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

We observed HCO+ J = 1 - 0 and (HCO+)-C-13 J = 1 - 0 emission toward the five protoplanetary disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480 as part of the MAPS project. HCO+ is detected and mapped at 0.'' 3 resolution in all five disks, while (HCO+)-C-13 is detected (S/N > 6 sigma) toward GM Aur and HD 163296 and tentatively detected (S/N > 3 sigma) toward the other disks by a matched filter analysis. Inside a radius of R similar to 100 au, the HCO+ column density is flat or shows a central dip. At outer radii (greater than or similar to 100 au), the HCO+ column density decreases outward, while the column density ratio of HCO+/CO is mostly in the range of similar to 10(-5)-10(-4). We derived the HCO+ abundance in the warm CO-rich layer, where HCO+ is expected to be the dominant molecular ion. At R greater than or similar to 100 au, the HCO+ abundance is similar to 3 x 10(-11) - 3 x 10(-10), which is consistent with a template disk model with X-ray ionization. At the smaller radii, the abundance decreases inward, which indicates that the ionization degree is lower in denser gas, especially inside the CO snow line, where the CO-rich layer is in the midplane. Comparison of template disk models with the column densities of HCO+, N2H+, and N2D+ indicates that the midplane ionization rate is greater than or similar to 10(-18) s(-1) for the disks around IM Lup, AS 209, and HD 163296. We also find hints of an increased HCO+ abundance around the location of dust continuum gaps in AS 209, HD 163296, and MWC 480. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.