High ions towards white dwarfs: Circumstellar line shifts and stellar temperature

Aims. Our aim is to gain new insights into highly ionized gas towards nearby hot white dwarfs (WDs). The detection of absorption lines of highly ionized interstellar (IS) species in their spectra is the main diagnostic tool of the hot IS gas phase. This requires disentangling IS ions from photospheric or circumstellar (CS) ions, and thus their simultaneous study. This is particularly timely due to the recent discovery of infra-red emitting disks around WDs and the recognition that accretion from those disks can explain the presence of photospheric heavy ions, thus opening the exciting prospect of measuring the composition of extra-solar disks and planetary systems. Methods. We present far UV spectra of three nearby white dwarfs recorded with the Cosmic Object Spectrograph on board the HST. Absorption lines of several ions at various ionization stages are detected, and we investigate their origin by means of their kinematical properties, in combination with previous measurements with the FUSE satellite. We supplement with these new results a large set of archival white dwarfs UV data and we analyze the line shifts. Results. Interstellar CIV, SiIV, NV and OVI ions are likely detected at distance from the Plane towards WD1040+492 (230 pc), but within 100 parsecs there are no detections of IS ions. Small absorptions generated at cloud-hot gas interfaces may be present but are within our detection limit. The main result is the detection of the CIV, SiIV and NV counterparts to the redshifted OVI lines measured towards WD2257-073 and WD1040+492. Based on their velocities, they very likely trace absorbing circumstellar material, extending earlier results. More surprisingly, CS absorption is also detected at small redshift towards WD1942+499, and potentially towards WD2257-073. Comparisons of the COS CIII and CIV data with atmospheric models preclude a stellar origin for these features. We thus present first evidence for a temporal variability of the CS lines and for a case of double absorption. Overall these results demonstrate the ubiquity of CS absorptions and favor a large interval of CS line shifts. Based on the compilation of OVI, CIV, SiIV and NV data from IUE, FUSE, GHRS, STIS, and COS, we derive an anti-correlation between the stellar temperature and the high ion velocity shift w.r.t. to the photosphere, with positive (resp. negative) velocity shifts for the cooler (resp. hotter) white dwarfs. This trend probably reflects more than a single process, however such a dependence on the WD’s temperature again favors a CS origin for a very large fraction of those ion absorptions, and a form of balance between radiation pressure and gravitation. This is consistent with ubiquitous evaporation of orbiting dusty material. In view of these results, ion measurements close to the photospheric or the IS velocity should be interpreted with caution, especially for stars at intermediate temperatures. While tracing CS gas, they may be erroneously attributed to photospheric material or to the ISM, explaining the difficulty of finding a coherent pattern of the high ions in the local IS 3D distribution