Metallicity, planet formation and disc lifetimes

The lifetime of protoplanetary discs is intimately linked to the mechanism responsible for their dispersal. Since the formation of planets within a disc must operate within the time frame of disc dispersal, it is crucial to establish what is the dominant process that disperses the gaseous component of discs around young stars. Planet formation itself as well as photoevaporation by energetic radiation from the central young stellar object has been proposed as plausible dispersal mechanisms. There is, however, still no consensus as what the dominant process may be. In this paper, we use the different metallicity dependence of X-ray photoevaporation and planet formation to discriminate between these two processes. We study the effects of metallicity, Z, on the dispersal time-scale, t(phot), in the context of a photoevaporation model, by means of detailed thermal calculations of a disc in hydrostatic equilibrium irradiated by extreme ultraviolet and X-ray radiation from the central source. Our models show t(phot) alpha Z(0.52) for a pure photoevaporation model. By means of analytical estimates, we derive instead a much stronger negative power dependence on metallicity of the disc lifetime for a dispersal model based on planet formation. A census of disc fractions in lower metallicity regions should therefore be able to distinguish between the two models. A recent study by Yasui et al. in low-metallicity clusters of the extreme outer Galaxy ([O/H] similar to -0.7 dex and dust-to-gas ratio of similar to 0.001) provides preliminary observational evidence for shorter disc lifetimes at lower metallicities, in agreement with the predictions of a pure photoevaporation model. While we do not exclude that planet formation may indeed be the cause of some of the observed discs with inner holes, these observational findings and the models and analysis presented in this work are consistent with X-ray photoevaporation as the dominant disc dispersal mechanism. We finally develop an analytical framework to study the effects of metallicity-dependent photoevaporation on the formation of gas giants in the core accretion scenario. We show that accounting for this effect strengthens the conclusion that planet formation is favoured at higher metallicity. We find, however, that the metallicity dependence of photoevaporation only plays a secondary role in this scenario, with the strongest effect being the positive correlation between the rate of core formation and the density of solids in the disc.