Unevolved Li-rich stars at low metallicity: A possible formation pathway through novae

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
² Institute of Astronomy (IvS), KU Leuven, Celestijnenlaan 200D, 3001, Leuven, Belgium
³ Center for Information Science, Fukui Prefectural University, 4-1-1 Matsuoka Kenjojima, Eiheiji-cho, Fukui 910-1195, Japan
⁴ Department of Astronomy, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA
⁵ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
⁶ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
⁷ CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Beijing 100101, China
⁸ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China ;
Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK

^⋆ Corresponding author: matsuno@uni-heideberg.de

Received: 25 February 2025
Accepted: 14 May 2025

Abstract

A small fraction of low-mass stars have been found to have anomalously high Li abundances. Although it has been suggested that mixing during the red giant branch phase can lead to Li production, this method of intrinsic Li production cannot explain Li-rich stars that have not yet undergone the first dredge-up. To obtain clues about the origin of such stars, we present a detailed chemical abundance analysis of four unevolved Li-rich stars with −2.1<[Fe/H]<−1.3 and 2.9<A(Li)<3.6, 0.7−1.4 dex higher Li abundances than the ones of typical unevolved metal-poor stars. One of the stars, Gaia DR3 6334970766103389824 (D25_6334), was serendipitously found in the stellar stream ED-3. The other three stars have been reported to have massive (M≳1.3 M_⊙) nonluminous companions. We show that three of the four stars exhibit abundance patterns similar to the ones of known unevolved Li-rich stars, namely normal abundances in most elements except for Li and Na. These abundance similarities suggest a common origin for the unevolved Li-rich stars and low-mass metal-poor stars with massive compact companions. We also made the first detection of N abundance to unevolved Li-rich stars in D25_6334, and found that it is significantly enhanced ([N/Fe] = 1.3). The observed abundance pattern of D25_6334, spanning from C to Si, indicates that its surface has been polluted by a former intermediate-mass companion star or a nova system that involves a massive ONe white dwarf. Using a population synthesis model, we show that the nova scenario can lead to the observed level of Li enhancement and also provide an explanation for Li-rich stars without companions and those with massive compact companions.