The Carbon Isotopic Ratio and Planet Formation

Abstract

We present the first detection of ^13 CCH in a protoplanetary disk (TW Hya). Using observations of C _2 H, we measure CCH/ ^13 CCH = 65 ± 20 gas with a CO isotopic ratio of ^12 CO/ ^13 CO = 21 ± 5. The TW Hya disk exhibits a gas phase C/O that exceeds unity, and C _2 H is the tracer of this excess carbon. We confirm that the TW Hya gaseous disk exhibits two separate carbon isotopic reservoirs, as noted previously. We explore two theoretical solutions for the development of this dichotomy. One model represents TW Hya today with a protoplanetary disk exposed to a cosmic-ray ionization rate that is below interstellar as consistent with current estimates. We find that this model does not have sufficient ionization in cold ( T < 40 K) layers to activate carbon isotopic fractionation. The second model investigates a younger TW Hya protostellar disk exposed to an interstellar cosmic-ray ionization rate. We find that the younger model has sources of ionization deeper in a colder disk that generates two independent isotopic reservoirs. One reservoir is ^12 C-enriched carried by methane/hydrocarbon ices, and the other is ^13 C-enriched carried by gaseous CO. The former potentially provides a source of methane/hydrocarbon ices to power the chemistry that generates the anomalously strong C _2 H emission in this (and other) disk systems in later stages. The latter provides a source of gaseous ^13 C-rich material to generate isotopic enrichments in forming giant planets, as recently detected in the super-Jupiter TYC 8998-760-1 b by Zhang et al.

Keywords

• Protoplanetary disks