Volatile-carbon locking and release in protoplanetary disks

Abstract

Aims. The composition of planetary solids and gases is largely rooted in the processing of volatile elements in protoplanetary disks. To shed light on the key processes, we carry out a comparative analysis of the gas-phase carbon abundance in two systems with a similar age and disk mass, but different central stars: HD 100546 and TW Hya. Methods. We combine our recent detections of C 0 in these disks with observations of other carbon reservoirs (CO, C + , C 2 H) and gas-mass and warm-gas tracers (HD, O 0 ), as well as spatially resolved ALMA observations and the spectral energy distribution. The disks are modelled with the DALI 2D physical-chemical code. Stellar abundances for HD 100546 are derived from archival spectra. Results. Upper limits on HD emission from HD 100546 place an upper limit on the total disk mass of ≤0.1 M . The gas-phase carbon abundance in the atmosphere of this warm Herbig disk is, at most, moderately depleted compared to the interstellar medium, with [C]/[H] gas = (0.1−1.5) × 10 −4 . HD 100546 itself is a λ Boötis star, with solar abundances of C and O but a strong depletion of rockforming elements. In the gas of the T Tauri disk TW Hya, both C and O are strongly underabundant, with [C]/[H] gas = (0.2−5.0)×10 −6 and C/O > 1. We discuss evidence that the gas-phase C and O abundances are high in the warm inner regions of both disks. Our analytical model, including vertical mixing and a grain size distribution, reproduces the observed [C]/[H] gas in the outer disk of TW Hya and allows to make predictions for other systems.

Far-infrared HD emission as a measure of protoplanetary disk mass

“…Figure 11), which favor a gas mass between 7.7 · 10 −3 M ≤ M disk ≤ 2.3 · 10 −2 M . Both estimates are lower than previous estimates (e.g., Bergin Kama et al 2016b). Note that the estimates lie closer to the disk mass estimated from C 18 O observations (Favre et al 2013), suggesting a smaller tension between the two.…”

“…It closely resembles a 4000 K blackbody with a 10000 K blackbody component to represent the observed UV excess (following the prescription in Kama et al 2016a, see also Figure 3 and Table 3 in Kama et al 2016b).…”

“…Bold numbers denote the best fit values for TW Hya from Kama et al (2016b). The deuterium fraction was taken from Prodanović et al (2010).…”

“…Section 2.2). In both cases, the mass and vertical structure of the disk were fixed to the values from Kama et al (2016b) Table 2). Figure 5 shows that the HD 1-0 flux increases with dust settling (i.e., when the large grains are more settled towards the midplane).…”

“…However, studies that include these effects find that overall gas phase carbon must still be additionally depleted by a factor 10- 100. This leads to low CO emission even for massive disks (e.g., Bruderer et al 2012;Favre et al 2013;Du et al 2015;Kama et al 2016b;Bergin et al 2016).…”