130 research outputs found
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
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
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
Chemistry in Evaporating Ices: Unexplored Territory
We suggest that three-body chemistry may occur in warm high density gas
evaporating in transient co\textendash desorption events on interstellar ices.
Using a highly idealised computational model we explore the chemical conversion
from simple species of the ice to more complex species containing several heavy
atoms, as a function of density and of adopted three body rate coefficients. We
predict that there is a wide range of densities and rate coefficients in which
a significant chemical conversion may occur. We discuss the implications of
this idea for the astrochemistry of hot cores.Comment: Accepted in Ap
Tracing the Mass during Low-Mass Star Formation. III. Models of the Submillimeter Dust Continuum Emission from Class 0 Protostars
Seven Class 0 sources mapped with SCUBA at 850 and 450 micron are modeled
using a one dimensional radiative transfer code. The modeling takes into
account heating from an internal protostar, heating from the ISRF, realistic
beam effects, and chopping to model the normalized intensity profile and
spectral energy distribution. Power law density models, n(r) ~ r^{-p}, fit all
of the sources; best fit values are mostly p = 1.8 +/- 0.1, but two sources
with aspherical emission contours have lower values (p ~ 1.1). Including all
sources, = 1.63 +/- 0.33. Based on studies of the sensitivity of the
best-fit p to variations in other input parameters, uncertainties in p for an
envelope model are \Delta p = +/- 0.2. If an unresolved source (e.g., a disk)
contributes 70% of the flux at the peak, p is lowered in this extreme case and
\Delta p = ^{+0.2}_{-0.6}. The models allow a determination of the internal
luminosity ( = 4.0 \lsun) of the central protostar as well as a
characteristic dust temperature for mass determination ( = 13.8 +/-
2.4 K). We find that heating from the ISRF strongly affects the shape of the
dust temperature profile and the normalized intensity profile, but does not
contribute strongly to the overall bolometric luminosity of Class 0 sources.
There is little evidence for variation in the dust opacity as a function of
distance from the central source. The data are well-fitted by dust opacities
for coagulated dust grains with ice mantles (Ossenkopf & Henning 1994). The
density profile from an inside-out collapse model (Shu 1977) does not fit the
data well, unless the infall radius is set so small as to make the density
nearly a power-law.Comment: Accepted to ApJ. 28 pages, 13 figures, uses emulateapj5.st
Tracing the Mass during Low-Mass Star Formation. II. Modelling the Submillimeter Emission from Pre-Protostellar Cores
We have modeled the emission from dust in pre-protostellar cores, including a
self-consistent calculation of the temperature distribution for each input
density distribution. Model density distributions include Bonnor-Ebert spheres
and power laws. The Bonnor-Ebert spheres fit the data well for all three cores
we have modeled. The dust temperatures decline to very low values (\Td \sim 7
K) in the centers of these cores, strongly affecting the dust emission.
Compared to earlier models that assume constant dust temperatures, our models
indicate higher central densities and smaller regions of relatively constant
density. Indeed, for L1544, a power-law density distribution, similar to that
of a singular, isothermal sphere, cannot be ruled out. For the three sources
modeled herein, there seems to be a sequence of increasing central
condensation, from L1512 to L1689B to L1544. The two denser cores, L1689B and
L1544, have spectroscopic evidence for contraction, suggesting an evolutionary
sequence for pre-protostellar cores.Comment: 22 pages, 9 figures, Ap. J. accepted, uses emulateapj5.st
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