2008A&A...478..739B

Luminous quasars are known to display a sharp steepening of the continuum near 1100Å. This spectral feature is not well fitted by current accretion disk models, unless Comptonization of the disk emission is invoked. Absorption by carbon crystalline dust has been proposed to account for this feature. Ton34 (z_q=1.928) exhibits the steepest far-UV decline (F_ν∝ν^–5.3) among the 183 quasar HST-FOS spectra analyzed by Telfer et al. (2002ApJ...565..773T). It is an ideal object to test the crystalline dust hypothesis as well as alternative interpretations of the UV break.We reconstruct the UV spectral energy distribution of Ton34 by combining HST, IUE and Palomar spectra. The far-UV continuum shows a very deep continuum trough, which is bounded by a steep far-UV rise. We fit the trough assuming nanodiamond dust grains.Extinction by carbon crystalline dust reproduces the deep absorption trough of Ton34 reasonably well, but not the observed steep rise in the extreme UV. We also study the possibility of an intrinsic continuum rollover. The dust might be part of a high velocity outflow (≃13000km/s), which is observed in absorption in the lines of CIVλ1549, OVIλ1035, NVλ1240 and Lyα.