Chemical composition of a sample of candidate post-asymptotic giant branch stars

Rao, S. Sumangala; Giridhar, Sunetra; Lambert, David L.

doi:10.1111/j.1365-2966.2011.19780.x

Cited by 36 publications

(45 citation statements)

References 91 publications

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“…B.6) are similar to the determined abundances from Van Winckel & Reyniers (2000) and Rao et al (2012). All three independent studies confirm the overabundance of ls elements with respect to the hs elements of IRAS 22223+4327.…”

Section: Appendix B: Comparison To Previous Studiessupporting

confidence: 75%

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Detailed homogeneous abundance studies of 14 Galactics-process enriched post-AGB stars: In search of lead (Pb)

Smedt

Winckel

Kamath

et al. 2016

A&A

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Context. This paper is part of a larger project in which we systematically study the chemical abundances of Galactic and extragalactic post-asymptotic giant branch (post-AGB) stars. The goal at large is to provide improved observational constraints to the models of the complex interplay between the AGB s-process nucleosynthesis and the associated mixing processes. Aims. Lead (Pb) is the final product of the s-process nucleosynthesis and is predicted to have large overabundances with respect to other s-process elements in AGB stars of low metallicities. However, Pb abundance studies of s-process enriched post-AGB stars in the Magellanic Clouds show a discrepancy between observed and predicted Pb abundances. The determined upper limits based on spectral studies are much lower than what is predicted. In this paper, we focus specifically on the Pb abundance of 14 Galactic s-process enhanced post-AGB stars to check whether the same discrepancy is present in the Galaxy as well. Among these 14 objects, two were not yet subject to a detailed abundance study in the literature. We apply the same method to obtain accurate abundances for the 12 others. Our homogeneous abundance results provide the input of detailed spectral synthesis computations in the spectral regions where Pb lines are located. Methods. We used high-resolution UVES and HERMES spectra for detailed spectral abundance studies of our sample of Galactic post-AGB stars. None of the sample stars display clear Pb lines, and we only deduced upper limits of the Pb abundance by using spectrum synthesis in the spectral ranges of the strongest Pb lines. Results. We do not find any clear evidence of Pb overabundances in our sample. The derived upper limits are strongly correlated with the effective temperature of the stars with increasing upper limits for increasing effective temperatures. We obtain stronger Pb constraints on the cooler objects. Moreover, we confirm the s-process enrichment and carbon enhancement of two unstudied 21 µm sources IRAS 13245-6428 and IRAS 14429-4539. The mildly s-process enhanced post-AGB star IRAS 17279-1119 is part of a binary system and may be the long sought precursor of extrinsic Ba stars. Conclusions. Stars with T eff > 7500 K do not provide strong constraints on the Pb abundance as the strongest line in the optical spectrum is only detectable at unrealistically high Pb atmospheric abundances. Combining the Pb abundance results from this study with abundances from our previous studies, we conclude that the discrepancy between theory and observation increases towards lower metallicities. The model predictions are consistent with the deduced upper limits on the Pb abundances for all stars with [Fe/H] > −0.7 dex. For stars with [Fe/H] < −0.7 dex, however, the model predictions overestimate the Pb abundances with respect to the other s-process elements. All objects, except IRAS 17279-1119, confirm the relation between neutron exposure [hs/ls] and third dredge-up efficiency [s/Fe], whereas no relation between metallicity and neut...

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Section: Appendix B: Comparison To Previous Studiessupporting

confidence: 75%

“…Comparison between our derived abundances and the results of Van Winckel & Reyniers (2000) and Rao et al (2012) for IRAS 22223+4327. We adopt a standard deviation of 0.2 dex for all results of Rao et al (2012). we compare our individual results for the two settings with these combined results.…”

Section: Appendix B: Comparison To Previous Studiesmentioning

confidence: 93%

Detailed homogeneous abundance studies of 14 Galactics-process enriched post-AGB stars: In search of lead (Pb)

Smedt

Winckel

Kamath

et al. 2016

A&A

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“…Only in the case of CC Lyr does it reach the extreme values more commonly seen in RV Tau stars (e.g., ≈−3.0 dex in HP Lyr and DY Ori, see Giridhar et al 2005). The RV Tau stars depleted in their refractory elements are known binaries for a large number of them, supporting the hypothesis of a circumbinary dusty disc (Rao et al 2012). Disentangling orbital velocities from pulsational velocities is very demanding in terms of observing time both in the case of RV Tau and W Vir stars, and indeed only four type II Cepheids are currently known as binaries: AU Peg, IX Cas, TX Del, and ST Pup.…”

Section: Chemical Compositionmentioning

confidence: 62%

Type II Cepheids in the Milky Way disc

Lemasle

Kovtyukh

Bono

et al. 2015

A&A

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Context. A robust classification of Cepheids into their different sub-classes and, in particular, between classical and Type II Cepheids, is necessary to properly calibrate the period-luminosity relations and for populations studies in the Galactic disc. Type II Cepheids are, however, very diverse, and classifications based either on intrinsic (period, light curve) or external parameters (e.g., [Fe/H], |z|) do not provide a unique classification. Aims. We want to ascertain the classification of two Cepheids, HQ Car and DD Vel, that are sometimes classified as classical Cepheids and sometimes as Type II Cepheids. Methods. To achieve this goal, we examine both their chemical composition and the presence of specific features in their spectra. Results. We find emission features in the Hα and in the 5875.64 Å He I lines that are typical of W Vir stars. The [Na/Fe] (or [Na/Zn]) abundances are typical of thick-disc stars, while BL Her stars are Na-overabundant ([Na/Fe] > +0.5 dex). Finally, the two Cepheids show a possible (HQ Car) or probable (DD Vel) signature of mild dust-gas separation that is usually observed only in long-period type II Cepheids and RV Tau stars. Conclusions. These findings clearly indicate that HQ Car and DD Vel are both Type II Cepheids from the W Vir sub-class. Several studies have reported an increase in the Cepheids' abundance dispersion towards the outer (thin) disc. A detailed inspection of the Cepheid classification, in particular for those located in the outer disc, will indicate whether this feature is real or simply an artefact of the inclusion of type II Cepheids belonging to the thick disc in the current samples.

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“…Only about 25% of the studied Galactic post-AGB stars indicate strong s-process enrichment (e.g. Rao et al 2012, and references therein). Although stars in the Galactic sample of post-AGB stars are well studied, their unknown distances hamper accurate luminosity determinations and hence accurate initial mass estimates, which are a key parameter in constraining single star evolution and nucleosynthesis AGB models.…”

Section: Introductionmentioning

confidence: 99%

Chemical abundance study of two stronglys-process enriched post-AGB stars in the LMC: J051213.81-693537.1 and J051848.86-700246.9

Smedt

Winckel

Kamath

et al. 2015

A&A

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Context. This paper is part of a larger project in which we systematically study the chemical abundances of extra-galactic postasymptotic giant branch (post-AGB) stars. The aim of our programme is to derive chemical abundances of stars covering a large range in luminosity and metallicity with the ultimate goal of testing, constraining, and improving our knowledge of the poorly understood AGB phase, especially the third dredge-up mixing processes and associated s-process nucleosynthesis. Aims. Post-AGB photospheres are dominated by atomic lines and indicate the effects of internal chemical enrichment processes over the entire stellar lifetime. In this paper, we study two carefully selected post-AGB stars: J051213.81-693537.1 and J051848.86-700246.9 in the Large Magellanic Cloud (LMC). Both objects show signs of s-process enhancement. The combination of favourable atmospheric parameters for detailed abundance studies and their known distances (and hence luminosities and initial masses) make these objects ideal probes of the AGB third dredge-up and s-process nucleosynthesis in that they provide observational constraints for theoretical AGB models. Methods. We use high-resolution optical UVES spectra to determine accurate stellar parameters and subsequently perform detailed elemental abundance studies of post-AGB stars. Additionally, we use available photometric data covering optical and IR bands to construct spectral energy distributions for reddening and luminosity determinations. We then estimate initial masses from theoretical post-AGB tracks. Results. We obtained accurate atmospheric parameters for J051213.81-693537.1 (T eff = 5875 ± 125 K, log g = 1.00 ± 0.25 dex, [Fe/H] = −0.56 ± 0.16 dex) and J051848.86-700246.9 (T eff = 6000 ± 125 K, log g = 0.50 ± 0.25 dex, [Fe/H] = −1.06 ± 0.17 dex). Both stars show extreme s-process enrichment associated with relatively low C/O ratios of 1.26 ± 0.40 and 1.29 ± 0.30 for J051213-693537.1 and J051848-700246.9, respectively. We could only derive upper limits of the lead (Pb) abundance. These upper limits show a possible very slight Pb overabundance with respect to heavy s-elements for J051213-693537.1, while J051848-700246.9 shows an upper limit of the Pb abundance similar to [hs/Fe]. A comparison with theoretical post-AGB evolutionary tracks in the HR-diagram reveals that both stars have low initial masses between 1.0 and 1.5 M . Conclusions. This study adds to the results obtained so far on a very limited number of s-process enriched stars in the Magellanic Clouds. With the addition of the two stars in this study, we find an increasing discrepancy between observed and predicted Pb abundances towards lower metallicities of the studied s-process rich post-AGB stars in the Magellanic Clouds. The more metal-rich J051213-693537.1 fits the theoretical Pb abundance predictions well, while the five other objects with [Fe/H] < 1, including J051848-700246.9, have much lower Pb overabundances than predicted. In all objects found so far, including the objects in this study, th...

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Chemical composition of a sample of candidate post-asymptotic giant branch stars

Cited by 36 publications

References 91 publications

Detailed homogeneous abundance studies of 14 Galactics-process enriched post-AGB stars: In search of lead (Pb)

Detailed homogeneous abundance studies of 14 Galactics-process enriched post-AGB stars: In search of lead (Pb)

Type II Cepheids in the Milky Way disc

Chemical abundance study of two stronglys-process enriched post-AGB stars in the LMC: J051213.81-693537.1 and J051848.86-700246.9

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