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Journal articles on the topic 'AGB stars'

Author: Grafiati
Published: 4 June 2021
Last updated: 9 March 2023

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1

Suh, Kyung-Won. "Infrared studies of post-AGB stars and PNe." Proceedings of the International Astronomical Union 12, S323 (October 2016): 28–31. http://dx.doi.org/10.1017/s1743921317002204.

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AbstractWe present infrared two color diagrams (2CDs) for a large sample of AGB stars, post-AGB stars, and planetary nebulae (PNe) to study their spectral evolution in infrared bands. We discuss possible evolutionary tracks from AGB stars to PNe on the 2CDs. We use catalogs from the available literature for the sample of AGB stars, post-AGB stars, and PNe in our Galaxy.
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2

Sargent, B., S. Srinivasan, M. Boyer, M. Feast, P. Whitelock, M. Marengo, M. A. T. Groenewegen, M. Meixner, J. L. Hora, and M. Otsuka. "Infrared Studies of the Variability and Mass Loss of Some of the Dustiest Asymptotic Giant Branch Stars in the Magellanic Clouds." Proceedings of the International Astronomical Union 14, S343 (August 2018): 498–99. http://dx.doi.org/10.1017/s1743921318007068.

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AbstractThe asymptotic giant branch (AGB) stars with the reddest colors have the largest amounts of circumstellar dust. AGB stars vary in their brightness, and studies show that the reddest AGB stars tend to have longer periods than other AGB stars and are more likely to be fundamental mode pulsators than other AGB stars. Such stars are difficult to study, as they are often not detected at optical wavelengths. Therefore, they must be observed at infrared wavelengths. Using the Spitzer Space Telescope, we have observed a sample of very dusty AGB stars in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) over Cycles 9 through 12 during the Warm Spitzer mission. For each cycle’s program, we typically observed a set of AGB stars at both 3.6 and 4.5 μm wavelength approximately monthly for most of a year. We present results from our analysis of the data from these programs.
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3

Parthasarathy, M. "Evolutionary connection between C-rich AGB stars and C-rich central stars of PNe." Symposium - International Astronomical Union 191 (1999): 475–80. http://dx.doi.org/10.1017/s0074180900203434.

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On the basis of C/O ratios the evolutionary connection between C-rich AGB/post-AGB stars and C-rich central stars of PNe is discussed. The proportion of carbon-rich post-AGB stars is comparable to that of carbon-rich PNe. The post-AGB stars with 21μm emission are found to be over abundant in carbon and s-process elements indicating that they have gone through the third dredge-up and carbon star phase on the AGB. As these stars evolve to higher temperature they will form carbon-rich PNe with carbon-rich central stars. Analysis of C/O ratios of PNe suggests that 68 percent of PNe have C/O > 1 indicating that the progenitors have gone through the third dredge-up and carbon-rich phase on the AGB. The Type I PNe can be divided into two groups. Group I Type I PNe are those with C/O < 1 indicating that their massive progenitors have gone through the third dredge-up and hot bottom burning on the AGB. Group II Type I PNe are those with C/O > 1. Their progenitors have gone through third dredge-up only and may not have experienced hot bottom burning. Carbon stars with silicate dust and or carbon-rich post-AGB stars (such as post-AGB stars with 21μm emission) may be the progenitors of PNe with [WC] central stars. The evolutionary sequence seems to be: carbon-rich AGB → carbon-rich post-AGB → [WC late type] → [WC early type] → [WC-PG 1159] → PG 1159.
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4

Kamath, Devika, and Hans Van Winckel. "Post-AGB Stars as Tracers of AGB Nucleosynthesis: An Update." Universe 8, no. 4 (April 11, 2022): 233. http://dx.doi.org/10.3390/universe8040233.

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The chemical evolution of galaxies is governed by the chemical yields from stars, and here we focus on the important contributions from asymptotic giant branch (AGB) stars. AGB nucleosynthesis is, however, still riddled with complexities. Observations from post-asymptotic giant branch (post-AGB) stars serve as exquisite tools to quantify and understand AGB nucleosynthesis. In this contribution, we review the invaluable constraints provided by post-AGB stars with which to study AGB nucleosynthesis, especially the slow neutron capture nucleosynthesis (i.e., the s-process).
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5

Parthasarathy, M., Marina Kounkel, and Keivan G. Stassun. "Evolutionary Status of Selected Post-AGB Single and Binary Stars in Gaia DR3." Research Notes of the AAS 6, no. 8 (August 26, 2022): 171. http://dx.doi.org/10.3847/2515-5172/ac8c46.

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Abstract Gaia data is helping to further understand the evolutionary status of post-AGB and related stars. In this paper we present an analysis of Gaia DR3 data of post-AGB stars and post-AGB binaries with accurate parallaxes. Gaia DR3 data of 44 post-AGB candidates are analyzed, including 16 post-AGB binary candidates. Of these, 19 stars have RUWE values >1.4. For several stars, the calculated absolute luminosities confirm that they are indeed in post-AGB evolutionary stage. We find that 12 stars have relatively lower luminosities; some of them may be post-RGB stars and some may be post-HB stars. We find that IRAS 01427+4633 (BD+46 442), IRAS 16230–3410, and IRAS 19199+3950 (HP Lyr) are evolved high velocity stars.
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6

Saberi, Maryam. "Tracing the role of AGB stars in the Galactic Fluorine budget." Proceedings of the International Astronomical Union 16, S366 (November 2020): 196–98. http://dx.doi.org/10.1017/s1743921322000412.

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AbstractThe cosmic origin of fluorine is still under debate. Asymptotic giant branch (AGB) stars are among the few suggested candidates to efficiently synthesis F in our Galaxy, however their relative contribution is not clear. In this paper, we briefly review the theoretical studies from stellar yield models of the F synthesis and chemical equilibrium models of the F-containing molecules in the outflow around AGB stars. Previous detections of the F-bearing species towards AGB and post-AGB stars are also highlighted. We suggest that high-resolution ALMA observations of the AlF, one of the two main carriers of F in the outflow of AGB stars, can provide a reliable tracer of the F-budget in AGB stars. This will be helpful to quantify the role of AGB stars in the Galactic F budget.
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7

Loup, C., E. Josselin, M. R. Cioni, H. J. Habing, J. A. D. L. Blommaert, N. R. Trams, M. A. T. Groenewegen, et al. "ISOCAM and DENIS Survey of 0.5 square degrees in the Bar of the LMC. Detection of the whole TP-AGB Star Population." Symposium - International Astronomical Union 191 (1999): 561–66. http://dx.doi.org/10.1017/s0074180900203550.

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We surveyed 0.5 square degrees in the Bar of the LMC with ISOCAM at 4.5 and 12 μm, and with DENIS in the I, J, and Ks bands. Our goal was to build a complete sample of Thermally-Pulsing AGB stars. Here we present the first analysis of 0.14 square degrees. In total we find about 300 TP-AGB stars. Among these TP-AGB stars, 9% are obscured AGB stars (high mass-loss rates); 9 of them were detected by IRAS, and only 1 was previously identified. Their luminosities range from 2 500 to 14 000 L⊙, with a distribution very similar to the one of optical TP-AGB stars (i.e. those with low mass-loss rates). Such a luminosity distribution, as well as the percentage of obscured stars among TP-AGB stars, is in very good agreement with the evolutionary models of Vassiliadis & Wood (1993) if most of the TP-AGB stars that we find have initial masses smaller than 1.5 to 2 M⊙.
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8

Waters, L. B. F. M., C. Waelkens, and H. Van Winckel. "Binary post-AGB stars." Symposium - International Astronomical Union 180 (1997): 313–18. http://dx.doi.org/10.1017/s0074180900131146.

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Low and intermediate mass stars leave the Asymptotic Giant Branch (AGB) when the mass in their H-rich envelope is less than about 0.01 M⊙, and the high mass loss drops several orders of magnitude. The central star rapidly evolves to the left part of the HR diagram along a track of constant luminosity (e.g. Schönberner 1983). In principle the evolution of the central star to higher Teff and the expansion and cooling of the AGB remnant are easy to calculate. In practice several complicating factors arise which make it much more difficult to predict the morphology and properties of post-AGB stars, such as binarity, post-AGB mass loss and aspherical AGB mass loss. Binarity of post-AGB stars affects the morphology of the circumstellar environment, and it affects evolutionary timescales and surface chemical abundances of the components in the system. This review discusses some properties of binary post-AGB stars.
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9

Frantsman, Yu L. "The Ratio of the Numbers of Carbon Stars to M Stars in Galaxies." International Astronomical Union Colloquium 106 (1989): 51. http://dx.doi.org/10.1017/s0252921100062606.

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Simulated populations of the AGB stars were calculated with different assumptions about mass loss, initial chemical composition and dredge-up efficiency. The early-AGB (E-AGB) phase was taken into account. The numbers of carbon and oxygen stars per 106 generated stars and the ratio (NC/NM) of these numbers were obtained. It is possible to match theoretically obtained NC/NM with the observations only if the luminosity of observed stars Mbol < -3.5; otherwise it is necessary to take into account the E-AGB phase. The data in the Table are for all AGB stars in the Galaxy and for stars with Mbol < -1.80 in the LMC.
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10

Parthasarathy, M. "Chemical Composition and Evolution of Post-AGB Stars." Symposium - International Astronomical Union 177 (2000): 225–36. http://dx.doi.org/10.1017/s0074180900002448.

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Analysis of the chemical compositions of post-AGB stars reveals the following abundance patterns: (i) Post-AGB stars which are extremely underabundant in Fe and other refractory elements, but which have nearly normal abundances of C, N, O, S, and Zn. The depleted refractory elements are locked up in circumstellar dust grains. Formation of dust close to the star, and dust-gas separation and dust-driven mass loss driving out mostly the dust may explain the abundances of these stars. (ii) High-latitude hot post-AGB stars which show an underabundance of carbon, indicating that they left the AGB before the third dredge-up occurred. (iii) Post-AGB stars with overabundances of carbon and s-process elements, indicating that they have gone through the third dredge-up and carbon-star phase on the AGB. The overabundance of Li, Al, C and s-process elements in some post-AGB stars indicate that they have gone through the dredge-up and Hot Bottom Burning nucleosynthesis at the base of the convective envelope. The observed characteristics of post-AGB stars indicate an evolutionary sequence in the transition region from the tip of the AGB into the young planetary nebula stage.
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11

Vlemmings, Wouter. "Magnetic fields of AGB and post-AGB stars." Proceedings of the International Astronomical Union 14, S343 (August 2018): 19–26. http://dx.doi.org/10.1017/s1743921318005367.

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AbstractThere is ample evidence for the presence of strong magnetic fields in the envelopes of (post-)Asymptotic Giant Branch (AGB) stars as well as supergiant stars. The origin and role of these fields are still unclear. This paper updates the current status of magnetic field observations around AGB and post-AGB stars, and describes their possible role during these stages of evolution. The discovery of magnetically aligned dust around a supergiant star is also highlighted. In our search for the origin of the magnetic fields, recent observations show the signatures of possible magnetic activity and rotation, indicating that the magnetic fields might be intrinsic to the AGB stars.
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12

Loup, Cécile, Mark Allen, Ariane Lançon, and Anais Oberto. "OH/IR stars versus YSOs in infrared photometric surveys." Proceedings of the International Astronomical Union 14, S343 (August 2018): 454–55. http://dx.doi.org/10.1017/s1743921318007901.

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AbstractAGB stars play a major role in the chemical evolution of the galaxies. It thus is important to establish reliable photometric selection criteria to count them, especially AGB stars at the last stages of AGB evolution like OH/IR stars. Here, we have identified about 1500 OH/IR stars and 500 YSOs with methanol masers, in all major mid– and far–infrared surveys (IRAS, MSX, AKARI, WISE, GLIMPSE, and Hi–Gal). We show that AGB stars with high mass-loss rates cannot be disentagled from YSOs with only mid–infrared photometry; far–infrared photometry is essential. In the region observed by GLIMPSE, we show that the proportion of AGB stars has been severely underestimated in previous works: about 70% of “intrinsically” red objects in GLIMPSE are AGB stars rather than YSOs.
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13

Boyer, Martha L. "The Impact of AGB Stars on Galaxies." Proceedings of the International Astronomical Union 14, S343 (August 2018): 321–29. http://dx.doi.org/10.1017/s1743921318005707.

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AbstractAt the end of their evolution, asymptotic giant branch (AGB) stars undergo strong pulsation, mass loss, and dust production. Their mass loss results in substantial chemical and dust enrichment of the interstellar medium. Dust evolution models and isotope abundances in presolar grains suggest that AGB stars play a key role in both dust evolution and the star formation process. They are also the brightest stars in galaxies, potentially dominating in the near-infrared. As a result, AGB stars have a significant influence on the evolution and appearance of their host galaxies and thus must be accounted for when interpreting a galaxy’s integrated light. I will highlight new results that describe the impact AGB stars have on galaxies, including how AGB stars are used to probe galaxy evolution.
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14

Goldman, Steven R., Martha L. Boyer, Julianne Dalcanton, Iain McDonald, Léo Girardi, Benjamin F. Williams, Sundar Srinivasan, and Karl Gordon. "A Census of Thermally Pulsing AGB Stars in the Andromeda Galaxy and a First Estimate of Their Contribution to the Global Dust Budget." Astrophysical Journal Supplement Series 259, no. 2 (March 23, 2022): 41. http://dx.doi.org/10.3847/1538-4365/ac4d9e.

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Abstract We present a near-complete catalog of the metal-rich population of thermally pulsing asymptotic giant branch (AGB) stars in the northwest quadrant of M31. This metal-rich sample complements the equally complete metal-poor Magellanic Cloud AGB catalogs produced by the SAGE program. Our catalog includes Hubble Space Telescope (HST) wide-band photometry from the Panchromatic Hubble Andromeda Treasury survey, HST medium-band photometry used to chemically classify a subset of the sample, and Spitzer mid- and far-IR photometry that we have used to isolate dust-producing AGB stars. We have detected 346,623 AGB stars; these include 4802 AGB candidates producing considerable dust, and 1356 AGB candidates that lie within clusters with measured ages, and in some cases metallicities. Using the Spitzer data and chemical classifications made with the medium-band data, we have identified both carbon- and oxygen-rich AGB candidates producing significant dust. We have applied color–mass-loss relations based on dusty-AGB stars from the LMC to estimate the dust injection by AGB stars in the PHAT footprint. Applying our color relations to a subset of the chemically classified stars producing the bulk of the dust, we find that ∼97.8% of the dust is oxygen-rich. Using several scenarios for the dust lifetime, we have estimated the contribution of AGB stars to the global dust budget of M31 to be 0.9%–35.5%, which is in line with previous estimates in the Magellanic Clouds. Follow-up observations of the M31 AGB candidates with the JWST will allow us to further constrain stellar and chemical evolutionary models, and the feedback and dust production of metal-rich evolved stars.
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15

Miller Bertolami, Marcelo M. "Evolutionary timescales from the AGB to the CSPNe phase." Proceedings of the International Astronomical Union 14, S343 (August 2018): 36–46. http://dx.doi.org/10.1017/s1743921318007330.

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AbstractThe transition from the asymptotic giant branch (AGB) to the final white dwarf (WD) stage is arguably the least understood phase in the evolution of single low- and intermediate-mass stars (0.8 ≲ MZAMS/M⊙ ≲ 8…10). Here we briefly review the progress in the last 50 years of the modeling of stars during the post-AGB phase. We show that although the main features, like the extreme mass dependency of post-AGB timescales were already present in the earliest post-AGB models, the quantitative values of the computed post-AGB timescales changed every time new physics was included in the modeling of post-AGB stars and their progenitors. Then we discuss the predictions and uncertainties of the latest available models regarding the evolutionary timescales of post-AGB stars.
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16

Wang, Y., F. Primas, C. Charbonnel, M. Van der Swaelmen, G. Bono, W. Chantereau, and G. Zhao. "Sodium abundances of AGB and RGB stars in Galactic globular clusters." Astronomy & Astrophysics 607 (November 2017): A135. http://dx.doi.org/10.1051/0004-6361/201730976.

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Aims. We investigate the Na abundance distribution of asymptotic giant branch (AGB) stars in Galactic globular clusters (GCs) and its possible dependence on GC global properties, especially age and metallicity. Methods. We analyze high-resolution spectra of a large sample of AGB and red giant branch (RGB) stars in the Galactic GCs NGC 104, NGC 6121, and NGC 6809 obtained with FLAMES/GIRAFFE at ESO/VLT, and determine their Na abundances. This is the first time that the AGB stars in NGC 6809 are targeted. Moreover, to investigate the dependence of AGB Na abundance dispersion on GC parameters, we compare the AGB [Na/H] distributions of a total of nine GCs, with five determined by ourselves with homogeneous method and four from literature, covering a wide range of GC parameters. Results. NGC 104 and NGC 6809 have comparable AGB and RGB Na abundance distributions revealed by the K−S test, while NGC 6121 shows a lack of very Na-rich AGB stars. By analyzing all nine GCs, we find that the Na abundances and multiple populations of AGB stars form complex picture. In some GCs, AGB stars have similar Na abundances and/or second-population fractions as their RGB counterparts, while some GCs do not have Na-rich second-population AGB stars, and various cases exist between the two extremes. In addition, the fitted relations between fractions of the AGB second population and GC global parameters show that the AGB second-population fraction slightly anticorrelates with GC central concentration, while no robust dependency can be confirmed with other GC parameters. Conclusions. Current data roughly support the prediction of the fast-rotating massive star (FRMS) scenario. However, considering the weak observational and theoretical trends where scatter and exceptions exist, the fraction of second-population AGB stars can be affected by more than one or two factors, and may even be a result of stochasticity.
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17

Moehler, S., W. B. Landsman, T. Lanz, and M. M. Miller Bertolami. "Hot UV-bright stars of galactic globular clusters." Astronomy & Astrophysics 627 (June 27, 2019): A34. http://dx.doi.org/10.1051/0004-6361/201935694.

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Context. We have performed a census of the UV-bright population in 78 globular clusters using wide-field UV telescopes. This population includes a variety of phases of post-horizontal branch (HB) evolution, including hot post-asymptotic giant branch (AGB) stars, and post-early AGB stars. There are indications that old stellar systems like globular clusters produce fewer post-(early) AGB stars than currently predicted by evolutionary models, but observations are still scarce. Aims. We wish to derive effective temperatures, surface gravities, and helium abundances of the luminous hot UV-bright stars in these clusters to determine their evolutionary status and compare the observed numbers to predictions from evolutionary theory. Methods. We obtained FORS2 spectroscopy of eleven of these UV-selected objects (covering a range of −2.3 < [Fe/H] < −1.0), which we (re-)analysed together with previously observed data. We used model atmospheres of different metallicities, including super-solar ones. Where possible, we verified our atmospheric parameters using UV spectrophotometry and searched for metal lines in the optical spectra. We calculated evolutionary sequences for four metallicity regimes and used them together with information about the HB morphology of the globular clusters to estimate the expected numbers of post-AGB stars. Results. We find that metal-rich model spectra are required to analyse stars hotter than 40 000 K. Seven of the eleven new luminous UV-bright stars are post-AGB or post-early AGB stars, while two are evolving away from the HB, one is a foreground white dwarf, and another is a white dwarf merger. Taking into account published information on other hot UV-bright stars in globular clusters, we find that the number of observed hot post-AGB stars generally agrees with the predicted values, although the numbers are still low. Conclusions. Spectroscopy is clearly required to identify the evolutionary status of hot UV-bright stars. For hotter stars, metal-rich model spectra are required to reproduce their optical and UV spectra, which may affect the flux contribution of hot post-AGB stars to the UV spectra of evolved populations. While the observed numbers of post-AGB and post-early AGB stars roughly agree with the predictions, our current comparison is affected by low number statistics.
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18

Oomen, Glenn-Michael, Hans Van Winckel, Onno Pols, and Gijs Nelemans. "Modelling depletion by re-accretion of gas from a dusty disc in post-AGB stars." Astronomy & Astrophysics 629 (September 2019): A49. http://dx.doi.org/10.1051/0004-6361/201935853.

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Many disc-type post-asymptotic giant branch (post-AGB) stars are chemically peculiar, showing underabundances of refractory elements in their photospheres that correlate with condensation temperature. The aim of this paper is to investigate how accretion from a circumbinary disc can cause this phenomenon of depletion and how this impacts the evolution of post-AGB stars. We used the MESA code to evolve stars in the post-AGB phase, while including accretion of metal-poor gas. We compared the models to a sample of 58 observed disc-type post-AGB stars with chemical abundance data. For each of these stars, we estimated the luminosity and the mass using the Gaia distance. We modelled the accretion rate onto the binary from a viscously evolving disc for a range of initial accretion rates and disc masses. We find that large initial accretion rates (≳3 × 10−7 M⊙ yr−1) and large initial disc masses (∼10−2 M⊙) are needed to reproduce the observed depleted post-AGB stars. Based on these high accretion rates, the evolution timescale of post-AGB stars can be significantly extended by a factor between two and five. We distinguish depletion patterns that are unsaturated (plateau profile) from those that are saturated, and we expect that post-red giant branch (post-RGB) stars are much more likely to show an unsaturated abundance pattern compared to post-AGB stars. Finally, because of the slower evolution of the low-mass post-RGB stars, we find that these systems can become depleted at lower effective temperatures (<5000 K). We conclude that accretion from a circumbinary disc successfully accounts for the chemical peculiarity of post-AGB stars.
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19

Decin, L., M. Montargès, A. M. S. Richards, C. A. Gottlieb, W. Homan, I. McDonald, I. El Mellah, et al. "(Sub)stellar companions shape the winds of evolved stars." Science 369, no. 6510 (September 17, 2020): 1497–500. http://dx.doi.org/10.1126/science.abb1229.

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Binary interactions dominate the evolution of massive stars, but their role is less clear for low- and intermediate-mass stars. The evolution of a spherical wind from an asymptotic giant branch (AGB) star into a nonspherical planetary nebula (PN) could be due to binary interactions. We observed a sample of AGB stars with the Atacama Large Millimeter/submillimeter Array (ALMA) and found that their winds exhibit distinct nonspherical geometries with morphological similarities to planetary nebulae (PNe). We infer that the same physics shapes both AGB winds and PNe; additionally, the morphology and AGB mass-loss rate are correlated. These characteristics can be explained by binary interaction. We propose an evolutionary scenario for AGB morphologies that is consistent with observed phenomena in AGB stars and PNe.
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20

Woodland, Michele, and Rodolfo Montez. "Commotion in their Motions: Proper Motion Anomalies of Nearby AGB Stars." Research Notes of the AAS 6, no. 7 (July 11, 2022): 142. http://dx.doi.org/10.3847/2515-5172/ac7f46.

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Abstract Proper motion anomalies (PMa) are often useful for identifying companions, but using this technique for large and luminous stars like Asymptotic Giant Branch (AGB) stars is problematic. We studied nearby AGB stars and their potential companions in a PMa catalog derived from Hipparcos and Gaia DR2 proper motion measurements. We found that companion configurations based on PMa for AGB stars are often implausible, such that the radius of the primary star is larger than the orbit of the putative companion. This may suggest an alternative origin of PMa in AGB stars, possibly surface brightness fluctuations due to their large convective surfaces.
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Karakas, Amanda I., Maria Lugaro, and Simon W. Campbell. "The slow-neutron capture process in low-metallicity asymptotic giant branch stars." Proceedings of the International Astronomical Union 5, S265 (August 2009): 57–60. http://dx.doi.org/10.1017/s1743921310000165.

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AbstractElements heavier than iron are produced in asymptotic giant branch (AGB) stars via the slow neutron capture process (s process). Recent observations of s-process-enriched Carbon Enhanced Metal-Poor (CEMP) stars have provided an unprecedented wealth of observational constraints on the operation of the s-process in low-metallicity AGB stars. We present new preliminary full network calculations of low-metallicity AGB stars, including a comparison to the composition of a few s-process rich CEMP stars. We also discuss the possibility of using halo planetary nebulae as further probes of low-metallicity AGB nucleosynthesis.
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22

Escorza, Ana, and Robert J. De Rosa. "Learning about AGB stars by studying the stars polluted by their outflows." Proceedings of the International Astronomical Union 16, S366 (November 2020): 253–58. http://dx.doi.org/10.1017/s1743921322000448.

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AbstractA rich zoo of peculiar objects forms when Asymptotic Giant Branch (AGB) stars, undergo interactions in a binary system. For example, Barium (Ba) stars are main-sequence and red-giant stars that accreted mass from the outflows of a former AGB companion, which is now a dim white dwarf (WD). Their orbital properties can help us constrain AGB binary interaction mechanisms and their chemical abundances are a tracer of the nucleosynthesis processes that took place inside the former AGB star. The observational constraints concerning the orbital and stellar properties of Ba stars have increased in the past years, but important uncertainties remained concerning their WD companions. In this contribution, we used HD 76225 to demonstrate that by combining radial-velocity data with Hipparcos and Gaia astrometry, one can accurately constrain the orbital inclinations of these systems and obtain the absolute masses of these WDs, getting direct information about their AGB progenitors via initial-final mass relationships.
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23

Abia, C., S. Cristallo, K. Cunha, P. de Laverny, and V. V. Smith. "Additional fluorine abundance determinations in evolved stars." Astronomy & Astrophysics 625 (May 2019): A40. http://dx.doi.org/10.1051/0004-6361/201935286.

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We present new fluorine abundance measurements for a sample of carbon-rich asymptotic giant branch (AGB) stars and two other metal-poor evolved stars of Ba/CH types. The abundances are derived from IR, K-band, high-resolution spectra obtained using GEMINI-S/Phoenix and TNG/Giano-b. Our sample includes an extragalactic AGB carbon star belonging to the Sagittarius dSph galaxy. The metallicity of our stars ranges from [Fe/H] = 0.0 down to − 1.4 dex. The new measurements, together with those previously derived in similar stars, show that normal (N-type) and SC-type AGB carbon stars of near solar metallicity present similar F enhancements, discarding previous hints that suggested that SC-type stars have larger enhancements. These mild F enhancements are compatible with current chemical-evolution models pointing out that AGB stars, although relevant, are not the main sources of this element in the solar neighbourhood. Larger [F/Fe] ratios are found for lower-metallicity stars. This is confirmed by theory. We highlight a tight relation between the [F/⟨s⟩] ratio and the average s-element enhancement [⟨s⟩/Fe] for stars with [Fe/H] > −0.5, which can be explained by the current state-of-the-art low-mass AGB models assuming an extended 13C pocket. For stars with [Fe/H] < −0.5, discrepancies between observations and model predictions still exist. We conclude that the mechanism of F production in AGB stars needs further scrutiny and that simultaneous F and s-element measurements in a larger number of metal-poor AGB stars are needed to better constrain the models.
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Husti, Laura, Roberto Gallino, Sara Bisterzo, Oscar Straniero, and Sergio Cristallo. "Barium Stars: Theoretical Interpretation." Publications of the Astronomical Society of Australia 26, no. 3 (2009): 176–83. http://dx.doi.org/10.1071/as08065.

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AbstractBarium stars are extrinsic Asymptotic Giant Branch (AGB) stars. They present the s-enhancement characteristic for AGB and post-AGB stars, but are in an earlier evolutionary stage (main sequence dwarfs, subgiants, red giants). They are believed to form in binary systems, where a more massive companion evolved faster, produced the s-elements during its AGB phase, polluted the present barium star through stellar winds and became a white dwarf. The samples of barium stars of Allen & Barbuy (2006) and of Smiljanic et al. (2007) are analysed here. Spectra of both samples were obtained at high-resolution and high S/N. We compare these observations with AGB nucleosynthesis models using different initial masses and a spread of 13C-pocket efficiencies. Once a consistent solution is found for the whole elemental distribution of abundances, a proper dilution factor is applied. This dilution is explained by the fact that the s-rich material transferred from the AGB to the nowadays observed stars is mixed with the envelope of the accretor. We also analyse the mass transfer process, and obtain the wind velocity for giants and subgiants with known orbital period. We find evidence that thermohaline mixing is acting inside main sequence dwarfs and we present a method for estimating its depth.
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Reindl, Nicole, Ellen Ringat, Thomas Rauch, Klaus Werner, and Jeffrey W. Kruk. "Spectral analysis of the O(He)-type central stars of the planetary nebulae K 1-27 and LoTr 4." Proceedings of the International Astronomical Union 7, S283 (July 2011): 482–83. http://dx.doi.org/10.1017/s1743921312012033.

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AbstractThe four known O(He) stars are the only amongst the hottest post-AGB stars whose atmospheres are composed of almost pure helium. Thus, their evolution deviates from the hydrogen-deficient post-AGB evolutionary sequence of carbon-dominated stars like e.g. PG 1159 stars. The origin of the O(He) stars is still not explained. They might be either post-early AGB stars or the progeny of R Coronae Borealis stars. We present preliminary results of a non-LTE spectral analysis based on FUSE and HST/COS observations.
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26

Karakas, Amanda I. "Globular cluster abundances: the role of asymptotic giant branch stars." Proceedings of the International Astronomical Union 5, S266 (August 2009): 161–68. http://dx.doi.org/10.1017/s1743921309991013.

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AbstractOne of the more popular theories to account for the abundance anomalies in globular cluster stars is the ‘self-pollution scenario,’ where the polluters were a previous generation of intermediate-mass asymptotic giant branch (AGB) stars. This idea has proved attractive because: (i) the hot-bottom burning experienced by these objects qualitatively provides an ideal proton-capture environment to produce helium and convert C and O to N, Ne to Na and Mg to Al, and (ii) the slow winds from these stars allow their retention by the cluster's gravitational potential. New stellar yields from low-metallicity AGB models are presented and compared to abundances derived in globular clusters. We also discuss external pollution and inhomogeneous-pollution models that use AGB stars as polluters. Current models of AGB stars cannot match all observational features of globular cluster stars. However, stellar modelling uncertainties are considerable and suggest AGB stars should not be ruled out just yet.
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Hollowell, David, and Icko Iben. "Nucleosynthesis and Mixing in Low- and Intermediate-Mass AGB Stars." International Astronomical Union Colloquium 108 (1988): 38–43. http://dx.doi.org/10.1017/s0252921100093374.

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AbstractThe existence of carbon stars brighter than Mbol=-4 can be understood in terms of dredge up in thermally pulsing asymptotic giant branch (AGB) stars. As a low- or intermediate-mass star evolves on the AGB, the large fluxes engendered in a helium shell flash cause the base of the convective envelope to extend into the radiative, carbon-rich region, and transport nucleosynthesis products to the stellar surface. Numerical models indicate that AGB stars with sufficiently massive stellar envelopes can become carbon stars via this standard dredge-up mechanism. AGB stars with less massive stellar envelopes can become carbon stars when carbon recombines in the cool, carbon-rich region below the convective envelope.Neutron capture occurs on iron-seed nuclei during a shell flash, and the products of this nucleosynthesis are also carried to the stellar surface. The conversion of 22Ne into 25Mg can initiate neutron capture nucleosynthesis in largecore mass AGB stars, but only if these stars can survive their large mass loss rates. The current estimates of nuclear reaction rates do not allow for appreciable neutron capture nucleosynthesis via the 22Ne source in lower mass AGB stars. The carbon recombination that induces dredge up in AGB stars of small envelope mass, however, also induces mixing of 1H and 12C in such a way that ultimately a 13C neutron source is activated in these stars. The 13C source can provide an abundant supply of neutrons for the nucleosynthesis of both light and heavy elements. While the existence of neutron-nucleosynthesis products in AGB stellar atmospheres can be understood qualitatively in terms of an active neutron source, the combination of nuclear reaction theory and evolutionary models has yet to provide quantitative agreement with stellar observations.
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28

Lattanzio, John. "AGB Stars: Remaining Problems." Proceedings of the International Astronomical Union 14, S343 (August 2018): 3–8. http://dx.doi.org/10.1017/s1743921318007019.

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AbstractI present a subjective list of what I think are the most serious problems in the modelling of AGB stars. Because AGB stars represent the last phase of stellar evolution, they suffer from the accumulation of the effects of uncertainties in all the earlier phases. The complexity of AGB evolution adds further uncertainties specific to the evolutionary behaviour of those stars. Most of the problems are associated with mixing, specifically the boundaries of mixed regions. The nature of the “extra-mixing” remains a mystery, let alone how to model it reliably. Other problems are briefly mentioned and I finish with some hopes of making progress in the future.
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29

Lebzelter, Thomas, Nami Mowlavi, Paola Marigo, Isabelle Lecoeur-Taibi, Michele Trabucchi, Giada Pastorelli, and Peter Wood. "AGB stars in Gaia DR2." Proceedings of the International Astronomical Union 14, S343 (August 2018): 73–76. http://dx.doi.org/10.1017/s1743921318005999.

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AbstractGaia Data Release 2 (DR2; April 25, 2018) provides astrometric and photometric data for more than a billion stars - among them many AGB stars. As part of DR2 the light curves of several hundreds of thousand variable stars, including many long-period variable (LPV) candidates, are made available. The publication of the light curves and LPV-specific attributes in addition to the standard DR2 products offers a unique opportunity to study AGB stars. In this contribution, we present the first results for AGB stars based on the analysis of the Gaia data performed after their release. As an immediate result of the Gaia DR2 LPV database we introduce a new photometric index capable of efficiently distinguishing AGB stars of different masses and chemical properties.
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30

Herwig, Falk. "AGB star models. Results from 1D stellar evolution and multi-dimensional hydrodynamics simulations." Proceedings of the International Astronomical Union 4, S252 (April 2008): 205–13. http://dx.doi.org/10.1017/s1743921308022795.

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AbstractIn this review I am discussing the current state of simulating the internal evolution of AGB stars. Recent work on AGB stars include the effect of rotation, magnetic fields and internal gravity waves, as well as thermohaline mixing induced by the 3He + 3He pp-chain reaction. Hydrodynamic simulations of the interior convection of AGB stars are now becoming available, giving insights to convective boundary mixing, for example for He-shell flash convection. At very low metallicity convective-reactive events are encountered in AGB stars (as well as in massive stars), and the necessity of hydrodynamic simulations to address this difficult phase of stellar evolution is emphasized.
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31

Van Winckel, Hans. "Post-AGB Stars." Annual Review of Astronomy and Astrophysics 41, no. 1 (September 2003): 391–427. http://dx.doi.org/10.1146/annurev.astro.41.071601.170018.

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32

Van Winckel, Hans. "The s-process in Post-AGB Stars." Symposium - International Astronomical Union 209 (2003): 91–98. http://dx.doi.org/10.1017/s0074180900208206.

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In this contribution an overview is given of recent accurate chemical abundance studies in post-AGB stars. The intrinsic nature of the enrichement and the spread in metallicity together with the absence of strong molecular veiling make post-AGB stars very useful to constrain AGB (chemical) evolutionary models. S-process enrichment is, however, not a general characteristic of post-AGB stars and a photospheric chemical study is not always a good tracer for the evolved status of an object.
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33

Stasińska, G., R. Szczerba, M. Schmidt, and N. Siódmiak. "Post-AGB stars as testbeds of nucleosynthesis in AGB stars." Astronomy & Astrophysics 450, no. 2 (April 10, 2006): 701–14. http://dx.doi.org/10.1051/0004-6361:20053553.

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34

Parthasarathy, M., Marina Kounkel, and Keivan G. Stassun. "Evolutionary Status of Selected Post-AGB Stars Based on Gaia DR3." Research Notes of the AAS 6, no. 10 (October 14, 2022): 210. http://dx.doi.org/10.3847/2515-5172/ac99dd.

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Abstract The evolutionary status of 24 post-AGB stars is presented based on Gaia DR3 data. All 24 stars have parallaxes accurate to better than 3σ and have RUWE values <1.4. Based on the Gaia DR3 distances the absolute luminosities are derived. For 14 of the stars, the luminosities confirm their post-AGB evolutionary stage. However, V1027 Cyg, which was previously classified as a post-AGB star, is found to have a higher luminosity than would be expected if it was truly a post-AGB star; thus it may be an evolved, massive, pulsating semi-regular variable star of type G7Ia. For 9 of the stars, the luminosities are lower than 1000 L ⊙, indicating that some are post-HB stars and some are post-RGB stars.
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35

Tanabé, Toshihiko, Yoshifusa Ita, Noriyuki Matsunaga, and Yoshikazu Nakada. "Properties of AGB variables in the intermediate-age cluster NGC 419 in the SMC." International Astronomical Union Colloquium 193 (2004): 295–303. http://dx.doi.org/10.1017/s0252921100010824.

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AbstractWe found that the OGLE database contains most AGB stars associated with the SMC cluster NGC 419. Combining these data with our near-infrared (NIR) data, we obtained the following results: 1) Almost all AGB stars above the TRGB are variables. 2) Large amplitude variables tend to show more regular light curves. 3) Only the tip AGB stars (in this case, carbon stars) increase their amplitudes with their J – K colors and only the most evolved ones can become large amplitude (Mira-like) variables. 4) AGB variables may explain the sequences found by Wood et al. (1999).
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36

Montez, Rodolfo, Sofia Ramstedt, Joel H. Kastner, and Wouter Vlemmings. "Updates on the Ultraviolet Emission from Asymptotic Giant Branch Stars." Proceedings of the International Astronomical Union 14, S343 (August 2018): 474–75. http://dx.doi.org/10.1017/s1743921318007160.

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AbstractA comprehensive study of UV emission from asymptotic giant branch (AGB) stars with the Galaxy Evolution Explorer (GALEX) revealed that out of the 316 observed AGB stars, 57% were detected in the near-UV (NUV) bandpass and 12% were detected in the far-UV (FUV) bandpass (Montez et al. 2017). A cross-match between our sample and Gaia DR2 results in parallax estimates for 90% of the sample of AGB stars, compared to only 30% from Hipparcos. This increase allowed us to further probe trends and conclusions of our initial study. Specifically, that the detection of UV emission from AGB stars is subject to proximity and favorable lines of sight in our Galaxy. These improved results support the notion that some of the GALEX-detected UV emission is intrinsic to AGB stars, likely due to a combination of photospheric and chromospheric emission.
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37

Wang, Yue, Francesca Primas, Corinne Charbonnel, Mathieu Van der Swaelmen, William Chantereau, and Giuseppe Bono. "Are Galactic globular cluster AGB stars rich or poor in Sodium? Sodium abundance of AGB and RGB stars in NGC 2808." Proceedings of the International Astronomical Union 12, S316 (August 2015): 361–62. http://dx.doi.org/10.1017/s1743921316008681.

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AbstractA spectroscopic study comparing the [Na/Fe] distributions of RGB and AGB stars in the Galactic globular cluster (GC) NGC 6752 found that there was no Na-rich, 2nd-generation star along the early-AGB of this cluster. This came as a surprise since in this GC, as well as other Galactic GCs studied so far, 1st- and 2nd-generation stars have usually been found from the main sequence turnoff up to the red giant branch. To investigate whether the failure of a significant fraction of stars to ascend the AGB also happens to other GCs, we studied a sample of AGB and RGB stars in NGC 2808 observed at the ESO/VLT with FLAMES. Contrary to NGC 6752, we find that the AGB and RGB stars we studied in NGC 2808 have comparable [Na/Fe] dispersions.
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38

Abia, C., I. Domínguez, R. Gallino, M. Busso, O. Straniero, P. de Laverny, and G. Wallerstein. "Understanding AGB Carbon Star Nucleosynthesis from Observations." Publications of the Astronomical Society of Australia 20, no. 4 (2003): 314–23. http://dx.doi.org/10.1071/as03021.

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AbstractRecent advances in the knowledge of the evolutionary status of asymptotic giant branch (AGB) stars and of the nucleosynthesis processes occurring in them are discussed, and used to interpret abundance determinations for s-process elements, lithium and CNO isotopes in several types of AGB stars. We focus our attention mainly on carbon-rich AGB stars. By combining these different constraints we conclude that most carbon stars in the solar neighborhood are of low mass (M≤3 M⊙), their abundances being a consequence of the operation of thermal pulses and the third dredge-up. However, the observed abundances in carbon stars of the R and J types cannot be explained by this standard scenario. These stars may not be on the AGB, but possibly in the core-He burning phases; their envelopes may have been polluted with nuclear ashes of the core-He flash, followed by CNO re-processing enhancing 13C. Observational evidence suggesting the operation of non-standard mixing mechanisms during the AGB phase is also discussed.
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39

Lattanzio, John, and Manuel Forestini. "Nucleosynthesis in AGB Stars." Symposium - International Astronomical Union 191 (1999): 31–40. http://dx.doi.org/10.1017/s0074180900202854.

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We give a qualitative review of the nucleosynthesis occurring in AGB stars. We summarise some new calculations of intermediate mass stars which include all thermal pulses until the star is about to leave the AGB, as well as a detailed nucleosynthesis network. We will show that hot bottom burning delays, rather than prevents, the formation of carbon stars; those that form are not optically visible.
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40

Kipper, Tônu. "Technetium in Stellar Atmospheres." Symposium - International Astronomical Union 145 (1991): 317–26. http://dx.doi.org/10.1017/s0074180900227460.

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The quantitative determinations of technetium abundances are discussed. Up to now Tc abundances were found for nearly 30 stars which all are thermally pulsing AGB stars. The technetium abundance values for these AGB stars atmospheres, where it was found to be present, cluster around −1.0 ÷ 1.7. The fact that Tc is not omnipresent in the atmospheres of AGB stars, which are highly enriched in other heavy elements produced in s-process, is discussed.
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41

Frantsman, Ju. "The Formation of Carbon Stars in the Magellanic Clouds from Mass Transfer in Close Binaries." Symposium - International Astronomical Union 190 (1999): 381–82. http://dx.doi.org/10.1017/s0074180900118339.

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The presence of the carbon stars in the MC with luminosities higher or lower than predicted for thermally-pulsing (TP) AGB stars, can be explained by processes that happen during the early AGB (E–AGB) stage. I examine this assumption by means of a population simulation technique. I find that there must be TP–AGB C and S stars in the MC that formed as a result of mass transfer in binary systems. Their presence may influence the age determinations of MC clusters.
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42

Leščinskaitė, Alina, Rima Stonkutė, and Vladas Vansevičius. "AGB and RGB stars in the dwarf irregular galaxy Leo A." Astronomy & Astrophysics 647 (March 2021): A170. http://dx.doi.org/10.1051/0004-6361/202037967.

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Context. Leo A is a gas-rich dwarf irregular galaxy of low stellar mass located in the outskirts of the Local Group. It has an extended star formation history with stellar populations spanning a wide age range (∼0.01−10 Gyr). As Leo A is a well-isolated dwarf galaxy, it is a perfect target to study a galactic structure formed entirely by processes of self-induced star formation. Aims. Our aim is to study populations of the brightest asymptotic giant branch (AGB) stars and red giant branch (RGB) stars over the entire extent of the Leo A galaxy. Methods. We analysed populations of AGB and RGB stars in the Leo A galaxy using multicolour photometry data obtained with the Subaru Suprime-Cam (B, V, R, I, Hα) and HST ACS (F475W, F814W) cameras. In order to separate the Milky Way and Leo A populations of red stars, we developed a photometric method that enabled us to study the spatial distribution of AGB and RGB stars within the Leo A galaxy. Results. We found a previously unknown sequence of 26 peculiar RGB stars which probably have a strong CN band in their spectra (∼380−390 nm). This conclusion is supported by the infrared CN spectral features observed in four of these stars with available spectra from the literature. Additionally, we present a catalogue of 32 luminous AGB stars and 3 candidate AGB stars. Twelve AGB stars (three of them might have dusty envelopes) from this sample are newly identified; the remaining 20 AGB stars were already presented in the literature based on near-infrared observations. By splitting the RGB sequence into blue and red parts, we revealed different spatial distributions of the two subsets, with the former being more centrally concentrated than the latter. Cross-identification with spectroscopic data available in the literature suggests that the bulk of blue and red RGB stars are, on average, similar in metallicity; however, the red RGB stars might have an excess of metal-deficient stars of [Fe/H] < −1.8. We also found that the distributions of luminous AGB and blue RGB stars have nearly equal scale lengths (0.′87 ± 0.′06 and 0.′89 ± 0.′09, respectively), indicating that they could belong to the same generation. This conclusion is strengthened by the similarities of the cumulative distributions of AGB and blue RGB stars, both showing more centrally concentrated populations compared to red RGB stars. There is also a prominent decline in the ratio of AGB to RGB stars with an increasing radius. These results suggest that the star-forming disk of Leo A is shrinking, which is in agreement with the outside-in star formation scenario of dwarf galaxy evolution.
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43

Lagadec, Eric. "Post-AGB nebular studies." Proceedings of the International Astronomical Union 12, S323 (October 2016): 20–27. http://dx.doi.org/10.1017/s1743921317000928.

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AbstractThis review presents the latest advances in the nebular studies of post-AGB objects. Post-AGB stars are great tools to test nucleosynthesis and evolution models for stars of low and intermediate masses, and the evolution of dust in harsh environment. I will present the newly discovered class of post-RGB stars, formed via binary interaction on the RGB. Binary systems can also lead to the formation of two class of aspherical post-AGB, the Proto-Planetary Nebulae and the naked post-AGBs (dusty RV Taus, a.k.a. Van Winckel’s stars).
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44

Herwig, Falk. "Current Models for the Evolution of AGB Stars." Symposium - International Astronomical Union 209 (2003): 61–68. http://dx.doi.org/10.1017/s0074180900208103.

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While the basic properties of AGB stellar evolution are well established, comprehensive observational studies of late phases of intermediate mass stars continue to generate puzzles for current stellar models. Here, I review current techniques to model AGB stars, and I discuss important aspects of current research of AGB (and post-AGB) stellar evolution with a particular focus on how these interrelate.
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45

Waters, L. B. F. M., and F. J. Molster. "Crystalline silicates in AGB and post-AGB stars." Symposium - International Astronomical Union 191 (1999): 208–20. http://dx.doi.org/10.1017/s0074180900203094.

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We discuss ISO spectroscopy of oxygen-rich dust shells surrounding evolved stars. The dust that condenses in the outflows of stars on the Asymptotic Giant Branch consists mainly of amorphous silicates and simple oxides. For high mass loss rates, crystalline silicates begin to appear at modest abundance. These crystalline silicates are cold and Fe-poor. ISO spectroscopy for the first time allows quantitative mineralogy of oxygen-rich circumstellar dust. Crystalline silicates are found at high abundance in sources with peculiar (disk) geometry, such as long-lived circum-binary disks. Some C-rich post-AGB stars, notably the Red Rectangle and several nebulae surrounding [WC] central stars of Planetary Nebulae, also show crystalline silicates. We speculate on the origin and evolution of the crystalline dust component in evolved stars.
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46

Manchado, Arturo, D. Anibal García-Hernández, Eva Villaver, and Jean Guironnet de Massas. "Morphological classification of post-AGB stars." Proceedings of the International Astronomical Union 7, S283 (July 2011): 83–86. http://dx.doi.org/10.1017/s1743921312010745.

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AbstractWe present a complete study of the morphology of post-Asymptotic Giant Branch (post-AGB) stars. The post-AGB stage is a very short evolutionary phase between the end of the AGB and the beginning of the Planetary Nebula (PN) stage (between 100 and 10,000 yrs). Post-AGB stars do not show variability and are not hot enough to fully ionize the hydrogen envelope. We have defined the end of the post-AGB phase and the beginning of the PN phase when the star has a temperature of 30000 K. Post-AGB stars have a circumstellar shell that is illuminated by the central stars or partially ionized. However, this circumstellar shell is too small to be resolved by ground-based observations. Thus, we have used the Hubble Space Telescope (HST) database to resolve these shells. 117 post-AGBs were found in this database. Here we present the preliminary results on their morphological classification and the correlation with the galactic latitude. Our preliminary results show that 38% of the sample are stellar-like (S), 31% bipolar (B), 12% multipolar (M) and 19% elliptical (E).
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47

Frantsman, Ju. "Expected Differences between AGB Stars in the LMC and the SMC Due to Differences in Chemical Composition." Symposium - International Astronomical Union 190 (1999): 383–84. http://dx.doi.org/10.1017/s0074180900118340.

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Certain aspects of the AGB population, such as the relative number of M and N stars, the mass loss rates, and the initial masses of carbon-oxygen cores, depend on the initial heavy element abundance Z. I have calculated synthetic populations of AGB stars for different initial Z values taking into consideration the evolution of single and close binary stars. I present the results of population syntheses of AGB stars in clusters as a function of different initial chemical compositions. The relation for the tip luminosity of AGB stars versus cluster age as a function of Z is presented and is used to determine the ages for a number of clusters in the LMC and the SMC, including clusters with no previous age determinations.
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48

Ramstedt, Sofia, Fredrik L. Schöier, and Hans Olofsson. "The physics and chemistry of circumstellar envelopes of S-stars on the AGB." Proceedings of the International Astronomical Union 4, S251 (February 2008): 167–68. http://dx.doi.org/10.1017/s1743921308021479.

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AbstractPresented here are the preliminary results of a long-term study of S-stars on the AGB. S-stars are important as possible transition objects between oxygen-rich M-stars and carbon stars. The aim of the study is to compare results from our newly gathered observational database for the S-stars with those already obtained for the M- and carbon stars. We can thus follow the changes as the stars evolve along the AGB and more firmly establish the suggested M-MS-S-SC-C evolutionary sequence. It will also allow us to determine the relative importance of processes such as non-equilibrium chemistry, grain formation, and photodissociation in regulating the chemistry in circumstellar envelopes of AGB stars.
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49

Lugaro, Maria A., and Susanne Höfner. "What can pre-solar grains tell us about AGB stars?" Proceedings of the International Astronomical Union 2, no. 14 (August 2006): 345–48. http://dx.doi.org/10.1017/s1743921307010940.

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AbstractThe vast majority of pre-solar grains recovered to date show the signature of an origin in asymptotic giant branch (AGB) stars. In AGB stars, the abundances of elements lighter than silicon and heavier than iron are largely affected by proton- and neutron-capture processes, respectively, while the compositions of the elements in between also carry the signature of the initial composition of the star. Dust is produced and observed around AGB stars and the strong mass loss experienced by these stars is believed to be driven by radiation pressure on dust grains. We briefly review the main developments that have occurred in the past few years in the study of AGB stars in relation to dust and pre-solar grains. From the nucleosynthesis point of view these include: more stringent constraints on the main neutron source nucleus,13C, for theslowneutron capture process (thesprocess); the possibility of pre-solar grains coming from massive AGB stars; and the unique opportunity to infer the ‘isotopic’ evolution of the Galaxy by combining pre-solar grain data and AGB model predictions. Concerning the formation of grains in AGB stars, considerable progress has been achieved in modelling. In particular, self-consistent models for atmospheres and winds of C-stars have reached a level of sophistication which allows direct quantitative comparison with observations. In the case of stars with C/O < 1, however, recent work points to serious problems with the dust-driven wind scenario. A current trend in atmosphere and wind modelling is to investigate the possible effects of inhomogenieties (e.g., due to giant convection cells) with 2D/3D models.
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50

García-Lario, P., A. Manchado, and S. R. Pottasch. "Planetary and Proto-Planetary Nebulae in the IRAS Two-Colour Diagram." Symposium - International Astronomical Union 155 (1993): 331. http://dx.doi.org/10.1017/s0074180900171207.

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We present a study of the distribution of post-AGB stars and planetary nebulae (PNe) in the IRAS two-colour diagram. From the analysis of the distribution of spectral types of post-AGB stars in this diagram we conclude that the position at which a star leaves the infrared sequence of colours observed in AGB stars depends only on the initial mass of the progenitor star. The evolution after the end of the AGB phase takes place, in a first approximation, at a constant value of [25] – [60].
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