[Rb/Zr] ratio in Ba stars as a diagnostic of the companion AGB stellar mass
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J. Astrophys. Astr. (2020)41:37 https://doi.org/10.1007/s12036-020-09654-7
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[Rb/Zr] ratio in Ba stars as a diagnostic of the companion AGB stellar mass J. SHEJEELAMMAL*
and ARUNA GOSWAMI*
Indian Institute of Astrophysics, Sarjapur Road, Koramangala, Bengaluru 560 034, India. *Corresponding author. E-mail: [email protected]; [email protected] MS received 30 August 2020; accepted 17 September 2020 Abstract. Understanding nucleosynthesis in and evolution of asymptotic giant branch (AGB) stars is of primary importance as these stars are the main producers of some of the key elements in the Universe. They are the predominant sites for slow-neutron-capture nucleosynthesis (s-process). The exact physical conditions and nucleosynthetic processes occurring in the interior of AGB stars are not clearly understood, and that hinders better understanding of the contribution of these stars to the Galactic chemical enrichment. Extrinsic-variable stars that are known to have received products of AGB phase of evolution via binary masstransfer mechanisms are vital tools in tracing AGB nucleosynthesis. The [Rb/Zr] ratio is an important diagnostic to understand the average neutron density at the s-process site and provides important clues to the mass of companion AGB stars in binaries. In this work we have presented estimates of [Rb/Zr] ratios based on high-resolution spectroscopic analysis for a sample of Ba stars, and discussed how the ratio can be used to understand the characteristics of the AGB star. Results from an analysis based on a parametric model to confirm the mass of the companion AGB star are also presented. Keywords. Stars—abundance—stars—chemically peculiar—stars—nucleosynthesis.
1. Introduction Barium (Ba II) stars are peculiar G and K type stars which were first identified by Bidelman and Keenan (1951). Their surface chemical composition is characterized by overabundance of elements heavier than iron and C/O \ 1 (Barbuy et al. 1992; Allen and Barbuy 2006; Drake and Pereira 2008; Pereira and Drake 2009). They exhibit abnormally strong lines of s-process (slowneutron-capture process) elements such as Ba II at k ˚ , Sr II at k 4077 A ˚ , as well as enhanced CH, CN 4554 A and C2 molecular bands. They are mostly in their mainsequence and giant phase of stellar evolution. Nucleosynthesis theories do not support occurrence of heavyelement nucleosynthesis during the stellar evolutionary phases to which these stars belong. It is known that asymptotic giant branch (AGB) stars are the major producers of s-process elements in
This article is part of the Topical Collection: Chemical elements in the Universe: Origin and evolution.
the Universe (Busso et al. 1999). The s-process-enriched materials produced in the interiors of AGB stars are brought to the surface through a third dredge-up (TDU) process. The observed overabundance of heavy elements at the surface of Ba stars could not be attributed to an intrinsic origin as they are not luminous enough to undergo s-p
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