Characterizing filaments in regions of high-mass star formation: High-resolution submilimeter imaging of the massive star-forming complex NGC 6334 with ArTeMiS

Context. Herschel observations of nearby molecular clouds suggest that interstellar filaments and prestellar cores represent two fundamental steps in the star formation process. The observations support a picture of low-mass star formation according to which filaments of ~0.1 pc width form first in the cold interstellar medium, probably as a result of large-scale compression of interstellar matter by supersonic turbulent flows, and then prestellar cores arise from gravitational fragmentation of the densest filaments. Whether this scenario also applies to regions of high-mass star formation is an open question, in part because the resolution of Herschel is insufficient to resolve the inner width of filaments in the nearest regions of massive star formation. Aims. In an effort to characterize the inner width of filaments in high-mass star-forming regions, we imaged the central part of the NGC 6334 complex at a resolution higher by a factor of >3 than Herschel at 350 μm. Methods. We used the large-format bolometer camera ArTéMiS on the APEX telescope and combined the high-resolution ArTéMiS data at 350 μm with Herschel/HOBYS data at 70–500 μm to ensure good sensitivity to a broad range of spatial scales. This allowed us to study the structure of the main narrow filament of the complex with a resolution of 8″ or <0.07 pc at d ~ 1.7 kpc. Results. Our study confirms that this filament is a very dense, massive linear structure with a line mass ranging from ~500 M⊙/pc to ~2000 M⊙/pc over nearly 10 pc. It also demonstrates for the first time that its inner width remains as narrow as W ~ 0.15 ± 0.05 pc all along the filament length, within a factor of <2 of the characteristic 0.1 pc value found with Herschel for lower-mass filaments in the Gould Belt. Conclusions. While it is not completely clear whether the NGC 6334 filament will form massive stars in the future, it is two to three orders of magnitude denser than the majority of filaments observed in Gould Belt clouds, and has a very similar inner width. This points to a common physical mechanism for setting the filament width and suggests that some important structural properties of nearby clouds also hold in high-mass star-forming regions

Experimental Evidence for Ultrafast Electronic Relaxation in Molecules, Mediated by Diffuse States

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Multifragmentation of the Au(H2O)n10+ Cluster Ions by Collision with Helium

International audienceA beam of mass selected Au(H2O)n+ (n = 1-10) cluster ions has been generated using a source that couples laser evaporation, supersonic expansion, and tandem time-of-flight mass spectrometry. A collision-induced-dissociation (CID) experiment has been performed with helium at energies in the range of 0.2-3 eV. A maximum of four water molecules is lost by the clusters. The key point is the data analysis where the total (loss of at least one water molecule) and partial (loss of a specified number of water molecules) CID cross sections have been simulated using a model describing the energy transfer between helium and the cluster. This has allowed us to fit the experimental data and to give insight into the structure and energetic of the Au(H2O)n+ clusters, unraveling the existence of two kinds of isomers for these clusters, one with two water molecules coordinating the metal ion, tentatively assigned to (H2O)p(H2O)Au+(H2O)(H2O)n-p-2, and a more compact one with three (or more) coordinating water molecules. Multifragmentation of Au(H2O)n6+ clusters seems to involve a competition between the sequential loss of several water molecules and the loss of a water dimer and possibly a trimer

Binding energies of first and second shell water molecules in the Fe(H

The fragmentation cross-section of the Fe(H2O)$_{1,2}^+$, Co(H2O)$_{1,2}^+$ and Au(H2O)$_{1,2}^+$ ions were measured, as a function of the collision energy. Threshold energies of $1.4\pm0.2$ eV, $1.4\pm0.2$ eVand $1.7\pm0.1$ eVwere measured for the monohydrated $\rm Fe^+$, $\rm Co^+$ and $\rm Au^+$ ions respectively, in fair agreement with the existing literature. Small threshold energies of $0.7\pm0.2$ eV, $0.7\pm0.2$ eVand $0.5\pm0.1$ eV were found for the Fe(H2O)$_{2}^+$, Co(H2O)$_{2}^+$ and Au(H2O)$_{2}^+$ clusters respectively. Secondary thresholds were observed on the cross-section, respectively at $1.7\pm0.3$ eV and $2.0\pm0.2$ eV for the Co(H2O)$_{2}^+$ and Au(H2O)$_{2}^+$ clusters. This double threshold behavior could be attributed to the presence of two kinds of isomers in the beam. The upper threshold is associated with clusters where both water molecules are linked to the metal ion (first solvation shell), whereas the lower threshold corresponds to clusters with one water molecule in the first solvation shell and the other in the second shell. Such an analysis documents the binding energy of either a first shell or a second shell water molecule in the M(H2O)$_{2}^+$ cluster ions

Probing several structures of Fe(H2O)n+ and Co(H2O)n+ (n=1,...,10) cluster ions

International audienceCo(H2O)n≤10+ and Fe(H2O)n≤10+ cluster ions were generated in a source combining laser ablation and a supersonic expansion. The clusters were fragmented to get insight into their structure. Two questions were addressed: first, the arrangement of the water molecules about the metal ion, and second, the electronic properties of the solvated metal ion. Collision induced dissociation by helium was used to answer the first question, especially for the smallest clusters with n=2 and 3. This revealed the existence of filament structures where one water molecule lies in the second solvation shell about the metal ion although the first shell is not filled. The binding energies of second shell water in Co(H2O)2+ and Fe(H2O)2+ are 0.45±0.1 and 0.5±0.1 eV, respectively. The answer to the second question was provided by photofragmentation experiments where the cluster ions are illuminated at 532, 355 and 266 nm. The most striking effect is seen with cobalt ions where increasing the number n of water molecules above n=7 allows one to built up an absorption band that is known when Co+ is solvated in liquid water. The two fragmentation techniques appear as complementary

Spectroscopic manifestations of phase changes in CaAr

The finite temperature optical spectroscopy of CaArn clusters in the range $6\leq n\leq 146$ is investigated using a Diatomics-In-Molecule (DIM) Hamiltonian and classical parallel tempering Monte Carlo simulations. The absorption spectrum is calculated in the vertical approximation at various temperatures between 2 K and 50 K. Several typical situations are reported. CaAr6 shows a strong thermal broadening and shift of the spectral lines, possibly associated with isomerization. CaAr13 only shows some broadening. CaAr37 exhibits features corresponding to coexisting isomers at low temperature. Finally, the abrupt changes in the absorption spectrum in CaAr146 at about 20 K are indicative of surface diffusion