2000A&A...357...37L

We report on Hubble Space Telescope Faint Object Spectrograph observations of the double QSO HS 1216+5032 AB (z_e(A)=1.455; z_e(B)=1.451, and angular separation θ=9.1"). The spectral coverage is 910Å to 1340Å in the QSO rest-frame. An unusual broad-absorption-line (BAL) system is observed only in the B component: maximum outflow velocity of ∼5000km/s; probably a mixture of broad and narrow components. Observed ions are: HI, CII, CIII, NIII, NV, OVI, and possibly SIV and SVI. We also discuss two outstanding intervening systems: (1) a complex CIV system at z=0.72 of similar strength in A and B, with a velocity span of 1500km/s along the lines of sight (LOSs; LOS separation: S_{{perpendicular}}[z=0.72]≃75 h₅₀^–1kpc); and (2) a possible strong MgII system at z=0.04 observed in B only, presumably arising in a damped Lyα system. We assume HS 1216+5032 is a binary QSO but discuss the possibility of a gravitational lens system. The size of Lyα forest clouds is constrained using S_{{perpendicular}}≃80h₅₀^–1kpc at redshifts between z=1.15 and 1.45. Four Lyα systems not associated with metal lines and producing lines with W₀>0.17Å are observed in both spectra, while five appear in only one spectrum. This sample, although scarce due to the redshift path blocked out by the BALs in B, allows us to place upper limits on the transverse cloud sizes. Modelling the absorbers as non-evolving spheres, a maximum-likelihood analysis yields a most probable cloud diameter D=256 h₅₀^–1kpc and 2σ bounds of 172<D<896 h₅₀^–1kpc. If the clouds are modelled as filamentary structures, the same analysis yields lower transverse dimensions by a factor of two. Independently of the maximum-likelihood approach, the equivalent width differences provide evidence for coherent structures. The suggestion that the size of Lyα forest clouds increases with decreasing redshift is not confirmed. Finally, we discuss two z_a≃z_e Lyα systems observed in both QSO spectra.