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 T \mathrm { \hskip { -1.72 pt } { } _ { eff } } \approx 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 .

Conclusions :