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| - HST/COS search for far-ultraviolet signatures
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| - The gaseous debris disk of the white dwarf SDSS J1228+1040
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| - Context. Gaseous and dust debris disks around white dwarfs (WDs) are formed from tidally disrupted planetary bodies. This offers an opportunity to determine the composition of exoplanetary material by measuring element abundances in the accreting WD’s atmosphere. A more direct way to do this is through spectral analysis of the disks themselves. Aims. Currently, the number of chemical elements detected through disk emission-lines is smaller than that of species detected through lines in the WD atmospheres. We assess the far-ultraviolet (FUV) spectrum of one well-studied object (SDSS J122859.93 +104032.9) to search for disk signatures at wavelengths < 1050 Å, where the broad absorption lines of the Lyman series effectively block the WD photospheric flux. In addition, we investigate the Ca ii infrared triplet (IRT) line profiles to constrain disk geometry and composition. Methods. We performed FUV observations (950 −1240 Å) with the Hubble Space Telescope/Cosmic Origins Spectrograph and used archival optical spectra. We compared them with non-local thermodynamic equilibrium model spectra. Results. No disk emission-lines were detected in the FUV spectrum, indicating that the disk effective temperature is Teff ≈ 5000 K. The long-time variability of the Ca ii IRT was reproduced with a precessing disk model of bulk Earth-like composition, having a surface mass density of 0.3 g cm -2 and an extension from 55 to 90 WD radii. The disk has a spiral shape that precesses with a period of approximately 37 years, confirming previous results.
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