Weak G-band stars, or carbon-deficient red giants (CDGs), characterised by weak or absent G-band absorption of the CH molecule driven by low carbon abundances, have remained mysterious for decades. Here, we present a significant increase in the known CDG population identified through systematic spectroscopic surveys. This expanded sample confirms their rarity (<1% of giants). Combining asteroseismology (Kepler, TESS), spectroscopy (APOGEE, LAMOST), and astrometry (Gaia), we unveil their properties: predominantly low-mass (M < 2 Msun) and residing in the red giant clump phase, contrasting prior suggestions of intermediate-mass progenitors (2.5 - 5.0 Msun). Distinct chemical patterns and luminosities subdivide CDGs into three groups. Normal-luminosity CDGs exhibit signatures of non-equilibrium CN-cycle burning. Over-luminous groups display varying degrees of CN-cycle burning, with one group additionally enriched in sodium and the other exhibiting He-burning product dredge-up followed by further CN(O) cycling, resulting in extreme carbon depletion and nitrogen enrichment. We propose a merger scenario involving a helium white dwarf and a red giant branch star for the over-luminous groups. The origin of normal-luminosity CDGs remains ambiguous, with possibilities including core He-flash pollution or lower-mass mergers. Finally, the strong correlation between CDGs and Li-rich giants suggests a potential link between their formation processes.
https://doi.org/10.48550/arXiv.2308.14303
https://doi.org/10.48550/arXiv.2308.15919
BIO
Sunayana Maben is a post-doctoral researcher at the National Astronomical Observatories of China (NAOC), where she also completed her Ph.D. in 2024 under the supervision of Professor Gang Zhao. Her research focuses on the chemical anomalies in stars, particularly addressing the weak G-band/carbon-deficiency puzzle within the Milky Way Galaxy. In addition to this, Sunayana's work extends to studying lithium-rich giants, driven by her passion for uncovering the chemical signatures in peculiar stars. This research aims to enhance our understanding of stellar evolution and the broader chemical history of our Galaxy.
Host: Pablo Renard
