Chemistry in the GG Tau A Disk: Constraints from H2D+, N2H+ and DCO+ high angular resolution ALMA observations

Highly resolved molecular line observations are essential to gain insight into the physical and chemical struc- ture of a protoplanetary disk. Specifically, the cold and highly dense regions are important as planets form and acquire their composition here. However, it is challenging to trace cold, high-density regions because most of the molecules freeze onto grain surfaces and are not available in the gas phase to be observed. We aimed to investigate the cold molecular chemistry in the protoplanetary disk around a triple star T Tauri system, GG Tau A, using the Atacama Large Millimeter/submillimeter Array (ALMA) band 7 observations. This work presents highly resolved emissions of N2 H+ and DCO+ and non-detection of H2 D+ , 13 CS, and SO2 . Radial intensity profiles of N2H+ reveal that the majority of the emission originates from the ring edge. DCO+ radial intensity distribution exhibits dual peaks—one at the ring center and another at the outer disk region—suggesting that DCO+ is tracing two distinct regions within the disk. We obtained upper limits of column density for the unde- tected molecules. We constrained molecular column densities using the rotational diagram method along with the radiative transfer code. Additionally, we utilized our state-of-the-art, in-house astrochemical code to model the physical and chemical structure of the disk. We ran grids of 360 astrochemistry models and found that the best-fit model requires a carbon-to-oxygen (C/O) ratio of 0.7 and a low cosmic ray ionization rate (ζX ) of 10−18 s−1. Our best-fit model shows good agreement with the radial distribution of column density for both N2H+ and DCO+. However, we could not accurately reproduce the observed temperature trends, which could be attributed to our limited understanding of the physical structure of the ring region