1,850 research outputs found
The origin of the molecular emission around the southern hemisphere Re 4 IRS - HH 188 region
We present SEST observations of the molecular environment ahead of the
southern Herbig-Haro object 188 (HH188), associated with the low-mass protostar
Re4 IRS. We have also used the SuperCosmos Halpha survey to search for Halpha
emission associated with the Re4 IRS - HH188 region. The aim of the present
work is to study the properties of the molecular gas and to better characterize
this southern star forming region. We mapped the HCO+ 3-2 and H13CO+ 1-0
emission around the YSO and took spectra of the CH3OH 2(0)-1(0)A+ and
2(-1)-1(-1)E and SO 6(5)-5(4) towards the central source. Column densities are
derived and different scenarios are considered to explain the origin of the
molecular emission. HCO+ arises from a relatively compact region around the
YSO; however, its peak emission is displaced to the south following the outflow
direction. Our chemical analysis indicates that a plausible scenario is that
most of the emission arises from the cold, illuminated dense gas ahead of the
HH188 object. We have also found that HH188, a high excitation object, seems to
be part of a parsec scale and highly collimated HH system. Re4 IRS is probably
a binary protostellar system, in the late Class 0 or Class I phase. One of the
protostars, invisible in the near-IR, seems to power the HH188 system.Comment: 9 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Chemistry of dense clumps near moving Herbig-Haro objects
Localised regions of enhanced emission from HCO+, NH3 and other species near
Herbig-Haro objects (HHOs) have been interpreted as arising in a photochemistry
stimulated by the HHO radiation on high density quiescent clumps in molecular
clouds. Static models of this process have been successful in accounting for
the variety of molecular species arising ahead of the jet; however recent
observations show that the enhanced molecular emission is widespread along the
jet as well as ahead. Hence, a realistic model must take into account the
movement of the radiation field past the clump. It was previously unclear as to
whether the short interaction time between the clump and the HHO in a moving
source model would allow molecules such as HCO+ to reach high enough levels,
and to survive for long enough to be observed. In this work we model a moving
radiation source that approaches and passes a clump. The chemical picture is
qualitatively unchanged by the addition of the moving source, strengthening the
idea that enhancements are due to evaporation of molecules from dust grains. In
addition, in the case of several molecules, the enhanced emission regions are
longer-lived. Some photochemically-induced species, including methanol, are
expected to maintain high abundances for ~10,000 years.Comment: 7 pages, 3 figure
Modelling the ArH emission from the Crab Nebula
We have performed combined photoionization and photodissociation region (PDR)
modelling of a Crab Nebula filament subjected to the synchrotron radiation from
the central pulsar wind nebula, and to a high flux of charged particles; a
greatly enhanced cosmic ray ionization rate over the standard interstellar
value, , is required to account for the lack of detected [C I]
emission in published Herschel SPIRE FTS observations of the Crab Nebula. The
observed line surface brightness ratios of the OH and ArH transitions
seen in the SPIRE FTS frequency range can only be explained with both a high
cosmic ray ionization rate and a reduced ArH dissociative recombination
rate compared to that used by previous authors, although consistent with
experimental upper limits. We find that the ArH/OH line strengths and
the observed H vibration-rotation emission can be reproduced by model
filaments with cm,
and visual extinctions within the range found for dusty globules in the Crab
Nebula, although far-infrared emission from [O I] and [C II] is higher than the
observational constraints. Models with cm
underpredict the H surface brightness, but agree with the ArH and
OH surface brightnesses and predict [O I] and [C II] line ratios consistent
with observations. These models predict HeH rotational emission above
detection thresholds, but consideration of the formation timescale suggests
that the abundance of this molecule in the Crab Nebula should be lower than the
equilibrium values obtained in our analysis.Comment: Accepted by MNRAS. Author accepted manuscript. Accepted on
05/09/2017. Deposited on 05/09/1
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
A study of methanol and silicon monoxide production through episodic explosions of grain mantles in the Central Molecular Zone
Methanol (CHOH) is found to be abundant and widespread towards the
Central Molecular Zone, the inner few hundred parsecs of our Galaxy. Its origin
is, however, not fully understood. It was proposed that the high cosmic ray
ionisation rate in this region could lead to a more efficient non-thermal
desorption of this species formed on grain surfaces, but it would also mean
that this species is destroyed in a relatively short timescale. In a first
step, we run chemical models with a high cosmic ray ionisation rate and find
that this scenario can only reproduce the lowest abundances of methanol derived
in this region (10-10). In a second step, we investigate
another scenario based on episodic explosions of grain mantles. We find a good
agreement between the predicted abundances of methanol and the observations. We
find that the dominant route for the formation of methanol is through
hydrogenation of CO on the grains followed by the desorption due to the grain
mantle explosion. The cyclic aspect of this model can explain the widespread
presence of methanol without requiring any additional mechanism. We also model
silicon monoxide (SiO), another species detected in several molecular clouds of
the Galactic Centre. An agreement is found with observations for a high
depletion of Si (Si/H 10) with respect to the solar abundance.Comment: Accepted in MNRA
The clumpiness of molecular clouds: HCO+ (3--2) survey near Herbig-Haro objects
Some well-studied Herbig Haro objects have associated with them one or more
cold, dense, and quiescent clumps of gas. We propose that such clumps near an
HH object can be used as a general measure of clumpiness in the molecular cloud
that contains that HH object. Our aim is to make a survey of clumps around a
sample of HH objects, and to use the results to make an estimate of the
clumpiness in molecular clouds. All known cold, dense, and quiescent clumps
near HH objects are anomalously strong HCO+ emitters. Our method is, therefore,
to search for strong HCO+ emission as an indicator of a clump near to an HH
object. The searches were made using JCMT and SEST in the HCO+ 3-2 and also
H13CO+ 1-0 lines, with some additional searches for methanol and sulphur
monoxide lines. The sources selected were a sample of 22 HH objects in which no
previous HCO+ emission had been detected. We find that half of the HH objects
have clumps detected in the HCO+ 3-2 line and that all searches in H13CO$+ 1-0
lines show evidence of clumpiness. All condensations have narrow linewidths and
are evidently unaffected dynamically by the HH jet shock. We conclude that the
molecular clouds in which these HH objects are found must be highly
heterogeneous on scales of less than 0.1 pc. An approximate calculation based
on these results suggests that the area filling factor of clumps affected by HH
objects is on the order of 10%. These clumps have gas number densities larger
than 3e4 cm-2.Comment: 11 pages, 14 figures. Accepted for publication in Astronomy and
Astrophysic
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