Roughness of stylolites: a stress-induced instability with non local interactions

We study the roughness of stylolite surfaces (i.e. natural pressure-dissolution surfaces in sedimentary rocks) from profiler measurements at laboratory scales. The roughness is shown to be nicely described by a self-affine scaling invariance. At large scales, the roughness exponent is $\zeta_1 \approx 0.5$ and very different from that at small scales where $\zeta_2 \approx 1.1$. A cross-over length scale at around $\lambda_c =1$mm is well characterized and interpreted as a possible fossil stress measurement if related to the Asaro-Tiller-Grinfeld stress-induced instability. Measurements are consistent with a Langevin equation that describes the growth of stylolite surfaces in a quenched disordered material with long range elastic correlations.Comment: 4 pages, 5 figure

Reactions forming C(0,+)n=2,10, Cn=2,4H(0,+) and C3H(0,+) in the gas phase: semi empirical branching ratios

The aim of this paper is to provide a new set of branching ratios for interstellar and planetary chemical networks based on a semi empirical model. We applied, instead of zero order theory (i.e. only the most exoergic decaying channel is considered), a statistical microcanonical model based on the construction of breakdown curves and using experimental high velocity collision branching ratios for their parametriza- tion. We applied the model to ion-molecule, neutral-neutral, and ion-pair reactions implemented in the few popular databases for astrochemistry such as KIDA, OSU and UMIST. We studied the reactions of carbon and hydrocarbon species with electrons, He+, H+, CH+, CH, C, and C+ leading to intermediate complexes of the type Cn=2,10, Cn=2,4 H, C3 H2, C+n=2,10, Cn=2,4 H+, or C3 H+2 . Comparison of predictions with measurements supports the validity of the model. Huge deviations with respect to database values are often obtained. Effects of the new branching ratios in time dependant chemistry for dark clouds and for photodissociation region chemistry with conditions similar to those found in the Horsehead Nebula are discussed

The IRAM-30m line survey of the Horsehead PDR: III. High abundance of complex (iso-)nitrile molecules in UV-illuminated gas

Complex (iso-)nitrile molecules, such as CH3CN and HC3N, are relatively easily detected in our Galaxy and in other galaxies. We constrain their chemistry through observations of two positions in the Horsehead edge: the photo-dissociation region (PDR) and the dense, cold, and UV-shielded core just behind it. We systematically searched for lines of CH3CN, HC3N, C3N, and some of their isomers in our sensitive unbiased line survey at 3, 2, and 1mm. We derived column densities and abundances through Bayesian analysis using a large velocity gradient radiative transfer model. We report the first clear detection of CH3NC at millimeter wavelength. We detected 17 lines of CH3CN at the PDR and 6 at the dense core position, and we resolved its hyperfine structure for 3 lines. We detected 4 lines of HC3N, and C3N is clearly detected at the PDR position. We computed new electron collisional rate coefficients for CH3CN, and we found that including electron excitation reduces the derived column density by 40% at the PDR position. While CH3CN is 30 times more abundant in the PDR than in the dense core, HC3N has similar abundance at both positions. The isomeric ratio CH3NC/CH3CN is 0.15+-0.02. In the case of CH3CN, pure gas phase chemistry cannot reproduce the amount of CH3CN observed in the UV-illuminated gas. We propose that CH3CN gas phase abundance is enhanced when ice mantles of grains are destroyed through photo-desorption or thermal-evaporation in PDRs, and through sputtering in shocks. (abridged)Comment: Accepted for publication in Astronomy & Astrophysic

Dense molecular globulettes and the dust arc towards the runaway O star AE Aur (HD 34078)

Some runaway stars are known to display IR arc-like structures around them, resulting from their interaction with surrounding interstellar material. The properties of these features as well as the processes involved in their formation are still poorly understood. We aim at understanding the physical mechanisms that shapes the dust arc observed near the runaway O star AEAur (HD34078). We obtained and analyzed a high spatial resolution map of the CO(1-0) emission that is centered on HD34078, and that combines data from both the IRAM interferometer and 30m single-dish antenna. The line of sight towards HD34078 intersects the outer part of one of the detected globulettes, which accounts for both the properties of diffuse UV light observed in the field and the numerous molecular absorption lines detected in HD34078's spectra, including those from highly excited H2 . Their modeled distance from the star is compatible with the fact that they lie on the 3D paraboloid which fits the arc detected in the 24 {\mu}m Spitzer image. Four other compact CO globulettes are detected in the mapped area. These globulettes have a high density and linewidth, and are strongly pressure-confined or transient. The good spatial correlation between the CO globulettes and the IR arc suggests that they result from the interaction of the radiation and wind emitted by HD 34078 with the ambient gas. However, the details of this interaction remain unclear. A wind mass loss rate significantly larger than the value inferred from UV lines is favored by the large IR arc size, but does not easily explain the low velocity of the CO globulettes. The effect of radiation pressure on dust grains also meets several issues in explaining the observations. Further observational and theoretical work is needed to fully elucidate the processes shaping the gas and dust in bow shocks around runaway O stars. (Abridged)Comment: Accepted for publication in Astronomy & Astrophysic

Chemical complexity in the Horsehead photodissociation region

The interstellar medium is known to be chemically complex. Organic molecules with up to 11 atoms have been detected in the interstellar medium, and are believed to be formed on the ices around dust grains. The ices can be released into the gas-phase either through thermal desorption, when a newly formed star heats the medium around it and completely evaporates the ices; or through non-thermal desorption mechanisms, such as photodesorption, when a single far-UV photon releases only a few molecules from the ices. The first one dominates in hot cores, hot corinos and strongly UV-illuminated PDRs, while the second one dominates in colder regions, such as low UV-field PDRs. This is the case of the Horsehead were dust temperatures are ~20-30K, and therefore offers a clean environment to investigate what is the role of photodesorption. We have carried-out an unbiased spectral line survey at 3, 2 and 1mm with the IRAM-30m telescope in the Horsehead nebula, with an unprecedented combination of bandwidth high spectral resolution and sensitivity. Two positions were observed: the warm PDR and a cold condensation shielded from the UV field (dense core), located just behind the PDR edge. We summarize our recently published results from this survey and present the first detection of the complex organic molecules HCOOH, CH2CO, CH3CHO and CH3CCH in a PDR. These species together with CH3CN present enhanced abundances in the PDR compared to the dense core. This suggests that photodesorption is an efficient mechanism to release complex molecules into the gas-phase in far-UV illuminated regions.Comment: 15 pages, 7 figures, 7 tables, Accepted in Faraday discussions 16

Experimental stylolites in quartz and modeled application to natural structures.

Experimental stylolites have been observed at stressed contacts between quartz grains loaded for a period of several months in presence of aqueous silica solution, at 350°C under 50 MPa of differential stress. Stereoscopic analysis of pairs of SEM images, processed in the same way as earth-surface elevation data gives the stylolites topography. Coupled with observations of closed interactions between dissolution pits and stylolitic peaks, these data illuminate the mechanism of stylolite formation. The complex geometry of stylolite surfaces is imposed by the interplay between the development of dissolution peaks in favored locations (fast dissolution pits) and the mechanical properties of the solid-fluid-solid interfaces. Simple mechanical modeling expresses the crucial competition that could rule the development of stylolites: (i) a stress related process (modeled as the stiffness of springs (N/m3) activates the heterogeneous dissolution rates of the solid interface that promotes the deflection. In parallel, (ii) the strength of the solid interface, modeled as the stiffness of a membrane (N/m) and equivalent to a surface tension) limits the deflection and is opposed to its development. The modeling produces stylolitic surfaces with characteristic geometries that vary from conical to columnar shaped stylolites when both the effect of dissolution-rate heterogeneity and the strength properties of the rock are included

Experimental microstylolites in quartz and modelling of natural stylolitic structures

International audienceExperimental microstylolites have been observed at stressed contacts between quartz grains loaded for several weeks in the presence of an aqueous silica solution, at 350 8C and 50 MPa of differential stress. Stereoscopic analysis of pairs of SEM images yielded a digital elevation model of the surface of the microstylolites. Fourier analyses of these microstylolites reveal a self-affine roughness (with a roughness exponent H of 1.2). Coupled with observations of close interactions between dissolution pits and stylolitic peaks, these data illustrate a possible mechanism for stylolite formation. The complex geometry of stylolite surfaces is imposed by the interplay between the development of dissolution peaks in preferential locations (fast dissolution pits) and the mechanical properties of the solid-fluid-solid interfaces. Simple mechanical modeling expresses the crucial competition that could rule the development of microstylolites: (i) a stress-related process, modeled in terms of the stiffness of springs that activate the heterogeneous dissolution rates of the solid interface, promotes the deflection. In parallel, (ii) the strength of the solid interface, modeled in terms of the stiffness of a membrane, is equivalent to a surface tension that limits the deflection and opposes its development. The modeling produces stylolitic surfaces with characteristic geometries varying from conical to columnar when both the effect of dissolution-rate heterogeneity and the strength properties of the rock are taken into account. A self-affine roughness exponent (Hz1.2) measured on modeled surfaces is comparable with natural stylolites at small length scale and experimental microstylolites

Infants' overlapping vocalizations during maternal humming : Contributions to the synchronization of preterm dyads

Abstract Despite the neurological vulnerability of premature newborns, there is evidence that they are able to process temporal aspects of the maternal voice, as a previous study observed more overlapping vocalizations during maternal humming versus speech. However, there is a lack of knowledge about the markers of the infants’ overlapping vocalizations. Our aim was to identify the location of overlapping vocalizations during the humming and the impacts of maternal antenatal and postnatal engagement of infant-directed singing on: (1) the features of humming and (2) the infants’ overlapping vocalizations during humming. Preterm dyads (N = 36) were observed in silent, speech, and humming conditions. Microanalysis was performed using the Elan Program to identify the location of the overlapping vocalizations during the humming phrase. Infants’ overlapping vocalizations were found predominantly at the ends of each humming phrase; almost half of the overlaps occurred on the final note. More overlapping vocalization in the final notes were observed in female infants. Antenatal and postnatal experiences of ID singing are influenced by the mothers’ nationality and contribute to maternal humming style. Preterm newborns synchronize with maternal humming, anticipating the end of musical phrases. The ability to synchronize seems to be phylogenetically associated with gender differences.info:eu-repo/semantics/publishedVersio

Single-contact pressure solution creep on calcite monocrystals

Pressure solution creep rates and interface structures have been measured by two methods on calcite single crystals. In the first kind of experiments, calcite monocrystals were indented at 40°C for six weeks using ceramic indenters under stresses in the 50-200 MPa range in a saturated solution of calcite and in a calcite-saturated aqueous solution of NH4Cl. The deformation (depth of the hole below the indenter) is measured ex-situ at the end of the experiment. In the second type of experiment, calcite monocrystals were indented by spherical glass indenters for 200 hours under stresses in the 0-100 MPa range at room temperature in a saturated aqueous solution of calcite. The displacement of the indenter was continuously recorded using a specially constructed differential dilatometer. The experiments conducted in a calcite-saturated aqueous solution of NH4Cl show an enhanced indentation rate owing to the fairly high solubility of calcite in this solution. In contrast, the experiments conducted in a calcite-saturated aqueous solution show moderate indentation rate and the dry control experiments did not show any measurable deformation. The rate of calcite indentation is found to be inversely proportional to the indenter diameter, thus indicating that the process is diffusion-controlled. The microcracks in the dissolution region under the indenter dramatically enhance the rate of calcite indentation by a significant reduction of the distance of solute transport in the trapped fluid phase. This result indicates that care should be taken in extrapolating the kinetic data of pressure solution creep from one mineral to another