180 research outputs found
The gas phase origin of complex organic molecules precursors in prestellar cores
No full textComplex organic molecules (COMs) have long been observed in the warm regions surrounding nascent protostars. The recent discovery of oxygen-bearing COMs like methyl formate or dimethyl ether in prestellar cores (Bacmann et al. [2]), where gas and dust temperatures rarely exceed 10–15 K, has challenged the previously accepted models according to which COM formation relied on the diffusion of heavy radicals on warm (∼30 K) grains. Following these detections, new questions have arisen: do non-thermal processes play a role in increasing radical mobility or should new gas-phase routes be explored? The radicals involved in the formation of the aforementioned COMs, HCO and CH3O represent intermediate species in the grain-surface synthesis of methanol which proceeds via successive hydrogenations of CO molecules in the ice. We present here observations of methanol and its grain-surface precursors HCO, H2CO, CH3O in a sample of prestellar cores and derive their relative abundances. We find that the relative abundances HCO:H2CO:CH3O:CH3OH are constant across the core sample, close to 10:100:1:100. Our results also show that the amounts of HCO and CH3O are consistent with a gas-phase synthesis of these species from H2CO and CH3OH via radical-neutral or ion-molecule reactions followed by dissociative recombinations. Thus, while grain chemistry is necessary to explain the abundances of the parent volatile CH3OH, and possibly H2CO, the reactive species HCO and CH3O might be daughter molecules directly produced in the gas-phase
Paramètres atomiques et structures magnétiques de UCrO4
No full textLa diffraction neutronique nucléaire permet de préciser les coordinences octaédriques de Cr et de U dans UCrO₄ orthorhombique (a = 4,87₁ Å ; b = 11,78₇ Å ; c = 5,05₃ Å ; groupe Pbcn). Cr est à l'état trivalent, U à l'état pentavalent. La diffraction neutronique magnétique à 4°,2 K montre un arrangement antiferromagnétique avec des liaisons Cr˗Cr et Cr˗O˗U négatives. Les valeurs observées des spins Sobs. (Cr) = 1,20 ± 0,05, Sobs. (U) = 0,15 ± 0,02 sont considérablement réduites par des effets de covalence par rapport aux valeurs théoriques de spin seul Sth (Cr³⁺) = 3/2 et Sth (U⁵⁺) = 1/2.Bacmann Madeleine, Lewy-Bertaut Erwin Félix, Bassi Gérard. Paramètres atomiques et structures magnétiques de UCrO4. In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 88, 2, 1965. pp. 214-218
The origin of gas-phase HCO and CH
No full textContext. The recent unexpected detection of terrestrial complex organic molecules in the cold (~10 K) gas has cast doubts on the commonly accepted formation mechanisms of these species. Standard gas-phase mechanisms are inefficient and tend to underproduce these molecules, and many of the key reactions involved are unconstrained. Grain-surface mechanisms, which were presented as a viable alternative, suffer from the fact that they rely on grain surface diffusion of heavy radicals, which is not possible thermally at very low temperatures.
Aims. One of the simplest terrestrial complex organic molecules, methanol is believed to form on cold grain surfaces following from successive H atom additions on CO. Unlike heavier species, H atoms are very mobile on grain surfaces even at 10 K. Intermediate species involved in grain surface methanol formation by CO hydrogenation are the radicals HCO and CH3O, as well as the stable species formaldehyde H2CO. These radicals are thought to be precursors of complex organic molecules on grain surfaces.
Methods. We present new observations of the HCO and CH3O radicals in a sample of prestellar cores and carry out an analysis of the abundances of the species HCO, H2CO, CH3O, and CH3OH, which represent the various stages of grain-surface hydrogenation of CO to CH3OH.
Results. The abundance ratios between the various intermediate species in the hydrogenation reaction of CO on grains are similar in all sources of our sample, HCO:H2CO:CH3O:CH3OH ~10:100:1:100. We argue that these ratios may not be representative of the primordial abundances on the grains but, rather, suggest that the radicals HCO and CH3O are gas-phase products of the precursors H2CO and CH3OH, respectively. Various gas-phase pathways are considered, including neutral-neutral and ion-molecule reactions, and simple estimates of HCO and CH3O abundances are compared to the observations. Critical reaction rate constants, branching ratios, and intermediate species are finally identified
Atomic structure of grain boundaries in semiconductors
No full textThis paper summarizes the electron microscope observations (high resolution, diffraction and α-fringes) on germanium bicrystals. Observed structures were found isomorphous to those of silicon. They tend to form a 2D-periodic medium with eventually additional linear defects. In pure tilt grain boundaries the tetrahedral coordination is always restored by dangling bond reconstruction. The electrical activity of such grain boundaries can therefore only be due to point defects (intrinsic defects or impurities).Cet article résume l'ensemble des observations en microscopie électronique (haute résolution, diffraction et franges-α) sur les bicristaux de germanium. Les structures observées sont totalement isomorphes à celles qui existent dans le silicium. Elles tendent à former des arrangements bidimensionnels périodiques sur lesquels se superposent éventuellement des défauts linéaires. Sur les joints de flexion il a été montré que l'environnement des atomes reste toujours tetraédrique par reconstruction des liaisons pendantes. L'activité électrique éventuelle de ces joints ne peut donc venir que de défauts ponctuels (intrinsèques ou impuretés)
Prestellar core modeling in the presence of a filament
No full textContext. Lacking a paradigm for the onset of star formation, it is
important to derive basic physical properties of prestellar cores and filaments like
density and temperature structures.
Aims. We aim to disentangle the spatial variation in density and
temperature across the prestellar core L1689B, which is embedded in a filament. We want to
determine the range of possible central densities and temperatures that are consistent
with the continuum radiation data.
Methods. We apply a new synergetic radiative transfer method: the
derived 1D density profiles are both consistent with a cut through the Herschel
PACS/SPIRE and JCMT SCUBA-2 continuum maps of L1689B and with a derived local
interstellar radiation field. Choosing an appropriate cut along the filament major axis,
we minimize the impact of the filament emission on the modeling.
Results. For the bulk of the core (5000−20 000 au) an isothermal sphere model with
a temperature of around 10 K provides the best fits. We show that the power law index of
the density profile, as well as the constant temperature can be derived directly from the
radial surface brightness profiles. For the inner region (<5000 au), we find a range of densities
and temperatures that are consistent with the surface brightness profiles and the local
interstellar radiation field. Based on our core models, we find that pixel-by-pixel single
temperature spectral energy distribution fits are incapable of determining dense core
properties.
Conclusions. We conclude that, to derive physical core properties, it is
important to avoid azimuthally-averaging core and filament. Correspondingly, derived core
masses are too high since they include some mass of the filament, and might introduce
errors when determining core mass functions. The forward radiative transfer methods also
avoids the loss of information owing to smearing of all maps to the coarsest spatial
resolution. We find the central core region to be colder and denser than estimated in
recent inverse radiative transfer modeling, possibly indicating the start of star
formation in L1689B
Excitation of Molecules and Atoms for Astrophysics (EMAA): A spectroscopic and collisional database
No full textInternational audienceContext . In astrophysical environments, the energy levels of molecules, atoms, and ions are rarely populated at local thermodynamic equilibrium (LTE), that is the level populations reflect the competition between radiative and collisional processes. Interpreting non-LTE spectra therefore requires knowing both the Einstein radiative coefficients and the collisional rate coefficients. For a long time, inelastic collision calculations were limited to the most abundant and simple species, but they have now entered a new era thanks to the increase of computer power and the development of high-accuracy potential energy surfaces. Aims . With the advent of observatories with powerful spectral capabilities, such as ALMA or the JWST, and the wealth of new species detected, obtaining collisional rate coefficients quickly has become essential. We aim to provide the community with atomic and molecular data available from the literature for an ever-increasing number of systems. Methods . We have developed a database hosting both the collisional and spectroscopic data necessary to interpret spectra of non-LTE environments such as the (extra)galactic interstellar media, star-forming regions, and cometary atmospheres. We provide data files that can be employed directly in widely used non-LTE radiative transfer codes such as RADEX . Results . To date, the database contains 106 targets, including nuclear-spin isomers and isotopologues and nine possible projectiles (ortho-H 2 , para-H 2 , H, H + , electrons, He, CO, ortho-H 2 O and para-H 2 O, depending on the targets), for a total of 311 target-projectile data files
Detection of complex organic molecules in a prestellar core: a new challenge for astrochemical models
No full textContext. Complex organic molecules (COMs) have long been detected in the interstellar medium, especially in hot cores and in the hot corinos of low-mass protostars. Their formation routes however remain uncertain. Both warm gas-phase reactions and warm grain-surface reactions have been invoked to account for their presence in low-mass protostars. In this latter scheme, COMs result from radical-radical reactions on the grains as radicals become mobile when the nascent protostar warms up its surroundings and the resulting molecules are subsequently desorbed into the gas phase at higher temperatures.
Aims. Prestellar cores are the direct precursors of low-mass protostars and offer a unique opportunity to study the formation of COMs before the warm-up phase. Their very low temperatures (≤ 10 K) and the absence of any heating source or outflow exclude any efficient warm gas phase or warm dust chemistry, so that the presence of COMs in prestellar cores would have to originate from non-thermal chemical processes.
Methods. We used the IRAM 30 m telescope to look for four O-bearing COMs (acetaldehyde CH3CHO, dimethyl ether CH3OCH3, methyl formate CH3OCHO, and ketene CH2CO) in the prestellar core L1689B.
Results. We report the unambiguous detection of all four molecules in the cold gas phase of L1689B. These detections support the role played by non-thermal (possibly photolytic) processes in COM formation and desorption, though the presence of dimethyl ether is so far unexplained by current grain formation scenarios. The data show univocally that COM synthesis has already started at the prestellar stage and suggests at least part of the COMs detected in hot corinos have a prestellar origin
The excitation of NH2 in the interstellar medium
No full textInternational audienceAccurate estimation of the abundance of the NH 2 radical in the interstellar medium requires accurate radiative and collisional rate coefficients. The calculation of hyperfine-resolved rate coefficients for the collisional (de-)excitation of NH 2 by both ortho-and para-H 2 are presented in this work. Hyperfine-resolved rate coefficients are calculated from pure rotational close-coupling rate coefficients using the M j randomizing approximation. Rate coefficients for temperatures ranging from 5 to 150 K were computed for all hyperfine transitions among the first 15 rotational energy levels of both ortho-and para-NH 2 in collisions with ortho-and para-H 2. The new data were then employed in radiative transfer calculations to simulate the excitation of NH 2 in typical star-forming regions such as W31C. We compared the excitation and brightness temperatures for different NH 2 transitions obtained using the new and the previously available collisional data. It is found that the new rate coefficients increase the simulated line intensities. As a consequence, NH 2 abundance derived from the observations will be significantly reduced by the use of the present rate coefficients
Recherche des orientations mutuelles optimales entre une phase CFC et une phase CC. Cas du système cuivre-chrome
No full textThe 0-lattice method has been used in order to predict possible mutual orientations between an f. c. c. phase (Cu) and a b. c. c. phase (Cr). The result of these calculations is in good enough agreement with previous experimental observations.La méthode du réseau 0 a été utilisée pour rechercher les orientations mutuelles préférentielles possibles entre les phases du système cuivre-chrome. Les orientations mutuelles favorables déduites des calculs présentent un accord satisfaisant avec les observations expérimentales
Detection of protonated formaldehyde in the prestellar core L1689B
No full textComplex organic molecules (COMs) are detected in many regions of the interstellar medium, including prestellar cores. However, their formation mechanisms in cold (~10 K) cores remain to this date poorly understood. The formyl radical HCO is an important candidate precursor for several O-bearing terrestrial COMs in cores, as an abundant building block of many of these molecules. Several chemical routes have been proposed to account for its formation: on grain surfaces, as an incompletely hydrogenated product of H addition to frozen-out CO molecules; and in the gas phase, either as the product of the reaction between H2CO and a radical or as a product of dissociative recombination of protonated formaldehyde H2COH+. The detection and abundance determination of H2COH+, if present, could provide clues as to whether this latter scenario might apply. We searched for protonated formaldehyde H2COH+ in the prestellar core L1689B using the IRAM 30 m telescope. The H2COH+ ion is unambiguously detected, for the first time, in a cold (~10 K) source. The derived abundance agrees with a scenario in which the formation of H2COH+ results from the protonation of formaldehyde. We use this abundance value to constrain the branching ratio of the dissociative recombination of H2COH+ towards the HCO channel to ~10−30%. This value could however be lower if HCO were efficiently formed from neutral-neutral reactions in the gas phase, and we stress the need for laboratory measurements of the rate constants of these reactions at 10 K. Given the experimental difficulties in measuring branching ratios experimentally, observations can place valuable constraints on these values and provide useful input for chemical networks
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