Density Effects on the Opacity of Hydrogen at White Dwarf Atmospheric Conditions

The Gaia mission’s discovery of thousands of ultra-cool white dwarf stars has revealed a puzzling

contradiction in stellar physics where these stellar remnants exhibit anomalously low inferred masses

(≈ 0.3 M⊙) and temperatures (≈ 3,000 K) that fundamentally challenge our understanding of stellar

evolution. As cosmic chronometers that help determine galactic age, accurate mass measurements

are crucial for constraining stellar lifetimes and galactic history. We hypothesize that the mass dis-

crepancies arise from inaccurate opacity calculations at high densities, particularly for the hydrogen

anion (H−), which is expected to be highly sensitive to density effects. Our computational model-

ing of hydrogen-dominated, isotropic planar atmospheres examines whether density-dependent H−

opacity variations can reconcile observational data with theoretical predictions, offering a pathway

toward resolving systematic errors in white dwarf mass determinations and improving our fun-

damental understanding of stellar evolution timescales. Our radiative transfer modeling predicts

density-dependent changes in absorption coefficients and light attenuation through white dwarf at-

mospheres, supporting experimental design to test our hypothesis. This work was performed under

the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under

Contract DE-AC52-07NA27344.