Infrared Study of Fullerene Planetary Nebulae
We present a study of sixteen Planetary Nebulae (PNe) where fullerenes have been detected in their Spitzer Space Telescope spectra. This large sample of objects offers an unique opportunity to test conditions of fullerene formation and survival under different metallicity environments as we are analyzing five sources in our own Galaxy, four in the Large Magellanic Cloud, and seven in the Small Magellanic Cloud. Among the sixteen PNe under study, we present the first detection of C (possibly also C) fullerenes in the PN M 1-60 as well as of the unusual 6.6, 9.8, and 20 m features (attributed to possible planar C) in the PN K 3-54. Although selection effects in the original samples of PNe observed with Spitzer may play a potentially significant role in the statistics, we find that the detection rate of fullerenes in C-rich PNe increases with decreasing metallicity (5% in the Galaxy, 20% in the LMC, and 44 % in the SMC) and we interpret this as a possible consequence of the limited dust processing occuring in Magellanic Cloud (MC) PNe. CLOUDY photoionization modeling matches the observed IR fluxes with central stars that display a rather narrow range in effective temperature (30,00045,000 K), suggesting a common evolutionary status of the objects and similar fullerene formation conditions. Furthermore, the data suggest that fullerene PNe likely evolve from low-mass progenitors and are usually of low-excitation. We do not find a metallicity dependence on the estimated fullerene abundances. The observed C intensity ratios in the Galactic sources confirm our previous finding in the MCs that the fullerene emission is not excited by the UV radiation from the central star. CLOUDY models also show that line- and wind-blanketed model atmospheres can explain many of the observed [NeIII]/[NeII] ratios by photoionization suggesting that possibly the UV radiation from the central star, and not shocks, are triggering the decomposition of the circumstellar dust grains. With the data at hand, we suggest that the most likely explanation for the formation of fullerenes and graphene precursors in PNe is that these molecular species are built from the photo-chemical processing of a carbonaceous compound with a mixture of aromatic and aliphatic structures similar to that of HAC dust.
The existence of fullerenes, highly resistant molecules formed exclusively by carbon atoms in a number of 60 and more, was predicted in the 70s as a possible variety of carbon assemblage in the Universe (Osawa 1970). However, it was only in the 80’s when the most common fullerenes C (buckyball) and C were synthetized in the laboratory for the first time (Kroto et al. 1985) in an effort to trace the carriers of the elusive diffuse interstellar bands (DIBs). Since then, they have been found on Earth (e.g., Buseck et al. 1992), on meteorites (e.g., Becker et al. 1999)111Note that the fullerene detections on Earth and meteorites are somewhat controversial and it is not firmly sure that these authors have really found fullerenes in their matrices., in Planetary Nebulae (Cami et al. 2010; García-Hernández et al. 2010, 2011), and in the interstellar medium (Sellgren et al. 2010). More recently, fullerenes have also been detected in a proto-planetary nebula (Zhang & Kwok 2011), around R Coronae Borealis (RCB) stars (García-Hernández, Rao & Lambert 2011a,b), in the Orion nebula (Rubin et al. 2011), in post-AGB circumbinary discs (Gielen et al. 2011), young stellar objects (YSOs), and around a Herbig Ae/Be star (Roberts, Smith & Sarre 2012).
From the variety of environments were fullerene molecules have been recently detected the picture is emerging that the conditions for fullerene formation cannot be too stringent since they have to be shared by star forming regions, evolved stars, and the interstellar medium (ISM). In that sense, the detection by the Spitzer Space Telescope of fullerenes in a remarkably large/considerable number of Planetary Nebulae (16 sources so far)222Note that instead, fullerenes have been detected towards a few - sometimes even only one - targets in other astrophysical environments such as RCB stars, YSOs, etc. offers a unique opportunity to test the conditions of fullerene formation and survival.
Planetary Nebulae (PNe) represent the evolutionary stage during which the slow, intense dust-driven winds, which are the signature of the late Asymptotic Giant Branch (AGB) evolution, become ionized. During the PN phase, the effective temperature of the central star can reach 200,000 K while a fast, low density post-AGB wind, shocks and shapes the inner parts of the previously ejected AGB envelope. Chemically, the envelope shredded from the AGB star can show a C-rich or O-rich chemistry, primarily as a direct consequence of the number and efficiency of dredge-up processes experienced by the central star. These processes ultimately are just determined by the stellar initial mass and its evolutionary stage (Herwig 2005). Dynamically, the evolution of the shocked ejected shell is mainly determined by both the stellar evolutionary stage and core mass (see e.g., Villaver, Manchado & García-Segura 2002; Villaver, García-Segura & Manchado 2002). In addition, during the PN phase, two fundamental chemical drivers, i.e., the UV radiation field from the central star and the shocks generated by the stellar winds, are present and quickly evolving (see e.g., Kwok 2004).
From laboratory experiments we can learn a lot on the formation process of fullerenes (Kroto et al. 1985; Kratschmer et al. 1990; de Vries et al. 1993) and, in fact, the first synthesis of fullerenes in the lab motivated the first searches in space. However, they can also be misleading. Although the laboratory synthesis of fullerenes shows that they are more efficiently produced under hydrogen-deficient conditions, this does not seem to be the case in space, where fullerenes seem to be efficiently formed in H-rich circumstellar environments (García-Hernández et al. 2010, 2011). This is manifested by the presence of C fullerene bands in Spitzer spectra of Galactic and extra-Galactic PNe with normal H-rich central stars that also display the classical aromatic infrared bands (AIBs) usually attributed to polycyclic aromatic hydrocarbons (PAHs) (García-Hernández et al. 2010, 2011) and that includes the PN Tc 1. Interestingly, fullerenes have been detected in PNe whose IR spectra are clearly dominated by aliphatic carbon-rich dust, represented by broad emissions such as those centered at 69, 913, 1520, and 2535 m333Note that the 1520 m emission may be also related with the C-C-C bending modes of relatively large PAHs or PAH clusters (e.g., Van Kerckhoven et al. 2000).. The different spectral properties of these broad features observed from source to source are quite consistent with the expected different properties of the hydrogenated amorphous carbon (HAC) grains (or their decompositon products), which may be present in the circumstellar envelopes of these stars, and that would explain the wide range of different spectra observed depending on their physical and chemical properties (e.g., size, shape, degree of hydrogenation; see García-Hernández et al. 2010, 2011). This raises the exciting possibility that considerable physical and chemical processing of dust occurs just after the AGB phase (see also Zhang & Kwok 2011), including the formation of fullerenes. Moreover, fullerenes, being relatively hardy molecules, may survive indefinitely in space playing an important role in circumstellar/interstellar chemistry and physics.
From the experimental point of view, only very recently, laboratory spectroscopy of C and C fullerenes (Iglesias-Groth et al. 2011) has provided the temperature dependence of wavelength shifts and molar absorptivity of the C and C infrared bands, which are fundamental for a firm identification of the observed bands as fullerenes and for a quantitative abundance determination of these molecules in space.
In this paper, we present a study of the infrared properties of sixteen fullerene PNe, including the first detection of fullerenes in the Galactic PN M 160 and of the unusual 6.6, 9.8, and 20 m features (attributed to possible planar C) in the PN K 3-54. Our sample, represents the largest (irrespective of the object type) in which fullerene molecules have been detected, and includes for the first time objects belonging to very different metallicity environments (the Galaxy and the Large and Small Magellanic Clouds), allowing the study of how metallicity influences the chemical route for fullerene formation. The Spitzer infrared data used in our study are described in Section 2 while the data analysis, including the estimation of C and C fullerene abundances, is presented in Section 3. Section 4 discusses the results of our study and we offer the conclusions of this work in Section 5.
2 The Data
We have searched for the presence of the infrared features atributed to the C and C molecules in the spectra of a large sample of PNe (mainly from our own Spitzer programs) and available in the Spitzer/IRS archive. This includes the infrared spectra obtained under Spitzer GO programs # 3633 (P.I.: M. Bobrowsky), and # 20443 and # 50261 (P.I.: L. Stanghellini). A total of 238 Spitzer/IRS spectra of PNe were obtained under these programs with the general goal of investigating the chemical properties and dust evolution during this evolutionary stage and has been published elsewhere in studies dealing with the original goals as proposed to the Spitzer observatory. All Spitzer programs have in common the spectral coverage in the 538 m range, making use of different combinations of the Short-Low (SL: 5.214.5 m; 64 R 128), Long Low (SL: 14.038 m; 64 R 128), Short-High (SH: 9.919.6 m; R600) and Long-High (LH: 18.737.2 m; R600) modules depending on the source brightness in the mid-IR444Note that a minimum S/N of 50 is usually reached.. Detailed descriptions of observations and the data reduction process can be found in Perea-Calderon et al. (2009) (program # 3633) for the 40 PNe observed in the Galactic Bulge, in Stanghellini et al. (2007) and Shaw et al. (2010) (program # 20443) for the 41 PNe in the Magellanic Clouds (MCs), and in Stanghellini et al. (2012) (program # 50261) for the 157 compact PNe observed in the Galactic Disk. An additional sample of 25 PNe in the MCs were observed under GTO program # 103 (see Bernard-Salas et al. 2009) and we have included these data in the analysis as well. In summary, a total of 263 PNe have been used in this paper in the search for fullerene emission and in the analysis of the characteristics of its emission that follows.
2.1 Spitzer/IRS Spectra of PNe with Fullerenes
The C fullerene molecule displays four infrared features located at 7.0, 8.5, 17.4, and 18.9 m (e.g., Kratschmer et al. 1990; Iglesias-Groth et al. 2011). In order to better isolate these spectral features we have performed a careful substraction of the dust continuum emission between 5 and 25 m in the spectra of the 263 PNe mentioned above. To do so, the dust continuum emission was represented by 3-7 order polynomials fitted at spectral locations that we are confident to be free from any dust or gas feature; e.g., the spectral points at 6, 9, 14, and 24 m are not affected by dust emission at 69, 913, 1520, and 2535 m. Note that we prefer to exclude the 25-38 m spectral region from the dust continuum subtraction because the 25-35 m emission sometimes extend well beyond 38 m, difficulting the location of the continuum. Then, by inspecting the dust continuum subtracted or residual spectra, we search for emission features at the four spectral positions corresponding to the bands that may be attributed to C fullerenes (see Figure 1). To illustrate the technique in Figure 1 we show examples of the dust continuum substracted spectra of the PNe Tc 1 and K 3-54. Tc 1 (see left panel in Figure 1) was observed at high-resolution and displays some of the strongest fullerene features in our sample. On the other hand, PN K 3-54 (see right panel in Figure 1) was observed at low resolution and displays much weaker fullerene bands than Tc 1.
We found obvious fullerene features in 16 objects out of the 263 PNe analyzed. They are distributed as follows: seven SMC PNe (six from program # 20443 and one from program # 103), four LMC PNe (three from program # 20443 and one from program # 103), and five Galactic PNe (three from program GO # 3633 and two of them from GO # 50261). Most of these detections have already been published: 3 Galactic and one SMC targets in García-Hernández et al. (2010); the ten detections of fullerenes in PNe of the Magellanic Clouds in García-Hernández et al. (2011); and the first fullerene detection in Tc 1 in Cami et al. (2010). In this paper, we report the detection of fullerenes in a new source, the Galactic PN M 1-60, and we provide an overall and detailed analysis of the whole sample of PNe with fullerenes.
Figures 2 to 4 display the Spitzer/IRS spectra of all fullerene PNe discovered so far grouped by location or metallicity environment, that is, we show in Fig 2 the targets in the Galaxy while Figs. 3 and 4 show the PNe in the LMC and SMC, respectively. Note that in these figures the strongest C feature at 18.9 m (sometimes also C 17.4 m) is distinctly present superimposed on the dust continuum thermal emission. Most of the PNe also show broad dust emission features centered at 913 and/or 2535 m (the so-called 30 m feature; e.g., Hrivnak, Volk & Kwok 2000). All C-detected PNe exhibit either the broad 30 m, or the 913 m feature, but if not, then they display a very prominent 69 m feature, like SMC 16 and SMC 24 (Figure 4) where this feature is the strongest among the three features mentioned above. A huge 69 m feature is also seen in LMC 02 (Fig. 3), which does not show any 913 nor 30 m features. SMC 20 (Fig. 4) is a peculiar case because it shows the strongest 913 and 1520 m features together with the complete lack of the 30 m feature.
M 1-60, a new fullerene PN555M 1-60 also displays the weak AIBs at 6.2, 7.7, and 11.3 m usually attributed to PAHs., displays an extremely weak 913 m feature together with a wider and sub-structured 30 m feature - narrower (FWHM0.5-3.5 m) emission features superimposed to the broad 30 m emission are seen at 25.9, 27.8, 29.7, and 34.1 m. The 30 m feature seen in M 1-60 resembles that of the PPN IRAS 225746609 (Hrivnak, Volk & Kwok 2000), which also shows a narrow and unidentified emission feature at 25.8 m. Interestingly, a similar pattern (although the features look weaker and narrower) of emission sub-features is also seen in the other Galactic fullerene PNe such as Tc 1, M 1-20, and M 1-12 (Figure 2) for which high-resolution (R600) Spitzer/IRS spectra are available. We believe that these unidentified 30 m sub-features are possibly related with the formation of fullerenes, in the sense that their carriers may be decomposition products of HACs (or a similar carbonaceus compound like coal, petroleum fractions, etc.) such as fullerene precursors.
It is worth to note here that the exact positions (and widths) of the 7.0, 8.5, 17.4, and 18.9 m C features are temperature dependent (see Iglesias-Groth et al. 2011 and references therein). Duley & Hu (2012) have very recently suggested that these four IR features may be due to proto-fullerenes rather than C in objects that also show the 16.4 m feature and other AIBs usually attributed to PAHs. The 16.4 m feature is usually seen when the classical AIBs at 6.2, 7.7, and 11.3 m are much stronger than the 7.0, 8.5, 17.4, and 18.9 m IR features, being the case for sources like reflection nebulae (NGC 7023, NGC 2023), RCB stars (DY Cen, V854 Cen), the proto-PN IRAS 010057910 and a few post-AGB stars (Gielen et al. 2011). Thus, it is possible that the 7.0, 8.5, 17.4, and 18.9 m IR features are due to proto-fullerenes in these sources (García-Hernández et al. 2012). However, the PN Tc 1 does not display strong AIBs nor the 16.4 m feature, being a proxy for a source where only C fullerenes are detected. The 16.4 m emission feature is not present either in any of the PN sources analyzed in this paper, and the AIBs are very weak–contrary to the case in reflection nebulae, post-AGB stars, and RCB stars– and therefore the most reasonable assumption is that the 7.0, 8.5, 17.4, and 18.9 m emission features are mostly due to C fullerenes.
Some of the PNe in our sample (e.g., SMC 16, M 1-12, K 3-54, SMC 24, LMC 02) are almost spectroscopic twins of Tc 1, showing the broad 69 m emission plateau and the 913, and 30 m dust emission features characteristic of aliphatic hydrocarbons (see e.g., Kwok & Zhang 2011) such as HAC aggregates or very small grains (VSG) (e.g., Tielens 2008). Thus, they should be in an evolutionary stage similar to Tc 1. This interpretation is supported by the narrow range of central star effective temperatures that all these sources have (see §3.1).
It is also important to consider that we lack the knowledge of the relative spatial distribution of the carrier of the AIBs with respect to C emission, which is an important aspect of the analysis and that could offer important clues on the formation mechanism of fullerenes in PNe. The PNe are spatially small and the Spitzer spectrum generally encompases the whole nebula. Tc 1 is the only exception, being the most extended source in our sample. In Tc 1 the fullerene and PAH emission seem to peak slightly offset from the central star (where the dust continuum emission peaks) and they look more extended by 1-2 pixels than the dust continuum emission. Indeed, a careful inspection of the Tc 1’s Spitzer spectrum reveals the presence of very weak PAH 6.2 and 11.3 m emission; see e.g., Figures 2 and 3 in García-Hernández et al. (2010). Unfortunately, the Spitzer/IRS observations contain a marginal amount of information at a spatial resolution of 2”. We believe that mid-IR images at much higher spatial resolution are needed in order to reach any firm conclusion about the relative spatial distribution of C, AIBs, and other emission in Tc 1.
Figure 5 displays the residual spectra of the galactic fullerene PNe Tc 1, M 1-20, M 1-12, and K 3-54 and the extragalactic PN LMC 02. The expected positions of the four C bands are marked with dashed vertical lines. We would like to remark that two additional MC PNe (SMC 17 and SMC 19; see Stanghellini et al. 2007 for the spectra) show a strong feature at 19.0 m that could be attributable to C. However, these PNe show very strong AIBs (e.g., those at 6.2, 7.7, 8.6, and 11.3 m usually attributed to PAHs) together with a yet unidentified feature at 17.0 m (see e.g., Boersma et al. 2010). Given that the other C features (at 7.0 and 17.4 m) are not obvious in their residual spectra, and that the 17.0 and 19.0 m features may be due to very large PAHs (Boersma et al. 2010) we considered that these two sources do not have solid fullerene detections, and therefore they have not been included in the analysis presented here.
For three (Tc 1, LMC 02, and SMC 24) of the C-containing PNe, high resolution (R600) Spitzer/IRS spectra are also available, in which relatively strong and isolated C features are present at 12.6, 14.9, 15.6, 17.8, 18.7, and 21.8 m (von Czarnowski & Meiwes-Broer 1995; see Figure 2 in García-Hernández et al. 2011). The C infrared features may be present in PNe other than Tc 1, LMC 02, and SMC 24. Only low resolution Spitzer/IRS spectra were obtained for the MC PNe in our sample, avoiding us to confirm/exclude the detection of these features in more extra-galactic sources. However, the Galactic PNe M 1-20, M 1-12, and M 1-60 were observed at high resolution as well and these three PNe show a weak but unblended feature at 21.8 m (see Figure 2) that we attribute to C. Consistently with our identification, M 1-60 and M 1-20 show also some C emission at 14.9 m (Figure 5). We do not estimate the C abundances and temperatures in these three sources due to the intrinsic weakness of these possible C features and because we only see a maximum of two unblended C features (see Section 3.2).
We report for the first time the presence of unusual IR features at 6.6, 9.8, and 20 m, which are coincident with the strongest transitions of planar C (graphene precursors or “proto-graphene”) in the Galactic PN K 3-54. These features have been detected in another two targets that also have fullerenes, LMC 02 and SMC 24 (García-Hernández et al. 2011). Among the other Galactic fullerene PNe, M 1-12 also displays a strong 6.6 m feature but not the other ones. Although more tentative, the 6.6 m feature seems to be present also in Tc 1, M 1-20, and M 1-60 (see Figure 5).
In Table 1 we give the positions and integrated fluxes for the C and C fullerene bands identified in the 16 fullerene PNe, as well as the possible planar C features. These measurements have been done in the residual spectra by fitting a baseline to the local continuum of each infrared band in order to subtract any possible extra-contribution from the underlying dust features at 69, 913, and 1520 m (see Figures 2 to 4) still present in the dust continuum subtracted spectra. We also list in Table 1 our estimated flux errors between brackets, which take into account the error in the measurement as well as the error introduced by the uncertainty of the determination of the local baseline selected for integrating the fluxes. Our estimated flux errors are typically 15% for the Galactic PNe as well as for the strongest fullerene transitions (e.g., at 17.4 and 18.9 m) in the PNe of the Magellanic Clouds. The estimated flux errors are, however, higher for the weaker fullerene transitions (e.g., at 7.0 and 8.5 m) observed in Magellanic Cloud PNe, which sometimes can be as high as 30-40% (Table 1). Note also that the observed 7.0 m emission is a blend of C, C, and [Ar II] 6.99 m emission. We estimate the C and C 7.0 m flux contributions from the corresponding Boltzmann excitation diagrams below (§3.2). The position and flux for the 15.6 m C band is only given when this feature is not blended with the usually stronger 15.6 m [Ne III] emission.
Table 2 gives some basic physical parameters when available in the literature such as the effective temperature of the central star, the H flux, carbon abundance, electron temperature and density, and total hydrogen and carbon mass of the nebula.
3.1 Cloudy Photoionizaton Models with Line- and Wind-blanketed Atmospheres
In a previous work we presented some basic CLOUDY models (Ferland et al. 1998) for the fullerene PNe in the Magellanic Clouds (García-Hernández et al. 2011). We were only interested in doing some general and basic model predictions to explore the possible origin of the 7.0 m emission as well as of the usually low [Ne III]/[Ne II] ratios observed in fullerene PNe. To do so we adopted for the stellar ionizing continuum blackbodies (BB) and stellar spectra taken from Rauch (2003) and concluded that the 7.0 m emission in fullerene PNe cannot be attributed solely to [Ar II] 6.99 m but to a combination of C and C. Furthermore, the fact that those models were unable of explaining the low [Ne III]/[Ne II] ratios by photoionization, and given that fast stellar winds from the central stars are ubiquitous in PNe (and the associated shocks) lead us to the suggestion of a possible shock-excited origin for the [Ne II] 12.8 m emission in fullerene PNe (see García-Hernández et al. 2011 for more details). We did not contemplate the possibility, however, that the observed [Ne III]/[Ne II] ratios might be also explained by using model atmospheres with line (and wind) blanketing (C. Morisset 2011, private communication; see also Morisset et al. 2004 and references therein). Here we present new radiation transfer modeling exploring this option.
We have used version C08.01 of the photoionization code CLOUDY to model the nebular emission in our fullerene sources. For the model atmosphere of the central star, this time we have used the grid of models provided by Pauldrach et al. (2001), which are available in CLOUDY, identified as tables WMBasic. These models include non-LTE, line- and wind-blanketed model atmospheres for O-B stars with solar and sub-solar metallicities. For all models we have assumed a constant luminosity of L of the star and a fixed distance to the nebula of 0.1 pc (inner radius). Variations of luminosity and/or inner radius closely mimics variations of the temperature of the central star. Thus, our sequences (varying central star temperature) represents the locus of a given model for any combination of luminosity, distance and temperature. We have computed models using solar and sub-solar metallicities for the atmosphere of the central star, but there are no large differences in the resulting line ratios. We have assumed for the abundances of the nebula those for the prototype fullerene PN Tc 1 and quoted by Pottasch et al. (2011) in Table 15. We have considered a spherical geometry for the nebula with a density of 1800 cm at the inner radius, and a dependency with the distance to central ionizing source as r. The chemical abundances of the photoionized gas are also those quoted by Pottasch et al. (2011) in their Table 15.
In Figure 6 we display our model predictions compared to the observed line ratios ([ArII]/HI vs. [ArIII]/HI and [SIV]/[SIII] vs. [NeIII]/[NeII]) in the fullerene PNe of our sample. In the right panel we present a combination of line ratios ([SIV]/[SIII] vs. [NeIII]/[NeII]), which permits to determine the effective temperature (T) of the central star, independently of the nebular abundances. It can be clearly seen that model atmospheres with line and wind blanketing can explain many of the observed [NeIII]/[NeII] ratios. In addition, all fullerene sources show a rather narrow range in effective temperature (T30,00045,000 K).
The fact that line and wind blanketing can explain many of the observed [NeIII]/[NeII] ratios weakens the hypothesis of a shock-excited origin for the [Ne II] 12.8 m emission. Unfortunately, the shock hypothesis cannot be directly tested with the present data.
It is to be noted here that the observed central star effective temperatures determinations in the literature (those listed in Table 2) came mostly from hydrogen Zanstra analysis, and thus although more reliable for optically thick PNe, they should always be used as lower limits for optically thin targets (see also §4.1). Moreover, none of the temperature determinations available are free from model assumptions. Indeed, from our CLOUDY modeling (see Figure 6) we infer effective temperatures higher than the hydrogen Zanstra temperatures (although always lower than 45,000 K) for all of these MC PNe. The only exception is LMC 02 for which the [S IV]/[S III] ratio is somewhat lower than expected for its T (39,000 K as derived from FUSE data); we atribute this difference to a contamination by C of the observed 18.7 m emission. From Figure 6 is also evident that for Galactic PNe M 1-20 and M 1-60 the models indicate T 40,000-45,000 K666The CLOUDY T for M 1-20 and M 1-60 are significantly lower thant the literature values (Table 2) and derived from the Zanstra HI method in M 1-20 (Kaler & Jacoby 1991) and from the Energy-Balance method in M 1-60 (Preite-Martínez et al. 1989)., whereas for Tc 1 T 35,000 K. Note that the Galactic PNe M 1-12 and K 3-54 are not shown in this diagram because they only present [Ne II] 12.8 m and [S IV] 10.5 m, emission, respectively. An anomalous high value of [Ne II] 12.8 m emission (e.g., as a consequence of strong shocks) in these sources cannot be discarded.
In the left panel of Figure 6 ([ArII]/HI vs. [ArIII]/HI) we illustrate how varying the abundance of Ar affects noticeably the line ratios involving these lines. It can be seen that the observed values of [Ar III]/H I can be explained with the model effective temperature, consistently with the values derived from the [SIV]/[SIII] vs. [NeIII]/[NeII] diagram shown in the right panel of Figure 6. However, none of the models can explain (even taking into account maximum errors of 40% in the observed line intensity ratios) the high observed values of the [Ar II]6.99 m/HI line flux ratio. For example, in the case of Tc 1 and M 1-12, the expected ratios are around 1, whereas in the case of M 1-20 and M 1-60 those are around 0.1. Note that PN K 3-54 does not appear in this diagram because the HI 7.5 m line is blended with the much stronger 7.7m AIB. Thus, the new CLOUDY modeling confirms that the 7.0 m emission in fullerene PNe cannot be attributed uniquely to [Ar II] 6.99 m and the C and C emission contributions have to be taken into account.
3.2 Boltzmann Excitation Diagrams of the Fullerene Emission
Our previous work indicates that the fullerene emission in PNe of the Magellanic Clouds is likely originated from solid-phase fullerene molecules, being formed on the surface of pre-existing very small grains (or HACs) (García-Hernández et al. 2011; Duley & Hu 2012). This conclusion was based on the fact that the observed C 7.0/18.9, 8.5/18.9, and 17.4/18.9 intensity ratios differ from the theoretical predictions for UV-excited gas-phase C molecules given by Sellgren et al. (2010). Here we have determined the fluxes of the 7 and 8.5 m C transitions (which are actually only due to C) in the Galactic PNe M 1-20, M 1-12, M 1-60, and K 3-54777Note that the observed 8.5 m fluxes cannot be completely explained by C emission in a few MC PNe (SMC 13, 24, LMC 02, 99; see García-Hernández et al. 2011) and in two Galactic PNe (K 3-54 and M 1-12) only. Thus, these objects may show some AIB emission (e.g., from PAHs) at this wavelength. and we have constructed the corresponding Boltzmann excitation diagrams (see below for more details) for these Galactic PNe.
In Table 3 we list the C intensity ratios in fullerene PNe of our Galaxy in comparison with the values derived for fullerene PNe in the Magellanic Clouds. Note that the listed C F(7.0)/F(18.9) flux ratios are obtained by correcting the observed emission fluxes at 7 m by the C contribution factors (f) derived from the Boltzmann excitation diagrams (see below) and also listed in Table 3. In addition, the listed C F(8.5)/F(18.9) intensity ratios in a few sample PNe (see above) were corrected by their corresponding contribution factors at 8.5m (not shown in Table 3). From the results obtained we can reach a similar conclusion about the fullerene excitation mechanism for the subsample of fullerene-detected PNe in our Galaxy (see Table 3). In particular, the C 7.0/18.9 and 8.5/18.9 ratios indicate photon energies of less than 5 eV, regardless of the central star temperatures (between 30,000 K and 45,000 K). In addition, the C 17.4/18.9 intensity ratio strongly varies from source to source (from 0.21 to 1.13), while this ratio is predicted to be unsensitive to the energy of the absorbed UV photons (a C 17.4/18.9 ratio of 0.280.38 is predicted by Sellgren et al. 2010) for gas-phase fullerene molecules. It should be noted here that our conclusion is different from that very recently drawn by Bernard-Salas et al. (2012). However, the other fullerene C also contributes to the observed 7 m flux (Iglesias-Groth et al. 2011; García-Hernández et al. 2011) - e.g., in Tc 1 the C flux contribution (40%) is even higher than that for C (25%). Our conclusion is supported by the fact that the same temperatures (within the errors) are needed to explain both the 7.0 and 8.5 m C band strengths. Finally, we note that taking into account the uncertainties of flux measurements and Einstein-coefficients, our average intensity C F(7.0)/F(18.9) (=0.28), F(8.5)/F(18.9) (=0.35), and F(17.4)/F(18.9) (=0.56) ratios would be more consistent with the C intrinsic strengths reported by Fabian (1996) and Iglesias-Groth et al. (2011). Thus, in the following we will assume that the fullerene emission in PNe is originated from solid-state fullerenes.
We can determine the fullerene excitation temperature for each PN from the flux measurements of the C and C transitions (see Table 1) and the absorptivity measurements recently obtained by Iglesias-Groth et al. (2011). Absorptivity for each transition led to an f-value from which the Einstein coefficient A, is derived which provides a relation between the emitted flux and the number of molecules populating the energy level associated with the transition (see e.g., Cami et al. 2010 for more details).
We adopted the temperature dependence of the absorptivity for each C and C transition given in Iglesias-Groth et al. (2011) and developed an iterative process to find the best possible correlation coefficient in the plane ln(N/g) vs. E/k independently for each fullerene. A thermal distribution over the excited states will be indicated by colinear datapoints in the Boltzmann excitation diagram. In this case, the slope and the intercept of the linear relation determines the temperature and the total number of emitting fullerene molecules. The initial temperatures for this iterative process were assumed in the range from 100 to 1000 K and only a few iterations were required for a quick convergence. The temperature values for C and C derived this way are listed in Table 3. The four mid-IR transitions of C were considered, allowing the fraction of the flux of the 7m transition - which is actually due to C - as a free parameter because of the possible contamination with C and the [Ar II] nebular line. The best linear fit is sensitive to this fraction, and thus we also iterated on this parameter in order to explore the full range of possible values (from 0 to 1). In the case of C, we considered the 3 or 4 (depending on the source and the S/N of the spectrum; see Table 1 for the C features used for each source) mid-infrared transitions less contaminated by other species, iterating also on the fraction of the 7m flux that may be due to C. We generally found very good correlation coefficients that are always higher than 0.89. Figure 7 displays illustrative examples of the Boltzmann excitation diagrams for the C (left panel) and C (right panel) fullerene emission detected in the MC PNe SMC 16 and LMC 02, respectively.
The fullerene PNe in the Magellanic Clouds were previously studied by us in García-Hernández et al. (2011). The sources in the MCs offer a more reliable estimation of the C and C abundances in PNe than in their Galactic counterparts. This is because the distances to the MCs (48.5 and 61 kpc for the LMC and SMC, respectively) are accurately known and reliable C abundances are also available in the literature (García-Hernández et al. 2011). However, this is not the case for the five fullerene PNe in our Galaxy - in addition, only Tc 1 has a reliable C abundance in the literature (Table 2). For consistency, we decided to assume the recent statistical distances by Stanghellini & Haywood (2010) for four of the Galactic sources: M 1-20 (8.0 kpc), M 1-12 (9.2 kpc), M 1-60 (9.5 kpc), and K 3-54 (23.4 kpc). The only exception is Tc 1888Stanghellini & Haywood (2010) give a statistical distance of 2.7 kpc towards Tc 1., for which we assumed a distance of 2 kpc for comparison with Cami et al. (2010). Following Milanova & Kholtygin (2009) we may assign a PN type (i.e., type I, IIa, IIb, III, IV)999For type I (z= 0.23 kpc) A(C)=8.32, for type IIa (z= 0.31) A(C)=8.82, for type IIb (z= 0.56) A(C)=8.55, for type III (z= 1.05) A(C)=8.60, and type IV (z= 1.35) A(C)=8.64 (see Table 5 in Milanova & Kholtygin 2009). and an average carbon abundance (A(C)=log(C/H)+12) to each Galactic PN from the height above the Galactic plane z. Therefore we assume a type IIa and A(C)=8.55 for M 1-20 (z=0.492 kpc) and K 3-54 (z=0.547 kpc) as well as type IIb and A(C)=8.83 for M 1-12 (z=0.258 kpc) and M 1-60 (z=0.290 kpc). The IIab PN types of the Galactic fullerene PNe, as derived by this very rough and limited method, seem to be consistent with that of Tc 1, which is identified as a slowly evolving low-mass type II PN (García-Hernández et al. 2010). In addition, as we will see in Section 4.1, all fullerene PNe are likely of low-mass and are slowly evolving towards the white dwarf stage. Table 2 lists all relevant information (e.g., H flux, C abundance, electron temperature and density, total hydrogen and carbon masses of the nebula) as well as the source of information used in each case from the literature for all fullerene PNe in our sample.
By using the above distances, the number of C and C molecules (or masses of pure C and C) was estimated for all sources in our sample and the C/C and C/C abundances listed in Table 3 were derived from the total carbon and hydrogen mass of the nebula. The total H mass of the nebula is obtained from the electronic temperature and density and the observed H flux, while the total carbon mass is obtained from the total H mass and C abundance of the nebula. The corresponding C/C ratios are also given in Table 3. We find C/C0.010.05% (average of 0.03%) and T=260660 K for the four Galactic fullerene PNe studied for the first time in this paper. The formal error of our fullerene abundance estimations are difficult to evaluate because we are using different literature sources for the C abundance, electron density, etc. However, it should be noted that by giving the C/C and C/C abundance ratios, the distance dependence is canceled. Thus, the main sources of error are the electronic density and carbon abundance. We estimate that the errors of our fullerene abundances in MC PNe are certainly lower than 50% (likely much lower than this value for most of the MC PNe sample) but the maximum error in the C/C ratio for the Galactic PNe (with the exception of Tc 1 for which the C abundance is more accurately known) may be a factor 2 or 3. Taking into account all assumptions made in the Galactic objects, our individual and average C/C abundances as well as the C temperatures for these sources are very similar to the C abundances obtained in the MC ones (see Table 3). Finally, note that we estimate C/C=0.04% and C/C=0.005% in Tc 1, which are a factor of 40 and 300, respectively, lower than the 1.5% estimates for both species by Cami et al. (2010).
4.1 Evolutionary Status of the Fullerene PNe
The temperature and luminosity of central stars of PNe are important parameters in order to determine the evolutionary status of the nebula. Central star temperatures are available for a sizable number of objects in the sample of PNe with fullerenes (9 PNe). However, most of the temperature determinations of the objects at hand rely on widely used indirect methods either Zanstra (Zanstra 1931), or the Energy Balance (Stoy 1933) with only one exception LMC 02 obtained from FUSE data (Herald & Bianchi 2004). Moreover, for those PNe for which Zanstra temperatures have been obtained (SMC 13, SMC 24, SMC 27 and LMC 25) only hydrogen Zanstra temperatures could be determined for which the optically thick nature of the nebulae to all the photons above the Lyman limit of H is not assured. A hint to the fact that the effective temperature of these stars cannot be much higher than 50,000 K is provided by the lack of detection of He II 4686 Å nebular line flux and confirmed by our new CLOUDY modeling (see Section 3.1).
Although not definitive conclusions can be drawn given the small number of objects and the uncertainties, the available data shows that all the PNe with fullerenes have central stars with low effective temperatures in the range between 30,000 and 45,000 K. Given the paths that these stars follow in the HR diagram a low temperature can imply a very old or a very young star, therefore a second parameter, stellar luminosity (or mass), is needed in order to understand their evolutionary status. Unfortunately, central star luminosity determinations rely on the availability of the distances which are accurately determined for a handful of objects in the Galaxy and none of the fullerene containing PNe are among them. The situation is quite different for the PNe located at the known distances of the Magellanic Clouds. In this case, determinations of the central star luminosities are possible with great accuracy provided that the central star flux is measured. Out of the 11 PNe in the clouds only 3 of them have meassured luminosities (see Villaver, Stanghellini & Shaw 2003, 2004); the reasons are either the objects have not been observed with the appropriate instrumentation in order to measure the stellar flux (6 PNe), the star was saturated (LMC 25), or the central star was below the nebular level and thus not detected (SMC 13).
PNe SMC 24, SMC 27, and LMC 02 are the only ones that we can analyze in full extent given that they have both luminosities and effective temperatures available (Villaver, Stanghellini & Shaw 2004; Herald & Bianchi 2004). Although conclusions from 3 objects cannot be extrapolated to the full sample, we find meaningful the fact that the 3 of them have luminosities that locate them in the horizontal part of the post-AGB tracks for central stars of PNe. Together with their low effective temperatures this indicate that, at least these 3 objects are truly unevolved stars for which less than 10,000 yr (central star time according to Vassiliadis & Wood 1994) could have passed since the moment the star left the AGB phase. Moreover, all three objects have central stars in the low mass range: 0.59 M for SMC 24; 0.60 M for SMC 27, and 0.56 M for LMC 02 implying that the three of them descend from a low mass progenitor with a main sequence mass 1.5 M. It is important to note that the masses of fullerene systems in the SMC are within the average mass of central stars of PNe in the SMC, 0.63 M (Villaver, Stanghellini & Shaw 2004), while the LMC object is significantly less massive than the average, 0.65 M central star mass in the LMC (Villaver, Stanghellini & Shaw 2003, 2007).
The central star and the nebular gas are interdependent systems, that is, the evolution of the nebulae is governed by the energetics provided by the central star mostly throught the stellar wind and the ionizing radiation field. In that sense, the physical radius of the PNe can provide an indirect way of evaluating the evolutionary status of the systems provided that for the objects in the MCs this quantity is available. If we take the photometric radius of the nebulae101010The photometric radius, R, corresponds to the size of a circular aperture that contains 85% of the flux in [O III] 5007. R gives an objective measurement of the nebular angular size that is insensitive to the S/N ratio of the image and is useful for evolutionary studies. measured from HST data in Stanghellini et al. (2003) and Shaw et al. (2006), then adopting a distance to the SMC of 61 kpc and to the LMC of 48.5 kpc we obtain the physical radius of the PNe in the MCs. With only one exception LMC 99 (with a physical radius of 0.1 pc) all the radius are 0.06 pc. The average radius of all the PNe in the MCs that contain fullerenes is 0.058 pc (or 0.054 pc if we exclude LMC 99). The majority of the PNe in the HST samples have small sizes; they were selected to avoid entangled line in the STIS spectroscopic observations. They have a median photometric radius of 0.10 pc in the LMC and 0.08 pc in the SMC (see Shaw et al. 2006). The sample of MC PNe that contain fullerenes have physical sizes even smaller (most of them 0.06 pc) than the average values. Irrespective of the mechanism for shell growth, at the expense of the energy provided by the hot bubble (the high velocity wind producing an adiabatic shock) or by the propagation of the ionization front, it is expected to increase with time (see Villaver, Manchado & García-Segura 2002). Thus, if we accept the physical size as an indicator of the evolutionary status of the nebulae, then the fullerene PNe in the MCs seem to be young (non-evolved) systems as suggested by their small physical radii; smaller than the average value of a full sample.
4.2 Fullerene Detection vs. Metallicity
Interestingly, the fraction of fullerene-detected PNe increases with decreasing metallicity (5% in the Galactic Disk versus 31% in the MCs). This fraction goes from 5% in the Galactic Disk (2 detections from 39 C-rich compact (4”) and young PNe; Stanghellini et al. 2012) to 20% in the LMC (4 detections from 20 C-rich LMC PNe) and 44% in the SMC (7 detections from 16 C-rich SMC PNe). In addition, the Spitzer/IRS spectra reveal that the dust processing is very limited in the low metallicity environments of the Magellanic Clouds. The size of the dust grains remains small (e.g., Stanghellini et al. 2012) and the formation of PAHs (as indicated by the presence of AIBs) is more difficult at low metallicities (Stanghellini et al. 2007). This indicates that the dust grains are less processed in low-metallicity PNe than in their Galactic counterparts, likely because of the probability of forming big carbon dust aggregates around the central star, which may be affected by the posterior PN evolution - i.e., the quickly changing UV radiation field and/or the post-AGB shocks from the strong stellar winds. This probability decreases with decreasing metallicity.
Although we find meaningful that probably the fraction of fullerene PNe depends upon the metallicity of the host environment, it is worth to mention here that selection effects in the Spitzer PN samples may play a potentially significant role in the statistics mentioned above. None of these Magellanic Cloud PN samples is complete, except that the surface brightness was high enough to be included in a HST or Spitzer sample. However, our estimation of the detection rate of fullerenes (5%) in our own Galaxy comes from the sample of 157 compact and young PNe in the the Galactic Disk (Stanghellini et al. 2012). This Galactic fullerene detection rate may be considered as an upper limit for environments of roughly solar metallicity. This is because the Stanghellini et al. (2012)’s PNe sample is sampling the early stages of PN evolution and dust processing (see below), when the detection rate of fullerenes is expected to be maximum; the fullerenes are not too cold (e.g., T 100 K) to escape detection at mid-infrared wavelengths.
In short, the high detection rate of fullerenes in MC PNe seems to be strongly linked to the limited dust processing (or the general presence of small dust grains) in low-metallicity circumstellar envelopes. This is consistent with the observational finding that the still unidentified 21 m feature (Kwok, Volk & Bernath 2001; see also García-Hernández 2012 for a recent review) is much more common in post-AGB stars of the Magellanic Clouds than in our own Galaxy (Volk et al. 2011), suggesting that the 21 m feature is possibly also related with the formation of fullerenes and its carrier may be a fragile intermediate product from the decomposition of HAC or a similar material (see below).
4.3 Broad 69, 913, 1520, and 2535 m Emission
As we have mentioned before, the dust processing seems to depend on metallicity. For example, MC PNe display infrared spectra, which are similar to many Galactic post-AGB stars where dust evolution is still in its early stages, and/or still taking place. However, already in galactic post-AGB stars we can see huge 2535 m features (e.g., Hrivnak, Volk & Kwok 2000) and very little 69, 913, and 1520 m features, contrary to what we observe in the MCs. It seems that it is now well established that the carrier of unidentified infrared emission (UIE) - both discrete and broad features - observed from 3 to 20 m in C-rich evolved stars should be carbon nanoparticles with mixed aliphatic and aromatic structures similar to that of HAC dust (Kwok, Volk & Bernath 2001; Kwok & Zhang 2011). Note that instead of HAC nanoparticles, other organic matter with a complex mix of aliphatic and aromatic structures (e.g., coal, petroleum fractions, quenched carbonaceous compounds, etc.) may be present in the circumstellar envelopes of evolved stars and might be responsible of the UIE (e.g., Kwok, Volk & Bernath 2001). For example, coal and petroleum fractions may explain a diverse set of aromatic and aliphatic features seen in proto-PNe (e.g., Cataldo, Keheyan & Heymann 2002). In this context, our Spitzer spectra of fullerene PNe also show that a complex mixture of aliphatic and aromatic species (e.g., HACs, PAH clusters, fullerenes, and small dehydrogenated carbon clusters) is present in their circumstellar envelopes.
Our interpretation is that the broad infrared features at 69, 913, 1520, and 2535 m are intimately related with the formation of fullerenes in PNe, being likely produced by a carbonaceous compound with a mixture of aromatic and aliphatic structures (e.g., HAC) and/or their decomposition products (e.g., fullerene precursors or intermediate products). It is to be noted here that Kwok, Volk & Bernath (2001) were the first to suggest that the broad 69 and 913 m features seen in proto-PNe may be due to a carbonaceous compound with a mixed aromatic/aliphatic structure. The 69 and 1520 m features, which are almost exclusively seen in low metallicity MC PNe (Figures 3 and 4), may be due to small dust grains (e.g., HAC nanoparticles) while the 913 and 2535 m that are seen in all metallicity environments (Figures 2 to 4) would be produced by big dust grains (e.g., HAC aggregates). The broad 69 m feature may be due to HACs, PAH clusters or VSGs (see e.g., Tielens 2008) while the 1520 m emission may be identified with the C-C-C bending modes of relatively large PAHs or PAH clusters (Van Kerckhoven et al. 2000). On the other hand, and although the broad 913 and 2535 m features are usually attributed to SiC (the so-called 11.5 m feature; e.g., Speck et al. 2009) and MgS (e.g., Hony et al. 2002), respectively, these features may be due to bigger dust grains (e.g., HAC aggregates). García-Hernández (2012) argued against the SiC and MgS identification of these features in fullerene PNe and it will not be repeated here. Basically, the 2535 m feature may be explained by HACs (see Grishko et al. 2001; García-Hernández et al. 2010) and the spectral characteristics (position, shape, see also Table 4) of the 913 m emission differ from those for the SiC 11.5m feature.
4.4 Fullerene Emission vs. other IR Features
In this Section we study the fullerene emission in our sample of fullerene PNe versus other solid-state/molecular infrared features. The flux emitted in the C 18.9m feature is roughly correlated with the fullerene content (or C/C abundance values in Table 3; see also Figure 8) and can be taken as representative of the fullerene emission. Almost all fullerene PNe in our sample show some emission in the broad 913 m feature111111The only exceptions are LMC 02 and LMC 99 with no significant 913 m emission; see Figure 3. and in the 11.3m PAH feature. However, not all PNe in our sample display evident AIB emissions at 6.2 and 7.7 m or in the very broad 69 and 2535 m features, something that difficults the exploration of any possible relation between the fullerene emission and the carriers of the latter emission features. In particular, we prefer to exclude (on a quantitative basis; see below) the very broad 69 and 2535 m features from our study. This is because very few fullerene PNe display the 69 m emission and the sensitivity of IRS drops dramatically for 36 m and previous Infrared Space Observatory (ISO) observations of PNe show that the 2535 m feature may go up to wavelenghts longer than 40 m (Hony et al. 2002). The central wavelengths and integrated fluxes of the AIBs (usually attributed to PAHs) at 6.2, 7.7, 11.3 m as well as the broad 913 and 1520 m features are given in Table 4.
Figure 8 displays the flux ratio C 18.9m/913m versus the fullerene abundances (in percentage), where we include the propagated error bars corresponding to an individual flux error estimation of 15%. Note that we do not include the error bars of the fullerene abundances in order to avoid dulling of Figure 8. The formal error in our estimation of the fullerene abundances is difficult to calculate (e.g., different literature sources for the C abundance, electron density, etc.) but it should be lower than 50% in the Magellanic Cloud sources - likely higher in the Galactic PNe (see §3.2). Although most of the sources are concentrated in the lower left corner of the diagram displayed in Figure 8, we find that the flux ratio C 18.9m/913m seems tentatively to increase with increasing fullerene abundances. Also, two apparent outliers in Figure 8 seem to be PNe Tc 1 and SMC 15. The position of Tc 1 in this diagram, if real, may involve a steeper correlation at the highest Galactic metallicities. However, the Tc 1’s Spitzer spectrum only covers the inner nebula while the Spitzer apertures covers the full nebula in the rest of fullerene PNe displayed in Figure 8. On the other hand, SMC 15 displays the highest electron density (N=7500 cm) among the PNe in the SMC (with an average N of 3500 cm; see Table 2). It is to be noted here that by using the average electron density for SMC 15 (3500 cm), then a lower C/C abundance of 0.03 % is obtained. Thus, if we do not take into account Tc 1 and SMC 15, then Figure 8 would show an even tighter trend between the C 18.9m/913m flux ratio and fullerene abundances in fullerene PNe. In summary, we conclude that there is a tentative observed trend in Figure 8 but more detections of fullerenes in PNe are needed in order to reach a definitive conclusion.
More interesting is that Figure 8 may be also interpreted as a consequence of the destruction of the carrier of the 913 m feature in those PNe dominated by the fullerene emission. This would be expected if the carrier of the broad 913 m feature is a precursor of the fullerene molecules121212Note that the small amount of C (1%, Table 3) is with respect to the total gaseous carbon in the nebulae. We do not know what fraction of carbon is locked in the carrier of the 913 m feature. For example, if the 913 m feature’s carrier is of solid-state origin (dust), then only a small fraction of C (in comparison with gaseous carbon) is also expected for the 913 m feature’s carrier. and it is intimately related with the fullerene formation process, as we have just interpreted in the previous Section. As we have already mentioned in the Introduction, the strong variability of the broad 913 m emission from source to source (see Table 4) is more consistent with a HACs identification.
Figure 9 compares the fullerene emission with the AIB emission at 6.2 and 11.3 m (e.g., the C 18.9m/AIB 11.3m flux ratio versus the AIB 6.2m/AIB 11.3m flux ratio). We find two different groups in Figure 9. A first group composed by three PNe (Tc 1, LMC 02, and SMC 24)131313Note that PN SMC 16 does not display the 6.2 and 7.7 m AIBs and it would also pertain to this first group of fullerene-dominated PNe., which are dominated by the fullerene emission (C 18.9m/AIB 11.3m 8) and show very weak AIB emission at 6.2 and 11.3 m (AIB 6.2m/AIB 11.3m 0.75). These fullerene-dominated PNe are also characterized by the weakness of the broad 913 m feature compared to the fullerene emission. This suggests that these three fullerene-dominated PNe may have a mix of aliphatic and aromatic hydrocarbon materials of similar chemical composition (e.g.,. size and degree of hydrogenation) and/or are under approximate physical conditions (e.g.., the intensity of the UV radiation field) in their circumstellar envelopes. A second group in Figure 9 is, however, formed by the rest of PNe in our sample, displaying similar levels of fullerene emission but covering a wide range of AIB 6.2m / AIB 11.3m flux ratios, likely reflecting quickly changing physical conditions (e.g., UV radiation field and/or the post-AGB shocks) and/or chemical composition. The latter is consistent with the relative strengths of the IR features of amorphous organic nanoparticles with mixed aliphatic and aromatic structures (e.g., HACs), which are known to be strongly dependent of physical conditions and chemical composition (e.g., Scott et al. 1997; Grishko et al. 2001). A similar result is obtained from Figure 10, where we display the C 18.9m/AIB 11.3m flux ratio versus the AIB 7.7m/AIB 11.3m flux ratio. Again, the same three fullerene-dominated PNe (although Tc 1 does not show the 7.7 m AIB) form a separated group in this latter diagram, while the rest of objects show a wide range of AIB 7.7m/AIB 11.3m intensity ratios.
Finally, a few MC PNe (LMC 25, 48 in Figure 3 and SMC 13, 15, 18, and 20 in Figure 4) show a broad 1520 m emission (see also Table 4). This broad emission feature is usually attributed to the C-C-C bending modes of relatively large PAHs or PAH clusters (e.g., Van Kerckhoven et al. 2000). The 1520m/11.3m and 1520m/6.2m flux ratios are displayed in Figure 11 and they may be used to estimate the size (number of C atoms) of the grains/PAH clusters responsible of this emission (see e.g., Van Kerckhoven et al. 2000; Tappe et al. 2006, 2012 for more details). All members of the 1520m subgroup of MC PNe, with the exception of SMC 20, show AIB 11.3m/1520m and AIB 6.2m/1520m ratios lower than 0.1 and 0.2, respectively, suggesting relatively large grains/PAHs clusters of more than 4000 C atoms (see Tappe et al. 2006, 2012 and references therein). Curiously, the observed characteristics of the 1520 m emission in these sources - shape, integrated strength relative to the 6.2 and 11.3 m AIBs, complete lack of the discrete emission features at 16.4, 17.0, and 17.8 m - are very similar to the 1520 m bump observed in the shock-induced dust processing environment of the supernova remnant N132D (Tappe et al. 2006, 2012). Our interpretation is that these PAH clusters/amorphous carbon nanoparticles survive the harsh conditions in the circumstellar envelope long enough to be detected only in the low metallicity environments of MC PNe. In our own Galaxy, however, the usual lack of PAH clusters-like 1520m emission in fullerene PNe indicates that these PAH clusters/amorphous carbon nanoparticles are quickly destroyed.
4.5 Formation route of Fullerenes in PNe
Some advances in our understanding of the fullerene formation process in PNe have been made since the discovery of C and C fullerenes in the PN Tc 1 by Cami et al. (2010). From the observational point of view, our recent detections of fullerenes and graphene precursors (possible planar C) in PNe have challenged our previous understanding on the formation of these complex molecules, showing that they can form in H-rich circumstellar environments and that they may coexist with PAH-like molecules (García-Hernández et al. 2010, 2011). The coexistente of a great diversity of aromatic and aliphatic species in PNe with fullerenes supports the idea that fullerenes may form from the photo-chemical processing (e.g., as a consequence of UV irradiation or shocks) of small solid particles similar to that of HAC; the so-called “HACs fullerene formation scenario”. The prevaling view was that the H-poor RCBs, resembling the experimental conditions on Earth (Kroto et al. 1985), would be the ideal astrophysical environments for fullerene formation. However, Spitzer observations of RCB stars (García-Hernández, Rao & Lambert 2011a,b) have shown that fullerene formation is inefficient in the highly hydrogen-deficient circumstellar environment of most RCBs. Fullerenes are seen in conjunction with strong PAH-like features in the two least H-deficient RCB stars DY Cen and V854 Cen. In addition, the V854 Cen’s IR spectrum evolved from HACs to AIBs (e.g., from PAHs) and C in a timescale of 10 years only, supporting the “HACs fullerene formation scenario”. The fact that previous CLOUDY photoionization models - by using BB and Rauch (2003) stellar atmospheres as ionizing continuum - could not explain the low [Ne III]/[Ne II] ratios observed in fullerene MC PNe with fast stellar winds (e.g., shocks) prompted the speculation that fullerenes may be formed by the shock-induced decomposition of HAC material. This interpretation was consistent with the low effective temperatures (25,000 K) found in the other astronomical environments such as the proto-PN IRAS 010057910, post-AGB and RCB stars, and reflection nebulae, where fullerenes have been found so far (see below). The UV radiation field in the latter astrophysical environments is unlikely intense enough for the photo-chemical processing of HAC (see discussion in García-Hernández, Rao & Lambert 2011a). Indeed, the PAH clusters-like 1520m emission observed in fullerene PNe show a resemblance to that observed in the shock-induced dust processing environment of the supernova remnant N132D (Tappe et al. 2006, 2012).
However, the new CLOUDY models presented in this paper (Section 3.1) show that many of the observed [Ne III]/[Ne II] ratios may be explained by photoionization when using model atmospheres with line and wind blanketing. This result weakens the hypothesis of a shock-excited origin for the [Ne II] 12.8 m emission. Although fast stellar winds (e.g., shocks) are generally present in fullerene PNe141414This is deduced from the common presence of P-cygni profiles in their UV lines (see algo García-Hernández et al. 2011)., the shock hypothesis cannot be directly tested with the actual data. Furthermore, the winds of central stars of PNe are radiation-driven (Pauldrach et al. 1986) and as such the wind velocity is expected to increase with the central star temperature, allowing the development of wind-driven shock regions when only the central star is sufficiently evolved (Villaver, Manchado & García-Segura 2002). Weaker shocks are also expected associated to the propagation of the ionization front within the nebula. Figure 6 suggests that the UV radiation from the central star can explain the low [Ne III]/[Ne II] ratios observed in many fullerene PNe, putting some doubt to the suggestion of the fullerenes being formed by the shock-induced HAC’s decomposition. In summary, this result together with the narrow range of T (30,00045,000 K) displayed by our sample sources in Figure 6 indicate common fullerene formation conditions in PNe and suggest that perhaps the UV irradiation is photo-chemically processing the small solid particles with mixed aromatic and aliphatic structures similar to that of HAC dust and that may form the fullerenes. This interpretation is consistent with the early suggestion by Kwok, Volk & Bernath (2001), who emphasizes the important role of photo-chemistry in the evolution from proto-PNe to PNe.
From the experimental point of view, we would like to emphasize the recent finding by Duley & Hu (2012) that HAC nanoparticles prepared under a variety of conditions can show IR features coincident with the four C IR features, and that may be attributed to fullerene precursors or proto-fullerenes. The presence of proto-fullerenes takes place in conjunction with the 16.4 m feature. The 16.4 m feature is usually present in the proto-PN IRAS 010057910, post-AGB and RCB stars, and reflection nebulae (all of them with T30,000 K), likely indicating the presence of proto-fullerenes (García-Hernández et al. 2012) as a previous stage in the evolution and dehydrogenation of UV irradiated HAC dust in fullerene PNe. More recently, Dunk et al. (2012) propose a Closed Network Growth (CNG) mechanism of fullerenes, where larger fullerenes may growth from preexisting C molecules and this formation route can work under the presence of hydrogen. CNG of fullerenes would be consistent with the “HAC’s fullerene formation scenario” and it may work in the complex circumstellar environments of fullerene PNe. In this context, the smaller C, C, and C would be supplied by the photo-chemical processing of HAC dust.
In this paper we present a detailed study of the Spitzer infrared spectra of sixteen fullerene PNe. We report the first detection of fullerenes in the Galactic PN M 1-60 as well as the presence of the unusual set of infrared features at 6.6, 9.8, and 20 m (attributed to possible planar C) in the PN K 3-54.
The observed C intensity ratios in the Galactic PNe with fullerene emission confirm our previous finding in the Magellanic Clouds that fullerenes - probably in solid-state - in PNe are not excited by the UV radiation from the central star. In addition, new CLOUDY photoionization models show that line- and wind-blanketed model atmospheres can explain many of the observed [NeIII]/[NeII] ratios by photoionization and that PNe containing fullerenes display a rather narrow range in effective temperature (30,00045,000 K). This shows that fullerene PNe share similar physical conditions for the fullerene formation and that perhaps is the UV radiation from the central star that is processing the circumstellar dust grains - e.g., small solid particles containing aromatic, aliphatic, fullerene, and grahene structures similar to that of HAC dust - and forming fullerenes.
We have studied the fullerene emission with respect to other infrared emission features such as the classical AIBs (usually attributed to PAHs) or the broad emission features at 69, 913, 1520, and 2535 m that we interpret as produced by a carbonaceous compound with a mixture of aromatic and aliphatic structures (e.g., HAC, coal, petroleum fractions, etc.) and/or their decomposition products (e.g., fullerene precursors or intermediate products). Thus, fullerenes and proto-graphene in PNe may be formed from the photo-chemical processing of such a carbonaceous compound, which should be very abundant in their circumstellar envelopes. Our interpretation is consistent with the early seminal work by Kwok, Volk & Bernath (2001).
The PNe targets that show fullerene emission belong to very different metallicity environments such as our own Galaxy and the Magellanic Clouds. The detection rate of fullerenes in carbon-rich PNe is found to increase with decreasing metallicity, something that seems to be strongly linked to the limited dust processing in Magellanic Cloud PNe when compared to the Galactic PNe. It is important, however, to acknowledge the fact that all the original samples of PNe from which the fullerene detections come from have their own selection biases and cannot be considered complete by any means. The fact that in general the Spitzer spectra of Magellanic Cloud PNe are usually dominated by small dust grains (e.g., HAC and/or PAH clusters or even HAC decomposition products), while their Galactic counterparts display dust emission mainly from big grains (e.g.,. HAC aggregates) also points towards a higher probability of fullerene detection with decreasing metallicity under our proposed mechanism for fullerene formation. Our estimated fullerene abundances and temperatures are similar in the various metallicity environments analyzed. When possible, we have argued that fullerene PNe likely evolve from low-mass progenitors (main sequence masses 1.5 M) and are usually low-excitation objects slowly evolving towards the white dwarf stage.
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|(K)||log(C/H)+12||(10 K)||(cm)||(M)||(10 M)|
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