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arXiv · 1907.10647

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

Chunhua Qi·Karin I. Öberg·Catherine C. Espaillat·Connor E. Robinson·Sean M. Andrews·David J. Wilner·Geoffrey A. Blake·Edwin A. Bergin·L. Ilsedore Cleeves
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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$''$-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$_2$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.

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Chunhua Qi, Karin I. Öberg, Catherine C. Espaillat, Connor E. Robinson, Sean M. Andrews, David J. Wilner, Geoffrey A. Blake, Edwin A. Bergin, L. Ilsedore Cleeves. 2019-07-24. Probing CO and N$_2$ Snow Surfaces in Protoplanetary Disks with N$_2$H$^+$ Emission. https://doi.org/10.3847/1538-4357%2Fab35d3

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