An ALMA Survey of Chemistry in Disks around M4-M5 Stars
  M-stars are the most common hosts of planetary systems in the Galaxy.
Protoplanetary disks around M-stars thus offer a prime opportunity to study the
chemistry of planet-forming environments. We present an ALMA survey of
molecular line emission toward a sample of five protoplanetary disks around
M4-M5 stars (FP Tau, J0432+1827, J1100-7619, J1545-3417, and Sz 69). These
observations can resolve chemical structures down to tens of AU. Molecular
lines of $^{12}$CO, $^{13}$CO, C$^{18}$O, C$_2$H, and HCN are detected toward
all five disks. Lines of H$_2$CO and DCN are detected toward 2/5 and 1/5 disks,
respectively. For disks with resolved C$^{18}$O, C$_2$H, HCN, and H$_2$CO
emission, we observe substructures similar to those previously found in disks
around solar-type stars (e.g., rings, holes, and plateaus). C$_2$H and HCN
excitation conditions estimated interior to the pebble disk edge for the bright
disk J1100-7619 are consistent with previous measurements around solar-type
stars. The correlation previously found between C$_2$H and HCN fluxes for
solar-type disks extends to our M4-M5 disk sample, but the typical C$_2$H/HCN
ratio is higher for the M4-M5 disk sample. This latter finding is reminiscent
of the hydrocarbon enhancements found by previous observational infrared
surveys in the innermost ($<$10AU) regions of M-star disks, which is intriguing
since our disk-averaged fluxes are heavily influenced by flux levels in the
outermost disk, exterior to the pebble disk edge. Overall, most of the
observable chemistry at 10-100AU appears similar for solar-type and M4-M5
disks, but hydrocarbons may be more abundant around the cooler stars.
