  Ultra-hot jupiters (UHJs) are giant planets on short orbital periods with
high equilibrium temperature (T_eq) values. Their hot, extended atmospheres are
perfect laboratories for transmission spectroscopy studies based on
high-resolution spectrographs. In recent years, a variety of atoms and
molecules were found in their atmospheres, using different methods such as
cross-correlation or transmission and emission spectroscopy. Here, we present
the studies of six ultra-hot Jupiters: HAT-P-57b, KELT-7b, KELT-17b, KELT-21b,
MASCARA-1b, and WASP-189b, based on high-resolution observations obtained with
HARPS-N and HARPS spectrographs. By applying line and cross-correlation
transmission spectroscopy methods, we searched for the absorption features of a
broad range of atomic and molecular species. We did not detect any absorption
features in our sample of UHJs, with the exception of WASP-189b, for which we
detected FeI, FeII, and TiI using cross-correlation. The transmission
spectroscopy of single lines for WASP-189b revealed several absorption features
(including Halpha, Hbeta and Ca H&K), but they remain tentative pending a
better modelling of the gravity darkening deformation of the
Rossiter-McLaughlin effect. The non-detections with regard to the rest of the
planets can be explained via a combination of stellar pulsations and the
Rossiter-McLaughlin effect, which mask possible planetary signals for most of
the planets, and by the low signal-to-noise ratios (S/N) of the observations
for KELT-21b. Here, we compare our results with the known population of planets
for which atmospheric detections have been reported in the literature. We find
that the empirical frontier between hot and ultra-hot planets, based on the
detection of atomic and ionized species in their atmospheres, can be
established as Teq = 2150 K.
