Optical and NIR spectroscopy of cool CEMP stars to probe the nucleosynthesis in low mass AGB binary system

TL;DR

This study analyzes the chemical abundances of seven CEMP stars using optical and NIR spectroscopy to understand their nucleosynthesis origins, revealing binary mass transfer and neutron-capture processes, including the i-process, in low-mass AGB systems.

Contribution

It demonstrates the effectiveness of combining optical and NIR spectroscopy for detailed abundance analysis of cool CEMP stars, clarifying their nucleosynthesis pathways and subclassifications.

Findings

NIR spectra complement optical data for oxygen and carbon isotope ratios.

Binary mass transfer from low-mass AGB stars explains observed abundances.

Some stars show signatures of the i-process in neutron-capture elements.

Abstract

We present the abundance analyses of 7 Carbon enhanced metal-poor (CEMP) stars to understand the origin of carbon in them. We used high-resolution optical spectra to derive abundances of various elements. We also used low-resolution Near-Infrared (NIR) spectra to derive the abundance of O and 12C/13C from the CO molecular band and compared their values with those derived from high-resolution optical spectra. We identified a good agreement between the values. Thus, in cool CEMP stars, the NIR observations complement the high-resolution optical observations to derive the oxygen abundance and the 12C/13C ratio. This enables us to probe fainter cool CEMP stars using NIR spectroscopy. C, N, O abundances of all the program stars in this study show abundances that are consistent with binary mass transfer from a low-mass low-metallicity Asymptotic Giant Branch (AGB) companion which is further supported by the presence of enhancement in neutron-capture elements and detection of radial velocity variation. One of the stars show abundance patterns similar to a CEMP-s star whereas the abundance pattern of the rest of the stars satisfy the criteria required to classify them as CEMP-r/s stars. The sub-classification of some of the stars studied here is revisited. The abundance of neutron capture elements in these CEMP-r/s stars resembles to that of i-process models where proton ingestion episodes in the companion low-mass low-metallicity AGB stars produce the necessary neutron density required for the onset of i-process.