Molecules with ALMA at Planet-forming Scales (MAPS). XIII. HCO⁺ and Disk Ionization Structure

We observed HCO+ J = 1 ? 0 and H13CO+ emission toward the five protoplanetary disks around IM Lup, GM Aur, AS 209, HD 163296, MWC 480 as part of MAPS project. is detected mapped at 0.?3 resolution in all disks, while (S/N > 6?) Aur 163296 tentatively 3?) other by a matched filter analysis. Inside radius R ? 100 au, column density flat or shows central dip. At outer radii (?100 au), decreases outward, ratio HCO+/CO mostly range ?10?5–10?4. derived abundance warm CO-rich layer, where expected to be dominant molecular ion. ? ?3 × 10?11 3 10?10, which consistent with template disk model X-ray ionization. smaller radii, inward, indicates that ionization degree lower denser gas, especially inside CO snow line, layer midplane. Comparison models densities HCO+, N2H+, N2D+ midplane rate ?10?18 s?1 for 163296. also find hints an increased location dust continuum gaps 480. This paper special issue Astrophysical Journal Supplement.