Peering into the Midplane of the DM Tau Disk Using N 2 H + and DCO +

Planets form in the midplanes of protoplanetary disks; thus, investigating the properties of the midplane region can shed light on the physical and chemical processes governing the evolution of planetary systems. In this work, we study the conditions near the midplane of the disk around DM Tau using 0 . ″ 3 (∼43 au) Atacama Large Millimeter/submillimeter Array ​​​​​observations of N 2 H + ( J = 3–2) and DCO + ( J = 4–3). The radial profiles reveal that DCO + is centrally peaked, with a depression near 120 au, while N 2 H + exhibits a central depression and a peak near 100 au. In addition, both molecules display extended emission out to 400 au. Using parametric models to interpret our observations, we note that a fiducial model based on CO-derived parameters captures the features within 200 au, but fails to reproduce the outer disk emission. We explore various mechanisms that could account for this extended emission and conclude that it is best reproduced by an increased external UV penetration and cosmic-ray/X-ray ionization rate, possibly enhanced by radial drift. These processes affect the precursors of N 2 H + and DCO + : increased UV leads to the photodesorption of CO and N 2 , while ionization enhances H 3 + abundance. Besides, the observed N 2 H + and DCO + features are broadly consistent with previous CO snowline constraints, but can also be shaped by processes such as ionization, CO depletion, and multiple chemical pathways. Thus, using these molecular ions as robust CO snowline tracers requires additional constraints from complementary molecular tracers and detailed chemical modeling.