Probing CO and N${}_2$ Snow Surfaces in Protoplanetary Disks with N${}_2$H${}^{+}$ Emission

Abstract

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${}_2$H${}^{+}$ 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${}_2$H${}^{+}$ at 0.2${}^{\prime \prime }$-0.4${}^{\prime \prime }$ 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${}_2$H${}^{+}$ 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${}_2$H${}^{+}$ 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${}_2$H${}^{+}$ emission peaks between the CO and N${}_2$ snowlines. Broad N${}_2$H${}^{+}$ 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.

Accepted for publication in ApJ

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