Constraining the X-ray and Cosmic Ray Ionization Chemistry of the TW Hya Protoplanetary Disk: Evidence for a Sub-interstellar Cosmic Ray Rate

TL;DR

This study combines observations and models to show that the TW Hya protoplanetary disk has a very low cosmic ray ionization rate, suggesting cosmic ray exclusion by magnetic fields or winds, which impacts disk chemistry and planet formation.

Contribution

It provides the first evidence of cosmic ray exclusion in an actively accreting T Tauri disk, constraining the ionization environment with new high-resolution molecular ion data.

Findings

TW Hya's ionization is dominated by X-rays, with a low cosmic ray rate of ≤10^{-19} s^{-1}.

A low turbulence dead-zone extends to 50-65 AU, aligning with dust concentration regions.

The inner disk may be a site of dust and planet growth due to low ionization and turbulence.

Abstract

We present an observational and theoretical study of the primary ionizing agents (cosmic rays 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 eleven 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 N$_2$H$^+$. We vary the model incident CR flux and stellar X-ray spectra and find that TW Hya's HCO$^+$ and N$_2$H$^+$ 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, $\zeta_{\rm CR}\lesssim10^{-19}$ 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.