2024ApJ...976..258K

Resolved molecular line observations are essential for gaining insight into the physical and chemical structure of protoplanetary disks, particularly in cold, dense regions where planets form and acquire their chemical compositions. However, tracing these regions is challenging because most molecules freeze onto grain surfaces and are not observable in the gas phase. We investigated cold molecular chemistry in the triple stellar T Tauri disk GG Tau A, which harbours a massive gas and dust ring and an outer disk, using Atacama Large Millimeter/submillimeter Array (ALMA) Band 7 observations. We present high angular resolution maps of N₂H⁺ and DCO⁺ emission, with upper limits reported for H₂D⁺, ¹³CS, and SO₂. The radial intensity profile of N₂H⁺ shows most emission near the ring's outer edge, while DCO⁺ exhibits a double peak, one near the ring's inner edge and the other in the outer disk. With complementary observations of lower-lying transitions, we constrained the molecular surface densities and rotation temperatures. We compared the derived quantities with model predictions across different cosmic-ray ionization (CRI) rates, carbon-to-oxygen (C/O) ratios, and stellar UV fluxes. Cold molecular chemistry, affecting the N₂H⁺, DCO⁺, and H₂D⁺ abundances, is most sensitive to the CRI rate, while the stellar UV fluxes and C/O ratios have minimal impact on these three ions. Our best model requires a low CRI rate of 10^–18 s^–1. However, it fails to match the low temperatures derived from N₂H⁺ and DCO⁺, 12-16 K, which are much lower than the CO freezing temperature.