Detection of complex organic molecules in a prestellar core: a new challenge for astrochemical models

Bacmann, A.; Taquet, V.; Fauré, Alexandre; Kahane, C.; Ceccarelli, C.

doi:10.1051/0004-6361/201219207

Cited by 351 publications

(387 citation statements)

References 27 publications

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“…Investigations into the efficacy of reactive desorption in dark cloud models by Garrod et al (2007) constrain the value for the probability of desorption to P rd = 0.01 and we adopt this value in our work. Recently, Vasyunin & Herbst (2013) suggested reactive desorption from grain surfaces followed by radiative association in the gas phase as a potential mechanism for the production of several complex molecules recently detected in dark clouds and prestellar cores (Bacmann et al 2012;Cernicharo et al 2012). The species detected include the methoxy radical (CH 3 O), ketene (CH 2 CO), acetaldehyde (CH 3 CHO), methyl formate (HCOOCH 3 ), and dimethyl ether (CH 3 OCH 3 ).…”

Section: Chemical Modelmentioning

confidence: 99%

Complex organic molecules in protoplanetary disks

Walsh

Millar

Nomura

et al. 2014

A&A

223

356

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Context. Protoplanetary disks are vital objects in star and planet formation, possessing all the material, gas and dust, which may form a planetary system orbiting the new star. Small, simple molecules have traditionally been detected in protoplanetary disks; however, in the ALMA era, we expect the molecular inventory of protoplanetary disks to significantly increase. Aims. We investigate the synthesis of complex organic molecules (COMs) in protoplanetary disks to put constraints on the achievable chemical complexity and to predict species and transitions which may be observable with ALMA. Methods. We have coupled a 2D steady-state physical model of a protoplanetary disk around a typical T Tauri star with a large gas-grain chemical network including COMs. We compare the resulting column densities with those derived from observations and perform ray-tracing calculations to predict line spectra. We compare the synthesised line intensities with current observations and determine those COMs which may be observable in nearby objects. We also compare the predicted grain-surface abundances with those derived from cometary comae observations. Results. We find COMs are efficiently formed in the disk midplane via grain-surface chemical reactions, reaching peak grain-surface fractional abundances ∼10 −6 -10 −4 that of the H nuclei number density. COMs formed on grain surfaces are returned to the gas phase via non-thermal desorption; however, gas-phase species reach lower fractional abundances than their grain-surface equivalents, ∼10 −12 -10 −7 . Including the irradiation of grain mantle material helps build further complexity in the ice through the replenishment of grain-surface radicals which take part in further grain-surface reactions. There is reasonable agreement with several line transitions of H 2 CO observed towards T Tauri star-disk systems. There is poor agreement with HC 3 N lines observed towards LkCa 15 and GO Tau and we discuss possible explanations for these discrepancies. The synthesised line intensities for CH 3 OH are consistent with upper limits determined towards all sources. Our models suggest CH 3 OH should be readily observable in nearby protoplanetary disks with ALMA; however, detection of more complex species may prove challenging, even with ALMA "Full Science" capabilities. Our grain-surface abundances are consistent with those derived from cometary comae observations providing additional evidence for the hypothesis that comets (and other planetesimals) formed via the coagulation of icy grains in the Sun's natal disk.

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Section: Chemical Modelmentioning

confidence: 99%

Complex organic molecules in protoplanetary disks

Walsh

Millar

Nomura

et al. 2014

A&A

223

356

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“…The above scenario requires that the grains spend some time at elevated temperatures so that the radicals become mobile (see § 5.2). The recent detection of some complex organic molecules in very cold sources that have never been heated much above 10 K therefore came as a surprise 101,[209][210][211] . Most of these species are the same as those identified as 'cold' complex molecules in the survey of hot cores by Bisschop et al 212 , i.e., molecules with excitation temperatures below 100 K originating in the colder outer envelope rather than the hot core.…”

Section: Protostars and Hot Cores: Complex Moleculesmentioning

confidence: 99%

Astrochemistry of dust, ice and gas: introduction and overview

Dishoeck

Ewine²

2014

Faraday Discuss.

140

102

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A brief introduction and overview of the astrochemistry of dust, ice and gas and their interplay is presented, aimed at non-specialists. The importance of basic chemical physics studies of critical reactions is illustrated through a number of recent examples. Such studies have also triggered new insight into chemistry, illustrating how astronomy and chemistry can enhance each other. Much of the chemistry in star- and planet-forming regions is now thought to be driven by gas-grain chemistry rather than pure gas-phase chemistry, and a critical discussion of the state of such models is given. Recent developments in studies of diffuse clouds and PDRs, cold dense clouds, hot cores, protoplanetary disks and exoplanetary atmospheres are summarized, both for simple and more complex molecules, with links to papers presented in this volume. In spite of many lingering uncertainties, the future of astrochemistry is bright: new observational facilities promise major advances in our understanding of the journey of gas, ice and dust from clouds to planets.Comment: Introductory paper for Faraday Discussions 168 conference, April 201

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“…It might also be that our approach to model CO hydrogenation on the dust surfaces does not include all the necessary details such as a multi-phase ice structure (chemically inert, bulk ice with chemically active ice layers), porosity, and the presence of surface binding sites of different energies (see Taquet et al 2012;Vasyunin & Herbst 2013a,b;Garrod 2013). This deficiency can be best demonstrated by the recent puzzling detections of complex organic molecules in cold, pre-stellar cores and infrared dark clouds, which is a challenge to explain without too much tuning of the key chemistry parameters such as reactive desorption efficiency, probabilities of radiative association between large molecules, and their least-destructive dissociative recombination (e.g., Bacmann et al 2012;Vasyunin & Herbst 2013b;Vasyunina et al 2014;Öberg et al 2014). Interestingly, with our other models with elemental C/O ratios >1 we can accurately reproduce the entire 1D RATRAN CH 3 OH profile, but then the H 2 CO abundances in these models become too high.…”

Section: Complex Organicsmentioning

confidence: 99%

The HIFI spectral survey of AFGL 2591 (CHESS)

Kaźmierczak-Barthel

Semenov

Tak

et al. 2015

A&A

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Aims. The aim of this work is to understand the richness of chemical species observed in the isolated high-mass envelope of AFGL 2591, a prototypical object for studying massive star formation. Methods. Based on HIFI and JCMT data, the molecular abundances of species found in the protostellar envelope of AFGL 2591 were derived with a Monte Carlo radiative transfer code (Ratran), assuming a mixture of constant and 1D stepwise radial profiles for abundance distributions. The reconstructed 1D abundances were compared with the results of the time-dependent gas-grain chemical modeling, using the best-fit 1D power-law density structure. The chemical simulations were performed considering ages of 1−5 × 10 4 years, cosmic ray ionization rates of 5−500 × 10 −17 s −1 , uniformly-sized 0.1−1 μm dust grains, a dust/gas ratio of 1%, and several sets of initial molecular abundances with C/O < 1 and >1. The most important model parameters varied one by one in the simulations are age, cosmic ray ionization rate, external UV intensity, and grain size. Results. Constant abundance models give good fits to the data for CO, CN, CS, HCO + , H 2 CO, N 2 H + , CCH, NO, OCS, OH, H 2 CS, O, C, C + , and CH. Models with an abundance jump at 100 K give good fits to the data for NH 3 , SO, SO 2 , H 2 S, H 2 O, HCl, and CH 3 OH. For HCN and HNC, the best models have an abundance jump at 230 K. The time-dependent chemical model can accurately explain abundance profiles of 15 out of these 24 species. The jump-like radial profiles for key species like HCO + , NH 3 , and H 2 O are consistent with the outcome of the time-dependent chemical modeling. The best-fit model has a chemical age of ∼10−50 kyr, a solar C/O ratio of 0.44, and a cosmic-ray ionization rate of ∼5 × 10 −17 s −1 . The grain properties and the intensity of the external UV field do not strongly affect the chemical structure of the AFGL 2591 envelope, whereas its chemical age, the cosmic-ray ionization rate, and the initial abundances play an important role. Conclusions. We demonstrate that simple constant or jump-like abundance profiles constrained with 1D Ratran line radiative transfer simulations are in agreement with time-dependent chemical modeling for most key C-, O-, N-, and S-bearing molecules. The main exceptions are species with very few observed transitions (C, O, C + , and CH) or with a poorly established chemical network (HCl, H 2 S) or whose chemistry is strongly affected by surface processes (CH 3 OH).

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Detection of complex organic molecules in a prestellar core: a new challenge for astrochemical models

Cited by 351 publications

References 27 publications

Complex organic molecules in protoplanetary disks

Complex organic molecules in protoplanetary disks

Astrochemistry of dust, ice and gas: introduction and overview

The HIFI spectral survey of AFGL 2591 (CHESS)

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