Probing CO and N_2 Snow Surfaces in Protoplanetary Disks with N_2H^+ Emission

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 a decreasing temperature with disk height. CO and N_2 are two of the most abundant carriers of C, N and O. N_2H^+ can be used to probe the snow surfaces of both molecules, because it is destroyed by CO and formed from N_2. Here we present Atacama Large Millimeter/submillimeter Array (ALMA) observations of N_2H^+ 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: N_2H^+ 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 N_2H^+ rings are expected in disks with a thick Vertically Isothermal Region above the Midplane (VIRaM) layer (LkCa 15, GM Aur, DM Tau) where the N_2H^+ emission peaks between the CO and N_2 snowlines. Broad N_2H^+ 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 N_2 snowline pairs and corresponding freeze-out temperatures towards the disks with a thick VIRaM layer. The results reveal a range of N_2 and CO snowline radii towards stars of similar spectral type, demonstrating the need for empirically determined snowlines in disks.