The role of OH in the chemical evolution of protoplanetary disks (original) (raw)

The role of OH in the chemical evolution of protoplanetary disks. I. The comet-forming region

2011

Context. Time-dependent gas-grain chemistry can help us understand the layered structure of species deposited onto the surface of grains during the lifetime of a protoplanetary disk. The history of trapping large quantities of carbon-and oxygen-bearing molecules onto the grains is especially significant for the formation of more complex (organic) molecules on the surface of grains. Aims. Among other processes, cosmic ray-induced UV photoprocesses can lead to the efficient formation of OH. Using a more accurate treatment of cosmic ray-gas interactions for disks, we obtain an increased cosmic ray-induced UV photon flux of 3.8 × 10 5 photons cm −2 s −1 for a cosmic-ray ionization rate of H 2 value of 5×10 −17 s −1 (compared to previous estimates of 10 4 photons cm −2 s −1 based on ISM dust properties). We explore the role of the enhanced OH abundance on the gas-grain chemistry in the midplane of the disk at 10 AU, which is a plausible location of comet formation. We focus on studying the formation/destruction pathways and timescales of the dominant chemical species. Methods. We solved the chemical rate equations based on a gas-grain chemical network and correcting for the enhanced cosmic ray-induced UV field. This field was estimated from an appropriate treatment of dust properties in a protoplanetary disk, as opposed to previous estimates that assume an ISM-like grain size distribution. We also explored the chemical effects of photodesorption of water ice into OH+H. Results. Near the end of the disk's lifetime our chemical model yields H 2 O, CO, CO 2 and CH 4 ice abundances at 10 AU (consistent with a midplane density of 10 10 cm −3 and a temperature of 20 K) that are compatible with measurements of the chemical composition of cometary bodies for a [C/O] ratio of 0.16. This comparison puts constraints on the physical conditions in which comets were formed.

Constraints on the Formation of Comets from D/H Ratios Measured in H2O and HCN

Icarus, 2000

This report is the follow-up of the paper of A. Drouart et al. (1999, Icarus 140, 129) in which it was demonstrated that appropriate models of the solar nebula permit us to interpret the deuterium enrichment in water with respect to the protosolar D/H ratio measured in LL3 meteorites and comets. In the present report, we show that the models selected by Drouart et al. are also able to explain D/H in HCN measured in Comet C/1995 O1 (Hale-Bopp). We find that the D/H ratio in HCN entering the nebula is ∼4 × 10 −3 , which is significantly less than values measured in cold dark clouds, but consistent with values found in hot molecular cores. Both H 2 O and HCN ices infalling from the presolar cloud onto the nebula discoid evaporated in the turbulent part of the nebula, isotopically exchanged with hydrogen, and mixed with water vapor coming from the inner part of the nebula. Subsequently, H 2 O and HCN ices with D/H ratios measured in Comet Hale-Bopp condensed, agglomerated and were incorporated in cometesimals. In the light of these results, we discuss the story of molecules detected in comets coming from Oort cloud. Most molecules detected in Comet Hale-Bopp originated from ices embedded in the presolar cloud. Ices vaporized prior to entering into the nebula or in the early nebula, and subsequently recondensed, except highly volatile molecules. According to A. Kouchi et al. (1994, Astron. Astrophys. 290, 1009), water ice condensed in crystalline form. We discuss the possibility that the most volatile species were then trapped in the form of clathrate hydrates. The oversolar C/N ratio and the strong depletion of Ne/O with respect to the solar abundance observed in comets are in agreement with the theory of clathrate hydrates of J. I. Lunine and D. J. . Comets formed in the Kuiper belt may contain amorphous water ice and have kept the isotopic signature of the presolar cloud. New published models of interiors of Uranus and Neptune permit us to calculate that the D/H ratios in proto-uranian and protoneptunian water ices are in agreement with those measured in comets. This confirms the current assumption that cometesimals and planetesimals that formed the cores of Uranus and Neptune had similar compositions.

Cometary Volatiles and the Origin of Comets

The Astrophysical Journal, 2012

We describe recent results on the CO/CO 2 /H 2 O composition of comets together with a survey of older literature (primarily for CO/H 2 O) and compare these with models of the protoplanetary disk. Even with the currently small sample, there is a wide dispersion in abundance ratios and little if any systematic difference between Jupiter-family comets (JFCs) and long-period and Halley-type comets (LPCs and HTCs). We argue that the cometary observations require reactions on grain surfaces to convert CO to CO 2 and also require formation of all types of comets in largely, but not entirely, overlapping regions, probably between the CO and CO 2 snow lines. Any difference in the regions of formation is in the opposite direction from the classical picture with the JFCs having formed closer to the Sun than the LPCs. In the classical picture, the LPCs formed in the region of the giant planets and the JFCs formed in the Kuiper Belt. However, these data suggest, consistent with suggestions on dynamical grounds, that the JFCs and LPCs formed in largely overlapping regions where the giant planets are today and with JFCs on average forming slightly closer to the Sun than did the LPCs. Presumably at least the JFCs passed through the scattered disk on their way to their present dynamical family.

Chemical Theories on the Origin of Comets

Astrophysics and Space Science Library, 1991

Firstly, observational data available at present to infer physical conditions of the formation environment of cometary matter are briefly surveyed. These include the chemical and isotopic composition of cometary matter, and the nuclear spin temperature derived from the ortho/para abundance ratio of H2O molecules. Secondly, theories on the origin of comets-theories based upon the chemical composition of the volatile component of cometary matter-are reviewed. The theories are classified into two types, distinguished by whether cometary volatiles originate as solar nebula condensates or as the sublimation residue of interstellar ices. Observational items helpful to test the theories are pointed out. Thirdly, discussion is given on the physical properties of ices relevant to the chemical theory of the origin of comets.

Isotopic ratios of H, C, N, O, and S in comets C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy)

Astronomy & Astrophysics, 2016

We detected 22 molecules and several isotopologues. The H 16 2 O and H 18 2 O production rates measured with Odin follow a periodic pattern with a period of 0.94 days and an amplitude of ∼25%. The inferred isotope ratios in comet Lovejoy are 16 O/ 18 O = 499 ± 24 and D/H = 1.4 ± 0.4 × 10 −4 in water, 32 S/ 34 S = 24.7 ± 3.5 in CS, all compatible with terrestrial values. The ratio 12 C/ 13 C = 109 ± 14 in HCN is marginally higher than terrestrial and 14 N/ 15 N = 145 ± 12 in HCN is half the Earth ratio. Several upper limits for D/H or 12 C/ 13 C in other molecules are reported. From our observation of HDO in comet C/2014 Q2 (Lovejoy), we report the first D/H ratio in an Oort Cloud comet that is not larger than the terrestrial value. On the other hand, the observation of the same HDO line in the other Oort-cloud comet, C/2012 F6 (Lemmon), suggests a D/H value four times higher. Given the previous measurements of D/H in cometary water, this illustrates that a diversity in the D/H ratio and in the chemical composition, is present even within the same dynamical group of comets, suggesting that current dynamical groups contain comets formed at very different places or times in the early solar system.

Variations in cometary dust composition from Giotto to Rosetta, clues to their formation mechanisms

Monthly Notices of the Royal Astronomical Society, 2016

This paper reviews the current knowledge on the composition of cometary dust (ice, minerals and organics) in order to constrain their origin and formation mechanisms. Comets have been investigated by astronomical observations, space missions (Giotto to Rosetta), and by the analysis of cometary dust particles collected on Earth, chondritic porous interplanetary dust particles (CP-IDPs) and ultracarbonaceous Antarctic micrometeorites (UCAMMs). Most ices detected in the dense phases of the interstellar medium (ISM) have been identified in cometary volatiles. However, differences also suggest that cometary ices cannot be completely inherited from the ISM. Cometary minerals are dominated by crystalline Mg-rich silicates, Fe sulphides and glassy phases including GEMS (glass with embedded metals and sulphides). The crystalline nature and refractory composition of a significant fraction of the minerals in comets imply a high temperature formation/processing close to the proto-Sun, resetting a possible presolar signature of these phases. These minerals were further transported up to the external regions of the disc and incorporated in comet nuclei. Cometary matter contains a low abundance of isotopically anomalous minerals directly inherited from the presolar cloud. At least two different kinds of organic matter are found in dust of cometary origin, with low or high nitrogen content. N-poor organic matter is also observed in primitive interplanetary materials (like carbonaceous chondrites) and its origin is debated. The N-rich organic matter is only observed in CP-IDPs and UCAMMs and can be formed by Galactic cosmic ray irradiation of N 2-and CH 4-rich icy surface at large heliocentric distance beyond a 'nitrogen snow line'.

The composition of the protosolar disk and the formation conditions for comets

Conditions in the protosolar nebula have left their mark in the composition of cometary volatiles, thought to be some of the most pristine material in the solar system. Cometary compositions represent the end point of processing that began in the parent molecular cloud core and continued through the collapse of that core to form the protosun and the solar nebula, and finally during the evolution of the solar nebula itself as the cometary bodies were accreting. Disentangling the effects of the various epochs on the final composition of a comet is complicated. But comets are not the only source of information about the solar nebula. Protostellar disks around young stars similar to the protosun provide a way of investigating the evolution of disks similar to the solar nebula while they are in the process of evolving to form their own solar systems. In this way we can learn about the physical and chemical conditions under which comets formed, and about the types of dynamical processing ...