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

Abstract

We observed HCO + J = 1 − 0 and H 13 CO + 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 H 13 CO + is detected (S/N > 6 σ ) toward GM Aur and HD 163296 and tentatively detected (S/N > 3 σ ) 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 + , N 2 H + , and N 2 D + 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.