Determination of trans-iron element abundances in the hitherto largest sample of hot white dwarfs

• Applicant: Dr. Nicole Reindl
• Subject Area: Astrophysics and Astronomy
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 578934844

The origins and distribution of chemical elements across the universe remain among the most compelling questions in physics. In particular, understanding the formation of trans-iron group elements (TIEs, Z > 28) via different neutron capture processes, and the astrophysical conditions enabling their synthesis, represent a vibrant, interdisciplinary research frontier. While TIEs are primarily thought to form through slow (s) or rapid (r) neutron capture, recent stellar abundance measurements challenged our view of heavy element production. Over the past decade, mainly driven by mounting observations of C-enhanced metal-poor stars, it has become evident that an intermediate neutron-capture process – the i-process – must also exist, though its astrophysical mechanism remains poorly understood. This proposal aims to derive TIE abundances in the largest sample of hot white dwarfs to date. These short-lived stars represent the beginning of the end of most stars. The majority has H-rich atmospheres, and either solar TIE abundance patterns or signatures of the s-process can be expected. Yet, ≈20% of the hottest white dwarfs possess He-dominated atmospheres as a result of a very late thermal pulse. Furthermore, H-rich white dwarfs with M<0.5M_sun may form after a late He-core flash. Both scenarios are promising i-process engines, marking He-rich and low mass H-rich white dwarfs as promising candidates to investigate i-process nucleosynthesis imprints. Detecting such signatures could lead to a transformation of our understanding of the still novel i-process and how it operates in stars. Thus far, the detection of TIEs has been confirmed in only ten hot white dwarfs, with lines of 19 TIEs identified. Yet, a comprehensive understanding of their abundance patterns remains elusive. This project will increase available TIE abundance data by a factor of five, transitioning from isolated cases to a broad, systematic study. It will leverage existing high-quality observational datasets, NLTE model atoms for highly ionized TIEs developed over the past decade, and a new model grid for H-rich white dwarfs. By computing additional atmospheric models, conducting kinematic analyses, and modeling diffusion processes, we will, for the first time, trace how TIE abundances evolve with temperature and stellar history. Prof. Dr. Falk Herwig will join as a Mercator Fellow, developing advanced stellar evolution and nucleosynthesis models tailored to our stars, incorporating both s- and i-processes. This collaboration will significantly reduce uncertainties in TIE abundance predictions from the i-process, crucial for disentangling the contributions of atomic, nuclear, and stellar physics to the peculiar TIE patterns observed in hot white dwarfs.

• DFG Programme: Research Grants
• International Connection: Canada
• Cooperation Partner: Professor Dr. Falk Herwig