2021ApJS..257...13A

We observed HCO⁺ J = 1 - 0 and H¹³CO⁺ 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 H¹³CO⁺ is detected (S/N > 6σ) toward GM Aur and HD 163296 and tentatively detected (S/N > 3σ) toward the other disks by a matched filter analysis. Inside a radius of R ∼ 100 au, the HCO⁺ column density is flat or shows a central dip. At outer radii (>=100 au), the HCO⁺ column density decreases outward, while the column density ratio of HCO⁺/CO is mostly in the range of ∼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 >= 100 au, the HCO⁺ abundance is ∼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 >=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.