Observations and modelling of CO and [CI] in protoplanetary disks First detections of [CI] and constraints on the carbon abundance

Kama, M. and Bruderer, S. and Carney, M. and Hogerheijde, M. and van Dishoeck, E. F. and Nagy, Zsófia (2016) Observations and modelling of CO and [CI] in protoplanetary disks First detections of [CI] and constraints on the carbon abundance. ASTRONOMY & ASTROPHYSICS, 588. ISSN 0004-6361

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Abstract

Context. The gas-solid budget of carbon in protoplanetary disks is related to the composition of the cores and atmospheres of the planets forming in them. The key gas-phase carbon carriers CO, C0 and C+ can now be observed regularly in disks. Aims. The gas-phase carbon abundance in disks has thus far not been well characterized observationally. We aim to obtain new con- straints on the [C]/[H] ratio in a large sample of disks, and to compile an overview of the strength of [C i] and warm CO emission. Methods. We carried out a survey of the CO 6–5 and [C i] 1–0 and 2–1 lines towards 37 disks with the APEX telescope, and supple- mented it with [C ii] data from the literature. The data are interpreted using a grid of models produced with the DALI disk code. We also investigate how well the gas-phase carbon abundance can be determined in light of parameter uncertainties. Results. The CO 6–5 line is detected in 13 out of 33 sources, the [C i] 1–0 in 6 out of 12, and the [C i] 2–1 in 1 out of 33. With separate deep integrations, the first unambiguous detections of the [C i] 1–0 line in disks are obtained, in TW Hya and HD 100546. Conclusions. Gas-phase carbon abundance reductions of a factor 5–10 or more can be identified robustly based on CO and [C i] detections, assuming reasonable constraints on other parameters. The atomic carbon detection towards TW Hya confirms a factor 100 reduction of [C]/[H]gas in that disk, while the data are consistent with an ISM-like carbon abundance for HD 100546. In addition, BP Tau, T Cha, HD 139614, HD 141569, and HD 100453 are either carbon-depleted or gas-poor disks. The low [C i] 2–1 detection rates in the survey mostly reflect insufficient sensitivity to detect T Tauri disks. The Herbig Ae/Be disks with CO and [C ii] upper limits below the models are debris disk like systems. A roughly order of magnitude increase in sensitivity compared to our survey is required to obtain useful constraints on the gas-phase [C]/[H] ratio in most of the targeted systems.

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