# Line Ratios Reveal N2H+ Emission Originates Above the Midplane in TW Hydrae

## TL;DR

This study uses ALMA line ratio observations of N2H+ in TW Hya to determine that the emission originates above the disk midplane, indicating a warmer, higher-altitude region influenced by photodissociation or chemical processes.

## Contribution

It demonstrates a novel application of line ratios to locate molecular emission regions in a protoplanetary disk, revealing emission above the midplane rather than from it.

## Key findings

- N2H+ emission region is at about 39 K, warmer than expected midplane temperatures.
- Emission originates from higher disk layers, not near the midplane.
- Photodissociation or chemical reprocessing likely influence the emission region.

## Abstract

Line ratios for different transitions of the same molecule have long been used as a probe of gas temperature. Here we use ALMA observations of the N2H+ J~=~1-0 and J~=~4-3 lines in the protoplanetary disk around TW Hya to derive the temperature at which these lines emit. We find an averaged temperature of 39~K with a one sigma uncertainty of 2~K for the radial range 0.8-2'', significantly warmer than the expected midplane temperature beyond 0.5'' in this disk. We conclude that the N2H+ emission in TW Hya is not emitting from near the midplane, but rather from higher in the disk, in a region likely bounded by processes such as photodissociation or chemical reprocessing of CO and N2 rather than freeze out.