Exploring the Complex Ionization Environment of the Turbulent DM Tau Disk

TL;DR

This study uses ALMA observations and advanced modeling to map ionization processes in the DM Tau protoplanetary disk, revealing complex ionization environments influenced by cosmic rays and stellar X-rays, with implications for disk chemistry and evolution.

Contribution

First 2D ionization constraints for DM Tau disk using forward-modeled chemical and radiative transfer analysis, incorporating multiple ionization sources and revealing complex ionization conditions.

Findings

Best fit with reduced cosmic ray ionization rate (~10^{-18} s^{-1})

Detection of a hard X-ray spectrum associated with stellar flaring

Inner disk emission under-produced, indicating additional ionization mechanisms

Abstract

Ionization drives important chemical and dynamical processes within protoplanetary disks, including the formation of organics and water in the cold midplane and the transportation of material via accretion and magneto-hydrodynamic (MHD) flows. Understanding these ionization-driven processes is crucial for understanding disk evolution and planet formation. We use new and archival ALMA observations of HCO+, H13CO+, and N2H+ to produce the first forward-modeled 2D ionization constraints for the DM Tau protoplanetary disk. We include ionization from multiple sources and explore the disk chemistry under a range of ionizing conditions. Abundances from our 2D chemical models are post-processed using non-LTE radiative transfer, visibility sampling, and imaging, and are compared directly to the observed radial emission profiles. The observations are best fit by a modestly reduced CR ionization rate ($\zeta_{CR}$ ~ 10$^{-18}$ s$^{-1}$) and a hard X-ray spectrum (hardness ratio [HR] = 0.3), which we associate with stellar flaring conditions. Our best-fit model under-produces emission in the inner disk, suggesting that there may be an additional mechanism enhancing ionization in DM Tau's inner disk. Overall, our findings highlight the complexity of ionization in protoplanetary disks and the need for high resolution multi-line studies.