It is a challenge to study the formation process of massive stars: their
formation time is short, they are few, often deeply embedded, and at relatively
large distances. Our strategy is to study the outcome of the star formation
process and to look for signatures remnant of the formation. We have access to
a unique sample of (massive) pre-main-sequence (PMS) stars in the giant HII
region M17, showing a photosphere and circumstellar disk. The aim is to
determine the variability properties of the hot gaseous disks to understand the
physical origin of the emission lines and identify dominant physical processes
in these disks. We have obtained multiple-epoch (4-5 epochs) VLT/X-shooter
spectra of six young stars in M17 covering about a decade. Using stacked
spectra we update the spectral classification and identify circumstellar
features. With the temporal variance method (TVS) we determine the extent and
amplitude of the spectral line variations. The double-peaked emission lines in
the PMS stars with gaseous disks are used to determine peak-to-peak velocities,
V/R-ratios and the radial velocity of the systems. We identify many disk
features, under which a new detection of CO bandhead and CI emission. In three
of the stars we detect spectral variability, mainly in lines originating in the
circumstellar disk, in a velocity range up to 320 km/s. In two PMS stars the
ratio between the blue and red peaks shows a correlation with the peak-to-peak
velocity, possibly explained by a spiral-arm structure in the disk. The PMS
stars with variability are at similar positions in the HRD but show significant
differences in disk lines and variability. The extent and timescale of the
variability differs for each star and per line (sets). We find indications for
an accretion flow, slow disk winds and/or disk structures in the hot gaseous
inner disk as the cause of the variability in these PMS stars.Comment: 27 pages, 24 figures, accepted for publication in Astronomy and
Astrophysics, abstract abbreviate