Molecules with ALMA at Planet-forming Scales (MAPS). XIII. HCO+ and disk ionization structure

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 × 10^-11 - 3 × 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.