899 research outputs found
The influence of normal stress and sliding velocity on the frictional behaviour of calcite at room temperature. Insights from laboratory experiments and microstructural observations
The presence of calcite in and near faults, as the dominant material, cement, or vein fill,
indicates that the mechanical behaviour of carbonate-dominated material likely plays an important role in shallow- and mid-crustal faulting. To better understand the behaviour of calcite,
under loading conditions relevant to earthquake nucleation, we sheared powdered gouge of
Carrara Marble, >98 per cent CaCO3, at constant normal stresses between 1 and 100 MPa
under water-saturated conditions at room temperature. We performed slide-hold-slide tests,
1–3000 s, to measure the amount of static frictional strengthening and creep relaxation, and
velocity-stepping tests, 0.1–1000 µm s–1, to evaluate frictional stability. We observe that the
rates of frictional strengthening and creep relaxation decrease with increasing normal stress
and diverge as shear velocity is increased from 1 to 3000 µm s–1 during slide-hold-slide experiments. We also observe complex frictional stability behaviour that depends on both normal
stress and shearing velocity. At normal stresses less than 20 MPa, we observe predominantly
velocity-neutral friction behaviour. Above 20 MPa, we observe strong velocity-strengthening
frictional behaviour at low velocities, which then evolves towards velocity-weakening friction
behaviour at high velocities. Microstructural analyses of recovered samples highlight a variety
of deformation mechanisms including grain size reduction and localization, folding of calcite grains and fluid-assisted diffusion mass transfer processes promoting the development of
calcite nanograins in the highly deformed portions of the experimental fault. Our combined
analyses indicate that calcite fault gouge transitions from brittle to semi-brittle behaviour at
high normal stress and slow sliding velocities. This transition has important implications for
earthquake nucleation and propagation on faults in carbonate-dominated lithologies
Modelling the sulphur chemistry evolution in Orion KL
We study the sulphur chemistry evolution in the Orion KL along the gas and
grain phases of the cloud. We investigate the processes that dominate the
sulphur chemistry and to determine how physical and chemical parameters, such
as the final star mass and the initial elemental abundances, influence the
evolution of the hot core and of the surrounding outflows and shocked gas (the
plateau). We independently modelled the chemistry evolution of both components
using the time-dependent gas-grain model UCL_CHEM and considering two different
phase calculations. Phase I starts with the collapsing cloud and the depletion
of atoms and molecules onto grain surfaces. Phase II starts when a central
protostar is formed and the evaporation from grains takes place. We show how
the gas density, the gas depletion efficiency, the initial sulphur abundance,
the shocked gas temperature and the different chemical paths on the grains
leading to different reservoirs of sulphur on the mantles affect
sulphur-bearing molecules at different evolutionary stages. We also compare the
predicted column densities with those inferred from observations of the species
SO, SO2, CS, OCS, H2S and H2CS. The models that reproduce the observations of
the largest number of sulphur-bearing species are those with an initial sulphur
abundance of 0.1 times the sulphur solar abundance and a density of at least
n_H=5x10^6 cm^-3 in the shocked gas region. We conclude that most of the
sulphur atoms were ionised during Phase I, consistent with an inhomogeneous and
clumpy region where the UV interstellar radiation penetrates leading to sulphur
ionisation. We also conclude that the main sulphur reservoir on the ice mantles
was H2S. In addition, we deduce that a chemical transition currently takes
place in the shocked gas, where SO and SO2 gas-phase formation reactions change
from being dominated by O2 to being dominated by OH.Comment: 14 pages, 28 figures, 6 table
Extended warm gas in Orion KL as probed by methyl cyanide
In order to study the temperature distribution of the extended gas within the
Orion Kleinmann-Low nebula, we have mapped the emission by methyl cyanide
(CH3CN) in its J=6_K-5_K, J=12_K-11_K, J=13_K-12_K, and J=14_K-13_K transitions
at an average angular resolution of ~10 arcsec (22 arcsec for the 6_K-5_K
lines), as part of a new 2D line survey of this region using the IRAM 30m
telescope. These fully sampled maps show extended emission from warm gas to the
northeast of IRc2 and the distinct kinematic signatures of the hot core and
compact ridge source components. We have constructed population diagrams for
the four sets of K-ladder emission lines at each position in the maps and have
derived rotational excitation temperatures and total beam-averaged column
densities from the fitted slopes. In addition, we have fitted LVG model spectra
to the observations to determine best-fit physical parameters at each map
position, yielding the distribution of kinetic temperatures across the region.
The resulting temperature maps reveal a region of hot (T > 350 K) material
surrounding the northeastern edge of the hot core, whereas the column density
distribution is more uniform and peaks near the position of IRc2. We attribute
this region of hot gas to shock heating caused by the impact of outflowing
material from active star formation in the region, as indicated by the presence
of broad CH3CN lines. This scenario is consistent with predictions from C-shock
chemical models that suggest that gas-phase methyl cyanide survives in the
post-shock gas and can be somewhat enhanced due to sputtering of grain mantles
in the passing shock front.Comment: 24 pages, 20 figures, accepted for publication in A&
The L1157-B1 astrochemical laboratory: testing the origin of DCN
L1157-B1 is the brightest shocked region of the large-scale molecular
outflow, considered the prototype of chemically rich outflows, being the ideal
laboratory to study how shocks affect the molecular gas. Several deuterated
molecules have been previously detected with the IRAM 30m, most of them formed
on grain mantles and then released into the gas phase due to the shock. We aim
to observationally investigate the role of the different chemical processes at
work that lead to formation the of DCN and test the predictions of the chemical
models for its formation. We performed high-angular resolution observations
with NOEMA of the DCN(2-1) and H13CN(2-1) lines to compute the deuterated
fraction, Dfrac(HCN). We detected emission of DCN(2-1) and H13CN(2-1) arising
from L1157-B1 shock. Dfrac(HCN) is ~4x10 and given the uncertainties, we
did not find significant variations across the bow-shock. Contrary to HDCO,
whose emission delineates the region of impact between the jet and the ambient
material, DCN is more widespread and not limited to the impact region. This is
consistent with the idea that gas-phase chemistry is playing a major role in
the deuteration of HCN in the head of the bow-shock, where HDCO is undetected
as it is a product of grain-surface chemistry. The spectra of DCN and H13CN
match the spectral signature of the outflow cavity walls, suggesting that their
emission result from shocked gas. The analysis of the time dependent gas-grain
chemical model UCL-CHEM coupled with a C-type shock model shows that the
observed Dfrac(HCN) is reached during the post-shock phase, matching the
dynamical timescale of the shock. Our results indicate that the presence of DCN
in L1157-B1 is a combination of gas-phase chemistry that produces the
widespread DCN emission, dominating in the head of the bow-shock, and
sputtering from grain mantles toward the jet impact region.Comment: Accepted for publication in A&A. 7 pages, 5 Figures, 1 Tabl
Non-Equilibrium Conformal Field Theories with Impurities
We present a construction of non-equilibrium steady states within conformal
field theory. These states sustain energy flows between two quantum systems,
initially prepared at different temperatures, whose dynamical properties are
represented by two, possibly different, conformal field theories connected
through an impurity. This construction relies on a real time formulation of
conformal defect dynamics based on a field scattering picture parallelizing -
but yet different from - the Euclidean formulation. We present the basic
characteristics of this formulation and give an algebraic construction of the
real time scattering maps that we illustrate in the case of SU(2)-based
conformal field theories.Comment: 12 pages + references, 1 figure. Published versio
A "Firework" of H2 Knots in the Planetary Nebula NGC 7293 (the Helix Nebula)
We present a deep and wide field-of-view (4' × 7') image of the planetary nebula (PN) NGC 7293 (the Helix Nebula) in the 2.12 μm H2 v = 1 → 0 S(1) line. The excellent seeing (0farcs4) at the Subaru Telescope, allows the details of cometary knots to be examined. The knots are found at distances of 2farcm2-6farcm4 from the central star (CS). At the inner edge and in the inner ring (up to 4farcm5 from the CS), the knot often show a "tadpole" shape, an elliptical head with a bright crescent inside and a long tail opposite to the CS. In detail, there are variations in the tadpole shapes, such as narrowing tails, widening tails, meandering tails, or multipeaks within a tail. In the outer ring (4farcm5-6farcm4 from the CS), the shapes are more fractured, and the tails do not collimate into a single direction. The transition in knot morphology from the inner edge to the outer ring is clearly seen. The number density of knots governs the H2 surface brightness in the inner ring: H2 exists only within the knots. Possible mechanisms which contribute to the shaping of the knots are discussed, including photoionization and streaming motions. A plausible interpretation of our images is that inner knots are being overrun by a faster wind, but that this has not (yet) reached the outer knots. Based on H2 formation and destruction rates, H2 gas can survive in knots from formation during the late asymptotic giant branch phase throughout the PN phase. These observations provide new constraints on the formation and evolution of knots, and on the physics of molecular gas embedded within ionized gas. Based on data taken with the Subaru Telescope, National Astronomical Observatory of Japan (proposal ID S07B-054).Peer reviewe
Diagnosing shock temperature with NH and HO profiles
In a previous study of the L1157 B1 shocked cavity, a comparison between
NH(1-) and HO(1--1) transitions showed a
striking difference in the profiles, with HO emitting at definitely higher
velocities. This behaviour was explained as a result of the high-temperature
gas-phase chemistry occurring in the postshock gas in the B1 cavity of this
outflow. If the differences in behaviour between ammonia and water are indeed a
consequence of the high gas temperatures reached during the passage of a shock,
then one should find such differences to be ubiquitous among chemically rich
outflows. In order to determine whether the difference in profiles observed
between NH and HO is unique to L1157 or a common characteristic of
chemically rich outflows, we have performed Herschel-HIFI observations of the
NH(1-0) line at 572.5 GHz in a sample of 8 bright low-mass outflow
spots already observed in the HO(1--1) line within
the WISH KP. We detected the ammonia emission at high-velocities at most of the
outflows positions. In all cases, the water emission reaches higher velocities
than NH, proving that this behaviour is not exclusive of the L1157-B1
position. Comparisons with a gas-grain chemical and shock model confirms, for
this larger sample, that the behaviour of ammonia is determined principally by
the temperature of the gas.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Societ
A combined IRAM and Herschel/HIFI study of cyano(di)acetylene in Orion KL: tentative detection of DC3N
We present a study of cyanoacetylene (HC3N) and cyanodiacetylene (HC5N) in
Orion KL, through observations from two line surveys performed with the IRAM
30m telescope and the HIFI instrument on board the Herschel telescope. The
frequency ranges covered are 80-280 GHz and 480-1906 GHz. We model the observed
lines of HC3N, HC5N, their isotopologues (including DC3N), and vibrational
modes, using a non-LTE radiative transfer code. To investigate the chemical
origin of HC3N and DC3N in Orion KL, we use a time-dependent chemical model. We
detect 40 lines of the ground state of HC3N and 68 lines of its 13C
isotopologues. We also detect 297 lines of six vibrational modes of this
molecule (nu_7, 2nu_7, 3nu_7, nu_6, nu_5, and nu_6+nu_7) and 35 rotational
lines of the ground state of HC5N. We report the first tentative detection of
DC3N in a giant molecular cloud with a DC3N/HC3N abundance ratio of 0.015. We
provide column densities and isotopic and molecular abundances. We also perform
a 2x2" map around Orion IRc2 and we present maps of HC3N lines and maps of
lines of the HC3N vibrational modes nu_6 and nu_7. In addition, a comparison of
our results for HC3N with those in other clouds allows us to derive
correlations between the column density, the FWHM, the mass, and the luminosity
of the clouds. The high column densities of HC3N obtained in the hot core, make
this molecule an excellent tracer of hot and dense gas. In addition, the large
frequency range covered reveals the need to consider a temperature and density
gradient in the hot core in order to obtain better line fits. The high D/H
ratio (comparable to that obtained in cold clouds) that we derive suggests a
deuterium enrichment. Our chemical models indicate that the possible deuterated
HC3N present in Orion KL is formed during the gas-phase. This fact provides new
hints concerning the processes leading to deuteration.Comment: 50 pages, 33 figures, 13 tables. Accepted for publication in A&
Broad N2H+ emission towards the protostellar shock L1157-B1
We present the first detection of N2H+ towards a low-mass protostellar
outflow, namely the L1157-B1 shock, at about 0.1 pc from the protostellar
cocoon. The detection was obtained with the IRAM 30-m antenna. We observed
emission at 93 GHz due to the J = 1-0 hyperfine lines. The analysis of the
emission coupled with the HIFI CHESS multiline CO observations leads to the
conclusion that the observed N2H+(1-0) line originates from the dense (> 10^5
cm-3) gas associated with the large (20-25 arcsec) cavities opened by the
protostellar wind. We find a N2H+ column density of few 10^12 cm-2
corresponding to an abundance of (2-8) 10^-9. The N2H+ abundance can be matched
by a model of quiescent gas evolved for more than 10^4 yr, i.e. for more than
the shock kinematical age (about 2000 yr). Modelling of C-shocks confirms that
the abundance of N2H+ is not increased by the passage of the shock. In summary,
N2H+ is a fossil record of the pre-shock gas, formed when the density of the
gas was around 10^4 cm-3, and then further compressed and accelerated by the
shock.Comment: ApJ, in pres
The B1 shock in the L1157 outflow as seen at high spatial resolution
We present high spatial resolution (750 AU at 250 pc) maps of the B1 shock in
the blue lobe of the L1157 outflow in four lines: CS (3-2), CH3OH (3_K-2_K),
HC3N (16-15) and p-H2CO (2_02-3_01). The combined analysis of the morphology
and spectral profiles has shown that the highest velocity gas is confined in a
few compact (~ 5 arcsec) bullets while the lowest velocity gas traces the wall
of the gas cavity excavated by the shock expansion. A large velocity gradient
model applied to the CS (3-2) and (2-1) lines provides an upper limit of 10^6
cm^-3 to the averaged gas density in B1 and a range of 5x10^3< n(H2)< 5x10^5
cm^-3 for the density of the high velocity bullets. The origin of the bullets
is still uncertain: they could be the result of local instabilities produced by
the interaction of the jet with the ambient medium or could be clump already
present in the ambient medium that are excited and accelerated by the expanding
outflow. The column densities of the observed species can be reproduced
qualitatively by the presence in B1 of a C-type shock and only models where the
gas reaches temperatures of at least 4000 K can reproduce the observed HC3N
column density.Comment: 13 pages, 12 figure
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