Studies on metal-poor stars and chemical enrichment of the Galaxy

Abstract

The low- and intermediate-mass stars are the major inhabitants of our Galaxy. They play an important role in the chemical evolution of the Galaxy. These stars have enriched the ISM with the products of various nucleosynthesis processes. They pass through the Asymptotic Giant Branch (AGB) phase during their evolution, and it is during this phase that the richest nucleosynthesis occurs. AGB stars are the predominant sites for s-process nucleosynthesis and major producers of 12C, 13C, 14N, F, Na, Mg, etc. in the Galaxy. So, understanding the nucleosynthesis and evolution of AGB stars is of primary importance. Studies on the Galactic chemical evolution remain incomplete without considering the yields from AGB stars. However, the theoretical uncertainties associated with the AGB nucleosynthesis and observational constraints of the AGB stars make their direct studies difficult. In this regard, the AGB binary system, where these stars have transferred their products of AGB nucleosynthesis to the now observed secondary through binary mass-transfer, could be of help. These extrinsic stars in the binary system, such as Ba stars, CH stars along with their more metal-poor counterparts, Carbon-Enhanced Metal-Poor (CEMP) stars, could be used as a tool to investigate the AGB nucleosynthesis. The abundance data available for metal-poor stars has been used extensively to constrain the Galactic chemical evolution. However, the sparse data available for the heavy element abundances in the lower metallicity underscores the need for a detailed study of a larger sample. A major motivation of our work is to understand the diverse abundance pattern observed for the heavy elements in the metal-poor stars that still remains poorly understood. We have attempted to understand the s-process nucleosynthesis, as well as the physical properties of the companion stars, through a detailed analysis of observed elemental abundances of a selected sample of metal-poor stars. The main objective of the thesis is to understand the role of metal-poor stars in i the chemical enrichment of the Galaxy and to provide observational constraints to the AGB nucleosynthesis theories from an analysis of a sample of extrinsic metal-poor stars at different metallicity. The problem is addressed through a spectroscopic analysis of three sets of metal-poor stellar samples: i) a comparatively metal-rich Ba stars sample; ii) a moderately metal-poor CH stars sample; and iii) a very metal-poor CEMP-s and CEMP-r/s stars sample. The procedures we have adopted to address the problem and the results obtained from each set of stellar samples investigated are briefly outlined here: • Stellar sample: As a first step towards our goal, we have performed an extensive literature survey of potential metal-poor stars including, Ba, CH, and CEMP stars. We have selected a sample of metal-poor star candidates from various sources in the literature and from the catalogue of carbon stars identified from the HES, HK and LAMOST surveys. The objects selected are then subjected to high-resolution spectroscopic analysis by obtaining their spectra from various observing facilities. The detailed chemical abundance analysis is performed using the recent version of the radiative transfer code MOOG that assumes Local Thermodynamic Equilibrium (LTE) conditions and using model atmospheres selected from the Kurucz grid of model atmospheres (Chapter 2). • We present the first time abundance analysis of the objects BD−19 132, BD−19 290, HD 30443, LAMOSTJ091608.81+230734.6, HE 1304−2111, HE 1354−2257, BD+19 3109 and HD 179832. Even though the abundances of Fe and C derived from the medium-resolution spectra are available in the literature for the stars HE 0457−1805, HE 0920−0506, and HE 1327−2116, we present the first time high-resolution abundance analysis for these objects. The abundances of a few light elements derived from high-resolution IR spectra for the object LAMOSTJ151003.74+305407.3 are available in the literature, but we present the first time detailed abundance analysis for this object based on high-resolution optical spectra. • Our sample contains 23 stars covering a metallicity [Fe/H] range of −2.86 to +0.23. The program stars are found to belong to the main-sequence and giant phase of stellar-evolution. Our analysis shows that they are enhanced in neutroncapture elements and are likely binaries, pointing to their surface chemical composition being influenced by pollution from the companions. The abundances of several elements C, N , O, Na, Al, α-elements, Fe-peak elements and several neutron-capture elements such as Rb, Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, and Eu are determined. The carbon isotopic ratio, 12C/13C, an important mixing indicator, is also measured whenever possible. • We have investigated the mass of companion AGB stars using several diagnostics, such as C, N, O, Na, and Mg abundances, [hs/ls] ratio, and [Rb/Zr] ratio. Rb plays a unique role as a diagnostic of the neutron density at the s-process site. In the low neutron density branch, Rb is the only element available to the stellar spectroscopists as a neutron densitometer. However, the important neutron density dependent abundance ratio, [Rb/Zr], is not explored much in the literature to determine the mass of the AGBs. We have explored this ratio for our stellar sample, to investigate the characteristics of their companion AGB stars. • Analysis of Ba star sample: In order to understand the chemical and kinematic properties of the program stars, as a first step, a detailed chemical and kinematic analysis of a sample of ten Ba star candidates based on high-resolution spectra obtained from HCT/HESP, VLT/UVES, and ESO-MPG/FEROS is carried out. The stellar sample covers the metallicity range −0.55 ≤ [Fe/H] ≤ +0.23, temperature range 4550 - 6350 K, log g range 2.20 - 4.28, and microturbulent velocity range 0.63 - 1.59 km s−1 . We have derived the neutron-density dependent [Rb/Zr] ratios to investigate the neutron source in the former companion AGB stars. The detection of the Rb I line at 7800.259 Å in the spectra of four program stars allowed us to determine the [Rb/Zr] ratio for these objects. The negative values obtained for this ratio in these stars indicate the operation of 13C(α, n)16O reaction. As this reaction occurs in the low-mass AGB stars, we confirm that the former companions of these stars are low-mass AGB stars with M ≤ 3 M⊙. A comparison of the observed abundances with the predictions from FRUITY models, also confirms low-mass for the former companion AGB stars. The lack of Na and Mg enhancement, combined with the positive [hs/ls] ratio values, rules out 22Ne(α, n)25Mg as the neutron-source and corroborate the low-mass AGB companions for the Ba stars. The kinematic analysis shows that Ba stars are members of the Galactic disk (Chapter 3). The main results of this study are published in the papers Shejeelammal and Goswami (2019); Shejeelammal et al. (2020); Shejeelammal and Goswami (2020). • Analysis of CH, CEMP-s and CEMP-r/s star sample: After completing the analysis of the sample of Ba stars, we have extended our analysis to the lowmetallicity regime to understand the role of low-metallicity stars in the Galactic chemical enrichment and have performed a detailed spectroscopic analysis of a selected sample of CH and CEMP stars. The metallicity ranges of CH stars (3 objects) and CEMP stars (10 objects) are −0.89 ≤ [Fe/H] ≤ −0.75 and −2.86 ≤ [Fe/H] ≤ −1.57 respectively. The stellar sample has a temperature in the range of 4005 - 5380 K, log g in the range of 0.61 - 2.65, and micro-turbulent velocity in the range of 0.63 - 3.45 km s−1 . The high-resolution spectra of these objects were obtained using HCT/HESP, Mercator/HERMES, and SUBARU/HDS. Our analysis based on different diagnostics confirms the low-mass companions of the program stars. The possible origin(s) of the CEMP-r/s stars in our sample are also investigated by carefully analyzing their observed abundance pattern. The continuity in CEMP-s and CEMP-r/s stars in terms of various abundance ratios with respect to metallicity indicates that the astrophysical site responsible for the origin of these two classes of stars may be the same. Our analysis confirms that a modified s-process, namely the intermediate neutron-capture process, the i-process, is responsible for the observed abundance patterns in CEMP-r/s stars. The abundance patterns of the CEMP-r/s stars in our sample are well reproduced by the model predictions of the i-process in low-mass, low-metallicity AGB stars. The parametric model-based analysis using the FRUITY models performed for the CH and CEMP-s stars also confirms low-mass for the former AGB companions. Kinematic analysis shows that CH stars belong to the Galactic disk and the majority of CEMP stars are members of the Galactic halo (Chapter 4). The main results of this study are published in the papers Purandardas et al. (2019); Shejeelammal et al. (2021); Shejeelammal and Goswami (2021).