Planned Laboratory Measurements Of N₂ And H₃⁺ Isotopologues

Prestellar cores and protoplanetary disks are important steps along the evolutionary pathway of baryonic matter from prestellar cores to the formation of a planetary system. Deuterated molecules are used as tracers for several properties, such as the temperature, chemistry, and thermal history of these objects. In the very dense cold regions found in prestellar cores and the outer midplane of protoplanetary disks, most molecules beside hydrogen freeze onto dust grains, leaving HD the primary deuterium reservoir in the gas phase. The HD can react with H₃⁺ to form deuterated isotopologues of the ion. Subsequent ion-neutral reactions pass on the deuteration to other gas-phase species. Of particular importance is the abundance ratio for N₂D⁺ and N₂H⁺. Their formation occurs near the N₂ snow line of prestellar cores and protoplanetary disks and they are commonly used to trace the properties of these objects. However, to reliably interpret observations of these ions, an accurate understanding of the formation process is needed. We will use our dual-source, ion-neutral, merged-fast-beams apparatus to measure the integral cross sections of the reaction of N₂ and H₃⁺ and its isotopologues, to an accuracy of about 15%. From these results, we will derive the thermal rate coefficients used in astrochemical models. In addition, our results will help the astrophysics community to determine the validity of the commonly assumed scaling of available kinetics data for H-bearing reactions to deuterated isotopologues and also of the assumed statistical branching ratios used for the relative fractions of H-bearing and D-bearing daughter products.

This project is supported, in part, by the NSF Astronomy and Astrophysics Grants program under AST-2002461.