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 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, ${\zeta }_{\mathrm{CR}}\lesssim 10^{-19}$ 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.

20 pages, 14 figures, Accepted in ApJ

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