208 research outputs found
Chemistry and Radiative Transfer of Water in Cold, Dense Clouds
The Herschel Space Observatory's recent detections of water vapor in the
cold, dense cloud L1544 allow a direct comparison between observations and
chemical models for oxygen species in conditions just before star formation. We
explain a chemical model for gas phase water, simplified for the limited number
of reactions or processes that are active in extreme cold ( 15 K). In this
model, water is removed from the gas phase by freezing onto grains and by
photodissociation. Water is formed as ice on the surface of dust grains from O
and OH and released into the gas phase by photodesorption. The reactions are
fast enough with respect to the slow dynamical evolution of L1544 that the gas
phase water is in equilibrium for the local conditions thoughout the cloud. We
explain the paradoxical radiative transfer of the HO ()
line. Despite discouragingly high optical depth caused by the large Einstein A
coefficient, the subcritical excitation in the cold, rarefied H causes the
line brightness to scale linearly with column density. Thus the water line can
provide information on the chemical and dynamical processes in the darkest
region in the center of a cold, dense cloud. The inverse P-Cygni profile of the
observed water line generally indicates a contracting cloud. This profile is
reproduced with a dynamical model of slow contraction from unstable
quasi-static hydrodynamic equilibrium (an unstable Bonnor-Ebert sphere).Comment: submitted to MNRA
Investigating the Efficiency of Explosion Chemistry as a Source of Complex Organic Molecules in TMC-1
Many species of complex organic molecules (COMs) have been observed in
several astrophysical environments but it is not clear how they are produced,
particularly in cold, quiescent regions. One process that has been proposed as
a means to enhance the chemical complexity of the gas phase in such regions is
the explosion of the ice mantles of dust grains. In this process, a build up of
chemical energy in the ice is released, sublimating the ices and producing a
short lived phase of high density, high temperature gas. The gas-grain chemical
code UCLCHEM has been modified to treat these explosions in order to model the
observed abundances of COMs towards the TMC-1 region. It is found that, based
on our current understanding of the explosion mechanism and chemical pathways,
the inclusion of explosions in chemical models is not warranted at this time.
Explosions are not shown to improve the model's match to the observed
abundances of simple species in TMC-1. Further, neither the inclusion of
surface diffusion chemistry, nor explosions, results in the production of COMs
with observationally inferred abundances.Comment: Accepted for publication in Ap
Towards a better understanding of ice mantle desorption by cosmic rays
The standard model of cosmic ray heating-induced desorption of interstellar ices is based on a continuous representation of the sporadic desorption of ice mantle components from classical (0.1μm) dust grains. This has been re-evaluated and developed to include tracking the desorption through (extended) grain cooling profiles, consideration of grain size-dependencies and constraints to the efficiencies. A model was then constructed to study the true, sporadic, nature of the process with possible allowances from species co-desorption and whole mantle desorption from very small grains. The key results from the study are that the desorption rates are highly uncertain, but almost certainly significantly larger than have been previously determined. For typical interstellar grain size distributions it is found that the desorption is dominated by the contributions from the smallest grains. The sporadic desorption model shows that, if the interval between cosmic ray impacts is comparable to, or less than, the freeze-out time-scale, the continuous representation is inapplicable; chemical changes may occur on very long time-scales, resulting in strong gas phase chemical enrichments that have very non-linear dependences on the cosmic ray flux. The inclusion of even limited levels of species co-desorption and/or the contribution from very small grains further enhances the rates, especially for species such as H2O. In general, we find that cosmic ray heating is the dominant desorption mechanism in dark environments. These results may have important chemical implications for protostellar and protoplanetary environments
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
Champagne Flutes and Brandy Snifters: Modelling Protostellar Outflow-Cloud Chemical Interfaces
A rich variety of molecular species has now been observed towards hot cores
in star forming regions and in the interstellar medium. An increasing body of
evidence from millimetre interferometers suggests that many of these form at
the interfaces between protostellar outflows and their natal molecular clouds.
However, current models have remained unable to explain the origin of the
observational bias towards wide-angled "brandy snifter" shaped outflows over
narrower "champagne flute" shapes in carbon monoxide imaging. Furthermore,
these wide-angled systems exhibit unusually high abundances of the molecular
ion HCO. We present results from a chemo-dynamic model of such regions
where a rich chemistry arises naturally as a result of turbulent mixing between
cold, dense molecular gas and the hot, ionized outflow material. The injecta
drives a rich and rapid ion-neutral chemistry in qualitative and quantitative
agreement with the observations. The observational bias towards wide-angled
outflows is explained naturally by the geometry-dependent ion injection rate
causing rapid dissociation of CO in the younger systems.Comment: Accepted to MNRAS. 12 pages, 8 Figure
Is acetylene essential for carbon dust formation?
We have carried out an investigation of the chemical evolution of gas in
different carbon-rich circumstellar environments. Previous studies have tended
to invoke terrestrial flame chemistries, based on acetylene (C2H2) combustion
to model the formation of carbon dust, via Polycyclic Aromatic Hydrocarbons
(PAHs). In this work we pay careful attention to the accurate calculation of
the molecular photoreaction rate coefficients to ascertain whether there is a
universal formation mechanism for carbon dust in strongly irradiated
astrophysical environments. A large number of possible chemical channels may
exist for the formation of PAHs, so we have concentrated on the viability of
the formation of the smallest building block species, C2H2, in a variety of
carbon-rich stellar outflows. C2H2 is very sensitive to dissociation by UV
radiation. This sensitivity is tested, using models of the time-dependent
chemistry. We find that C2H2 formation is sensitive to some of the physical
parameters and that in some known sources of dust-formation it can never attain
appreciable abundances. Therefore multiple (and currently ill-defined)
dust-formation channels must exist.Comment: 10 pages, 4 figures, 5 table
The dynamics of collapsing cores and star formation
Low-mass stars are generally understood to form by the gravitational collapse
of the dense molecular clouds known as starless cores. Continuum observations
have not been able to distinguish among the several different hypotheses that
describe the collapse because the predicted density distributions are the
almost the same, as they are for all spherical self-gravitating clouds.
However, the predicted contraction velocities are different enough that the
models can be discriminated by comparing the velocities at large and small
radii. This can be done by observing at least two different molecular line
transitions that are excited at different densities. For example, the spectral
lines of the H2O (110 - 101) and C18O (1-0) have critical densities for
collisional de-excitation that differ by 5 orders of magnitude. We compare
observations of these lines from the contracting starless core L1544 against
the spectra predicted for several different hypothetical models of contraction
including the Larson-Penston flow, the inside-out collapse of the singular
isothermal sphere, the quasi-equilibrium contraction of an unstable
Bonnor-Ebert sphere, and the non-equilibrium collapse of an over-dense
Bonnor-Ebert sphere. Only the model of the unstable quasi-equilibrium
Bonnor-Ebert sphere is able to produce the observed shapes of both spectral
lines. This model allows us to interpret other observations of molecular lines
in L1544 to find that the inward velocities seen in observations of CS(2-1) and
N2H+ are located within the starless core itself, in particular in the region
where the density profile follows an inverse square law. If these conclusions
were to hold in the analysis of other starless cores, this would imply that the
formation of hydrostatic clouds within the turbulent interstellar medium is not
only possible but not exceptional and may be an evolutionary phase in low-mass
star formation.Comment: 11 pages,7 figure
A Pre-Protostellar Core in L1551. II. State of Dynamical and Chemical Evolution
Both analytic and numerical radiative transfer models applied to high
spectral resolution CS and N2H+ data give insight into the evolutionary state
of L1551 MC. This recently discovered pre-protostellar core in L1551 appears to
be in the early stages of dynamical evolution. Line-of-sight infall velocities
of >0.1km/s are needed in the outer regions of L1551 MC to adequately fit the
data. This translates to an accretion rate of ~ 1e-6 Msun/yr, uncertain to
within a factor of 5 owing to unknown geometry. The observed dynamics are not
due to spherically symmetric gravitational collapse and are not consistent with
the standard model of low-mass star formation. The widespread, fairly uniform
CS line asymmetries are more consistent with planar infall. There is modest
evidence for chemical depletion in the radial profiles of CS and C18O
suggesting that L1551 MC is also chemically young. The models are not very
sensitive to chemical evolution. L1551 MC lies within a quiescent region of
L1551 and is evidence for continued star formation in this evolved cloud.Comment: 27 pages, 7 figures, ApJ accepte
Observations of CHOH and CHCHO in a Sample of Protostellar Outflow Sources
Iram 30-m Observations towards eight protostellar outflow sources were taken
in the 96-\SI{176}{\giga\hertz} range. Transitions of CHOH and CHCHO
were detected in seven of them. The integrated emission of the transitions of
each species that fell into the observed frequency range were measured and fit
using RADEX and LTE models. Column densities and gas properties inferred from
this fitting are presented. The ratio of the A and E-type isomers of CHOH
indicate that the methanol observed in these outflows was formed on the grain
surface. Both species demonstrate a reduction of terminal velocity in their
line profiles in faster outflows, indicating destruction in the post-shock gas
phase. This destruction, and a near constant ratio of the CHOH and
CHCHO column densities imply it is most likely that CHCHO also forms on
the grain surface.Comment: Accepted for publication in Ap
- …