2019ApJ...882..160Q

Snowlines of major volatiles regulate the gas and solid C/N/O ratios in the planet-forming midplanes of protoplanetary disks. Snow surfaces are the 2D extensions of snowlines in the outer disk regions, where radiative heating results in an increasing temperature with disk height. CO and N₂ are two of the most abundant carriers of C, N, and O. N₂H⁺ can be used to probe the snow surfaces of both molecules, because it is destroyed by CO and formed from N₂. Here we present Atacama Large Millimeter/submillimeter Array (ALMA) observations of N₂H⁺ at ∼0.''2-0.''4 resolution in the disks around LkCa 15, GM Aur, DM Tau, V4046 Sgr, AS 209, and IM Lup. We find two distinctive emission morphologies: N₂H⁺ is either present in a bright, narrow ring surrounded by extended tenuous emission, or in a broad ring. These emission patterns can be explained by two different kinds of vertical temperature structures. Bright, narrow N₂H⁺ rings are expected in disks with a thick Vertically Isothermal Region above the Midplane (VIRaM) layer (LkCa 15, GM Aur, DM Tau) where the N₂H⁺ emission peaks between the CO and N₂ snowlines. Broad N₂H⁺ rings come from disks with a thin VIRaM layer (V4046 Sgr, AS 209, IM Lup). We use a simple model to extract the first sets of CO and N₂ snowline pairs and corresponding freeze-out temperatures toward the disks with a thick VIRaM layer. The results reveal a range of N₂ and CO snowline radii toward stars of similar spectral type, demonstrating the need for empirically determined snowlines in disks.