Abundance Patterns in Hot Subdwarf B Stars: The Case of Carbon

One of the most puzzling phenomena observed at the surface of sdB stars is their abundance anomalies. For instance, high‐resolution optical spectroscopy shows that the C abundance is lower than the solar abundance by about a factor of ten on average. A few stars, however, show a solar abundance or an abundance higher than solar. Although optical spectroscopy allows the measurement of relatively high C abundance, it fails to provide stringent upper limits when no C lines are detected. The Far Ultraviolet Spectroscopic Explorer (FUSE) comes to the rescue when no C lines are detected in the optical, because it is 1,000 times more sensitive than the VLT for the purpose of measuring very small amounts of C in the atmospheres of sdB stars. We measure the C abundance in the FUSE spectra of 30 stars. We confirm the C abundance trend reported in previous studies, but report the disappearance of C in five stars. The upper limits that we measure for these stars are about four orders of magnitude lower than the solar...