scholarly journals Isotopic Abundances in Presolar SiC Grains accounted by s-Processing from MHD-induced Mixing in low mass AGB stars

2020 ◽  
Vol 1668 ◽  
pp. 012032
Author(s):  
S. Palmerini ◽  
M. Busso ◽  
D. Vescovi
Keyword(s):  
Agb Stars ◽  
Isotopic Abundances ◽  
Low Mass

Rubidium in Barium Stars

2020 ◽  
Author(s):  
M P Roriz ◽  
M Lugaro ◽  
C B Pereira ◽  
N A Drake ◽  
S Junqueira ◽  
...  
Keyword(s):  
Neutron Source ◽  
Slow Neutron ◽  
Asymptotic Giant Branch ◽  
Neutron Density ◽  
Data Sets ◽  
Agb Stars ◽  
Branching Points ◽  
Theoretical Predictions ◽  
Low Mass ◽  
Chemically Peculiar Stars

Abstract Barium (Ba) stars are chemically peculiar stars that display in their atmospheres the signature of the slow neutron-capture (the s-process) mechanism that occurs in asymptotic giant branch (AGB) stars, a main contributor to the cosmic abundances. The observed chemical peculiarity in these objects is not due to self-enrichment, but to mass transfer between the components of a binary system. The atmospheres of Ba stars are therefore excellent astrophysical laboratories providing strong constraints for the nucleosynthesis of the s-process in AGB stars. In particular, rubidium (Rb) is a key element for the s-process diagnostic because it is sensitive to the neutron density and therefore its abundance can reveal the main neutron source for the s-process in AGB stars. We present Rb abundances for a large sample of 180 Ba stars from high resolution spectra (R = 48000), and we compare the observed [Rb/Zr] ratios with theoretical predictions from AGB s-process nucleosynthesis models. The target Ba stars in this study display [Rb/Zr] <0, showing that Rb was not efficiently produced by the activation of branching points. Model predictions from the Monash and FRUITY data sets of low-mass (≲ 4 M⊙) AGB stars are able to cover the Rb abundances observed in the target Ba stars. These observations indicate that the 13C(α,n)16O reaction is the main neutron source of the s-process in the low-mass AGB companions of the observed Ba stars. We have not found in the present study candidate companion for IR/OH massive AGB stars.

Nucleosynthesis of fluorine in low-mass AGB stars during the thermal pulse phase

1996 ◽  
Vol 20 (2) ◽  
pp. 231-238
Author(s):  
Yong-xing Liu ◽  
Bo Zhang ◽  
Qiu-he Peng
Keyword(s):  
Agb Stars ◽  
Thermal Pulse ◽  
Pulse Phase ◽  
Low Mass

scholarly journals Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis

Universe ◽  
2021 ◽  
Vol 7 (2) ◽  
pp. 25
Author(s):  
Umberto Battino ◽  
Claudia Lederer-Woods ◽  
Borbála Cseh ◽  
Pavel Denissenkov ◽  
Falk Herwig
Keyword(s):  
Stellar Structure ◽  
Process Efficiency ◽  
Agb Stars ◽  
Mixing Processes ◽  
Data Set ◽  
Convective Envelope ◽  
Capture Process ◽  
Low Metallicity ◽  
Low Mass ◽  
The Impact

The slow neutron-capture process (s-process) efficiency in low-mass AGB stars (1.5 < M/M⊙ < 3) critically depends on how mixing processes in stellar interiors are handled, which is still affected by considerable uncertainties. In this work, we compute the evolution and nucleosynthesis of low-mass AGB stars at low metallicities using the MESA stellar evolution code. The combined data set includes models with initial masses Mini/M⊙=2 and 3 for initial metallicities Z=0.001 and 0.002. The nucleosynthesis was calculated for all relevant isotopes by post-processing with the NuGrid mppnp code. Using these models, we show the impact of the uncertainties affecting the main mixing processes on heavy element nucleosynthesis, such as convection and mixing at convective boundaries. We finally compare our theoretical predictions with observed surface abundances on low-metallicity stars. We find that mixing at the interface between the He-intershell and the CO-core has a critical impact on the s-process at low metallicities, and its importance is comparable to convective boundary mixing processes under the convective envelope, which determine the formation and size of the 13C-pocket. Additionally, our results indicate that models with very low to no mixing below the He-intershell during thermal pulses, and with a 13C-pocket size of at least ∼3 × 10−4 M⊙, are strongly favored in reproducing observations. Online access to complete yield data tables is also provided.

Metallicity, pulsation and mass loss in globular cluster low-mass AGB stars

2009 ◽  
Author(s):  
Iain McDonald ◽  
Jacco Th. van Loon ◽  
Martha L. Boyer ◽  
Eric Stempels
Keyword(s):  
Mass Loss ◽  
Globular Cluster ◽  
Agb Stars ◽  
Low Mass

scholarly journals Impact of rotation and magnetic fields in low mass AGB stars

2018 ◽  
Vol 940 ◽  
pp. 012038
Author(s):  
J W den Hartogh ◽  
R Hirschi ◽  
C Georgy ◽  
P Eggenberger ◽  
F Herwig ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Agb Stars ◽  
Low Mass

scholarly journals The Production of Low Mass Carbon Stars; Carbon-rich Dredge Up or Oxygen-rich Mass Loss?

1989 ◽  
Vol 106 ◽  
pp. 229-231
Author(s):  
R.E. Stencel ◽  
J.E. Pesce ◽  
K.M. MacGregor
Keyword(s):  
Mass Loss ◽  
Selective Removal ◽  
Secondary Effect ◽  
Agb Stars ◽  
Radiative Force ◽  
Conventional Theory ◽  
Carbon Stars ◽  
Solar Mass ◽  
Low Mass ◽  
Momentum Coupling

AbstractConventional theory explains the origin of carbon stars as due to dredge up of carbon enriched material from the stellar core during helium flash events late in the life of solar mass AGB stars (e.g. Boothroyd and Sackmann 1988). This relatively efficient process however, seems to produce a larger C/O ratio than observed (Lambert et al. 1987). A secondary effect which could contribute to the appearance of carbon stars, is the selective removal of oxygen from the atmosphere by radiative force expulsion of oxygen rich dust grains (e.g. silicates like [Mg, Fe2SiO4]). We present calculations for this scenario which evaluate the degree of momentum coupling between the grains and gas under the thermodynamical conditions of AGB star atmospheres.

scholarly journals ISOCAM and DENIS Survey of 0.5 square degrees in the Bar of the LMC. Detection of the whole TP-AGB Star Population

1999 ◽  
Vol 191 ◽  
pp. 561-566
Author(s):  
C. Loup ◽  
E. Josselin ◽  
M.-R. Cioni ◽  
H.J. Habing ◽  
J.A.D.L. Blommaert ◽  
...  
Keyword(s):  
Mass Loss ◽  
High Mass ◽  
Evolutionary Models ◽  
Agb Stars ◽  
Complete Sample ◽  
High Mass Loss ◽  
Low Mass ◽  
The One ◽  
Good Agreement ◽  
Loss Rates

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⊙.

scholarly journals A detailed look at chemical abundances in the Magellanic Clouds

2011 ◽  
Vol 7 (S283) ◽  
pp. 502-503
Author(s):  
Richard A. Shaw ◽  
Ting-Hui Lee ◽  
Letizia Stanghellini ◽  
James E. Davies ◽  
D. Anibal García-Hernández ◽  
...  
Keyword(s):  
Direct Methods ◽  
Abundance Ratio ◽  
Type I ◽  
Large Set ◽  
Agb Stars ◽  
Correction Factors ◽  
Chemical Abundances ◽  
Elemental Abundances ◽  
Low Metallicity ◽  
Low Mass

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

Magnetic fields in PNe and other evolved low-mass and intermediate-mass stars

2016 ◽  
Vol 12 (S323) ◽  
pp. 136-140
Author(s):  
Laurence Sabin ◽  
Qizhou Zhang ◽  
Gregg A. Wade ◽  
Agnès Lèbre ◽  
Roberto Vázquez
Keyword(s):  
Magnetic Fields ◽  
Planetary Nebulae ◽  
Agb Stars ◽  
Intermediate Mass ◽  
Efficient Mechanism ◽  
Intermediate Mass Stars ◽  
Low Mass

AbstractMagnetic fields are likely to be an efficient mechanism which can affect evolved intermediate mass stars (i.e. post-AGB stars and planetary nebulae) in different ways such as via the shaping of their envelope. However, observational probes for the presence of those fields are still scarce. I will present a summary of the works, including those from our group, on the detection and measurement of magnetic fields in various evolved objects.

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