25 research outputs found
Chemical Tracers of Pre-Brown Dwarf Cores Formed Through Turbulent Fragmentation
A gas-grain time dependent chemical code, UCL\_CHEM, has been used to
investigate the possibility of using chemical tracers to differentiate between
the possible formation mechanisms of brown dwarfs. In this work, we model the
formation of a pre-brown dwarf core through turbulent fragmentation by
following the depth-dependent chemistry in a molecular cloud through the step
change in density associated with an isothermal shock and the subsequent
freefall collapse once a bound core is produced. Trends in the fractional
abundance of molecules commonly observed in star forming cores are then
explored to find a diagnostic for identifying brown dwarf mass cores formed
through turbulence. We find that the cores produced by our models would be
bright in CO and NH but not in HCO. This differentiates them from
models using purely freefall collapse as such models produce cores that would
have detectable transitions from all three molecules.Comment: 7 page, 3 figures, Accepted for publication in MNRA
Nitrogen Fractionation in External Galaxies
In star forming regions in our own Galaxy, the 14N/15N ratio is found to vary
from 100 in meteorites, comets and protoplanetary disks up to
1000 in pre-stellar and star forming cores, while in external galaxies the very
few single-dish large scale measurements of this ratio lead to values of
100-450. The extent of the contribution of isotopic fractionation to these
variations is, to date, unknown. In this paper we present a theoretical
chemical study of nitrogen fractionation in external galaxies in order to
determine the physical conditions that may lead to a spread of the 14N/15N
ratio from the solar value of 440 and hence evaluate the contribution of
chemical reactions in the ISM to nitrogen fractionation. We find that the main
cause of ISM enrichment of nitrogen fractionation is high gas densities, aided
by high fluxes of cosmic rays.Comment: Accepted by 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
Discovering New Variable Stars at Key Stage 3
Details of the London pilot of the `Discovery Project' are presented, where
university-based astronomers were given the chance to pass on some real and
applied knowledge of astronomy to a group of selected secondary school pupils.
It was aimed at students in Key Stage 3 of their education, allowing them to be
involved in real astronomical research at an early stage of their education,
the chance to become the official discoverer of a new variable star, and to be
listed in the International Variable Star Index database, all while learning
and practising research-level skills. Future plans are discussed.Comment: 10 pages, 1 figur
UCLCHEMCMC: A MCMC Inference tool for Physical Parameters of Molecular Clouds
We present the publicly available, open source code UCLCHEMCMC, designed to
estimate physical parameters of an observed cloud of gas by combining Monte
Carlo Markov Chain (MCMC) sampling with chemical and radiative transfer
modeling. When given the observed values of different emission lines,
UCLCHEMCMC runs a Bayesian parameter inference, using a MCMC algorithm to
sample the likelihood and produce an estimate of the posterior probability
distribution of the parameters. UCLCHEMCMC takes a full forward modeling
approach, generating model observables from the physical parameters via
chemical and radiative transfer modeling. While running UCLCHEMCMC, the created
chemical models and radiative transfer code results are stored in an SQL
database, preventing redundant model calculations in future inferences. This
means that the more UCLCHEMCMC is used, the more efficient it becomes. Using
UCLCHEM and RADEX, the increase of efficiency is nearly two orders of
magnitude, going from 5185.33 \pm 1041.96 s for ten walkers to take one
thousand steps when the database is empty, to 68.89 \pm 45.39 s when nearly all
models requested are in the database. In order to demonstrate its usefulness we
provide an example inference of UCLCHEMCMC to estimate the physical parameters
of mock data, and perform two inferences on the well studied prestellar core,
L1544, one of which show that it is important to consider the substructures of
an object when determining which emission lines to use.Comment: 14 pages, 8 figures, 4 tables accepted by Ap
Investigating the impact of reactions of C and CH with molecular hydrogen on a glycine gas-grain network
The impact of including the reactions of C and CH with molecular hydrogen in a gas-grain network is assessed via a sensitivity analysis. To this end, we vary three parameters, namely, the efficiency for the reaction C + H2 −→ CH2, and the cosmic ray ionization rate, with the third parameter being the final density of the collapsing dark cloud. A grid of 12 models is run to investigate the effect of all parameters on the final molecular abundances of the chemical network. We find that including reactions with molecular hydrogen alters the hydrogen economy of the network; since some species are hydrogenated by molecular hydrogen, atomic hydrogen is freed up. The abundances of simple molecules produced from hydrogenation, such as CH4, CH3OH, and NH3, increase, and at the same time, more complex species such as glycine and its precursors see a significant decrease in their final abundances. We find that the precursors of glycine are being preferentially hydrogenated, and therefore glycine itself is produced less efficiently
On the Formation of Deuterated Methyl Formate in Hot Corinos
Methyl formate, HCOOCH, and many of its isotopologues have been detected
in astrophysical regions with considerable abundances. However, the recipe for
the formation of this molecule and its isotopologues is not yet known. In this
work, we attempt to investigate, theoretically, the successful recipe for the
formation of interstellar HCOOCH and its deuterated isotopologues. We used
the gas-grain chemical model, UCLCHEM, to examine the possible routes of
formation of methyl formate on grain surfaces and in the gas-phase in low-mass
star-forming regions. Our models show that radical-radical association on
grains are necessary to explain the observed abundance of DCOOCH in the
protostar IRAS~16293--2422. H-D substitution reactions on grains significantly
enhance the abundances of HCOOCHD, DCOOCHD, and HCOOCD. The
observed abundance of HCOOCHD in IRAS 16293--2422 can only be reproduced if
H-D substitution reactions are taken into account. However, HCOOCHD remain
underestimated in all of our models. The deuteration of methyl formate appears
to be more complex than initially thought. Additional studies, both
experimentally and theoretically, are needed for a better understanding of the
interstellar formation of these species.Comment: 13 pages , 3 figures, 5 tables , accepted for publications in MNRA
Exploiting Network Topology for Accelerated Bayesian Inference of Grain Surface Reaction Networks
In the study of grain-surface chemistry in the interstellar medium, there
exists much uncertainty regarding the reaction mechanisms with few constraints
on the abundances of grain-surface molecules. Bayesian inference can be
performed to determine the likely reaction rates. In this work, we consider
methods for reducing the computational expense of performing Bayesian inference
on a reaction network by looking at the geometry of the network. Two methods of
exploiting the topology of the reaction network are presented. One involves
reducing a reaction network to just the reaction chains with constraints on
them. After this, new constraints are added to the reaction network and it is
shown that one can separate this new reaction network into sub-networks. The
fact that networks can be separated into sub-networks is particularly important
for the reaction networks of interstellar complex organic molecules, whose
surface reaction networks may have hundreds of reactions. Both methods allow
the maximum-posterior reaction rate to be recovered with minimal bias
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