First Detections of [CI] and Constraints on the Carbon Abundance

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

Authors:

Kama et al

Abstract:

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 in disks. The gas-phase carbon abundance in disks has not yet been well characterized, we aim to obtain new constraints on the [C]/[H] ratio in a sample of disks, and to get an overview of the strength of [CI] and warm CO emission.

We carried out a survey of the CO6--5 and [CI]1--0 and 2--1 lines towards 37 disks with APEX, and supplemented it with [CII] data from the literature. The data are interpreted using a grid of models produced with the DALI code. We also investigate how well the gas-phase carbon abundance can be determined in light of parameter uncertainties.

The CO6--5 line is detected in 13 out of 33 sources, the [CI]1--0 in 6 out of 12, and the [CI]2--1 in 1 out of 33. With deep integrations, the first unambiguous detections of [CI]~1--0 in disks are obtained, in TW~Hya and HD~100546.

Gas-phase carbon abundance reductions of a factor 5--10 or more can be identified robustly based on CO and [CI] detections. The atomic carbon detection in 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 [CI]~2--1 detection rates in the survey mostly reflect insufficient sensitivity to detect T~Tauri disks. The Herbig~Ae/Be disks with CO and [CII] 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.