Molecules with ALMA at Planet-forming Scales (MAPS). XIII. HCO+ and Disk Ionization Structure
Abstract
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.
- Publication:
-
The Astrophysical Journal Supplement Series
- Pub Date:
- November 2021
- DOI:
- 10.3847/1538-4365/ac143c
- arXiv:
- arXiv:2109.06419
- Bibcode:
- 2021ApJS..257...13A
- Keywords:
-
- Astrochemistry;
- Planetary system formation;
- Protoplanetary disks;
- 75;
- 1257;
- 1300;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - Earth and Planetary Astrophysics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- accepted to ApJS, 33 pages, 20 figures