The chemical connection between 67P/C-G and IRAS 16293-2422

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 ...

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.

Protostars and Planets VI, 2014

We review some of the major findings in cosmochemistry since the Protostars and Planets V meeting: (1) the results of the sample-return space missions Genesis, Stardust, and Hayabusa, which yielded the oxygen and nitrogen isotopic composition of the Sun, evidence for significant radial transport of solids in the protoplanetary disk and the chondrite-comet connection, and the connection between ordinary chondrites and S-type asteroids, respectively; (2) the D/H ratio of chondritic water as a test of the Nice and Grand Tack dynamical models and implications for the origin of the Earth's volatiles; (3) the origin, initial abundances, and distribution of the short-lived radionuclides 10 Be, 26 Al, 36 Cl, 41 Ca, 53 Mn, 60 Fe in the early Solar System; (4) the absolute (U-Pb) and relative (short-lived radionuclide) chronology of early Solar System processes; (5) the astrophysical setting of the Solar System formation; (6) the formation of chondrules under highly nonsolar conditions; and (7) the origin of enstatite chondrites.

2010

In this chapter we review recent advances in our understanding of the chemical and isotopic evolution of protoplanetary disks and the solar nebula. Current observational and meteoritic constraints on physical conditions and chemical composition of gas and dust in these systems are presented. A variety of chemical and photochemical processes that occur in planet-forming zones and beyond, in both gas phase and on grain surfaces, are overviewed. The discussion is based upon radio-interferometric, meteoritic, space-born, and laboratory-based observations,measurements and theories. Linkage between cosmochemical and astrochemical data are presented, and interesting research puzzles are discussed.

Astronomy & Astrophysics, 2012

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.

The Astrophysical Journal

Comets provide a valuable window into the chemical and physical conditions at the time of their formation in the young solar system. We seek insights into where and when these objects formed by comparing the range of abundances observed for nine molecules and their average values across a sample of 29 comets to the predicted midplane ice abundances from models of the protosolar nebula. Our fiducial model, where ices are inherited from the interstellar medium, can account for the observed mixing ratio ranges of each molecule considered, but no single location or time reproduces the abundances of all molecules simultaneously. This suggests that each comet consists of material processed under a range of conditions. In contrast, a model where the initial composition of disk material is “reset,” wiping out any previous chemical history, cannot account for the complete range of abundances observed in comets. Using toy models that combine material processed under different thermal conditio...

Astronomy & Astrophysics, 2011

It remains a key challenge to establish the molecular content of different components of low-mass protostars, like their envelopes and disks, and how this depends on the evolutionary stage and/or environment of the young stars. Observations at submillimeter wavelengths provide a direct possibility to study the chemical composition of low-mass protostars through transitions probing temperatures up to a few hundred K in the gas surrounding these sources. This paper presents a large molecular line survey of the deeply embedded protostellar binary IRAS 16293-2422 from the Submillimeter Array (SMA)-including images of individual lines down to ≈ 1.5-3 ′′ (190-380 AU) resolution. More than 500 individual transitions are identified related to 54 molecular species (including isotopologues) probing temperatures up to about 550 K. Strong chemical differences are found between the two components in the protostellar system with a separation between, in particular, the sulfur-and nitrogen-bearing species and oxygen-bearing complex organics. The action of protostellar outflow on the ambient envelope material is seen in images of CO and SiO and appear to influence a number of other species, including (deuterated) water, HDO. The effects of cold gas-phase chemistry is directly imaged through maps of CO, N 2 D + and DCO + , showing enhancements of first DCO + and subsequently N 2 D + in the outer envelope where CO freezes-out on dust grains.

The Astrophysical Journal Supplement Series

We present molecular line observations of the high-mass molecular clump IRAS 16562−3959 taken at 3 mm using the Atacama Large Millimeter/submillimeter Array at 1 7 angular resolution (0.014 pc spatial resolution). This clump hosts the actively accreting high-mass young stellar object (HMYSO) G345.4938+01.4677, which is associated with a hypercompact H II region. We identify and analyze emission lines from 22 molecular species (encompassing 34 isomers) and classify them into two groups, depending on their spatial distribution within the clump. One of these groups gathers shock tracers (e.g., SiO, SO, HNCO) and species formed in dust grains like methanol (CH 3 OH), ethenone or ketene (H 2 CCO), and acetaldehyde (CH 3 CHO). The second group collects species closely resembling the dust continuum emission morphology and are formed mainly in the gas phase, like hydrocarbons (CCH, c-C 3 H 2 , CH 3 CCH), cyanopolyynes (HC 3 N and HC 5 N), and cyanides (HCN and CH 3 C 3 N). Emission from complex organic molecules (COMs) like CH 3 OH, propanenitrile (CH 3 CH 2 CN), and methoxymethane (CH 3 OCH 3 ) arise from gas in the vicinity of a hot molecular core (T100 K) associated with the HMYSO. Other COMs such as propyne (CH 3 CCH), acrylonitrile (CH 2 CHCN), and acetaldehyde seem to better trace warm (T80 K) dense gas. In addition, deuterated ammonia (NH 2 D) is detected mostly in the outskirts of IRAS 16562−3959 and associated with near-infrared dark globules, probably gaseous remnants of the clump's prestellar phase. The spatial distribution of molecules in IRAS 16562−3959 supports the view that in protostellar clumps, chemical tracers associated with different evolutionary stages-starless to hot cores/H II regions-exist coevally.

Space Science Reviews, 2007

Comets are thought to preserve the most pristine material currently present in the solar system, as they are formed by agglomeration of dust particles in the solar nebula, far from the Sun, and their interiors have remained cold. By approaching the Sun, volatile components and dust particles are released forming the cometary coma. During the phase of Heavy Bombardment, 3.8-4 billion years ago, cometary matter was delivered to the Early Earth. Precise knowledge on the physico-chemical composition of comets is crucial to understand the formation of the Solar System, the evolution of Earth and particularly the starting conditions for the origin of life on Earth. Here, we report on the COSAC instrument, part of the ESA cometary mission Rosetta, which is designed to characterize, identify, and quantify volatile cometary compounds, including larger organic molecules, by in situ measurements of surface and subsurface cometary samples. The technical concept of a multi-column enantio-selective gas chromatograph (GC) coupled to a linear reflectron time-of-flight mass-spectrometer instrument is presented together with its realisation under the scientific guidance of the Max-Planck-Institute for Solar System Research in Katlenburg-Lindau, Germany. The instrument's technical data are given; first measurements making use of standard samples are presented. The cometary science community is looking forward to receive fascinating data from COSAC cometary in situ measurements in 2014.