Metal Enrichment Leads to Low Atmospheric C/O Ratios in Transiting Giant Exoplanets

We predict the carbon-to-oxygen (C/O) ratios in the hydrogen-helium envelope and atmospheres of a sample of nearly 50 relatively cool ($T_{\mathrm eq}<$ 1000 K) transiting gas giant planets. The method involves planetary envelope metallicity estimates that use the structure models of Thorngren et al. (2016) and the disk and planetary accretion model of \"Oberg et al. (2011). We find that nearly all of these planets are strongly metal-enriched which, coupled with the fact that solid material is the main deliverer of metals in the protoplanetary disk, implies that the substellar C/O ratios of their accreted solid material dominate compared to the enhanced C/O ratio of their accreted gaseous component. We predict that these planets will have atmospheres that are typically reduced in their C/O compared to parent star values independent of the assessed formation locations, with C/O $<1$ a nearly universal outcome within the framework of the model. We expect water vapor absorption features to be ubiquitous in the atmospheres of these planets, and by extension, other gas giants.