We report observations of resolved C 2 H emission rings within the gas-rich protoplanetary disks of TWHya and DMTau using the Atacama Large Millimeter Array. In each case the emission ring is found to arise at the edge of the observable disk of millimeter-sized grains (pebbles) traced by submillimeter-wave continuum emission. In addition, we detect a C 3 H 2 emission ring with an identical spatial distribution to C 2 H in the TWHya disk. This suggests that these are hydrocarbon rings (i.e., not limited to C 2 H). Using a detailed thermo-chemical model we show that reproducing the emission from C 2 H requires a strong UV field and C/O>1 in the upper disk atmosphere and outer disk, beyond the edge of the pebble disk. This naturally arises in a disk where the ice-coated dust mass is spatially stratified due to the combined effects of coagulation, gravitational settling and drift. This stratification causes the disk surface and outer disk to have a greater permeability to UV photons. Furthermore the concentration of ices that transport key volatile carriers of oxygen and carbon in the midplane, along with photochemical erosion of CO, leads to an elemental C/O ratio that exceeds unity in the UV-dominated disk. Thus the motions of the grains, and not the gas, lead to a rich hydrocarbon chemistry in disk surface layers and in the outer disk midplane.

Identifying the source of Earth's water is central to understanding the origins of life-fostering environments and to assessing the prevalence of such environments in space. Water throughout the solar system exhibits deuterium-tohydrogen enrichments, a fossil relic of low-temperature, ion-derived chemistry within either (i) the parent molecular cloud or (ii) the solar nebula protoplanetary disk. Utilizing a comprehensive treatment of disk ionization, we find that ion-driven deuterium pathways are inefficient, curtailing the disk's deuterated water formation and its viability as the sole source for the solar system's water. This finding implies that if the solar system's formation was typical, abundant interstellar ices are available to all nascent planetary systems.Water is ubiquitous across the solar system, in cometary ices, terrestrial oceans, the icy 1 moons of the giant planets, and in the shadowed basins of Mercury (1, 2). Water has left its mark in hydrated minerals in meteorites, in lunar basalts (3) and in martian melt-inclusions (4).The presence of liquid water facilitated the emergence of life on Earth, and thus understanding the origin(s) of water throughout the solar system is a key goal of astrobiology. Comets and asteroids (traced by meteorites) remain the most primitive objects, providing a natural "time capsule" of the conditions present during the epoch of planet formation. Their compositions reflect those of the gas, dust, and -most importantly -ices encircling the Sun at its birth, i.e., the solar nebula protoplanetary disk. There remains an open question, however, as to when and where these ices formed, whether they i) originated in the dense interstellar medium (ISM) in the cold molecular cloud core prior to the Sun's formation, or ii) are products of reprocessing within the solar nebula (5-7). Scenario i) would imply that abundant interstellar ices, including water and presolar organic material, are incorporated into all planet-forming disks. By contrast, local formation within the solar nebula in scenario ii) would potentially result in large water abundance variations from stellar system to system, dependent upon the properties of the star and disk.In this work, we aim to constrain the formation environment of the solar system's water using deuterium fractionation as our chemical tracer. Water is enriched in deuterium relative to hydrogen (D/H) compared to the initial bulk solar composition across a wide range of solar system bodies, including comets, (8, 9), terrestrial and ancient Martian water (4), and hydrated minerals in meteorites (10). The amount of deuterium relative to hydrogen of a molecule depends on its formation environment, and thus the D/H fraction in water, [D/H] H 2 O , can be used to differentiate between the proposed source environments. Interstellar ices, as revealed by sublimation in close proximity to forming young stars, also exhibit high degrees of deuteriumenrichment, ∼ 2 − 30× that of terrestrial water (11)(12)(13)(14). It is unknown to what extent these extreme...

An abundance decrease in carbon-and oxygen-bearing species relative to dust has been frequently found in planet-forming disks, which can be attributed to an overall reduction of gas mass. However, in the case of TW Hya, the only disk with gas mass measured directly with HD rotational lines, the inferred gas mass ( 0.005 solar mass) is significantly below the directly measured value ( 0.05 solar mass). We show that this apparent conflict can be resolved if the elemental abundances of carbon and oxygen are reduced in the upper layers of the outer disk but are normal elsewhere (except for a possible enhancement of their abundances in the inner disk). The implication is that in the outer disk, the main reservoir of the volatiles (CO, water, . . . ) resides close to the midplane, locked up inside solid bodies that are too heavy to be transported back to the atmosphere by turbulence. An enhancement in the carbon and oxygen abundances in the inner disk can be caused by inward migration of these solid bodies. This is consistent with estimates based on previous models of dust grain dynamics. Indirect measurements of the disk gas mass and disk structure from species such as CO will thus be intertwined with the evolution of dust grains, and possibly also with the formation of planetesimals.Subject headings: astrochemistry -circumstellar matter -molecular processes -planetary systems -planet-disk interactions -planets and satellites: atmospheres arXiv:1506.03510v1 [astro-ph.SR]

We performed very deep searches for 2 ground-state water transitions in 13 protoplanetary disks with the HIFI instrument on board the Herschel Space Observatory, with integration times up to 12 hr per line. We also searched for, with shallower integrations, two other water transitions that sample warmer gas. The detection rate is low, and the upper limits provided by the observations are generally much lower than predictions of thermo-chemical models with canonical inputs. One ground-state transition is newly detected in the stacked spectrum of AATau, DMTau, LkCa15, and MWC480. We run a grid of models to show that the abundance of gas-phase oxygen needs to be reduced by a factor of at least 100 to be consistent with the observational upper limits (and positive detections) if a dust-to-gas mass ratio of 0.01 were to be assumed. As a continuation of previous ideas, we propose that the underlying reason for the depletion of oxygen (hence the low detection rate) is the freeze-out of volatiles such as water and CO onto dust grains followed by grain growth and settling/migration, which permanently removes these gas-phase molecules from the emissive upper layers of the outer disk. Such depletion of volatiles is likely ubiquitous among different disks, though not necessarily to the same degree. The volatiles might be returned back to the gas phase in the inner disk ( 15  au), which is consistent with current constraints. Comparison with studies on disk dispersal due to photoevaporation indicates that the timescale for volatile depletion is shorter than that of photoevaporation.