  This thesis presents the results of a high spectral resolution study of the
circumstellar (CS) gas around the intermediate mass, pre-main sequence UXOR
stars BF Ori, SV Cep, UX Ori, WW Vul and XY Per. The results are based on a set
of 38 echelle spectra covering the spectral range 3800-5900 Angstroms,
monitoring the stars on time scales of months, days and hours.
  All spectra show a large number of Balmer and metallic lines with variable
blueshifted and redshifted absorption features superimposed to the photospheric
stellar spectra. Synthetic Kurucz models are used to estimate rotational
velocities, effective temperatures and gravities of the stars. The best
photospheric models are subtracted from each observed spectrum to determine the
variable absorption features due to the circumstellar gas; those features are
characterized, via multigaussian fitting, in terms of their velocity, v,
dispersion velocity, Delta_v, and residual absorption, R_max.
  The absorption components detected in each spectrum can be grouped by their
similar radial velocities and are interpreted as the signature of the dynamical
evolution of gaseous clumps. Most of the events undergo
accelerations/decelerations at a rate of tenths of ms-2. The typical timescale
for the duration of the events is a few days. The dispersion velocity and the
relative absorption strength of the features do not show drastic changes during
the lifetime of the events, which suggests that they are gaseous blobs
preserving their geometrical and physical identity.
  See full abstract in the document
