Measurements of Water Surface Snow Lines in Classical Protoplanetary Disks

Abstract

We present deep Herschel-PACS spectroscopy of far-infrared water lines from a sample of four protoplanetary disks around solar-mass stars, selected to have strong water emission at mid-infrared wavelengths. By combining the new Herschel spectra with archival Spitzer-IRS spectroscopy, we retrieve a parameterized radial surface water vapor distribution from 0.1-100 AU using two-dimensional dust and line radiative transfer modeling. The surface water distribution is modeled with a step model comprising of a constant inner and outer relative water abundance and a critical radius at which the surface water abundance is allowed to change. We find that the four disks have critical radii of ∼ 3 − 11 AU, at which the surface water abundance decreases by at least 5 orders of magnitude. The measured values for the critical radius are consistently smaller than the location of the surface snow line, as predicted by the observed spectral energy distribution. This suggests that the sharp drop-off of the surface water abundance is not solely due to the local gas-solid balance, but may also be driven by the de-activation of gas-phase chemical pathways to water below 300 K. Assuming a canonical gas-to-dust ratio of 100, as well as coupled gas and dust temperatures T gas = T dust , the best-fit inner water abundances become implausibly high (0.01-1.0 H 2 −1 ). Conversely, a model in which the gas and dust temperatures are decoupled leads to canonical inner disk water abundances of ∼ 10 −4 H −1 2 , while retaining gas-to-dust ratios of 100. That is, the evidence for gas-dust decoupling in disk surfaces is stronger than for enhanced gas-to-dust ratios.

Survey of Cold Water Lines in Protoplanetary Disks: Indications of Systematic Volatile Depletion

“…This survey is mostly sensitive to cold (T 20  K) water vapor from the outer part of the disk (for a quantitative view, see Section 4.2, where we show that the inner few astronomical units contribute less than 10% to the transitions studied in this survey). In contrast, starting with Carr et al (2004), many papers have probed the warm-hot water reservoir in the inner few au of disks via infrared lines (e.g., Carr & Najita 2008Salyk et al 2008Salyk et al , 2011Salyk et al , 2015Pontoppidan et al 2010aPontoppidan et al , 2010bBanzatti et al 2012Banzatti et al , 2015and Blevins et al 2016).…”

The Depletion of Water During Dispersal of Planet-Forming Disk Regions

“…The decrease of water emission is found as a sequential intrinsic decrease of the observed line fluxes and of the line-to-continuum strength, where water line fluxes at short wavelengths decrease when water line fluxes at longer wavelengths are still strong. Since the emission probes hotter to colder gas from short to long wavelengths (following a global trend in E u energies that decrease with increasing wavelength; see Table 3, Figure 6, and Blevins et al 2016), this wavelength dependence of the line flux decrease may reflect the physical removal of molecular gas in the disk from the inside out (as found in the disk of TW Hya; Zhang et al 2013).…”

Hydrocarbon Emission Rings in Protoplanetary Disks Induced by Dust Evolution

“…However, this is a two way street in the sense that when dissociated, CO provides an oxygen atom that readily destroys C 2 H (going back to CO), it also leaves a reactive C + ion in the gas, which can produce hydrocarbons (e.g., Bergin et al 2014). The H 2 O sublimation front has been studied by Blevins et al (2016) and are inferred to exist within the inner few au of the disk surface for TTauri disk systems. CO 2 is more difficult to study but is known to be present in ices (Öberg et al 2011a) and comets (Mumma & Charnley 2011); based on estimated bond strengths it should evaporate between the snow-lines of water and CO (Martín-Doménech et al 2014) in between ∼5-10 au.…”