ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT) II:
  Vertical stratification of CO, CS, CN, H$_2$CO, and CH$_3$OH in a Class I
  disk
  The chemical composition of planets is inherited from that of the
protoplanetary disk at the time of planet formation. Increasing observational
evidence suggests that planet formation occurs in less than 1 Myr. This
motivates the need for spatially resolved spectral observations of Class I
disks, as carried out by the ALMA chemical survey of Disk-Outflow sources in
Taurus (ALMA-DOT). In the context of ALMA-DOT, we observe the edge-on disk
around the Class I source IRAS 04302+2247 (the butterfly star) in the 1.3mm
continuum and five molecular lines. We report the first tentative detection of
methanol (CH$_3$OH) in a Class I disk and resolve, for the first time, the
vertical structure of a disk with multiple molecular tracers. The bulk of the
emission in the CO 2-1, CS 5-4, and o-H$_2$CO 3(1,2)-2(1,1) lines originates
from the warm molecular layer, with the line intensity peaking at increasing
disk heights, $z$, for increasing radial distances, $r$. Molecular emission is
vertically stratified, with CO observed at larger disk heights (aperture
$z/r\sim0.41-0.45$) compared to both CS and H$_2$CO, which are nearly cospatial
($z/r\sim0.21-0.28$). In the outer midplane, the line emission decreases due to
molecular freeze-out onto dust grains (freeze-out layer) by a factor of >100
(CO) and 15 (CS). The H$_2$CO emission decreases by a factor of only about 2,
which is possibly due to H$_2$CO formation on icy grains, followed by a
nonthermal release into the gas phase. The inferred [CH$_3$OH]/[H$_2$CO]
abundance ratio is 0.5-0.6, which is 1-2 orders of magnitude lower than for
Class 0 hot corinos, and a factor ~2.5 lower than the only other value inferred
for a protoplanetary disk (in TW Hya, 1.3-1.7). Additionally, it is at the
lower edge but still consistent with the values in comets. This may indicate
that some chemical reprocessing occurs in disks before the formation of planets
and comets.
