Constraining the x-ray and cosmic-ray ionization chemistry of the TW Hya protoplanetary disk: Evidence for a sub-interstellar cosmic-ray rate

Abstract

We present an observational and theoretical study of the primary ionizing agents (cosmic rays (CRs) and X-rays) in the TW Hya protoplanetary disk. We use a set of resolved and unresolved observations of molecular ions and other molecular species, encompassing 11 lines total, in concert with a grid of disk chemistry models. The molecular ion constraints comprise new data from the Submillimeter Array on HCO+, acquired at unprecedented spatial resolution, and data from the literature, including ALMA observations of N2H+. We vary the model incident CR flux and stellar X-ray spectra and find that TW Hya's HCO+ and N2H+ emission are best-fit by a moderately hard X-ray spectra, as would be expected during the "flaring" state of the star, and a low CR ionization rate, ζCR ≲ 10-19 s-1. This low CR rate is the first indication of the presence of CR exclusion by winds and/or magnetic fields in an actively accreting T Tauri disk system. With this new constraint, our best-fit ionization structure predicts a low turbulence "dead-zone" extending from the inner edge of the disk out to 50-65 AU. This region coincides with an observed concentration of millimeter grains, and we propose that the inner region of TW Hya is a dust (and possibly planet) growth factory as predicted by previous theoretical work.