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tokens: scenario is that the gas and ice chemically evolve while accreting onto the disk ( Visser et al. 2009 , 2011 ; Drozdovskaya et al. 2014 , 2016 ). A final scenario is known as reset; this is when all of the molecules are dissociated into their base elements ( Visser et al. 2009 ; Pontoppidan et al. 2014 ; Eistrup et al. 2016 ). The distinction between any of the scenarios is not important to the discussion of C/O, after all, the number of carbon and oxygen atoms in the gas and ice does not change. Nonetheless, which scenario initializes the chemical state of the gas disk can have a drastic impact on the chemical evolution of its volatiles ( Eistrup et al. 2016 ). In this work we follow the initial volatile elemental abundance of Eistrup et al. (2016) , where C/O = 0.34, and assume thatthe volatile molecules are inherited from the pre-stellar core. For now, we similarly assume that the initial chemical state of the refractories follows the ISM, meaning that there was no chemical interaction between the gas and refractory dust during the collapse of the cloud. If the chemical composition of the dust is representative of the ISM then it has an atomic carbon-to-silicon ratio C/Si ISM = 6 ( Bergin et al. 2015 ). Based on chemical equilibrium simulations of condensing refractories in protoplanetary disks, Alessi et al. (2017) found that the most abundant silicates have silicon-to-oxygen ratios no lower than ~ 1∕4, i.e., mostly enstatite (MgSiO 3 ) or forsterite (MgSiO 4 ). While they produce oxygen-bearing iron species (fayalite – Fe 2 SiO 4 , ferrosilite – FeSiO 3 ) within 5 AU, the majority of iron-bearing materials are troilite (FeS) and pure iron. When iron-oxides become more abundant, it is at the expense of enstatite, meaning that C/O remains unchanged, implying that refractories are generally carbon rich relative to oxygen (C/O > 1). If we assume that the dust is composed of only enstatite (Si/O = 1∕3, C/O = 2) and the Sun accretes gas and dustwith the characteristic gas-to-dust ratio (100) then we can estimate the photospheric C/O. The mean molecular weight of the gas is 2.4 g mol −1 , while the molecular weight of the dust is 172.4 g mol −1 (6 carbon atoms per enstatite molecule − m mol,enstatite = 100.4 g mol −1 ). Hence, Table 1 Carbon-to-hydrogen (C/H) and carbon-to-oxygen (C/O) ratios for the volatiles (gas), refractories (gr), and combined (tot) given the simple model (mdl) where dust mass is dominated by enstatite and solid carbon (with C/Si = 6). Notes. We note that C/O tot = (C gas + C ref )/(O gas + O ref ). where n dust is the number of groupings of six carbon atoms and one enstatite molecule. Therefore (see a summary in Table 1 ), and hence The volatile carbon and oxygen abundances of Eistrup et al. (2016) are n C,gas = 1.8 × 10 −4 n H , and n O,gas = 5.2 × 10 −4 n H . Combining these values with our computed elemental abundances from our simple refractory model, we predict a solar photosphere with C/O = 0.52, and C/Si = 15,