Extended Data Fig. 6: The retrieved thermal profile for W1534 extended to deeper pressures to extrapolate to the SiH₄ quench region.

The black line with grey error bar represents the retrieved thermal profile for W1534. The colored contours represent the equilibrium abundance of silane calculated for an atmosphere with [M/H]=-2.22 and C/O=0.26, with the red dashed line corresponding to the  ~ 20 ppb obtained from the retrieval. The condensation curves of enstatite and forsterite are overplotted, and demonstrate the rapid decrease in the SiH₄(g) abundance at lower temperatures (i.e., above the cloud base). The thermal profile was extended to deeper pressures in three ways: by extrapolation from the trend of the retrieved profile (lower dashed line), an adiabatic extension from the deepest point of the retrieved profile (middle dashed line), and an adiabatic extension from the  ~ 10 bar level of the retrieved profile (top dashed line). The white dashed curve indicates the SiO-SiH₄ equal abundance boundary (cf.¹⁴). Highlighted as filled ovals are the vertical mixing rates parameterized by K_zz (labelled in units of cm² s⁻¹) that indicate where quenching will occur along each thermal profile.