Abstract
We study the chemical abundances of a wide sample of 142 Galactic planetary nebulae (PNe) with good-quality observations, for which the abundances have been derived more or less ...homogeneously, thus allowing a reasonable comparison with stellar models. The goal is the determination of mass, chemical composition and formation epoch of their progenitors through comparison of the data with results from asymptotic giant branch (AGB) evolution. The dust properties of PNe, when available, were also used to further support our interpretation. We find that the majority (∼60 per cent) of the Galactic PN
studies have nearly solar chemical composition, while ∼40 per cent of the sources investigated have subsolar metallicities. About half of the PNe have carbon star progenitors, in the 1.5 M⊙ < M < 3 M⊙ mass range, which have formed between 300 Myr and 2 Gyr ago. The remaining PNe are almost equally distributed among PNe enriched in nitrogen, which we interpret as the progeny of M > 3.5 M⊙ stars, younger than 250 Myr, and a group of oxygen-rich PNe, descending from old (>2 Gyr) low-mass (M < 1.5 M⊙) stars that never became C-stars. This analysis confirms the existence of an upper limit to the amount of carbon that can be accumulated at the surface of carbon stars, probably due to the acceleration of mass-loss in the late AGB phases. The chemical composition of the present sample suggests that in massive AGB stars of solar (or slightly subsolar) metallicity, the effects of third dredge-up combine with hot bottom burning, resulting in nitrogen-rich – but not severely carbon depleted – gaseous material to be ejected.
Context. Nearby galaxies are ideal places to study metallicity gradients in detail and their time evolution. Aims. We analyse the spatial distribution of metals in M 33 using a new sample and the ...literature data on H ii regions, and constrain a model of galactic chemical evolution with H ii region and planetary nebula (PN) abundances. Methods. We consider chemical abundances of a new sample of H ii regions complemented with previous data sets. We compared H ii region and PN abundances obtained with a common set of observations taken at MMT. With an updated theoretical model, we followed the time evolution of the baryonic components and chemical abundances in the disk of M 33, assuming that the galaxy is accreting gas from an external reservoir. Results. From the sample of H ii regions, we find that i) the 2D metallicity distribution has an off-centre peak located in the southern arm; ii) the oxygen abundance gradients in the northern and southern sectors, as well as in the nearest and farthest sides, are identical within the uncertainties, with slopes around -0.03-4 dex kpc-1; iii) bright giant H ii regions have a steeper abundance gradient than the other H ii regions; iv) H ii regions and PNe have O/H gradients very close within the errors; v) our updated evolutionary model is able to reproduce the new observational constraints, as well as the metallicity gradient and its evolution. Conclusions. Supported by a uniform sample of nebular spectroscopic observations, we conclude that i) the metallicity distribution in M 33 is very complex, showing a central depression in metallicity probably due to observational bias; ii) the metallicity gradient in the disk of M 33 has a slope of -0.037 ± 0.009 dex kpc-1 in the whole radial range up to ~8 kpc, and -0.044 ± 0.009 dex kpc-1 excluding the central kpc; iii) there is little evolution in the slope with time from the epoch of PN progenitor formation to the present.
We present a study of 16 planetary nebulae (PNe) where fullerenes have been detected in their Spitzer Space Telescope spectra. Among the 16 PNe studied, we present the first detection of C sub(60) ...(and possibly also C sub(70)) fullerenes in the PN M 1-60 as well as of the unusual ~6.6, 9.8, and 20 mum features (attributed to possible planar C sub(24)) in the PN K 3-54. We do not find a metallicity dependence on the estimated fullerene abundances. The observed C sub(60) 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. 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 photochemical processing of a carbonaceous compound with a mixture of aromatic and aliphatic structures similar to that of hydrogenated amorphous carbon dust.
ABSTRACT
We present a novel approach to address dust production by low- and intermediate-mass stars. We study the asymptotic giant branch (AGB) phase, during which the formation of dust takes place, ...from the perspective of post-AGB and planetary nebula (PN) evolutionary stage. Using results from stellar evolution and dust formation modelling, we interpret the spectral energy distribution of carbon-dust-rich sources currently evolving through different evolutionary phases, believed to descend from progenitors of similar mass and chemical composition. Comparing the results of different stages along the AGB to PNe transition, we can provide distinct insights on the amount of dust and gas released during the very late AGB phases. While the post-AGB traces the history of dust production back to the tip of the AGB phase, investigating the PNe is important to reconstruct the mass-loss process experienced after the last thermal pulse. The dust surrounding the post-AGB was formed soon after the tip of the AGB. The PNe dust-to-gas ratio is ∼10−3, 2.5 times smaller than what expected for the same initial mass star during the last AGB interpulse, possibly suggesting that dust might be destroyed during the PN phase. Measuring the amount of dust present in the nebula can constrain the capacity of the dust to survive the central star heating.
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.
Aims.
We study the population of Galactic planetary nebulae (PNe) and their central stars (CSPNe) through the analysis of their distances and Galactic distribution. The PN distances are obtained by ...means of a revised statistical distance scale, based on an astrometrically-defined sample of their central stars from the third
Gaia
Data Release (DR3) as calibrators. The new statistical distances, together with the proper motion of the CSPNe (also from DR3) with published PN abundances as well as radial velocities, are used to characterize the PN populations in the Galaxy and to derive the radial metallicity gradient.
Methods.
The statistical scale was applied to infer the distances of a significant number (∼850) of Galactic PNe, for which we deliver a new catalog of PN distances. By adopting a circular velocity curve of the Galaxy, we also obtained peculiar 3D velocities for a large sample of PNe (∼300). The elemental abundances of the PNe were culled from the literature for an updated catalog, to be used in our analysis and other external applications.
Results.
The radial chemical gradient of the Galactic disk is traced by PNe with available chemical abundances and distances, and kinematic data of the CSPNe are employed to identify the halo PN population. We date PN progenitors based both on abundances and kinematic properties, finding a confirmation of the first method with the second. For all PNe with at least one oxygen determination in the literature, we find a slope of the radial oxygen gradient equal to Δ log(O/H)/Δ
R
G
= −0.0144 ± 0.00385 dex kpc
−1
. Furthermore, we estimate radial oxygen gradients for the PNe with old (> 7.5 Gyr) and young (< 1 Gyr) progenitors to be Δ log(O/H)/Δ
R
G
= −0.0121 ± 0.00465 and −0.022 ± 0.00758 dex kpc
−1
, respectively, thus disclosing a mild steepening of the gradient since Galaxy formation, with a slope change of 0.01 dex. The time evolution is slightly higher (∼0.015 dex) when we select the best available abundances in the literature. This result broadly agrees with previous PN results, but is now based on
Gaia
DR3 analysis, and it also agrees with what has been traced by most other Galactic probes. We also find a moderate oxygen enrichment when comparing the PNe with young and old progenitors.
Planetary nebulae in the Small Magellanic Cloud Ventura, P; Stanghellini, L; Di Criscienzo, M ...
Monthly notices of the Royal Astronomical Society,
08/2016, Letnik:
460, Številka:
4
Journal Article
Recenzirano
Odprti dostop
We analyse the planetary nebulae (PNe) population of the Small Magellanic Cloud (SMC), based on evolutionary models of stars with metallicities in the range 10−3 ≤ Z ≤ 4 × 10−3 and mass 0.9 ...M⊙ < M < 8 M⊙, evolved through the asymptotic giant branch (AGB) phase. The models used account for dust formation in the circumstellar envelope. To characterize the PNe sample of the SMC, we compare the observed abundances of the various species with the final chemical composition of the AGB models: this study allows us to identify the progenitors of the PNe observed, in terms of mass and chemical composition. According to our interpretation, most of the PNe descend from low-mass (M < 2 M⊙) stars, which become carbon rich, after experiencing repeated third dredge-up episodes, during the AGB phase. A fraction of the PNe showing the signature of advanced CNO processing are interpreted as the progeny of massive AGB stars, with mass above ∼6 M⊙, undergoing strong hot bottom burning. The differences with the chemical composition of the PNe population of the Large Magellanic Cloud is explained on the basis of the diverse star formation history and age–metallicity relation of the two galaxies. The implications of this study for some still highly debated points regarding the AGB evolution are also commented.
We used a new generation of asymptotic giant branch (AGB) stellar models that include dust formation in the stellar winds to find the links between evolutionary models and the observed properties of ...a homogeneous sample of Large Magellanic Cloud (LMC) planetary nebulae (PNe). Comparison between the evolutionary yields of elements such as CNO and the corresponding observed chemical abundances is a powerful tool to shed light on evolutionary processes such as hot bottom burning (HBB) and third dredge-up (TDU). We found that the occurrence of HBB is needed to interpret the nitrogen-enriched (log (N/H) + 12 > 8) PNe. In particular, N-rich PNe with the lowest carbon content are nicely reproduced by AGB models of mass M ≥ 6 M⊙, whose surface chemistry reflects the pure effects of HBB. PNe with log (N/H) + 12 < 7.5 correspond to ejecta of stars that have not experienced HBB, with initial mass below ∼3 M⊙. Some of these stars show very large carbon abundances, owing to the many TDU episodes experienced. We found from our LMC PN sample that there is a threshold to the amount of carbon accumulated at AGB surfaces, log (C/H) + 12 < 9. Confirmation of this constraint would indicate that, after the C-star stage is reached, AGBs experience only a few thermal pulses, which suggests a rapid loss of the external mantle, probably owing to the effects of radiation pressure on carbonaceous dust particles present in the circumstellar envelope. The implications of these findings for AGB evolution theories and the need to extend the PN sample currently available are discussed.
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