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Register a new userComplex organic molecules in protostellar environments in the SKA era
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Molecular complexity builds up at each step of the Sun-like star formation process, starting from simple molecules and ending up in large polyatomic species. Complex organic molecules (COMs; such as methyl formate, HCOOCH$_3$, dymethyl ether, CH$_3$OCH$_3$, formamide, NH$_2$CHO, or glycoaldehyde, HCOCH$_2$OH) are formed in all the components of the star formation recipe (e.g. pre-stellar cores, hot-corinos, circumstellar disks, shocks induced by fast jets), due to ice grain mantle sublimation or sputtering as well as gas-phase reactions. Understanding in great detail the involved processes is likely the only way to predict the ultimate molecular complexity reached in the ISM, as the detection of large molecules is increasingly more difficult with the increase of the number of atoms constituting them. Thanks to the recent spectacular progress of astronomical observations, due to the Herschel (sub-mm and IR), IRAM and SMA (mm and sub-mm), and NRAO (cm) telescopes, an enormous activity is being developed in the field of Astrochemistry, extending from astronomical observatories to chemical laboratories. We are involved in several observational projects providing unbiased spectral surveys (in the 80-300 and 500-2000 GHz ranges) with unprecedented sensitivity of templates of dense cores and protostars. Forests of COM lines have been detected. In this chapter we will focus on the chemistry of both cold prestellar cores and hot shocked regions, (i) reviewing results and open questions provided by mm-FIR observations, and (ii) showing the need of carrying on the observations of COMs at lower frequencies, where SKA will operate. We will also emphasize the importance of analysing the spectra by the light of the experimental studies performed by our team, who is investigating the chemical effects induced by ionising radiation bombarding astrophysically relevant ices.
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(Abridged) Protoplanetary disks are vital objects in star and planet formation, possessing all the material which may form a planetary system orbiting the new star. We investigate the synthesis of complex organic molecules (COMs) in disks to constrain the achievable chemical complexity and predict species and transitions which may be observable with ALMA. We have coupled a 2D model of a protoplanetary disk around a T Tauri star with a gas-grain chemical network including COMs. We compare compare synthesised line intensities and calculated column densities with observations and determine those COMs which may be observable in future. COMs are efficiently formed in the disk midplane via grain-surface chemical reactions, reaching peak grain-surface fractional abundances 1e-6 - 1e-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, 1e-12 - 1e-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 H2CO observed towards several T Tauri star-disk systems. The synthesised line intensities for CH3OH are consistent with upper limits determined towards all sources. Our models suggest CH3OH should be readily observable in nearby protoplanetary disks with ALMA; however, detection of more complex species may prove challenging. 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 Suns natal disk.
During the formation of stars, the accretion of the surrounding material toward the central object is thought to undergo strong luminosity outbursts, followed by long periods of relative quiescence, even at the early stages of star formation when the protostar is still embedded in a large envelope. We investigated the gas phase formation and the recondensation of the complex organic molecules (COMs) di-methyl ether and methyl formate, induced by sudden ice evaporation processes occurring during luminosity outbursts of different amplitudes in protostellar envelopes. For this purpose, we updated a gas phase chemical network forming complex organic molecules in which ammonia plays a key role. The model calculations presented here demonstrate that ion-molecule reactions alone could account for the observed presence of di-methyl ether and methyl formate in a large fraction of protostellar cores, without recourse to grain-surface chemistry, although they depend on uncertain ice abundances and gas phase reaction branching ratios. In spite of the short outburst timescales of about one hundred years, abundance ratios of the considered species with respect to methanol higher than 10 % are predicted during outbursts due to their low binding energies relative to water and methanol that delay their recondensation during the cooling. Although the current luminosity of most embedded protostars would be too low to produce these complex species in hot core regions that can be observable with current sub-millimetric interferometers, previous luminosity outburst events would induce a formation of COMs in extended regions of protostellar envelopes with sizes increasing by up to one order of magnitude.
Although the Cassini Spacecraft and the Huygens lander provided numerous information about Titan atmospheric chemistry and the formation of its aerosols, the exact composition of these aerosols still remains unknown. A fruitful proxy to investigate these aerosols is the use of laboratory experiments that allow producing and studying analogs of Titan aerosol, the so35 called tholins. Even when produced in the laboratory, unveiling the exact composition of the aerosol remains problematic due to the high complexity of the material. Numerous advances have been recently made using high-resolution mass spectrometry (HRMS) (Pernot et al. 2010, Somogyi et al. 2012, Gautier et al. 2014) that allowed the separation of isobaric compounds and a robust identification of chemical species composing tholins regarding their molecular formulae. Nevertheless isomeric species cannot be resolved by a simple mass measurement. We propose here an analysis of tholins by high performance liquid chromatography (HPLC) coupled to HRMS to unveil this isomeric ambiguity for some of the major tholins compounds. By comparing chromatograms obtained when analyzing tholins and chemical standards, we strictly identified seven molecules in our tholins samples: melamine, cyanoguanidine, 6-methyl-1,3,5-triazine-2,4-diamine, 2,4,6-triaminopyrimidine, 3-amino- 1,2,4-triazole, 3,5-Dimethyl-1,2,4-triazole and 2,4-diamino-1,3,5-triazine. Several molecules, including hexamethylenetriamine (HMT) were not present at detectable levels in our sample. The use for the first time of a coupled HPLC-HRMS technique applied to tholins study demonstrated the interest of such a technique compared to single high-resolution mass spectrometry for the study of tholins composition.
Cassini discovered a plethora of neutral and ionised molecules in Titans ionosphere including, surprisingly, anions and negatively charged molecules extending up to 13,800 u/q. In this letter we forward model the Cassini electron spectrometer response function to this unexpected ionospheric component to achieve an increased mass resolving capability for negatively charged species observed at Titan altitudes of 950-1300 km. We report on detections consistently centered between 25.8-26.0 u/q and between 49.0-50.1 u/q which are identified as belonging to the carbon chain anions, CN$^-$/C$_3$N$^-$ and/or C$_2$H$^-$/C$_4$H$^-$, in agreement with chemical model predictions. At higher ionospheric altitudes, detections at 73-74 u/q could be attributed to the further carbon chain anions C$_5$N$^-$/C$_6$H$^-$ but at lower altitudes and during further encounters, extend over a higher mass/charge range. This, as well as further intermediary anions detected at $>$100 u, provide the first evidence for efficient anion chemistry in space involving structures other than linear chains. Furthermore, at altitudes below $sim$1100 km, the low mass anions ($<$150 u/q) were found to deplete at a rate proportional to the growth of the larger molecules, a correlation that indicates the anions are tightly coupled to the growth process. This study adds Titan to an increasing list of astrophysical environments where chain anions have been observed and shows that anion chemistry plays a role in the formation of complex organics within a planetary atmosphere as well as in the interstellar medium.
(Abridged) The birth environment of the Sun will have influenced the conditions in the pre-solar nebula, including the attainable chemical complexity, important for prebiotic chemistry. The formation and distribution of complex organic molecules (COMs) in a disk around a T Tauri star is investigated for two scenarios: (i) an isolated disk, and (ii) a disk irradiated externally by a nearby massive star. The chemistry is calculated along the accretion flow from the outer disk inwards using a comprehensive network. Two simulations are performed, one beginning with complex ices and one with simple ices only. For the isolated disk, COMs are transported without major alteration into the inner disk where they thermally desorb into the gas reaching an abundance representative of the initial assumed ice abundance. For simple ices, COMs efficiently form on grain surfaces under the conditions in the outer disk. Gas-phase COMs are released into the molecular layer via photodesorption. For the irradiated disk, complex ices are also transported inwards; however, they undergo thermal processing caused by the warmer conditions in the irradiated disk which tends to reduce their abundance along the accretion flow. For simple ices, grain-surface chemistry cannot synthesise COMs in the outer disk because the necessary grain-surface radicals, which tend to be particularly volatile, are not sufficiently abundant on the grain surfaces. Gas-phase COMs are formed in the inner region of the irradiated disk via gas-phase chemistry induced by the desorption of strongly bound molecules such as methanol; hence, the abundances are not representative of the initial molecular abundances injected into the outer disk. These results suggest that the composition of comets formed in isolated disks may differ from those formed in externally irradiated disks with the latter composed of more simple ices.
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