  Solar-like oscillations are stochastically excited by turbulent convection at
the surface layers of the stars. We study the role of the surface metal
abundance on the efficiency of the stochastic driving in the case of the CoRoT
target HD 49933. We compute two 3D hydrodynamical simulations representative --
in effective temperature and gravity -- of the surface layers of the CoRoT
target HD 49933, a star that is rather metal poor and significantly hotter
compared to the Sun. One 3D simulation has a solar metal abundance and the
other has a surface iron-to-hydrogen, [Fe/H], abundance ten times smaller. For
each 3D simulation we match an associated global 1D model and we compute the
associated acoustic modes using a theoretical model of stochastic excitation
validated in the case of the Sun and Alpha Cen A. The rate at which energy is
supplied per unit time into the acoustic modes associated with the 3D
simulation with [Fe/H]=-1 are found about three times smaller than those
associated with the 3D simulation with [Fe/H]=0. As shown here, these
differences are related to the fact that low metallicity implies surface layers
with a higher mean density. In turn, a higher mean density favors smaller
convective velocities and hence less efficient driving of the acoustic modes.
Our result shows the importance of taking the surface metal abundance into
account in the modeling of the mode driving by turbulent convection. A
comparison with observational data is presented in a companion paper using
seismic data obtained for the CoRoT target HD 49933.
