Origin of lithium enrichment in K giants

TL;DR

This study identifies and analyzes lithium-rich K giants, revealing their concentration at the luminosity of the clump and suggesting internal stellar processes like the Cameron-Fowler mechanism as the origin of lithium enrichment.

Contribution

First large low-resolution spectroscopic survey of Li-rich K giants across the entire luminosity range, discovering new Li-rich giants and analyzing their distribution and possible origins.

Findings

15 new Li-rich K giants discovered

Li-rich giants concentrated at the luminosity of the clump

Li enhancement unlikely due to planet swallowing

Abstract

In this Letter, we report on a low-resolution spectroscopic survey for Li-rich K giants among 2000 low mass (M <= 3 Msun) giants spanning the luminosity range from below to above the luminosity of the clump. Fifteen new Li-rich giants including four super Li-rich K giants (A(Li) >= 3.2) were discovered. A significant finding is that there is a concentration of Li-rich K giants at the luminosity of the clump or red horizontal branch. This new finding is partly a consequence of the fact that our low-resolution survey is the first large survey to include giants well below and above the RGB bump and clump locations in the HR diagram. Origin of the lithium enrichment may be plausibly attributed to the conversion of 3He via 7Be to 7Li by the Cameron-Fowler mechanism but the location for onset of the conversion is uncertain. Two possible opportunities to effect this conversion are discussed: the bump in the first ascent of the red giant branch (RGB) and the He-core flash at the tip of the RGB. The finite luminosity spread of the Li-rich giants serves to reject the idea that Li enhancement is, in general, a consequence of a giant swallowing a large planet.