Chandra and FUSE spectroscopy of the hot bare stellar core H 1504+65

H 1504+65 is an extremely hot hydrogen-deficient white dwarf with an effective temperature close to 200 000 K. We present new FUV and soft X-ray spectra obtained with FUSE and Chandra, which confirm that H 1504+65 has an atmosphere primarily composed of carbon and oxygen. The Chandra LETG spectrum (60–160 Å) shows a wealth of photospheric absorption lines from highly ionized oxygen, neon, and – for the first time identified in this star – magnesium and suggests relatively high Ne and Mg abundances. This corroborates an earlier suggestion that H 1504+65 represents a naked C/O stellar core or even the C/O envelope of an O-Ne-Mg white dwarf.