arXiv subjects

L. Stanghellini

Publications and source records attributed to L. Stanghellini.

33 records · Page 2Linked to original sources

arXiv2009

Carbon abundance in Small Magellanic Cloud planetary nebulae through Advanced Camera for Surveys prism spectroscopy: constraining stellar evolution at low metallicity

We perform near ultraviolet ACS prism spectroscopy of 11 Small Magellanic Cloud (SMC) planetary nebulae (PNe) with the main aim of deriving the abundance of carbon. The analysis of the ACS spectra provide reliable atomic carbon abundances for all but a couple of our targets; ionic C^(2+) abundances are calculated for all target PNe. With the present paper we more than double the number of SMC PNe with known carbon abundances, providing a good database to study the elemental evolution in low- and intermediate-mass stars at low metallicity. We study carbon abundances of Magellanic Cloud PNe in the framework of stellar evolution models and the elemental yields. Constraining SMC and LMC stellar evolutionary models is now possible with the present data, through the comparison of the final yields calculated and the CNO abundances observed. We found that SMC PNe are almost exclusively carbon rich, and that for the most part they have not undergone the hot-bottom burning phase, contrary to half of the studied LMC PNe. The yields from stellar evolutionary models with LMC and SMC metallicities broadly agree with the observations. In particular, evolutionary yields for M$_{\rm to}<3.5 {\rm M}_{\odot}$ well encompass the abundances of round and elliptical PNe in the SMC. We found that the carbon emission lines are major coolants for SMC PNe, more so than in their LMC counterparts, indicating that metallicity has an effect on the physics of PNe, as predicted by Stanghellini et al. (2003).

astro-ph.SR↗

arXiv2007

Spitzer Infrared Spectrograph Observations of Magellanic Cloud Planetary Nebulae: the nature of dust in low metallicity circumstellar ejecta

We present 5 - 40 micron spectroscopy of 41 planetary nebulae (PNe) in the Magellanic Clouds, observed with the Infrared Spectrograph on board the Spitzer Space Telescope. The spectra show the presence of a combination of nebular emission lines and solid-state features from dust, superimposed on the thermal IR continuum. By analyzing the 25 LMC and 16 SMC PNe in our sample we found that the IR spectra of 14 LMC and 4 SMC PNe are dominated by nebular emission lines, while the other spectra show solid-state features. We observed that the solid-state features are compatible with carbon-rich dust grains (SiC, polycyclic aromatic hydrocarbons (PAHs), etc.) in most cases, except in three PNe showing oxygen-rich dust features. The frequency of carbonaceous dust features is generally higher in LMC than in SMC PNe. The spectral analysis allowed the correlations of the dust characteristics with the gas composition and morphology, and the properties of the central stars. We found that: 1) all PNe with carbonaceous dust features have C/O>1, none of these being bipolar or otherwise highly asymmetric; 2) all PNe with oxygen-rich dust features have C/O<1, with probable high mass progenitors if derived from single-star evolution (these PNe are either bipolar or highly asymmetric); 3) the dust temperature tracks the nebular and stellar evolution; and 4) the dust production efficiency depends on metallicity, with low metallicity environments not favoring dust production.

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arXiv2006

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

We present ESO NTT 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 $10^2 - 10^7$ 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 $\approx 20,000$ G to 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 \ion{He}{1} $λ$5876 and $λ$6678, corresponding to an expanding photosphere with velocity $\sim$ 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.

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arXiv2006

Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way

This paper presents a homogeneous study of abundances in a sample of 79 northern galactic planetary nebulae whose morphological classes have been uniformly determined. Ionic abundances and plasma diagnostics were derived from selected optical line strengths in the literature, and elemental abundances were estimated with the Ionization Correction Factor developed by Kingsbourgh & Barlow (1994). We compare the elemental abundances to the final yields obtained from stellar evolution models of low-and intermediate-mass stars, and we confirm that most Bipolar planetary nebulae have high nitrogen and helium abundance, and are the likely progeny of stars with main-sequence mass larger than 3 solar masses. We derive =0.27, and discuss the implication of such a high ratio in connection with the solar neon abundance. We determine the galactic gradients of oxygen and neon, and found Delta log (O/H)/Delta R=-0.01 dex/kpc$ and Delta log (Ne/H)/Delta R=-0.01 dex/kpc. These flat PN gradients do not reconcile with galactic metallicity gradients flattening with time.

astro-ph↗

arXiv2006

Carbon Abundances in the Small Magellanic Cloud Planetary Nebulae

As an ongoing study of Magellanic Cloud PNe we have obtained UV spectra of 9 PNe in the SMC to measure their carbon abundances. The spectra have been acquired with ACS HRC/PR200L and SBC/PR130L. The ACS prisms give a reasonable resolution in the range of 1200 -- 2500 A to detect the C IV, C III], and C II] nebular emission, essential for chemical studies of the PNe. The carbon abundances of SMC PNe, together with those of the LMC previously determined with STIS spectroscopy, will allow a comparative study of nebular enrichment and provide the basis for comparison with stellar evolution models at various metallicity.

astro-ph↗

arXiv2006

Spitzer Spectra of Magellanic Cloud PNe

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 hinder Galactic PN studies. The emission shown by PNe in the 5-40 $μ$m range is characterized by the presence of a combination of solid state features (from the dust grains) and nebular emission lines over-imposed on a strong dust continuum. We acquired low resolution IRS spectroscopy of a selected sample of LMC and SMC PNe whose morphology, size, central star brightness, and chemical composition are known. The data have been acquired and reduced, and the IRS spectra show outstanding quality as well as very interesting features. The preliminary analysis presented here allows to determine strong correlations between gas and dust composition, and nebular morphology. More detailed analysis in the future will deepen our knowledge of mass-loss mechanism, its efficiency, and its relation to PN morphology.

astro-ph↗

arXiv2004

Space telescope Imaging Spectrograph ultraviolet spectra of LMC planetary nebulae. A study of carbon abundances and stellar evolution

We acquired spectra of 24 LMC PNe in the 1150--3000 Årange in order to determine carbon and other ionic abundances. The sample more than doubles the number of LMC PNe with good quality UV spectra in this wavelength range, and whose optical images are available in the {\it HST} archive. The {\it Space Telescope Imaging Spectrograph} was used with a very large aperture to obtain virtually slit-less spectra, thus the monochromatic images in the major nebulae emission lines are also available. The analysis of the data shows extremely good 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, and 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 PNe is one order of magnitude larger than that of the bipolar PNe, while elliptical and round PNe with a bipolar core have a bimodal behavior. This results confirm that bipolarity in LMC PNe is tightly correlated with high mass progenitors. When compared to predicted yields, we found that the observed abundance ratio show a shift toward higher carbon abundances, that may be due to initial conditions assumed in the models not appropriate for LMC PNe.

astro-ph↗

arXiv2002

The 12C/13C ratio in the Planetary Nebula NGC3242 from Hubble Space Telescope STIS observations

We present high resolution HST ultraviolet spectroscopy of NGC3242, the only planetary nebula with a measured abundance of 3He. The Space Telescope Imaging Spectrograph (STIS) has been used to observe the CIII multiplet near 1908 A. The presence of 12C and 13C lines at these wavelengths allows a direct estimate of the carbon isotopic ratio in the ionized gas of the nebula. We have detected the 12C doublet and obtained an upper limit on the 13C 1909.6 A line, resulting in a carbon isotopic ratio 12C/13C > 38, in agreement with standard stellar models. The lack of the 13C line and the presence of the 3He line in the spectrum indicate that the progenitor star did not undergo a phase of deep mixing during the last stages of its evolution. The significance of this result for studies of stellar nucleosynthesis and Galactic chemical evolution is discussed.

astro-ph↗

arXiv2000

The Morphological and Structural Classification of Planetary Nebulae

We present a statistical analysis of a complete sample (255) of northern planetary nebulae (PNe). Our analysis is based on morphology as a main parameter. The major morphological classes are: round (26 % of the sample), elliptical (61 %), and bipolar (13 %) PNe. About a half of the round and 30 % of the elliptical PNe present multiple shells. Round PNe have higher galactic latitude (| b| = 12) and galactic height ( = 753 pc), than the elliptical (|b| = 7, = 308 pc) and bipolar (|b| = 3, =179 pc). This possibly implies a different progenitor mass range across morphology, as a different stellar population would suggest.

astro-ph↗

arXiv1999

Measurements of 12C/13C ratio in planetary nebulae: implications on stellar and Galactic chemical evolution

We present the results of a study aimed at determining the 12C/13C ratio in two samples of planetary nebulae (PNe) by means of millimeter wave observations of 12CO and 13CO. The first group includes six PNe which have been observed in the 3He+ hyperfine transition by Balser et al. (1997); the other group consists of 22 nebulae with rich molecular envelopes. We have determined the carbon isotopic ratio in 14 objects, 9 of which are new detections. The results indicate a range of values of 12C/13C between 9 and 23. We estimate the mass of the progenitors of the PNe of our sample and combine this information with the derived 12C/13C isotopic ratios to test the predictions of stellar nucleosynthesis models. We find that the majority of PNe have isotopic ratios below the values expected from current standard asymptotic giant branch models in the mass range of interest. We suggest that the progenitors of the PNe must have undergone a non-standard mixing process during their red giant phase and/or asymptotic giant phase, resulting in a significant enhancement of the 13C abundance in the surface layers. Our study confirms a similar behaviour inferred from spectroscopic observations of field population II stars and globular cluster giants, and extends it to the final stages of stellar evolution. Finally, we discuss the implications of our results on models of Galactic chemical evolution of 3He and 12C/13C.

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arXiv1999

Magellanic Cloud Planetary Nebulae: A Fresh Look at the Relations between Nebular and Stellar Evolution

Studies of the relationship between planetary nebula morphology and the evolution of the central stars has long suffered from uncertainties in distance determinations, and from the bias of interstellar absorption, that are typical for Galactic PNe. We will be able to eliminate the distance errors and be assured of the sample homogeneity by studying Large Magellanic Cloud (LMC) PNe with images from the Hubble Space Telescope. In this talk we present the first observations in our new sample. The data consist of broad-band images and medium dispersion, slit-less spectra obtained with STIS, and are of excellent quality. Indeed, these data show great promise for subsequent analysis, which will centered on the relationship between nebular morphology and stellar and nebular evolution. While the most intensive analysis of the sample must await the completion of the survey, the data obtained so far show that we will learn a lot along the way.

astro-ph↗

arXiv1998

Hubble Space Telescope Images of Magellanic Cloud Planetary Nebulae: Data and Correlations across Morphological Classes

The morphology of planetary nebulae (PNe) provides an essential tool for understanding their origin and evolution, as it reflects both the dynamics of the gas ejected during the TP-AGB phase, and the central star energetics. Here we study the morphology of 27 Magellanic Cloud planetary nebulae (MCPNe) and present an analysis of their physical characteristics across morphological classes. Similar studies have been successfully carried out for galactic PNe, but were compromised by the uncertainty of individual PN distances. We present our own HST/FOC images of 15 Magellanic Cloud PNe (MCPNe) acquired through a narrow-band lambda 5007 [O III] filter. We use the Richardson-Lucy deconvolution technique on these pre-COSTAR images to achieve post-COSTAR quality. Three PNe imaged before and after COSTAR confirm the high reliability of our deconvolution procedure. We derive morphological classes, dimensions, and surface photometry for all these PNe. We have combined this sample with HST/PC1 images of 15 MCPNe, three of which are in common with the FOC set, acquired by Dopita et al. (1996), to obtain the largest MCPN sample ever examined from the morphological viewpoint. By using the whole database, supplemented with published data from the literature, we have analyzed the properties of the MCPNe and compared them to a typical, complete galactic sample. Morphology of the MCPNe is then correlated with PN density, chemistry, and evolution.

astro-ph↗

arXiv1996

3He in Planetary Nebulae: A Challenge to Stellar Evolution Models

The discrepancy between the observed abundances of 3He in the ISM and those predicted by stellar and galactic chemical evolution remains largely unexplained. In this paper, we attempt to shed some light on this unsolved problem by presenting a quantitative comparison of the 3He abundances recently measured in six planetary nebulae (PNe) with the corresponding predictions of stellar evolution theory. The determination of the mass of the PNe progenitors allows us to dismiss, to a good degree of confidence, the hypothesis that the abundance of 3He in the envelope of all low-mass stars is strongly reduced with respect to the standard theoretical values by some mixing mechanism acting in the latest phases of stellar evolution. The abundance versus mass correlation, allowance made for the limitation of the sample, is in fact found to be fully consistent with the classical prediction of stellar evolution. We examine the implications of this result on the galactic evolution of 3He with the help of a series of models with standard and non-standard nucleosynthesis prescriptions. The results are found to be consistent with the observed galactic abundances only if the vast majority of low-mass stars follows non-standard prescriptions. This implies that either the sample of PNe nebulae under exam is highly biased, or the solution to the 3He problem lies elsewhere.

astro-ph↗

arXiv1995

Search for new pulsating O VI central stars of planetary nebulae

A photoelectric monitoring program has been applied, during the last four years, to five central stars of planetary nebulae (PNNs) with strong O\,VI $λ$ 3811--34 Å\hspace{0.1mm} emission. NGC\,6905 and, marginally, NGC\,2452, show intrinsic luminosity variations, while NGC\,7026, IC\,2003 and NGC\,1501 have constant luminosity within a few mmag. Photometric data have been analyzed with the best available packages for power--spectra reductions. Both pulsators have periods and physical characteristics well encompassed by the theoretical pulsational models relative to these stars.

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arXiv1993

Filling Factors and Ionized Masses in Planetary Nebulae

We calculate filling factors (${\varepsilon}$) and ionized masses (M$_{\rm i}$) for a total of 84 galactic and extragalactic planetary nebulae (PNe) at known distances. To do these calculations, we have chosen forbidden line electron densities, observed angular diameters, and H$β$ fluxes, from the most recent measurements available in the literature. Statistical analysis on the distributions of ${\varepsilon}$ and ${\rm M_i}$ show that (1) the ranges of values of these parameters is wider than what was previously found; (2) the mean value of the filling factor is between $0.3$ and $0.4$, for the different sets; (3) the mean value of the ionized mass is between $0.1$ and $0.25$ $M_{\odot}$; (4) a clear correlation between the filling factors and the dimensions of the PNe was not found when distance-independent sets of PNe were used; (5) for extragalactic PNe, where distance errors are not a factor, the filling factors and the ionized masses anticorrelate tightly with the electron densities. The results indicate that the modified Shklowsky distance method is correct.

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