Li-rich Giants in LAMOST Survey. III. The statistical analysis of Li-rich giants

TL;DR

This study analyzes the properties and evolutionary stages of Li-rich giant stars using LAMOST survey data, revealing insights into their origins and the processes affecting lithium enrichment.

Contribution

It introduces a neural network method to classify giant stars into evolutionary stages and provides a comprehensive statistical analysis of Li-rich giants.

Findings

Most Li-rich RGB stars are descendants of Li-rich pre-RGB stars or result from planet engulfment.

Massive Li-rich SRC stars likely result from Li depletion in high-mass RGB stars.

Li-rich phenomena on PRC can be explained by internal mixing processes near the helium flash.

Abstract

The puzzle of Li-rich giant is still unsolved, contradicting the prediction of the standard stellar models. Although the exact evolutionary stages play a key role in the knowledge of Li-rich giants, a limited number of Li-rich giants have been taken with high-quality asteroseismic parameters to clearly distinguish the stellar evolutionary stages. Based on the LAMOST Data Release 7 (DR7), we applied a data-driven neural network method to derive the parameters for giant stars, which contain the largest number of Li-rich giants. The red giant stars are classified into three stages of Red Giant Branch (RGB), Primary Red Clump (PRC), and Secondary Red Clump (SRC) relying on the estimated asteroseismic parameters. In the statistical analysis of the properties (i.e. stellar mass, carbon, nitrogen, Li-rich distribution, and frequency) of Li-rich giants, we found that: (1) Most of the Li-rich RGB stars are suggested to be the descendants of Li-rich pre-RGB stars and/or the result of engulfment of planet or substellar companions; (2) The massive Li-rich SRC stars could be the natural consequence of Li depletion from the high-mass Li-rich RGB stars. (3) Internal mixing processes near the helium flash can account for the phenomenon of Li-rich on PRC that dominated the Li-rich giants. Based on the comparison of [C/N] distributions between Li-rich and normal PRC stars, the Li-enriched processes probably depend on the stellar mass.