Letizia Stanghellini

Formation of Fullerenes in H-containing Planetary Nebulae

Hydrogen depleted environments are considered an essential requirement for the formation of fullerenes. The recent detection of C60 and C70 fullerenes in what was interpreted as the hydrogen-poor inner region of a post-final helium shell flash planetary nebula (PN) seemed to confirm this picture. Here, we present strong evidence that challenges the current paradigm regarding fullerene formation, showing that it can take place in circumstellar environments containing hydrogen. We report the simultaneous detection of polycyclic aromatic hydrocarbons (PAHs) and fullerenes toward C-rich and H-containing PNe belonging to environments with very different chemical histories such as our own Galaxy and the Small Magellanic Cloud. We suggest that PAHs and fullerenes may be formed by the photochemical processing of hydrogenated amorphous carbon. These observations suggest that modifications may be needed to our current understanding of the chemistry of large organic molecules as well as the chemical processing in space.

The Formation of Fullerenes: Clues from New C60, C70, and (Possible) Planar C24 Detections in Magellanic Cloud Planetary Nebulae

We present 10 new Spitzer detections of fullerenes in Magellanic Cloud Planetary Nebulae, including the first extragalactic detections of the C70 molecule. These new fullerene detections together with the most recent laboratory data permit us to report an accurate determination of the C60 and C70 abundances in space. Also, we report evidence for the possible detection of planar C24 in some of our fullerene sources, as indicated by the detection of very unusual emission features coincident with the strongest transitions of this molecule at ~6.6, 9.8, and 20 μm. The infrared spectra display a complex mix of aliphatic and aromatic species such as hydrogenated amorphous carbon grains (HACs), polycyclic aromatic hydrocarbon clusters, fullerenes, and small dehydrogenated carbon clusters (possible planar C24). The coexistence of such a variety of molecular species supports the idea that fullerenes are formed from the decomposition of HACs. We propose that fullerenes are formed from the destruction of HACs, possibly as a consequence of shocks driven by the fast stellar winds, which can sometimes be very strong in transition sources and young planetary nebulae (PNe). This is supported by the fact that many of our fullerene-detected PNe show altered [Ne III]/[Ne II] ratios suggestive of shocks as well as P-Cygni profiles in their UV lines indicative of recently enhanced mass loss.

Space Telescope Imaging Spectrograph Ultraviolet Spectra of Large Magellanic Cloud Planetary Nebulae: A Study of Carbon Abundances and Stellar Evolution*

We acquired spectra of 24 LMC planetary nebulae (PNs) in the 1150-3000 A range in order to determine carbon and other ionic abundances. The sample more than doubles the number of LMC PNs with high-quality UV spectra in this wavelength range and whose optical images are available in the Hubble Space Telescope archive. The Space Telescope Imaging Spectrograph was used with a very large aperture to obtain virtually slitless spectra; thus, the monochromatic images in the major nebula emission lines are also available. The analysis of the data shows extremely high quality spectra. This paper presents the emission lines identified and measured and the calculation of the ionic abundances of the emitting carbon and other ions, as well as total carbon abundance. P Cygni profiles have been found in a fraction of the nebulae, and the limiting velocities of the stellar winds estimated. The total carbon abundance can be inferred reliably in most nebulae. We found that the average carbon abundance in round and elliptical PNs is one order of magnitude larger than that of the bipolar PNs, while elliptical and round PNs with a bipolar core have a bimodal behavior. This results confirm that bipolarity in LMC PNs is tightly correlated with high-mass progenitors. When compared with predicted yields, we found that the observed abundance ratio shows a shift toward higher carbon abundances, which may be due to initial conditions assumed in the models not appropriate for LMC PNs.

The Low- and Intermediate-Mass Stellar Population in the Small Magellanic Cloud: The Central Stars of Planetary Nebulae

We present a study on the central stars (CSs) of planetary nebulae (PNs) observed in the Small Magellanic Cloud (SMC) with the Space Telescope Imaging Spectrograph (STIS) instrument on board the Hubble Space Telescope (HST). The stellar magnitudes have been measured using broadband photometry, and Zanstra analysis of the nebulae provided the stellar temperatures. From the location of the CSs on the H-R diagram, and by comparing the observed CSs with current models of stellar evolution, we infer the CS masses. We examine closely the possibility of light contamination in the bandpass from an unrecognized stellar companion, and we establish strong constraints on the existence and nature of any binary companion. We find an average mass of 0.63 M☉, which is similar to the mass obtained for a sample of CSs in the LMC (0.65 M☉). However, the SMC and LMC CS mass distributions differ slightly, with the SMC sample lacking an intermediate-mass stellar population (0.65-0.75 M☉). We discuss the significance and possible reasons for the difference between the two mass distributions. In particular, we consider the differences in the star formation history between the clouds and the mass-loss rate dependence on metallicity.

High-resolution spectra of bright central stars of bipolar planetary nebulae and the question of magnetic shaping

We present ESO New Technology Telescope high-resolution echelle spectroscopy of the central stars (CSs) of eight southern bipolar planetary nebulae (PNe) selected for their asymmetry. Our aim was to determine or place limits on the magnetic fields of the CSs of these nebulae, and hence to explore the role played by magnetic fields in nebular morphology and PN shaping. If magnetic fields do play a role, we expect these CSs to have fields in the range 102-107 G from magnetic flux conservation on the reasonable assumption that they must evolve into the high-field magnetic white dwarfs. We were able to place an upper limit of ≈20,000 G on the magnetic fields of the central stars of He 2-64 and MyCn 18. The spectrum of He 2-64 also shows a P Cygni profile in He I λ5876 and λ6678, corresponding to an expanding photosphere with velocity ~100 km s-1. The detection of helium absorption lines in the spectrum of He 2-36 confirms the existence of a hot stellar component. We did not reach the necessary line detection for magnetic field analysis in the remaining objects. Overall, our results indicate that if magnetic fields are responsible for shaping bipolar planetary nebulae, these are not required to be greater than a few tens of kilogauss.

A detailed look at chemical abundances in the Magellanic Clouds

We determine elemental abundances of He, N, O, Ne, S, and Ar in Magellanic Cloud planetary nebulae (PNe) using direct methods and a large set of observed ions, minimizing the need for ionization correction factors. In contrast to prior studies, we find a clear separation between Type I and non-Type I PNe in these low-metallicity environments, and no evidence that the O-N nucleosynthesis cycle is active in low-mass progenitors. We find that the S/O abundance ratio is anomalously low compared to H ii regions, confirming the “sulfur anomaly” found for Galactic PNe. We also found that Ne/O is elevated in some cases, raising the possibility that Ne yields in low-mass AGB stars may be enhanced at low metallicity.

DIVISION VI / COMMISSION 24 / WORKING GROUP PLANETARY NEBULAE

The aims of this Working Group are: • To ensure that scientific symposia on planetary nebulae take place regularly, ideally every 5 years. These symposia would preferably be sponsored by the IAU; • To organize and coordinate the Joint Discussions on the subject at the IAU General Assemblies. These discussions should address topics of interest not only to our Division VI but to other Divisions as well; and • To maintain a Web page with general information about the WG, the activities related to planetary nebulae, and the future meetings and symposia.

Planetary Nebulae and Their Central Stars in the Magellanic Clouds

During the last decade the Hubble Space Telescope (HST) has allowed us to extend late stellar evolution studies to nearby galaxies where the effect of the environment can be quantified. Using HST we have observed over a hundred Planetary Nebulae (PNe) in the Magellanic Clouds, where its known distance has allowed us to determine accurate masses for their central stars. We find an average central star mass of 〈MCS,LMC〉=0.65±0.07 M⊙ in the Large Magellanic Cloud, higher than the one reported in the literature for both white dwarfs and the central stars of PNe in the Galaxy. Higher central star masses are expected in a lower metallicity environment as a consequence of the reduced mass-loss rates during the Asymptotic Giant Branch. We present the first observational evidence from PNe progenitors of this effect.

The IRS Spitzer Spectra of the Magellanic Cloud Planetary Nebulae: Revealing the Dust and Gas Chemistry

Planetary nebulae (PNe) in the Magellanic Clouds (LMC, SMC) offer a unique opportunity to study both the population and evolution of low- and intermediate-mass stars in an environment which is free of the distance scale bias that hinders Galactic PN studies. The emission shown by PNe in the 5–40

Recent HST Surveys of Planetary Nebulae in the Magellanic Clouds

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