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

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 1σ uncertainty of 2 K for the radial range 0.″8-2″, which is 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.