ABUNDANCES OF NEUTRON-CAPTURE ELEMENTS IN THE HOT EXTREME HELIUM STARS V1920 CYGNI AND HD 124448

摘要

Analysis of Hubble Space Telescope Space Telescope Imaging Spectrograph ultraviolet spectra of two hot extreme helium (EHe) stars, V1920 Cyg and HD 124448, provides the first measurements of abundances of neutron-capture elements for EHe stars. Although the two stars have similar abundances for elements up through the iron group, they differ strikingly in their abundances of heavier elements: V1920 Cyg is enriched by a factor of 30 in light neutron-capture elements (Y/Fe and Zr/Fe) relative to HD 124448. These differences in abundances of neutron-capture elements among EHe stars are exhibited by the R CrB stars and are evidence supporting the view that there is an evolutionary connection between these two groups of hydrogen-deficient stars. Also, the abundances of Y and Zr in V1920 Cyg provide evidence that at least one EHe star went through an s-process synthesis episode in its earlier evolution.