  The so-called transition discs provide an important tool to probe various
mechanisms that might influence the evolution of protoplanetary discs and
therefore the formation of planetary systems. One of these mechanisms is
photoevaporation due to energetic radiation from the central star, which can in
principal explain the occurrence of discs with inner cavities like transition
discs. Current models, however, fail to reproduce a subset of the observed
transition discs, namely objects with large measured cavities and vigorous
accretion. For these objects the presence of (multiple) giant planets is often
invoked to explain the observations. In our work we explore the possibility of
X-ray photoevaporation operating in discs with different gas-phase depletion of
carbon and show that the influence of photoevaporation can be extended in such
low-metallicity discs. As carbon is one of the main contributors to the X-ray
opacity, its depletion leads to larger penetration depths of X-rays in the disc
and results in higher gas temperatures and stronger photoevaporative winds. We
present radiation-hydrodynamical models of discs irradiated by internal
X-ray+EUV radiation assuming Carbon gas-phase depletions by factors of 3,10 and
100 and derive realistic mass-loss rates and profiles. Our analysis yields
robust temperature prescriptions as well as photoevaporative mass-loss rates
and profiles which may be able to explain a larger fraction of the observed
diversity of transition discs.
