  Models of core accretion assume that in the radiative zones of accreting gas
envelopes, radiation diffuses. But super-Earths/sub-Neptunes (1-4$R_\oplus$,
2-20$M_\oplus$) point to formation conditions that are optically thin: their
modest gas masses are accreted from short-lived and gas-poor nebulae
reminiscent of the transparent cavities of transitional disks. Planetary
atmospheres born in such environments can be optically thin to both incident
starlight and internally generated thermal radiation. We construct
time-dependent models of such atmospheres, showing that
super-Earths/sub-Neptunes can accrete their $\sim$1%-by-mass gas envelopes, and
super-puffs/sub-Saturns their $\sim$20%-by-mass envelopes, over a wide range of
nebular depletion histories requiring no fine tuning. Although nascent
atmospheres can exhibit stratospheric temperature inversions effected by atomic
Fe and various oxides that absorb strongly at visible wavelengths, the rate of
gas accretion remains controlled by the radiative-convective boundary (rcb) at
much greater pressures. For dusty envelopes, the temperature at the rcb $T_{\rm
rcb} \simeq 2500$ K is still set by ${\rm H}_2$ dissociation; for dust-depleted
envelopes, $T_{\rm rcb}$ tracks the temperature of the visible or thermal
photosphere, whichever is deeper, out to at least $\sim$5 AU. The rate of
envelope growth remains largely unchanged between the old radiative diffusion
models and the new optically thin models, reinforcing how robustly super-Earths
form as part of the endgame chapter in disk evolution.
