[Paper Review] Chemistry in Disks. IX. Observations and modeling of HCO+ and DCO+ in DM Tau

Richard Teague, D. Semenov+6 more|arXiv (Cornell University)|Jan 5, 2015

Astrophysics and Star Formation StudiesPhysics and Astronomy106 references18 citations

TL;DR

This study combines ALMA-like interferometric observations and chemical modeling to investigate deuteration and ionization in the DM Tau protoplanetary disk, revealing a radial increase in the DCO+/HCO+ ratio (R_D) from 0.1 at 50 AU to 0.2 at 450 AU, indicating ongoing deuterium fractionation. The ionization fraction is constrained at x(e⁻) ~ 10⁻⁷, with atomic ions like C⁺ and H⁺ dominating charge carriers, challenging steady-state assumptions in disk ionization modeling.

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+.

Motivation & Objective

• To understand the chemical and ionization structure of the DM Tau protoplanetary disk using high-resolution observations of HCO⁺ and DCO⁺.
• To investigate the origin and radial evolution of deuteration in the disk, particularly the DCO⁺/HCO⁺ ratio (R_D).
• To constrain the electron fraction (x(e⁻)) in the molecular layer using a steady-state approximation and compare it with chemical model predictions.
• To assess the influence of X-ray and UV radiation, CO depletion, and disk temperature on HCO⁺ and DCO⁺ abundances.

Proposed method

• Interferometric observations of HCO⁺ (1-0), HCO⁺ (3-2), and DCO⁺ (3-2) lines were conducted with the Plateau de Bure Interferometer at ~1.5'' angular and ~0.2 km s⁻¹ spectral resolution.
• Column density profiles were derived using a power-law fitting approach to model the radial distribution of molecular emission.
• Radiative transfer models were fitted using χ²-minimization and MCMC techniques to reproduce observed visibilities and line intensities.
• Chemical modeling explored the sensitivity of HCO⁺ and DCO⁺ abundances to physical parameters such as X-ray luminosity, UV radiation, and CO depletion.
• A steady-state approximation was applied to estimate the electron fraction (x(e⁻)) in the molecular layer.
• Charge balance analysis quantified the contribution of molecular ions (e.g., HCO⁺) versus atomic ions (e.g., C⁺, H⁺) to total ionization in the disk.

Experimental results

Research questions

• RQ1 What is the radial distribution of the DCO⁺/HCO⁺ deuteration ratio (R_D) in the DM Tau disk, and what does it reveal about ongoing fractionation processes?
• RQ2 How do X-ray and interstellar UV radiation influence the abundances of HCO⁺ and DCO⁺ in the disk?
• RQ3 What is the electron fraction (x(e⁻)) in the molecular layer, and how does it compare to steady-state estimates?
• RQ4 Why is HCO⁺ emission suppressed in an inner region (~50 AU), and what causes the observed 'chemical hole'?
• RQ5 To what extent do atomic ions dominate the charge balance in the disk, and how does this affect ionization constraints based on molecular ion abundances?

Key findings

• A radial increase in the DCO⁺/HCO⁺ ratio (R_D) from 0.1 at 50 AU to 0.2 at 450 AU was observed, indicating sustained deuterium fractionation in the disk beyond the prestellar phase.
• The observed column densities at 100 AU were (9.8⁺⁰.³₋₀.⁷)×10¹² cm⁻² for HCO⁺ and (1.2±0.7)×10¹² cm⁻² for DCO⁺.
• A chemical hole in HCO⁺ and DCO⁺ emission was identified, extending up to 50 AU from the star, requiring further investigation into its origin.
• X-ray luminosity and interstellar UV radiation were found to be the dominant physical parameters controlling HCO⁺ and DCO⁺ abundances.
• The electron fraction in the molecular layer was constrained at x(e⁻) ~ 10⁻⁷, consistent with MRI-active conditions.
• Atomic ions (C⁺, H⁺, S⁺) dominate the charge balance, contributing >50–90% of the total ionization, even within the molecular layer, challenging steady-state assumptions based on molecular ions alone.