Molecules with ALMA at Planet-forming Scales (MAPS) IV: Emission
  Surfaces and Vertical Distribution of Molecules
  The Molecules with ALMA at Planet-forming Scales (MAPS) Large Program
provides a unique opportunity to study the vertical distribution of gas,
chemistry, and temperature in the protoplanetary disks around IM Lup, GM Aur,
AS 209, HD 163296, and MWC 480. By using the asymmetry of molecular line
emission relative to the disk major axis, we infer the emission height ($z$)
above the midplane as a function of radius ($r$). Using this method, we measure
emitting surfaces for a suite of CO isotopologues, HCN, and C$_2$H. We find
that $^{12}$CO emission traces the most elevated regions with $z/r > 0.3$,
while emission from the less abundant $^{13}$CO and C$^{18}$O probes deeper
into the disk at altitudes of $z/r \lesssim 0.2$. C$_2$H and HCN have lower
opacities and SNRs, making surface fitting more difficult, and could only be
reliably constrained in AS 209, HD 163296, and MWC 480, with $z/r \lesssim
0.1$, i.e., relatively close to the planet-forming midplanes. We determine peak
brightness temperatures of the optically thick CO isotopologues and use these
to trace 2D disk temperature structures. Several CO temperature profiles and
emission surfaces show dips in temperature or vertical height, some of which
are associated with gaps and rings in line and/or continuum emission. These
substructures may be due to local changes in CO column density, gas surface
density, or gas temperatures, and detailed thermo-chemical models are necessary
to better constrain their origins and relate the chemical compositions of
elevated disk layers with those of planet-forming material in disk midplanes.
This paper is part of the MAPS special issue of the Astrophysical Journal
Supplement.
