Chemical Evolution of Cool White Dwarf’s Atmospheres

Abstract

Spectroscopy and photometry (B,V,R,I,J,H,K) of 110 cool white dwarfs have been analyzed with state-of-the-art model atmospheres appropriate for cool white dwarfs with pure hydrogen, pure helium, as well as mixed H/He compositions. The observed energy distributions are obtained from a combination of both optical BV RI and infrared J H K photometric data, and used to derive the effective temperature and the atmospheric composition of each star. Masses for 60 white dwarfs with known trigonometric parallaxes, are also derived.

Most cool white dwarfs turned out to have energy distributions which are well reproduced by either pure hydrogen or pure helium models, with very few objects showing mixed atmospheric compositions. Our results reveal an inhomogeneous temperature distribution of hydrogen-rich and helium-rich white dwarfs, with the presence of a non-DA gap in the range 5000 ≲ Teff ≲ 6000 K. Hydrogen-rich white dwarfs reappear below 5000 K, and are present down to the lowest temperatures covered by this sample.