653 research outputs found
Crack fronts and damage in glass at the nanometer scale
We have studied the low speed fracture regime for different glassy materials
with variable but controlled length scales of heterogeneity in a carefully
mastered surrounding atmosphere. By using optical and atomic force microscopy
(AFM) techniques we tracked in real-time the crack tip propagation at the
nanometer scale on a wide velocity range (mm/s - pm/s and below). The influence
of the heterogeneities on this velocity is presented and discussed. Our
experiments reveal also -for the first time- that the crack progresses through
nucleation, growth and coalescence of nanometric damage cavities within the
amorphous phase. This may explain the large fluctuations observed in the crack
tip velocities for the smallest values. This behaviour is very similar to what
is involved, at the micrometric scale, in ductile fracture. The only difference
is very likely due to the related length scales (nanometric instead of
micrometric). Consequences of such a nano-ductile fracture mode observed at a
temperature far below the glass transition temperature in glass is finally
discussed.Comment: 12 pages, 8 figures, submitted to Journal of Physics: Condensed
Matter; Invited talk at Glass and Optical Materials Division Fall 2002
Meeting, Pittsburgh, Pa, US
Mumford dendrograms and discrete p-adic symmetries
In this article, we present an effective encoding of dendrograms by embedding
them into the Bruhat-Tits trees associated to -adic number fields. As an
application, we show how strings over a finite alphabet can be encoded in
cyclotomic extensions of and discuss -adic DNA encoding. The
application leads to fast -adic agglomerative hierarchic algorithms similar
to the ones recently used e.g. by A. Khrennikov and others. From the viewpoint
of -adic geometry, to encode a dendrogram in a -adic field means
to fix a set of -rational punctures on the -adic projective line
. To is associated in a natural way a
subtree inside the Bruhat-Tits tree which recovers , a method first used by
F. Kato in 1999 in the classification of discrete subgroups of
.
Next, we show how the -adic moduli space of
with punctures can be applied to the study of time series of
dendrograms and those symmetries arising from hyperbolic actions on
. In this way, we can associate to certain classes of dynamical
systems a Mumford curve, i.e. a -adic algebraic curve with totally
degenerate reduction modulo .
Finally, we indicate some of our results in the study of general discrete
actions on , and their relation to -adic Hurwitz spaces.Comment: 14 pages, 6 figure
Shock excitation of H in the James Webb Space Telescope era
(Abridged) H2 is the most abundant molecule in the Universe. Thanks to its
widely spaced energy levels, it predominantly lights up in warm gas, T > 100 K,
such as shocked regions, and it is one of the key targets of JWST observations.
These include shocks from protostellar outflows, all the way up to starburst
galaxies and AGN. Shock models are able to simulate H2 emission. We aim to
explore H2 excitation using such models, and to test over which parameter space
distinct signatures are produced in H2 emission. We present simulated H2
emission using the Paris-Durham shock code over an extensive grid of 14,000
plane-parallel stationary shock models, a large subset of which are exposed to
an external UV radiation field. The grid samples 6 input parameters: preshock
density, shock velocity, transverse magnetic field strength, UV radiation field
strength, cosmic-ray-ionization rate, and PAH abundance. Physical quantities,
such as temperature, density, and width, have been extracted along with H2
integrated line intensities. The strength of the transverse magnetic field, set
by the scaling factor, b, plays a key role in the excitation of H2. At low
values of b (<~ 0.3, J-type shocks), H2 excitation is dominated by
vibrationally excited lines; at higher values (b >~ 1, C-type shocks),
rotational lines dominate the spectrum for shocks with an external radiation
field comparable to (or lower than) the solar neighborhood. Shocks with b >= 1
can be spatially resolved with JWST for nearby objects. When the input kinetic
energy flux increases, the excitation and integrated intensity of H2 increases
similarly. An external UV field mainly serves to increase the excitation,
particularly for shocks where the input radiation energy is comparable to the
input kinetic energy flux. These results provide an overview of the energetic
reprocessing of input kinetic energy flux and the resulting H2 line emission.Comment: Published in A&
FUSE Observations of the HD Molecule toward HD 73882
The Lyman and Werner band systems of deuterated molecular hydrogen (HD) occur
in the far UV range below 1200 A. The high sensitivity of the FUSE mission can
give access, at moderate resolution, to hot stars shining through translucent
clouds, in the hope of observing molecular cores in which deuterium is
essentially in the form of HD. Thus, the measurement of the HD/H2 ratio may
become a new powerful tool to evaluate the deuterium abundance, D/H, in the
interstellar medium. We report here on the detection of HD toward the high
extinction star HD 73882 [E(B-V)=0.72]. A preliminary analysis is presented.Comment: 4 pages + 4 .ps figures. This paper will appear in a special issue of
Astrophysical Journal Letters devoted to the first scientific results from
the FUSE missio
Photoelectric effect on dust grains across the L1721 cloud in the rho Ophiuchi molecular complex
We present ISO-LWS measurements of the main gas cooling lines, C+ 158 mum and
O 63 mum towards a moderate opacity molecular cloud (Av=3), L1721, illuminated
by the B2 star nu Sco (X = 5-10). These data are combined with an extinction
map and IRAS dust emission images to test our understanding of gas heating and
cooling in photo-dissociation regions (PDRs). This nearby PDR is spatially
resolved in the IRAS images; variations in the IRAS colors across the cloud
indicate an enhanced abundance of small dust grains within the PDR. A spatial
correlation between the gas cooling lines and the infrared emission from small
dust grains illustrates the dominant role of small dust grains in the gas
heating through the photoelectric effect. The photoelectric efficiency,
determined from the observations by ratioing the power radiated by gas and
small dust grains, is in the range 2 to 3% in close agreement with recent
theoretical estimates. The brightness profiles across the PDR in the C+ 158 mum
and O 63 mum lines are compared with model calculations where the density
profile is constrained by the extinction data and where the gas chemical and
thermal balances are solved at each position. We show that abundance variations
of small dust grains across the PDR must be considered to account for the LWS
observations.Comment: 10 pages, 15 figure
CN and HCN in Dense Interstellar Clouds
We present a theoretical investigation of CN and HCN molecule formation in
dense interstellar clouds. We study the gas-phase CN and HCN production
efficiencies from the outer photon-dominated regions (PDRs) into the opaque
cosmic-ray dominated cores. We calculate the equilibrium densities of CN and
HCN, and of the associated species C+, C, and CO, as functions of the
far-ultraviolet (FUV) optical depth. We consider isothermal gas at 50 K, with
hydrogen particle densities from 10^2 to 10^6 cm^-3. We study clouds that are
exposed to FUV fields with intensities 20 to 2*10^5 times the mean interstellar
FUV intensity. We assume cosmic-ray H2 ionization rates ranging from 5*10^-17
s^-1, to an enhanced value of 5*10^-16 s^-1. We also examine the sensitivity of
the density profiles to the gas-phase sulfur abundance.Comment: Accepted for publication in ApJ, 33 pages, 8 figure
Multifluid, Magnetohydrodynamic Shock Waves with Grain Dynamics II. Dust and the Critical Speed for C Shocks
This is the second in a series of papers on the effects of dust on
multifluid, MHD shock waves in weakly ionized molecular gas. We investigate the
influence of dust on the critical shock speed, v_crit, above which C shocks
cease to exist. Chernoff showed that v_crit cannot exceed the grain
magnetosound speed, v_gms, if dust grains are dynamically well coupled to the
magnetic field. We present numerical simulations of steady shocks where the
grains may be well- or poorly coupled to the field. We use a time-dependent,
multifluid MHD code that models the plasma as a system of interacting fluids:
neutral particles, ions, electrons, and various ``dust fluids'' comprised of
grains with different sizes and charges. Our simulations include grain inertia
and grain charge fluctuations but to highlight the essential physics we assume
adiabatic flow, single-size grains, and neglect the effects of chemistry. We
show that the existence of a phase speed v_phi does not necessarily mean that C
shocks will form for all shock speeds v_s less than v_phi. When the grains are
weakly coupled to the field, steady, adiabatic shocks resemble shocks with no
dust: the transition to J type flow occurs at v_crit = 2.76 v_nA, where v_nA is
the neutral Alfven speed, and steady shocks with v_s > 2.76 v_nA are J shocks
with magnetic precursors in the ion-electron fluid. When the grains are
strongly coupled to the field, v_crit = min(2.76 v_nA, v_gms). Shocks with
v_crit < v_s < v_gms have magnetic precursors in the ion-electron-dust fluid.
Shocks with v_s > v_gms have no magnetic precursor in any fluid. We present
time-dependent calculations to study the formation of steady multifluid shocks.
The dynamics differ qualitatively depending on whether or not the grains and
field are well coupled.Comment: 43 pages with 17 figures, aastex, accepted by The Astrophysical
Journa
Glass breaks like metals, but at the nanometer scale
We report in situ Atomic Force Microscopy experiments which reveal the
presence of nanoscale damage cavities ahead of a stress-corrosion crack tip in
glass. Their presence might explain the departure from linear elasticity
observed in the vicinity of a crack tip in glass. Such a ductile fracture
mechanism, widely observed in the case of metallic materials at the micrometer
scale, might be also at the origin of the striking similarity of the
morphologies of fracture surfaces of glass and metallic alloys at different
length scales.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Lett, few minor
corrections, Fig. 1b change
Sulphur-bearing species in the star forming region L1689N
We report observations of the expected main S-bearing species (SO, SO2 and
H2S) in the low-mass star forming region L1689N. We obtained large scale
(~300''x200'') maps of several transitions from these molecules with the goal
to study the sulphur chemistry, i.e. how the relative abundances change in the
different physical conditions found in L1689N. We identified eight interesting
regions, where we carried out a quantitative comparative study: the molecular
cloud (as reference position), five shocked regions caused by the interaction
of the molecular outflows with the cloud, and the two protostars IRAS16293-2422
and 16293E. In the cloud we carefully computed the gas temperature and density
by means of a non-LTE LVG code, while in other regions we used previous
results. We hence derived the column density of SO, SO2 and H2S, together with
SiO and H2CO - which were observed previously - and their relevant abundance
ratios. We find that SiO is the molecule that shows the largest abundance
variations in the shocked regions, whereas S-bearing molecules show more
moderate variations. Remarkably, the region of the brightest SiO emission in
L1689N is undetected in SO2, H2S and H2CO and only marginally detected in SO.
In the other weaker SiO shocks, SO2 is enhanced with respect to SO. We propose
a schema in which the different molecular ratios correspond to different ages
of the shocks. Finally, we find that SO, SO2 and H2S have significant abundance
jumps in the inner hot core of IRAS16293-2422 and discuss the implications of
the measured abundances.Comment: Accepted 08/10/0
Deuterated Thioformaldehyde in the Barnard 1 Cloud
We present observations of the singly and doubly deuterated species of
thioformaldehyde, HDCS and DCS, towards the dark cloud Barnard 1. This is
the first detection of DCS in Space and in dense and cold prestellar
regions. Column densities obtained using rotational diagrams and a Large
Velocity Gradient model show an extremely high D-enhancement in
thioformaldehyde in Barnard 1. Although the column density of HCS is
smaller than that of HCO, both species show similar D-enhancements in their
singly and doubly deuterated species. A chemical model -including multiply
deuterated species- has been used in order to interpret the observations.
Predicted rotational frequencies from laboratory data for HDCS and DCS
are significantly in error when compared to the observed frequencies in Space.
Consequently, we have derived new rotational constants for these two species
and for HCS and HCS using the observed frequencies in Barnard 1.
The new rotational constants allow to predict the rotational transitions of
these species with the accuracy needed for the narrow line emerging from dark
clouds. Rotational constants for HDCS and DCS have been obtained from the
observed transitions in the laboratory and in Space.Comment: 33 pages, 6 figures, accepted for publication in Ap
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