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

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.

Accepted for publication in ApJ Letters, 5 pages, 1 figure

References in corpus (6)

• The NumPy array: a structure for efficient numerical computation
• Direct mapping of the temperature and velocity gradients in discs. Imaging the vertical CO snow line around IM Lupi
• Setting the volatile composition of (exo)planet-building material. Does chemical evolution in disk midplanes matter?
• Robustness of N2H+ as tracer of the CO snowline
• Stacking Spectra in Protoplanetary Disks: Detecting Intensity Profiles from Hidden Molecular Lines in HD 163296
• Analytical Formulas of Molecular Ion Abundances and N2H+ Ring in Protoplanetary Disks

Cited by in corpus (4)

• A three-phase approach to grain surface chemistry in protoplanetary disks: Gas, ice surfaces and ice mantles of dust grains
• Discovery of Molecular Line Polarization in the Disk of TW Hya
• An inherited complex organic molecule reservoir in a warm planet-hosting disk
• ALMA-IMF XVIII: The assembly of a star cluster: Dense N${}_2$H${}^{+}$ (1-0) kinematics in the massive G351.77 protocluster