Polarization in the B band (left) and standard deviation of difference Hα profile (right) as a function of disk-base density ρ₀ for the residual disk models from Table 5 (model 1: full line; model 2: dashed line). The standard deviation is the root mean square of the disk model spectrum minus the pure photospheric profile. The horizontal dot-dashed lines represent the observational limits of average polarization and difference profile determined from observations performed at the epoch of interferometric observations, as explained in Sect. 3. While the measured Hα difference profile level does not impose a limit to ρ₀, the measured average polarization sets a strict upper limit to this quantity: ρ₀< 0.52 × 10^-12 g cm^-3 for the thin-disk model and ρ₀< 0.65 × 10^-12 g cm^-3 for the thick-disk model.