scholarly journals VLBI monitoring of Mira variables with VERA

2007 ◽  
Vol 3 (S242) ◽  
pp. 326-327
Author(s):  
A. Nakagawa ◽  
T. Omodaka ◽  
K. M. Shibata ◽  
T. Kurayama ◽  
H. Imai ◽  
...  
Keyword(s):  
Monitoring Program ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Current Analysis ◽  
Mira Variables ◽  
Vlbi Observations ◽  
The Galaxy ◽  
Absolute Magnitudes ◽  
Infrared Monitoring ◽  
Good Coverage

AbstractWe have started a VLBI monitoring program for Asymptotic Giant Branch (AGB) stars at 22 and 43 GHz as part of a project of the VLBI Exploration of Radio Astrometry (VERA) for precisely obtaining the period-luminosity (PL) relation of Galactic Mira variables. Using accurate distances measured with VERA, we reveal PL relation in the Galaxy based on the absolute magnitudes of the sources. We selected the sources for VLBI monitoring so that they have a good coverage of various pulsation periods. Photometry in the infrared J, H, and K bands for over 600 AGB stars has also started since 2003 with the 1m telescope of Kagoshima University to obtain the pulsation periods and magnitudes. Current analysis of the phase referencing VLBI observations of S Crt shows that the parallax of 2.3±0.2 milliarcsec (mas) corresponds to a distance of 435+41−35 pc. From the infrared monitoring data, pulsation periods and magnitudes in K band for 248 sources were obtained.

scholarly journals Distances of Stars by mean of the Phase-lag Method

2017 ◽  
Vol 13 (S336) ◽  
pp. 381-384
Author(s):  
Sandra Etoka ◽  
Dieter Engels ◽  
Eric Gérard ◽  
Anita M. S. Richards
Keyword(s):  
Monitoring Program ◽  
High Sensitivity ◽  
Mass Range ◽  
Asymptotic Giant Branch ◽  
Circumstellar Envelope ◽  
Phase Lag ◽  
Agb Stars ◽  
The Galaxy ◽  
Term Monitoring

AbstractVariable OH/IR stars are Asymptotic Giant Branch (AGB) stars with an optically thick circumstellar envelope that emit strong OH 1612 MHz emission. They are commonly observed throughout the Galaxy but also in the LMC and SMC. Hence, the precise inference of the distances of these stars will ultimately result in better constraints on their mass range in different metallicity environments. Through a multi-year long-term monitoring program at the Nancay Radio telescope (NRT) and a complementary high-sensitivity mapping campaign at the eMERLIN and JVLA to measure precisely the angular diameter of the envelopes, we have been re-exploring distance determination through the phase-lag method for a sample of stars, in order to refine the poorly-constrained distances of some and infer the currently unknown distances of others. We present here an update of this project.

scholarly journals On the Mass and Metallicity Distributions of the Parent AGB Stars of O-Rich Presolar Stardust Grains

2009 ◽  
Vol 26 (3) ◽  
pp. 271-277 ◽  
Author(s):  
Larry R. Nittler
Keyword(s):  
Molecular Clouds ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Isotopic Data ◽  
Intermediate Mass ◽  
Presolar Grains ◽  
Model Predictions ◽  
The Galaxy ◽  
Low Metallicity ◽  
The Masses

AbstractPresolar grains in meteorites formed in a sample of Asymptotic Giant Branch (AGB) stars that ended their lives within ≈1 Gyr of the origin of the Solar System 4.6 Gyr ago. The O-isotopic compositions of presolar O-rich stardust reflect the masses and metallicities of their parent stars. We present simple Monte Carlo simulations of the parent AGB stars of presolar grains. Comparison of model predictions with the grain data allow some broad conclusions to be drawn: (1) Presolar O-rich grains formed in AGB stars of mass ∼1.15–2.2 M⊙. The upper-mass cutoff reflects dredge-up of C in more massive AGB stars, leading to C-rich dust rather than O-rich, but the lack of grains from intermediate-mass AGB stars (>4 M⊙) is a major puzzle; (2) The grain O-isotopic data are reproduced well if the Galaxy in presolar times was assumed to have a moderate age-metallicity relationship, but with significant metallicity scatter for stars born at the same time; (3) The Sun appears to have a moderately low metallicity for its age and/or unusual 17O/16O and 18O/16O ratios for its metallicity; and (4) The Solar 17O/18O ratio, while unusual relative to present-day molecular clouds and protostars, was not atypical for the presolar disk and does not require self-pollution of the protosolar molecular cloud by supernova ejecta.

scholarly journals Chemical Peculiarities, Mass Loss, and Final Evolution of AGB Stars in the Magellanic Clouds

1988 ◽  
Vol 108 ◽  
pp. 31-37
Author(s):  
P.R. Wood
Keyword(s):  
Mass Loss ◽  
Great Majority ◽  
Magellanic Clouds ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Long Period ◽  
Evolved Stars ◽  
The Galaxy ◽  
Shed Light ◽  
Final Evolution

The Magellanic Clouds are sufficiently close that evolved stars which exhibit chemical peculiarities and the effects of mass loss can be readily observed. Such objects include carbon stars, S stars, long-period variables, OH/IR stars and planetary nebulae. Because of the relatively well-known distances of the Magellanic Clouds, the intrinsic luminosities of these objects can be accurately determined, in contrast to the situation in the Galaxy where the great majority of asymptotic giant branch (AGB) stars occur in the field population. In this review, observations of AGB stars in the Magellanic Clouds will be discussed with particular reference to those features which can shed light on mass loss and chemical peculiarities resulting from stellar evolution.

scholarly journals Implications of Including AGB Dust in SPS Models

2015 ◽  
Vol 11 (A29B) ◽  
pp. 169-170
Author(s):  
Alexa Villaume ◽  
Charlie Conroy ◽  
Benjamin Johnson
Keyword(s):  
Galaxy Evolution ◽  
Dust Emission ◽  
Asymptotic Giant Branch ◽  
Circumstellar Dust ◽  
Agb Stars ◽  
Population Synthesis ◽  
Star Forming ◽  
The Galaxy ◽  
Ir Emission ◽  
Ir Light

AbstractThe IR emission from galaxies is a unique window into multiple aspects of galaxy evolution including star-formation rates, the age of galaxies, and galactic-scale dust processes. However, asymptotic giant branch (AGB) stars continue to introduce uncertainty into stellar population synthesis (SPS) models and limit our ability to interpret the IR light of galaxies. Here we focus on incorporating circumstellar dust around AGB stars in SPS models and understanding the extent to which they influence the IR light of galaxies. We find that the significance of the AGB dust contribution depends on the characteristics of the galaxy. For quiescent galaxies and metal-poor star forming galaxies, circumstellar dust emission can have a large effect, whereas for dusty star-forming galaxies the circumstellar emission is dwarfed by emission from dust in the ISM. The models with circumstellar dust also suggest, in agreement with previous work, that IR colors can be a powerful age diagnostic for older stellar systems. Models such as these will be essential for interpreting data that will be provided by JWST and other next generation IR facilities.

scholarly journals Characterizing the companion AGBs using surface chemical composition of barium stars

2019 ◽  
Vol 492 (3) ◽  
pp. 3708-3727 ◽  
Author(s):  
J Shejeelammal ◽  
Aruna Goswami ◽  
Partha Pratim Goswami ◽  
Rajeev Singh Rathour ◽  
Thomas Masseron
Keyword(s):  
Neutron Capture ◽  
Detailed Comparison ◽  
Slow Neutron ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Capture Process ◽  
Origin And Evolution ◽  
Element Abundances ◽  
The Galaxy ◽  
Low Mass

ABSTRACT Barium stars are one of the important probes to understand the origin and evolution of slow neutron-capture process elements in the Galaxy. These are extrinsic stars, where the observed s-process element abundances are believed to have an origin in the now invisible companions that produced these elements at their asymptotic giant branch (AGB) phase of evolution. We have attempted to understand the s-process nucleosynthesis, as well as the physical properties of the companion stars through a detailed comparison of observed elemental abundances of 10 barium stars with the predictions from AGB nucleosynthesis models, FRUITY. For these stars, we have presented estimates of abundances of several elements, C, N, O, Na, Al, α-elements, Fe-peak elements, and neutron-capture elements Rb, Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, and Eu. The abundance estimates are based on high resolution spectral analysis. Observations of Rb in four of these stars have allowed us to put a limit to the mass of the companion AGB stars. Our analysis clearly shows that the former companions responsible for the surface abundance peculiarities of these stars are low-mass AGB stars. Kinematic analysis has shown the stars to be members of Galactic disc population.

scholarly journals What can pre-solar grains tell us about AGB stars?

2006 ◽  
Vol 2 (14) ◽  
pp. 345-348
Author(s):  
Maria A. Lugaro ◽  
Susanne Höfner
Keyword(s):  
Neutron Capture ◽  
Current Trend ◽  
3D Models ◽  
Point Of View ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Capture Process ◽  
Model Predictions ◽  
The Galaxy ◽  
Isotopic Evolution

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.

scholarly journals Molecular Evolution from AGB Stars to Planetary Nebulae

2011 ◽  
Vol 7 (S280) ◽  
pp. 203-215 ◽  
Author(s):  
Sun Kwok
Keyword(s):  
Chemical Synthesis ◽  
Stellar Evolution ◽  
Planetary Nebulae ◽  
Active Phase ◽  
Molecular Ions ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
The Galaxy ◽  
Short Time ◽  
Late Stages

AbstractThe late stages of stellar evolution from the Asymptotic Giant Branch (AGB) to planetary nebulae represent the most active phase of molecular synthesis in a star's life. Over 60 molecular species, including inorganics, organics, radicals, chains, rings, and molecular ions have been detected in the circumstellar envelopes of evolved stars. Most interestingly, complex organic compounds of aromatic and aliphatic structures are synthesized over very short time intervals after the end of the AGB. Also appeared during the post-AGB evolution are the unidentified 21 and 30 μm emission features, which are believed to originate from carbonaceous compounds.The circumstellar environment is an ideal laboratory for the testing of theories of chemical synthesis. The distinct spectral behavior among AGB stars, proto-planetary nebulae (PPN), and planetary nebulae (PN) and the short evolutionary time scales that separate these stages pose severe constraints on models. In this paper, we will present an observational summary of the chemical synthesis in the late stages of stellar evolution, discuss chemical and physical processes at work, and speculate on the possible effects these chemical products have on the Galaxy and the Solar System.

scholarly journals The Missing Lead: Developments in the Lead (Pb) Discrepancy in Intrinsically s-Process Enriched Single Post-AGB Stars

Universe ◽  
2021 ◽  
Vol 7 (11) ◽  
pp. 446
Author(s):  
Devika Kamath ◽  
Hans Van Winckel
Keyword(s):  
Magellanic Clouds ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Chemical Abundance ◽  
Major Development ◽  
Upper Limits ◽  
The Galaxy ◽  
Process Elements ◽  
Detailed Chemical ◽  
Theoretical Developments

Lead (Pb) is predicted to have large over-abundances with respect to other s-process elements in Asymptotic Giant Branch (AGB) stars, especially of low metallicities. However, our previous abundance studies of s-process enriched post-Asymptotic Giant Branch (post-AGB) stars in the Galaxy and the Magellanic Clouds show a discrepancy between observed and predicted Pb abundances. For the subset of post-AGB stars with low metallicities the determined upper limits based on detailed chemical abundance studies are much lower than what is predicted. Recent theoretical studies have pointed to the occurrence of the i-process to explain the observed chemical patterns, especially of Pb. A major development, in the observational context, is the release of the GAIA EDR3 parallaxes of the post-AGBs in the Galaxy, which has opened the gateway to systematically studying the sample of stars as a function of current luminosities (which can be linked to their initial masses). In this paper, we succinctly review the Pb discrepancy in post-AGB stars and present the latest observational and theoretical developments in this research landscape.

scholarly journals Observational Evidence Points at AGB Stars as Possible Progenitors of CEMP-s and CEMP-r/s Stars*

2021 ◽  
Vol 922 (1) ◽  
pp. 28
Author(s):  
Meenakshi Purandardas ◽  
Aruna Goswami
Keyword(s):  
Asymptotic Giant Branch ◽  
Chemical Compositions ◽  
Agb Stars ◽  
Surface Chemical ◽  
Chemical Enrichment ◽  
Group I ◽  
Internal Mixing ◽  
The Galaxy ◽  
Halo Population ◽  
First Time

Abstract The origin of enhanced abundance of heavy elements observed in the surface chemical composition of carbon-enhanced metal-poor (CEMP) stars still remains poorly understood. Here, we present detailed abundance analysis of seven CEMP stars based on high-resolution (R ∼ 50,000) spectra that reveal enough evidence of asymptotic giant branch (AGB) stars being possible progenitors for these objects. For the objects HE 0110−0406, HE 1425−2052, and HE 1428−1950, we present for the first time a detailed abundance analysis. Our sample is found to consist of one metal-poor ([Fe/H] < −1.0) and six very metal-poor ([Fe/H] < −2.0) stars with enhanced carbon and neutron-capture elements. We have critically analyzed the observed abundance ratios of [O/Fe], [Sr/Ba], and [hs/ls] and examined the possibility of AGB stars being possible progenitors. The abundance of oxygen estimated in the program stars is characteristic of AGB progenitors except for HE 1429−0551 and HE 1447+0102. The estimated values of [Sr/Ba] and [hs/ls] ratios also support AGB stars as possible progenitors. The locations of the program stars in the absolute carbon abundance A(C) versus [Fe/H] diagram, along with the Group I objects, hint at the binary nature of the object. We have studied the chemical enrichment histories of the program stars based on abundance ratios [Mg/C], [Sc/Mn], and [C/Cr]. Using [C/N] and 12C/13C ratios, we have examined whether any internal mixing had modified their surface chemical compositions. Kinematic analysis shows that the objects HE 0110−0406 and HE 1447+0102 are thick-disk objects and the remaining five objects belong to the halo population of the Galaxy.

scholarly journals Lithium factories in the Galaxy: novae and AGB stars

2009 ◽  
Vol 5 (S268) ◽  
pp. 395-404 ◽  
Author(s):  
Francesca D'Antona ◽  
Paolo Ventura
Keyword(s):  
Globular Clusters ◽  
Second Generation ◽  
State Of The Art ◽  
The State ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Preliminary Results ◽  
The Galaxy

AbstractWe review the state of the art in modelling lithium production, through the Cameron–Fowler mechanism, in two stellar sites: during nova explosions and in the envelopes of massive asymptotic giant branch (AGB) stars. We also show preliminary results concerning the computation of lithium yields from super–AGBs, and suggest that super–AGBs of metallicity close to solar may be the most important galactic lithium producers. Finally, we discuss how lithium abundances may help to understand the modalities of formation of the “second generation” stars in globular clusters.

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