744 research outputs found
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
and very different from that at small scales where
. A cross-over length scale at around 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
Modeling the growth of stylolites in sedimentary rocks
[1] Stylolites are ubiquitous pressure solution seams found in sedimentary rocks. Their morphology is shown to follow two self-affine regimes. Analyzing the scaling properties of their height over their average direction shows that (1) at small scale, they are self-affine surfaces with a Hurst exponent around 1, and (2) at large scale, they follow another self-affine scaling with Hurst exponent around 0.5. In the present paper, we show theoretically the influence of the main principal stress and the local geometry of the stylolitic interface on the dissolution reaction rate. We compute how it is affected by the deviation between the principal stress axis and the local interface between the rock and the soft material in the stylolite. The free energy entering in the dissolution reaction kinetics is expressed from the surface energy term and via integration from the stress perturbations due to these local misalignments. The resulting model shows the interface evolution at different stress conditions. In the stylolitic case, i.e., when the main principal stress is normal to the interface, two different stabilizing terms dominate at small and large scales which are linked respectively to the surface energy and to the elastic interactions. Integrating the presence of small-scale heterogeneities related to the rock properties of the grains in the model leads to the formulation of a Langevin equation predicting the dynamic evolution of the surface. This equation leads to saturated surfaces obeying the two observed scaling laws. Analytical and numerical analysis of this surface evolution model shows that the crossover length separating both scaling regimes depends directly on the applied far-field stress magnitude. This method gives the basis for the development of a paleostress magnitude marker. We apply the computation of this marker, i.e., the morphological analysis, on a stylolite found in the Dogger limestone layer located in the neighborhood of the ANDRA Underground Research Laboratory at Bure (eastern France). The results are consistent with the two scaling regimes expected, and the practical determination of the major principal paleostress, from the estimation of a crossover length, is illustrated on this example
Giant Molecular Clouds in the Local Group Galaxy M33
We present an analysis of the systematic CO(2-1) survey at 12" resolution
covering most of the local group spiral M 33 which, at a distance of 840 kpc,
is close enough that individual giant molecular clouds (GMCs) can be
identified. The goal of this work is to study the properties of the GMCs in
this subsolar metallicity galaxy. The CPROPS (Cloud Properties) algorithm
(Rosolowsky & Leroy 2006) was used to identify 337 GMCs in M 33, the largest
sample to date in an external galaxy. The sample is used to study the GMC
luminosity function, or mass spectrum under the assumption of a constant
N(H2)/ICO ratio. We find that n(L)dL = K*L^(-2.0\pm0.1) for the entire sample.
However, when the sample is divided into inner and outer disk samples, the
exponent changes from 1.6 \pm 0.2 for the centre 2 kpc to 2.3 \pm 0.2 for
galactocentric distances larger than 2 kpc. Based on the emission in the FUV,
Halpha, 8mu, and 24mu bands, each cloud was classified in terms of its star
forming activity - no star formation, embedded, or exposed star formation
(visible in FUV and Halpha). At least one sixth of the clouds had no (massive)
star formation, suggesting that the average time required for star formation to
start is about one sixth of the total time for which the object is identifiable
as a GMC. The clouds without star formation have significantly lower CO
luminosities than those with star formation, whether embedded or exposed,
presumably related to the lack of heating sources. Taking the cloud sample as a
whole, the main non-trivial correlation was the decrease in cloud CO brightness
(or luminosity) with galactocentric radius. The complete cloud catalog,
including CO and HI spectra and the CO contours on the FUV, Halpha, 8mu, and
24mu images is presented in the appendix.Comment: Accepted for publication in Astronomy & Astrophysics. Language
correction and change to figure 1. Figure 6 was shifted and cropped in the
previous versio
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
The hyperfine structure in the rotational spectrum of CF+
Context. CF+ has recently been detected in the Horsehead and Orion Bar
photo-dissociation regions. The J=1-0 line in the Horsehead is double-peaked in
contrast to other millimeter lines. The origin of this double-peak profile may
be kinematic or spectroscopic. Aims. We investigate the effect of hyperfine
interactions due to the fluorine nucleus in CF+ on the rotational transitions.
Methods. We compute the fluorine spin rotation constant of CF+ using high-level
quantum chemical methods and determine the relative positions and intensities
of each hyperfine component. This information is used to fit the theoretical
hyperfine components to the observed CF+ line profiles, thereby employing the
hyperfine fitting method in GILDAS. Results. The fluorine spin rotation
constant of CF+ is 229.2 kHz. This way, the double-peaked CF+ line profiles are
well fitted by the hyperfine components predicted by the calculations. The
unusually large hyperfine splitting of the CF+ line therefore explains the
shape of the lines detected in the Horsehead nebula, without invoking intricate
kinematics in the UV-illuminated gas.Comment: 2 pages, 1 figure, Accepted for publication in A&
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
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