2.6 kpc and a not- reduced using standard starlink routines, corrected to he- unreasonable radius of the giant of 11.5 R⊙. Alternatively, liocentric velocity and then summed. Unfortunately, because one can use the average value of V derived from SuperWASP of the large interorders in the spectra5, it is not possible to (see figure 4), i.e. V = 12.44, to derive a distance of 2.1 kpc. assess the barium abundance, as none of the available orders Note that this observed SuperWASP magnitude is likely to be contaminated by line emission and close field stars be- ′′ cause of the broad observing band and large (14 ) pixels. 5 UCLES operated in this mode gave non-continuous wavelength Therefore, we think it is preferable to use the distance de- coverage from roughly 5200A˚ to 8900A˚ across 19 orders with in- rived from the flux calibrated spectra. terorder spacing of 100–200A.˚

(a) Slit position 1 (b) Slit position 3

Figure 9. PV arrays showing reduced VLT-UVES spectra in [O iii]λ5007 A˚ from A70. Figures (a) and (c) show emission from the major axis, figures (b) and (d) are from the minor axis. Positive spatial offsets are to the northern (1 and 2) or eastern (3 and 4) ends of the slits.The velocity axis is heliocentric velocity, Vhel. The display scale has been modified to highlight the spatio-kinematic features referred to in the text. Cross-section 0′′ defines where the central star is found. contain the Baii lines at 6141.7A˚ and 6496.9A.˚ We have ver- ments are not overabundant, confirming the lack of s-process ified, however, that the iron lines are well fitted with a solar enhancement in LoTr 1. abundance and our preferred model, in agreement with what we derived from the FORS2 spectrum. Looking at the La ii The AAT spectra allowed us to find that the stellar 6390A˚ and Yii 6222A˚ lines, it is also clear that these ele- component of Hα is in emission (see figure 7). The line is very broad and is clearly double-peaked, with an equivalent width of about 6A˚ and the velocity spread is 572 km/s.

Using the method of Hodgkin et al. (1995), we derive an Hα luminosity LHα = 0.044 L⊙. With the above estimated total luminosity of the star (L∗ ∼ 60 L⊙), this gives a value log LHα/L∗ = −3.12. Such Hα double-peaked emission lines have been found in the other stars we are concerned with here, LoTr 5 (Jasniewicz et al. 1994; Strassmeier et al. 1997) and A35 (Acker & Jasniewicz 1990; Jasniewicz et al. 1992), but the origin is still unknown. Rapidly rotating giants, such as RS CVn or FK Com stars, are known to have high chromospheric activity which is often revealed by emission cores in some lines. And indeed, Acker & Jasniewicz (1990) find a modulation of the Hα emission line with the rotation phase, while Jasniewicz et al. (1992) postulate that the variable double-peaked emission line is the result of an overlap between an absorption and an emission line at Hα, with the (a) Slit position 5, [Oiii] (b) Slit position 5, Hα possibility for the absorption component to be formed in the photosphere or through a self-absorption process as in Figure 10. PV arrays showing reduced SPM-MES spectra of Be stars. However, the FWHM velocity we measure seems A70. Emission is through the horizontal axis (slit 5 in figure 8). too high to be caused by mass motions inside the chro- The velocity axis is heliocentric velocity, Vhel. The display scale mosphere, while the luminosity is too small to be due to has been modified to highlight the spatio-kinematic features re- accretion. Such double-peaked emission are also sometimes ferred to in the text. Cross-section 0′′ defines where the central found in symbiotic stars – detached systems which interact star is found. via wind accretion. A few of them, out of an outburst event, produce bipolar nebulae very similar to planetary nebulae. Jones et al. 2012). Furthermore, this image shows in detail It turns out that many symbiotic stars show double Hα pro- the low-ionisation knots first identified by Miszalski et al. files (e.g. Schild et al. 1996; Burmeister & Leedjärv 2009), (2012). Many of these structures seem to be akin to the which might be caused by a narrow absorption component cometary globules seen in the Helix Nebula (dense conden- from the giant overlaid with very broad Halpha emission sations of molecular gas embedded in the ionised nebula, from high-velocity jets at the core of the system, or by disc- see Meaburn et al. 1992), with knotty heads closest to the like structures. It would thus be of interest to further study nebula centre and extended tails reaching out towards the the possible link between LoTr 1 and symbiotic stars. outer rim. Extended material is also visible outside the east- A spectrum of the central star system of LoTr 1 was southeasterly edge of the nebular ring (the emission visible acquired using the International Ultraviolet Explorer (IUE) to the north of the ring originates from a background field satellite by Bond et al. (1989). Presented here in figure 5 galaxy). (b), it indicates a strong UV continuum of teff > 100 kK. Assuming the “ring” is a physical structure rather than Comparing this IUE spectrum to a known WD within PN +27 a projection effect, one can deduce the inclination of the NGC 7293 (d = 219 pc, mv = +13.5; Harris et al. 2007) −21 nebula by deprojection. The angular size of the nebula was gave us a reasonable fit: teff > 123 kK, and R = 0.017 R⊙. ′′ ′′ ′′ ′ ′′ ′′ −1 determined to be 44 × 38 ±2 , falling in line with the pre- Based on values of D = 2 22 ±4 , and Vexp = 25±4km s viously given value of 45.2′′ x 37.8′′ by Tylenda et al. (2003), at a distance of 2.1–2.6 kpc, we derived a kinematical age of giving an inclination of 30◦±10◦. 33,000±9,000 years for the outer nebular shell. For the inner ′′ −1 shell, given values of D = 47±2 and Vexp = 17±4 km s , the age is derived as 16,000±6,500 years. Using the white 2.2.2 Nebular Spectroscopy dwarf evolutionary curves from Bloecker (1995) and assuming an average remnant mass of 0.6 M⊙, we derived a stellar On 2011 June 10-11, high resolution data of the nebula temperature at the age of the PN of approximately 120,000 of A70 were acquired in [O iii] using grating #3 on the K. The derived radius and age of LoTr 1 are consistent with visual-to-red arm of the Ultraviolet and Visual Echelle Bloecker’s evolutionary curves. Spectrograph (UVES) on Kueyen Unit (UT2) of the VLT (Dekker et al. 2000), under program ID 087.D-0174(A). UVES was operated in its 30′′ longslit mode with a 0.6′′ 2.2 A70 slitwidth (R ∼ 70,000, 4.3 km s−1 pixel−1) to give a spatial scale of 0.17′′ pixel−1. A filter was used to isolate the [O iii] 2.2.1 Imaging emission lines and prevent contamination from overlapping The Hα+[N ii]λ6584 A˚ image of A70 shown in figure 8 was orders. The seeing was between 0.5′′ and 0.7′′ for all obser- acquired on 2012 July 08, using FORS2 under program ID vations. Four 1200s exposures were taken over four different 0.89.D-0453(A), with an exposure time of 60s and seeing of slit positions. Slits 1 and 2 were taken with VLT-UVES at 0.8′′. At first glance the image shows a general ring-like ap- a P.A. of 160◦ and slits 3 and 4 at a P.A. of 70◦, to line pearance similar to that of other ring-like PNe (e.g. SuWt 2: up with the major and minor axis of the nebula, respec- Jones et al. 2010); however, just as noted by Miszalski et al. tively. Slit 5 was acquired in both [O iii] and Hα on 2011 (2012), a closer inspection reveals a “ridged” profile more May 15 using the Manchester Echelle Spectrograph (MES) like that of a bipolar nebula viewed end-on (e.g. Sp 1: mounted on the 2.1-m San Pedro Martir (SPM) telescope

© 2013 RAS, MNRAS 000, 1–16 The Planetary Nebula LoTr 1 11 based at the Observatorio Astronomico Nacional in Mexico sion around its entire circumference – the ring is particularly (Meaburn et al. 2003; López et al. 2012). The full slit length diffuse in [O iii]λ5007 A,˚ as shown by the lack of a visible in- of 5′ was used with a slitwidth of 150µm(≡ 2′′,R ∼ 30,000), ner edge. Similar extended emission is also found in SuWt 2 and taken at a P.A. of 90◦. The data were 2x2 binned to give (Jones et al. 2010) and HaTr 10 (Tajitsu et al. 1999), where a spatial scale of 0.75′′ pixel−1. The seeing was ∼ 3′′ the ring is actually the waist of an extended bipolar struc- The nebular spectra acquired from VLT-UVES shown ture, possibly indicating that WeBo 1 may display the same in figure 9 show two highly filamentary components, one morphology but with as yet undetected, very faint lobes. In red-shifted and one blue-shifted, joined by bright knots of SuWt 2, Jones et al. (2010) attribute this extended material emission where the slits cross the nebular ring, to form to structural and brightness variations across an irregular a closed velocity ellipse in both axes. These filamentary toroidal structure; however (particularly in the light of the and irregular structures are typical of disrupted nebulae, Hα+[N ii] spectra acquired - see figure 12), WeBo 1 shows where instabilities have begun to structurally deform the a much more regular and even ring-like shape, indicating shell (Guerrero & Miranda 2012), and clearly show that A70 that this is more likely an intrinsic structural property (i.e. is not simply an inclined ring but instead has “bubbles” a tear-drop rather than circular shaped cross-section). extending in the line of sight. It is reasonable to assume Smith et al. (2007) deprojected the ring of WeBo 1, de- that these bubbles form a closed (as the velocity ellipses termining that it is seen almost edge-on with an inclination are closed along both axes) and axisymmetric (the blue- of 75◦±3◦ with an inner-ring radius of ∼ 25′′, which is consis- and red-shifted components are roughly symmetrical) struc- tent with the dimensions of the ring as cited by Bond et al. ture. The bright emission at the extremes of each slit indi- 2003 (64′′x 22′′), and as measured from the images presented cate that the nebula may have a cusped waist, with slits here (65′′×20′′±4′′). 1 and 2 showing the “crow’s foot”-like structure typical of narrow waisted nebulae viewed along their symmetry axis (Jones et al. 2012). However, there is a clear brightening in 2.3.2 Nebular Spectroscopy these regions and those of slits 3 and 4, consistent with a On 2010 December 10, spectra were acquired of WeBo 1 bright ring. We therefore deduce that A70 comprises of such in both Hα+[N ii] and [O iii]λ5007 A˚ using SPM-MES. The a bright ring, encircling the waist of a disrupted and faint maximum slit length of 5′ was used with a slitwidth of bipolar shell. 150µm (≡ 2′′,R ∼ 30000) for each filter. The data were Using the same spectra, a polar expansion velocity Vexp 2x2 binned to give a spatial scale of 0.75′′ pixel−1. Slit 1 ± −1 for A70 was calculated to be 39 10 km s . This is in agree- was taken at a P.A. of 353◦ and slit 2 was taken at a P.A. ment with the value for expansion velocity of Vexp = 38 km ◦ −1 of 263 to cover the major and minor axes of the nebula, re- s given by Meatheringham et al. (1988), although no er- spectively. The seeing was ∼1.5′′. Dueto theHα profiles hav- − ± −1 ror was quoted. A Vsys of 73 4kms was determined for ing high galactic background emission, only the background- − ± the nebula, which is consistent with the value of 72 3 km ii iii ˚ −1 subtracted [N ] and [O ]λ5007 A are presented here. The s by Miszalski et al. (2012). The kinematical age of A70 reduced PV arrays are presented in figure 12. ± −1 was determined to be 2700 950 years kpc . Taking the dis- Just as in the imagery (see section 2.3.1), the tance to the nebula to be 5 kpc, as given by Miszalski et al. [N ii]λ6584 A˚ profiles show well defined emission originat- (2012), this gives an overall kinematical age for A70 of order iii ˚ ± ing from the ring, while the [O ]λ5007 A is much more 13,400 4,700 years. diffuse. The [N ii]λ6584 A˚ profiles from both slit positions The limited depth and resolution of the MES-SPM spec- show strong emission at the inner edge of the ring, with tra offer little extra to the discussion of the nebular mor- slightly fainter material (also with a lower velocity disper- phology and kinematics, other than to show that there is sion) reaching out to greater angular extents – consistent no major difference between the emission in Hα and that with the “tear-drop” cross-section interpretation presented iii ˚ in [O ]λ5007 A. They are shown here in figure 10 mainly in section 2.3.1. The [N ii]λ6584 A˚ profile from slit 1 shows for comparison with similar data acquired from PN WeBo 1, little velocity difference between the emission originating which will be discussed in section 2.3. from the two opposing sides of the ring, indicating that the slit is roughly perpendicular to the P.A. of the nebular symmetry axis; therefore, the deprojected velocity difference of 2.3 WeBo 1 the two sides of the ring in slit 2 should offer a good measure 2.3.1 Imaging of the ring’s expansion velocity. Taking the nebula inclination to be 75◦, the deprojected expansion velocity for the ii ˚ −1 The deep Hα+[N ]λ6584 A image shown in figure 11 (a) is ring was calculated to be Vexp = 22.6±10 km s . × −1 the result of coadding 2 120s exposures, each with see- A Vsys of -6±4 km s was calculated for the nebula. ′′ ing better than 1.2 , acquired as part of the IPHAS survey The kinematical age of WeBo 1 was determined to be of (Drew et al. 2005) using the Wide Field Camera (WFC) on order 7300±3700 years kpc−1. Taking the distance to the the 2.5m Isaac Newton Telescope based at the Observato- nebula to be 1.6 kpc (Bond et al. 2003), this gives an overall iii 6 rio Roque de los Muchachos, La Palma. The [O ]λ5007 A˚ age for WeBo 1 of 11,700±5,900 years. image shown in figure 11 (b) was acquired using the same No clear evidence of lobes or extended nebular struc- instrument on 2010 September 9, with an exposure time of ture are detected in the spectra, unlike for A70 (see section 1200s and under seeing of 1.4′′. The images show another ring-like morphology, although structurally different to A70 (see section 2.2), with 6 Bond et al. (2003) assume an expansion velocity of 20 km s−1 a pronounced inner edge and fainter, more extended emis- to derive a similar age of 12,000±6,000 yrs.

(a) (b)

Figure 11. Images of PN WeBo 1 in (a) Hα+[N ii]λ6584 A,˚ and (b) [O iii]λ5007 A,˚ with (b) showing the two slit positions. Both slits were acquired in [N ii] and [O iii] using SPM-MES. The central star is visible at α = 02:40:13.0, δ = +61:09:17.0. North is to the top of the images, East is left.

2.2.2); however, faint material detected inside the ring on • Systemic velocity: The Vsys of the central star is con- the [N ii]λ6584 A˚ PV array of slit 1 could be consistent with sistent with the Vsys of the nebula for both LoTr 1 and A70 such a structure. Deeper, higher resolution spectra are re- (see section 2.1.4, and Miszalski et al. 2012), and so we can quired to confirm the nature of this emission, but it is safe to say that the observed emission comes from a true planetary say that if any lobes are present they are significantly fainter nebula. No Vsys for the central star of WeBo 1 has been than those of A70, as its lobes were still clearly detected by published. SPM-MES spectra (see figure 10). • Nebula expansion: All three nebulae have been shown to have an expansion velocity typical for a PN (see Table 1). • Nebular diameter: Using the values stated in this paper, the physical diameters of LoTr 1, A70 and WeBo 1 are all 3 DISCUSSION of order ∼1 pc (see Table 1): a sensible value for a PN. 3.1 The Abell-35 group and PN mimics • Galactic latitude: Two of the three nebulae are found at high galactic latitudes of -22◦ and -25◦ for LoTr 1 and Abell 35 - the archetype of the class of PNe discussed in ◦ A70, respectively (WeBo 1 is at and +1 ). PNe are more this paper – has recently been shown to be a PN mimic. likely to be found away from the Galactic plane than isolated It is, therefore, critical to establish the true nature of the Strömgren spheres. objects considered here before beginning a comparison. We restrict our analysis to the three PNe presented in this work, We can thus be confident that the three objects studied and exclude both Abell 35 (which is no longer considered a in this paper show the characteristics of bona-fide planetary true PN) and LoTr 5 (which is a considerably more complex nebulae, although, as mentioned earlier, their link with the case and discussed in more detail in Frew 2008; see also class of symbiotic stars should also be investigated further. Graham et al. 2004). Frew & Parker (2010) present a ‘recipe’ for determining whether we can classify an object as a true PN or not, parts 3.2 Conclusions of which we can apply to the nebulae presented here: From the study conducted in this paper, we have been able • Presence of a hot, blue central star: All three nebulae to show that LoTr 1 possesses a double-shelled, slightly el- presented here show evidence of excess UV flux that point liptical morphology of age of 35,000±7,000 years for the towards the existence of a hot companion – see Fig. 5a for outer shell, and 17,000±5,500 years for the inner. We have LoTr 1, Miszalski et al. 2012 for A70, and Siegel et al. 2012 been able to infer the presence of a K1 III-type giant (teff ∼ for WeBo 1. 4500 K) and hot white dwarf (teff ∼ 123 kK, R = 0.017 R⊙) • Nebular morphology: Each PN possesses what we would binary system at its core. The cool star has been shown to classify as a ‘typical’ PN shape, with rings (A70 and be kinematically associated with the nebula, and to have a WeBo 1) and shells (LoTr 1). A mimic is often more dif- rotation period of 6.4 days. Although it was not possible to fuse. accurately determine the [Ba/Fe] value for the central star

(b) Slit position 1, (a) Slit position 1, [Nii] [Oiii]

Figure 12. PV arrays showing reduced [N ii]λ6584 A˚ and [O iii]λ5007 A˚ SPM-MES spectra of WeBo 1. Figures (a) and (b) show the [N ii] and [O iii] emission from Slit 1, (c) and (d) from Slit 2 (see figure 11). North is to the top of the array. The velocity axis is heliocentric ′′ velocity, Vhel. The display scale has been modified to highlight the spatio-kinematic features referred to in the text. Cross-section 0 defines where the central star is found. The continuum of a field star is visible at cross-section +15′′ in figures (a) and (b).

Table 1. Physical parameters of LoTr 1, A 70 and WeBo 1

−1 Nebula vexp ( km s ) Kinematical age (yrs) Physical size (pc) Morphology LoTr 1 (inner) 17±4 17,000±5,500 0.59±0.05 spherical/elliptical

± ± +0.05 LoTr 1 (outer) 25 4 35,000 7,000 1.86−0.09 spherical/elliptical

± ± +0.06 A 70 39 10 13,400 4,700 1.10−0.04 Ringed-waist with detected lobes

WeBo 1 23±10 11,700±5,900 0.50±0.03 Ringed-waist, no lobes detected system, we were able to say with confidence that LoTr 1 2003, 2004), which are extended, generally spherical, struc- does not show any evidence for an over-abundance of Ba ii. tures attached to the inner shell, and detached outer shells, LoTr 1 also presents double-peaked emission lines, which with the latter being far less common. This is critical as the have been seen in the other PNe with cool central stars. haloes are generally understood to be the ionised remnant of Unlike LoTr 1, the PNe A70 and WeBo 1 have both been mass lost on the AGB which is now being swept up to form previously confirmed to contain a Ba ii-enriched central star the inner shell, while the formation mechanism for multiple, system at their core. The two nebulae are also shown here to detached shells is still a mystery. Schönberner et al. (1997) display morphologies distinct to that of LoTr 1, with both show that this may be possible via a combination of photo- possessing ring-like waists and possible extended lobes. The ionisation and wind interaction, or, alternatively, a binary similar morphologies and chemical enrichment strongly im- evolution might be responsible for rapid changes in mass-loss ply that the two have undergone very similar evolutionary that could form two distinct shells, such as with Abell 65 or mass-loss processes. It is possible that the wind-accretion (Huckvale et al. 2013). process involved in the formation of Ba ii stars is also re- LoTr 1 clearly shows a detached outer shell which may sponsible for the formation of these ring-like morphologies. have been produced by rapid changes in mass-loss/transfer Although the CSPN of LoTr 1 does share some common in the CSPN. The difference in kinematical ages between the traits with those of A70 and WeBo 1 – namely binarity two shells, of roughly 18,000-20,000 years, is much shorter with a hot- and cool-components, and rapid rotation of the than the single star evolutionary timescales on which these secondary – both the lack of a significant over-abundance changes might occur. of Ba ii and the marked difference in nebular morphology Perhaps the nature of LoTr 5 should also be investigated would imply a difference in the evolution of this system. further, as it too does not have an apparent ring-like mor- The lack of Ba ii enhancement could be explained by a dif- phology despite possessing a rapidly rotating, G5 III-type, ii ference in progenitor mass, metallicity, or simply quantity of Ba -rich central star. As mentioned earlier, Montez et al. mass transferred via the same wind-accretion process (the (2010) have carried out a study into the x-ray emission em- amount of material accreted is strongly dependent on orbital anating from this system and concluded that it is most likely separation – Boffin & Jorissen 1988). However, as shown chromospheric in origin, implying the presence of a spun-up by Boffin & Zacs (1994) only a small amount of matter is companion. needed to be accreted to make a star appear as a barium The results presented here show that we still have some star and some mass must have been transferred as it is re- way to go to fully constrain mass-transfer mechanisms in quired in order to spin up the secondary to its rapid rotator intermediate-period binary and post-AGB systems, and fur- state. The most obvious explanation, therefore, is that the ther study of other such similar systems would be highly ii mass was transferred at an earlier stage in the evolution of beneficial, in particular with regards to Ba pollution. the primary, i.e. before the thermally-pulsing AGB phase, when the s-process elements are created and brought to the surface. This would allow us to infer that the AGB evolution ACKNOWLEDGMENTS of the primary was cut short by this mass-transfer episode, AAT gratefully acknowledges the support of STFC and signs of which should be detectable in the properties of the ESO through her respective studentships. This work was WD. We strongly encourage follow-up observations of the co-funded under the Marie Curie Actions of the Euro- system in order to confirm this hypothesis, and in particular pean Commission (FP7-COFUND). Based on observations it would be crucial to determine the orbital period of these made with ESO telescopes at the La Silla Paranal Observa- systems. It is, however, important to note that given the in- tory under programme IDs: 0.74.D-0373(A), 087.D-0174(A), clination of the LoTr 1 nebula (very close to pole-on), any 087.D-0205(A), 088.D-0750(A), 0.88D-0573(A), and 0.89D- radial velocity variations of the central star system would be 0453(A). We thank the staff at the ESO La Silla Paranal very difficult to detect, particularly for the expected period Observatory and the Anglo-Australian telescope for their ∼ of 1–3 years, and this therefore requires very high spectral support in the acquisition of the observations. WeBo 1 im- resolution and stable instruments. ages were acquired with the 2.5m Isaac Newton Telescope Coming back to the nebula, multiple shells are not un- located in the Spanish Observatorio del Roque de los Mucha- common in planetary nebulae, with 25-60% found to show chos on La Palma, Canary Islands, which is operated by the outer structures (Chu et al. 1987). However, it is important Instituto de Astrof´sica de Canarias (IAC). Both A70 and to distinguish here between ‘halo-like’ shells (Corradi et al. WeBo 1 analyses are based on observations obtained at the

Observatorio Astronómico Nacional at San Pedro Martir, ence Series, ESO’s Multimode Instrument for the Nas- Baja California, Mexico, operated by the Instituto de As- myth focus of the 3.5 M New Technology Telescope. pp tronomá, Universidad Nacional Autónoma de México. 339–348 Some of the data presented in this paper were obtained Dekker H., D’Odorico S., Kaufer A., Delabre B., Kot- from the Mikulski Archive for Space Telescopes (MAST). zlowski H., 2000, in M. Iye & A. F. Moorwood ed., So- STScI is operated by the Association of Universities for ciety of Photo-Optical Instrumentation Engineers (SPIE) Research in Astronomy, Inc., under NASA contract NAS5- Conference Series Vol. 4008 of Society of Photo-Optical 26555. 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