Chemistry in Disks. IX. Observations and modeling of HCO+ and DCO+ in DM Tau

TL;DR

This study uses high-resolution observations and chemical modeling to analyze HCO+ and DCO+ molecules in the DM Tau disk, revealing a chemical inner hole, ionization sources, and molecular abundance sensitivities.

Contribution

It provides new observational data and modeling insights into the chemical composition, ionization processes, and molecular distributions in the DM Tau protoplanetary disk.

Findings

Detected a ~50 AU chemical inner hole in HCO+ and DCO+

Derived an isotopic ratio R_D ranging from 0.1 to 0.2

Identified X-rays as the dominant ionization source in the disk

Abstract

We present resolved Plateau de Bure Array observations of DM Tau in lines of HCO+ (3-2), (1-0) and DCO+ (3-2). A power-law fitting approach allowed a derivation of column densities of these two molecules. A chemical inner hole of ~50 AU was found in both HCO+ and DCO+ with DCO+ emission extending to only 450 AU. An isotopic ratio of R_D = N(DCO+) / N(HCO+) was found to range from 0.1 at 50 AU and 0.2 at 450 AU. Chemical modeling allowed an exploration of the sensitivity of these molecular abundances to physical parameters out with temperature, finding that X-rays were the domination ionization source in the HCO+ molecular region and that R_D also is sensitive to the CO depletion. The ionization fraction, assuming a steady state system, was found to be x(e-) ~ 10$^{-7}$. Modeling suggests that HCO+ is the dominant charged molecule in the disk but its contribution to ionization fraction is dwarfed by atmoic ions such as C+, S+ and H+.