Tracing the origin of heavy elements through metal-poor stars

Abstract

The hierarchical mass assembly of galaxies and the formation timescales of their substructures are important topics in astrophysics and cosmology. These aspects can be explored in the local universe by studying resolved stars. By analyzing individual stars and stellar populations, one can estimate their ages. Although the color-magnitude diagram of co-evolved stellar populations aids in age determination, dating field stars is more complex. Typically, the iron content (or metallicity) of stars serves as an age indicator. However, metallicity is influenced by the history and rate of star formation. Analyzing multiple chemical elements can provide additional insights. Yet, using elements heavier than iron, particularly those formed by rapid neutron capture, hinges on understanding their astrophysical origins. Identifying these origins remains a significant challenge and a key question in nuclear astrophysics. This thesis investigates chemical compositions of metal-poor stars, aiming to understand their origins and chemical evolution. Halo stars, which are some of the Galaxy’s oldest, offer insights into the astrophysical sites of element production due to their minimal pollution by progenitor stars. Utilizing high-resolution spectroscopic analysis through telescopes such as the 10-m GTC, 8.4-m VLT, 10-m KECK, and 2-m HCT, and supplementing with low-resolution spectra from LAMOST, this study analyzes the detailed abundances of approximately 50 stars. These stars, with metallicities ranging from −3.2 to −1.8, include many classified as very metal-poor and extremely metalpoor, with their chemical compositions examined for the first time. The findings are systematically presented across the thesis. The initial findings of the thesis include a comprehensive abundance analysis of four r-process enhanced (RPE) stars, utilizing the HORuS spectrograph at 10-m GTC. These stars are in the metallicity range of −2.3 to −1.9. The high SNR of the spectrograph enable determining the abundances of 16 light elements and 20 neutroncapture elements, including thir peak element, Os. We identified Th in two objects, with [Th/Fe] values of 0.65 and 0.6, respectively, which helped us estimate their ages. The study discusses the metallicity trends of elements such as Mg, Sr, Ba, Eu, Os, and Th in both r-II and r-I objects, using a compilation of RPE objects from existing literature. We carried out a detailed line-by-line differential analysis comparing a moderately RPE object (r-I: HD107752) with an extremely RPE object (r-II: CS31082-0001) to explore the potential shared origins of their heavy element nucleosynthesis. This part of the study utilized high-resolution and high SNR spectra from the ESO-VLT’s UVES instrument, sourced from the ESO data archive. We identified three distinct patterns in the differential abundance analysis. The similar abundances of light elements up to zinc in both stars suggest a shared origin for these elements, with no odd-even variation observed. In the case of neutron-capture elements, r-I stars exhibit slightly depleted light r-process elements and more significantly depleted heavier r-process elements, challenging the theory of a single production site. We also provide plausible scenarios for their r-process enrichment. Additionally, we performed a kinematic analysis of nearly 466 r-process-enhanced stars compiled from literature, examining their origins and locations within the Milky Way. We compare the significance of our orbit-based classification of stars into different Galactic components with the Toomre diagram-based classification. Our findings indicate that RPE stars are equally distributed between the disk and halo of the Galaxy. We also utilized archival data from the ESO and KECK telescopes to explore similarities between CEMP-r/s and r-process objects and to investigate the origin of thorium in CEMP-r/s stars using machine learning algorithms. We observed that r-I and r-II stars do not distinctly separate into two groups; rather, there is an intermediate group that may consist of diluted or mixed stars. Our analysis indicate that CEMP-r/s and RPE stars are separate classes of objects. Additionally, we used the HESP, installed on the Himalayan Chandra Telescope to observe some metal-poor stars (the HESP-GOMPA Survey). This led to the discovery of numerous very metal-poor, extremely metal-poor, CEMP-r, and RPE stars. We also study the kinematics of these stars. These findings are detailed in Chapter 7 of the thesis. Our study indicates that within our range of metallicities, neutron stars mergers and supernovae are the primary sites contributing to the chemical composition of halo stars.