Molecules with ALMA at Planet-forming Scales (MAPS). IX. Distribution
  and Properties of the Large Organic Molecules HC$_3$N, CH$_3$CN, and
  $c$-C$_3$H$_2$
  The precursors to larger, biologically-relevant molecules are detected
throughout interstellar space, but determining the presence and properties of
these molecules during planet formation requires observations of protoplanetary
disks at high angular resolution and sensitivity. Here we present 0.3"
observations of HC$_3$N, CH$_3$CN, and $c$-C$_3$H$_2$ in five protoplanetary
disks observed as part of the Molecules with ALMA at Planet-forming Scales
(MAPS) Large Program. We robustly detect all molecules in four of the disks (GM
Aur, AS 209, HD 163296 and MWC 480) with tentative detections of $c$-C$_3$H$_2$
and CH$_3$CN in IM Lup. We observe a range of morphologies -- central peaks,
single or double rings -- with no clear correlation in morphology between
molecule nor disk. Emission is generally compact and on scales comparable with
the millimetre dust continuum. We perform both disk-integrated and
radially-resolved rotational diagram analysis to derive column densities and
rotational temperatures. The latter reveals 5-10 times more column density in
the inner 50-100 au of the disks when compared with the disk-integrated
analysis. We demonstrate that CH$_3$CN originates from lower relative heights
in the disks when compared with HC$_3$N, in some cases directly tracing the
disk midplane. Finally, we find good agreement between the ratio of small to
large nitriles in the outer disks and comets. Our results indicate that the
protoplanetary disks studied here are host to significant reservoirs of large
organic molecules, and that this planet- and comet-building material can be
chemically similar to that in our own Solar System. This paper is part of the
MAPS special issue of the Astrophysical Journal Supplement Series.
