Ultra-low-mass and small-radius white dwarfs made of heavy elements

Seven possible ultra-low-mass and small-radius white dwarfs have been recently identified, with masses ranging from ∼0.02 M⊙ to ∼0.08 M⊙ and radii ranging from ∼ 4,270 km to 10670 km. The mass–radius measurements of these white dwarfs pose challenges to traditional white dwarf models, assuming they are mostly made of nuclei lighter than 56Fe. In this work, we consider the possibility that those white dwarfs are made of heavier elements. Due to the small charge-to-mass ratios in heavy elements, the electron number density in white dwarf matter is effectively reduced, which reduces the pressure with additional contributions of lattice energy and electron polarization corrections. This consequently leads to white dwarfs with much smaller masses and radii, which coincide with the seven ultra-low-mass and small-radius white dwarfs. The mass of the most massive white dwarfs is effectively reduced and could possibly account for the sub-Chandrasekhar progenitors in underluminous Type Ia supernovae. The corresponding equation of state and matter contents of dense stellar matter with and without reaching the cold-catalyzed ground state are presented, which are obtained using the latest Atomic Mass Evaluation (AME 2020). Further observations are necessary to unveil the actual matter contents in those white dwarfs via, e.g., spectroscopy, asteroseismology, and the discoveries of other ultra-low-mass and small-radius white dwarfs.