  The gaseous envelope of an accreting rocky planet becomes hot enough to
sublimate silicates and other refractory minerals. For this work, we studied
the effect of the resulting envelope enrichment with a heavy vapor species on
the composition and temperature of the envelope. For simplification, we used
the gas-phase molecule SiO to represent the sublimation of silicate material.
We solved the equilibrium structure equations in 1D for planets in the mass
range of $0.1$ to $3\,M_\oplus$. The convective stability criterion was
extended to take the stabilizing effect of the condensation of SiO clouds into
account. We assumed that the envelope is both in hydrostatic equilibrium and in
vapor equilibrium with the underlying magma ocean. This means that pebbles do
not undergo sublimation in the envelope and therefore survive until they plunge
into the magma ocean. We find that the emergence of an inner radiative region,
where SiO condensation suppresses convection, increases the pressure and
temperature in the inner envelope compared to pure H$_2$/He envelopes once
$M_\mathrm{pl} \gtrsim 0.3\,M_\oplus$. For $M_\mathrm{pl}>0.75\,M_\oplus$, the
temperature and pressure close to the surface reach the supercritical point of
SiO. The amount of SiO stored in the envelope is lower than the total planet
mass for low mass planets. However, for $M_\mathrm{pl}>2.0\,M_\oplus$, all
accreted pebble material must contribute to maintain the vapor equilibrium in
the envelope. Therefore, the non-vapor mass of the planet ceases to increase
beyond this threshold. Overall, our vapor equilibrium model of the planetary
envelope allows for direct core growth by pebble accretion up to much higher
masses than previously thought.
