  The presence of titanium oxide (TiO) and vanadium oxide (VO) gas phase
species is searched for in the atmosphere of the hot Jupiter HD209458b. We
compared a model for the planets transmitted spectrum to multi-wavelength
eclipse-depth measurements (from 3000 to 10000 Angstrom), obtained by Sing et
al. (2008a) using archived HST-STIS time series spectra. We make use of these
observations to search for spectral signatures from extra absorbers in the
planet atmosphere between 6000 and 8000 Angstrom. Along with sodium depletion
and Rayleigh scattering recently published for this exoplanet atmosphere, an
extra absorber of uncertain origin, redward of the sodium lines, resides in the
atmosphere of the planet. Furthermore, this planet has a stratosphere
experiencing a thermal inversion caused by the capture of optical stellar flux
by absorbers that resides at altitude. Recent models have predicted that the
presence of TiO and VO in the atmosphere of HD209458b may be responsible for
this temperature inversion. Although no specific TiO and VO spectral band head
signatures have been identified unambiguously in the observed spectrum, we
suggest here that the opacities of those molecules are possible candidates to
explain the remaining continuous broad band absorption observed between 6200
and 8000 Angstrom. To match reasonably well the data, the abundances of TiO and
VO molecules are evaluated from ten to one thousand times below solar. This
upper limit result is in agreement with expected variations with altitude due
to depletion effects such as condensation.
