ABUNDANCE OF ELEMENTS BEYOND THE IRON GROUP IN COOL DO WHITE DWARFS

摘要

We report the presence of elements beyond the iron group in the atmospheres of the cool DO white dwarfs HD 149499 B and HZ 21. Photospheric lines of germanium (Z = 32), arsenic (33), selenium (34), tin (50), tellurium (52), iodine (53), and perhaps bromine (35) are observed in ultraviolet spectra of HD 149499 B obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE), the Goddard High Resolution Spectrograph (GHRS), and the International Ultraviolet Explorer. Germanium, arsenic, and tellurium are also observed in FUSE and GHRS spectra of HZ 21. Light elements such as carbon, silicon, phosphorus, and sulfur are present in the atmospheres of both stars. Nitrogen is detected in HZ 21 but not in HD 149499 B. We calculated a grid of synthetic spectra covering the range of effective temperatures, surface gravities, and element abundances appropriate for a detailed line profile analysis. The measured mass fractions of elements heavier than iron reveal overabundances ranging from about a factor of 3 to a factor of 1000 relative to the Sun. The heavy-element enrichment in both stars could be the result of slow neutron capture nucleosynthesis, which would have occurred during the asymptotic giant branch phase of their evolution.