The lithium-rich giant stars puzzle: New observational trends for a general-mass-loss scenario

Observatorio Nacional (ON), MCT, Rua José Cristino 77, Sao Cristovao, Rio de Janeiro, Brazil

^⋆ Corresponding author; ramirodelareza@yahoo.com

Received: 31 July 2024
Accepted: 18 November 2024

Abstract

The existence of one percent of lithium-rich giant stars among normal, lithium-poor giant stars continues to be poorly explained. By merging two catalogues – one containing 10,535 lithium-rich giant stars with lithium abundances ranging from 1.5 to 4.9 dex, and the other detecting infrared sources – we have found 421 clump giant stars and 196 first-ascending giant stars with infrared excesses indicating stellar mass losses. The clump stars are the most lithium-rich. Approximately 5.8 percent of these stars appear to episodically lose mass in periods of approximately 10⁴ years or less, while the remaining stars ceased their mass loss and maintained their lithium for nearly 10⁷ years. We propose a scenario in which all giant stars with masses below two solar masses undergo prompt lithium enrichment with mass-ejection episodes. We suggest that the mass loss results from internal angular-momentum transport. It is possible that a transitory instability, perhaps of magnetic origin, rapidly transports the nuclear material responsible for the lithium enrichment to the stellar surface and triggers shell ejections. Additionally, the strong mass loss in some lithium-rich stars during their evolution activates their chromospheres, as observed in ultraviolet spectra. Furthermore, intense episodical mass losses in these stages led to the observable formation of complex organic and inorganic particles, as detected in near-infrared spectra. In contrast to first-ascending giant stars, helium flashes during the clump can contribute to additional lithium enrichment alongside the aforementioned process. The combination of these two lithium sources may explain the much higher observed lithium abundances in clump stars, as well as their observed infrared excesses. If our scenario – based on a universal and rapid lithium enrichment episode process – is correct, it could explain the rarity of lithium-rich giant stars.