386 research outputs found
Formation rates of complex organics in UV irradiated CH3OH-rich ices I: Experiments
(Abridged) Gas-phase complex organic molecules are commonly detected in the
warm inner regions of protostellar envelopes. Recent models show that
photochemistry in ices followed by desorption may explain the observed
abundances. This study aims to experimentally quantify the broad-band
UV-induced production rates of complex organics in CH3OH-rich ices at 20-70 K
under ultra-high vacuum conditions. The reaction products are mainly identified
by RAIRS and TPD experiments. Complex organics are readily formed in all
experiments, both during irradiation and during a slow warm-up of the ices to
200 K after the UV lamp is turned off. The relative abundances of photoproducts
depend on the UV fluence, the ice temperature, and whether pure CH3OH ice or
CH3OH:CH4/CO ice mixtures are used. C2H6, CH3CHO, CH3CH2OH, CH3OCH3, HCOOCH3,
HOCH2CHO and (CH2OH)2 are all detected in at least one experiment. The derived
product-formation yields and their dependences on different experimental
parameters, such as the initial ice composition, are used to estimate the CH3OH
photodissociation branching ratios in ice and the relative diffusion barriers
of the formed radicals. The experiments show that ice photochemistry in CH3OH
ices is efficient enough to explain the observed abundances of complex organics
around protostars and that ratios of complex molecules can be used to constrain
their formation pathway.Comment: Accepted for publication in A&A. 65 pages including appendice
Pore evolution in interstellar ice analogues: simulating the effects of temperature increase
Context. The level of porosity of interstellar ices - largely comprised of
amorphous solid water (ASW) - contains clues on the trapping capacity of other
volatile species and determines the surface accessibility that is needed for
solid state reactions to take place. Aims. Our goal is to simulate the growth
of amorphous water ice at low temperature (10 K) and to characterize the
evolution of the porosity (and the specific surface area) as a function of
temperature (from 10 to 120 K). Methods. Kinetic Monte Carlo simulations are
used to mimic the formation and the thermal evolution of pores in amorphous
water ice. We follow the accretion of gas-phase water molecules as well as
their migration on surfaces with different grid sizes, both at the top growing
layer and within the bulk. Results. We show that the porosity characteristics
change substantially in water ice as the temperature increases. The total
surface of the pores decreases strongly while the total volume decreases only
slightly for higher temperatures. This will decrease the overall reaction
efficiency, but in parallel, small pores connect and merge, allowing trapped
molecules to meet and react within the pores network, providing a pathway to
increase the reaction efficiency. We introduce pore coalescence as a new solid
state process that may boost the solid state formation of new molecules in
space and has not been considered so far.Comment: 9 pages, 8 figures Accepted for publication in A&
Deuterium enrichment of ammonia produced by surface N+H/D addition reactions at low temperature
The surface formation of NH3 and its deuterated isotopologues – NH_2D, NHD_2, and ND_3 – is investigated at low temperatures through the simultaneous addition of hydrogen and deuterium atoms to nitrogen atoms in CO-rich interstellar ice analogues. The formation of all four ammonia isotopologues is only observed up to 15 K, and drops below the detection limit for higher temperatures. Differences between hydrogenation and deuteration yields result in a clear deviation from a statistical distribution in favour of deuterium enriched species. The data analysis suggests that this is due to a higher sticking probability of D atoms to the cold surface, a property that may generally apply to molecules that are formed in low temperature surface reactions. The results found here are used to interpret ammonia–deuterium fractionation as observed in pre-protostellar cores
Detection of vibronic bands of C in a translucent cloud towards HD 169454
We report the detection of eight vibronic bands of C, seven of which have
been hitherto unobserved in astrophysical objects, in the translucent cloud
towards HD~169454. Four of these bands are also found towards two additional
objects: HD~73882 and HD~154368. Very high signal-to-noise ratio (1000
and higher) and high resolving power () UVES-VLT spectra (Paranal,
Chile) allow for detecting novel spectral features of C, even revealing
weak perturbed features in the strongest bands. The work presented here
provides the most complete spectroscopic survey of the so far largest carbon
chain detected in translucent interstellar clouds. High-quality laboratory
spectra of C are measured using cavity ring-down absorption spectroscopy in
a supersonically expanding hydrocarbon plasma, to support the analysis of the
identified bands towards HD~169454. A column density of N(C) = cm is inferred and the excitation of the molecule
exhibits two temperature components; K for the low-
states and K for the high- tail. The rotational
excitation of C is reasonably well explained by models involving a
mechanism including inelastic collisions, formation and destruction of the
molecule, and radiative pumping in the far-infrared. These models yield gas
kinetic temperatures comparable to those found for . The assignment of
spectral features in the UV-blue range 3793-4054 \AA\ may be of relevance for
future studies aiming at unravelling spectra to identify interstellar molecules
associated with the diffuse interstellar bands (DIBs).Comment: 15 pages, 13 figures, submitted to MNRA
Benefits of collaborative learning
AbstractCollaborative learning is an educational approach to teaching and learning that involves groups of learners working together to solve a problem, complete a task, or create a product. This review article outlines benefits of learning in collaboration style, begins with the concept of the term and continues with the advantages created by collaborative methods. This paper sets out major benefits of collaborative learning into four categories of; social, psychological, academic, and assessment benefits. Each of them is further subdivided to more specific themes
Relevance of the H_2 + O reaction pathway for the surface formation of interstellar water. Combined experimental and modeling study
The formation of interstellar water is commonly accepted to occur on the surfaces of icy dust grains in dark molecular clouds at low temperatures (10–20 K), involving hydrogenation reactions of oxygen allotropes. As a result of the large abundances of molecular hydrogen and atomic oxygen in these regions, the reaction H_2 + O has been proposed to contribute significantly to the formation of water as well. However, gas-phase experiments and calculations, as well as solid-phase experimental work contradict this hypothesis. Here, we use precisely executed temperature-programmed desorption (TPD) experiments in an ultra-high vacuum setup combined with kinetic Monte Carlo simulations to establish an upper limit of the water production starting from H_2 and O. These reactants were brought together in a matrix of CO_2 in a series of (control) experiments at different temperatures and with different isotopological compositions. The water detected with the quadrupole mass spectrometer upon TPD was found to originate mainly from contamination in the chamber itself. However, if water is produced in small quantities on the surface through H_2 + O, this can only be explained by a combined classical and tunneled reaction mechanism. An absolutely conservative upper limit for the reaction rate was derived with a microscopic kinetic Monte Carlo model that converts the upper limit into the highest possible reaction rate. Incorporating this rate into simulation runs for astrochemically relevant parameters shows that the upper limit to the contribution of the reaction H_2 + O in OH, and hence water formation, is 11% in dense interstellar clouds. Our combined experimental and theoretical results indicate, however, that this contribution is most likely much lower
Reaction Networks For Interstellar Chemical Modelling: Improvements and Challenges
We survey the current situation regarding chemical modelling of the synthesis
of molecules in the interstellar medium. The present state of knowledge
concerning the rate coefficients and their uncertainties for the major
gas-phase processes -- ion-neutral reactions, neutral-neutral reactions,
radiative association, and dissociative recombination -- is reviewed. Emphasis
is placed on those reactions that have been identified, by sensitivity
analyses, as 'crucial' in determining the predicted abundances of the species
observed in the interstellar medium. These sensitivity analyses have been
carried out for gas-phase models of three representative, molecule-rich,
astronomical sources: the cold dense molecular clouds TMC-1 and L134N, and the
expanding circumstellar envelope IRC +10216. Our review has led to the proposal
of new values and uncertainties for the rate coefficients of many of the key
reactions. The impact of these new data on the predicted abundances in TMC-1
and L134N is reported. Interstellar dust particles also influence the observed
abundances of molecules in the interstellar medium. Their role is included in
gas-grain, as distinct from gas-phase only, models. We review the methods for
incorporating both accretion onto, and reactions on, the surfaces of grains in
such models, as well as describing some recent experimental efforts to simulate
and examine relevant processes in the laboratory. These efforts include
experiments on the surface-catalysed recombination of hydrogen atoms, on
chemical processing on and in the ices that are known to exist on the surface
of interstellar grains, and on desorption processes, which may enable species
formed on grains to return to the gas-phase.Comment: Accepted for publication in Space Science Review
A Spectroscopic Survey of Electronic Transitions of CH, CH, and CD
Electronic spectra of CH are measured in the cm
domain using cavity ring-down spectroscopy of a supersonically expanding
hydrocarbon plasma. In total, 19 (sub)bands of CH are presented, all
probing the vibrational manifold of the B electronically excited state.
The assignments are guided by electronic spectra available from matrix
isolation work, isotopic substitution experiments (yielding also spectra for
CH and CD), predictions from ab initio calculations as well as
rotational fitting and vibrational contour simulations using the available
ground state parameters as obtained from microwave experiments. Besides the
origin band, three non-degenerate stretching vibrations along the
linear backbone of the CH molecule are assigned: the mode
associated with the C-C bond vibration and the and modes
associated with CC triple bonds. For the two lowest and
bending modes, a Renner-Teller analysis is performed identifying the
() and both () and
() components. In addition, two higher lying bending
modes are observed, which are tentatively assigned as ()
and () levels. In the excitation region below the first
non-degenerate vibration (), some transitions are
observed that are assigned as even combination modes of low-lying bending
vibrations. The same holds for a transition found above the
level. From these spectroscopic data and the vibronic analysis a
comprehensive energy level diagram for the B state of CH is derived
and presented.Comment: Accepted for publication in The Journal of Physical Chemistry A (26
July 2016
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