1,315 research outputs found
An infrared measurement of chemical desorption from interstellar ice analogues
In molecular clouds at temperatures as low as 10 K, all species except
hydrogen and helium should be locked in the heterogeneous ice on dust grain
surfaces. Nevertheless, astronomical observations have detected over 150
different species in the gas phase in these clouds. The mechanism by which
molecules are released from the dust surface below thermal desorption
temperatures to be detectable in the gas phase is crucial for understanding the
chemical evolution in such cold clouds. Chemical desorption, caused by the
excess energy of an exothermic reaction, was first proposed as a key molecular
release mechanism almost 50 years ago. Chemical desorption can, in principle,
take place at any temperature, even below the thermal desorption temperature.
Therefore, astrochemical net- work models commonly include this process.
Although there have been a few previous experimental efforts, no infrared
measurement of the surface (which has a strong advantage to quantify chemical
desorption) has been performed. Here, we report the first infrared in situ
measurement of chemical desorption during the reactions H + H2S -> HS + H2
(reaction 1) and HS + H -> H2S (reaction 2), which are key to interstellar
sulphur chemistry. The present study clearly demonstrates that chemical
desorption is a more efficient process for releasing H2S into the gas phase
than was previously believed. The obtained effective cross-section for chemical
desorption indicates that the chemical desorption rate exceeds the
photodesorption rate in typical interstellar environments
Tunnelling dominates the reactions of hydrogen atoms with unsaturated alcohols and aldehydes in the dense medium
Hydrogen addition and abstraction reactions play an important role as surface
reactions in the buildup of complex organic molecules in the dense interstellar
medium. Addition reactions allow unsaturated bonds to be fully hydrogenated,
while abstraction reactions recreate radicals that may undergo radical-radical
recombination reactions. Previous experimental work has indicated that double
and triple C--C bonds are easily hydrogenated, but aldehyde -C=O bonds are not.
Here, we investigate a total of 29 reactions of the hydrogen atom with
propynal, propargyl alcohol, propenal, allyl alcohol, and propanal by means of
quantum chemical methods to quantify the reaction rate constants involved.
First of all, our results are in good agreement with and can explain the
observed experimental findings. The hydrogen addition to the aldehyde group,
either on the C or O side, is indeed slow for all molecules considered.
Abstraction of the H atom of the aldehyde group, on the other hand, is among
the faster reactions. Furthermore, hydrogen addition to C--C double bonds is
generally faster than to triple bonds. In both cases, addition on the terminal
carbon atom that is not connected to other functional groups is easiest.
Finally, we wish to stress that it is not possible to predict rate constants
based solely on the type of reaction: the specific functional groups attached
to a backbone play a crucial role and can lead to a spread of several orders of
magnitude in the rate constant.Comment: Accepted for publication in A&
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
Massive Star Cluster Formation and Destruction in Luminous Infrared Galaxies in GOALS
We present the results of a {\it Hubble Space Telescope} ACS/HRC FUV, ACS/WFC
optical study into the cluster populations of a sample of 22 Luminous Infrared
Galaxies in the Great Observatories All-Sky LIRG Survey. Through integrated
broadband photometry we have derived ages and masses for a total of 484 star
clusters contained within these systems. This allows us to examine the
properties of star clusters found in the extreme environments of LIRGs relative
to lower luminosity star-forming galaxies in the local Universe. We find that
by adopting a Bruzual \& Charlot simple stellar population (SSP) model and
Salpeter initial mass function, the age distribution of clusters declines as
, consistent with the age distribution derived
for the Antennae Galaxies, and interpreted as evidence for rapid cluster
disruption occuring in the strong tidal fields of merging galaxies. The large
number of young clusters identified in the sample also
suggests that LIRGs are capable of producing more high-mass clusters than what
is observed to date in any lower luminosity star-forming galaxy in the local
Universe. The observed cluster mass distribution of is consistent with the canonical -2 power law used to describe the
underlying initial cluster mass function (ICMF) for a wide range of galactic
environments. We interpret this as evidence against mass-dependent cluster
disruption, which would flatten the observed CMF relative to the underlying
ICMF distribution.Comment: 63 pages, 58 Figures, 56 Tables, Accepted for publication in Ap
The long-acting somatostatin analogue octreotide alleviates symptoms by reducing posttranslational conversion of prepro-glucagon to glucagon in a patient with malignant glucagonoma, but does not prevent tumor growth
A 52-year-old female with metastatic glucagonoma secreting glucagon and chromogranin A was treated with the somatostatin analogue octreotide for 2 years without any additional tumor-reducing interventions. Before therapy plasma glucagon was above 8 μg/l (normal <0.2) and within 2 days 3 × 200 μg octreotide daily suppressed plasma glucagon to 2.2-2.5 μg/l. Concomitantly, chromogranin A dropped from 0.85 mg/l (normal <0.1) to 0.2. After 3 weeks the preexisting disabling necrolytic migratory erythema had vanished completely, and weight loss was temporarily stopped. During therapy chromogranin A and plasma glucagon rose, exceeding pretreatment levels after 3 and 14 months, respectively. After 1 year the erythema recurred, responding only transiently to increasing doses of octreotide. The patient died after 2 years of therapy of tumor cachexy despite very highdosesof octreotide (4 × 600 μg/day). Throughout treatment octreotide did not prevent tumor growth, as demonstrated by computed tomography and sonography. Determination of immunoreactive glucagon before and during octreotide therapy in fractions of plasma samples subjected to gel chromatography revealed a reduction in the ratio of glucagon to preproglucagon from 1.83 (before) to 0.56 (during therapy), indicating inhibition of posttranslational processing of preproglucagon by octreotide, thereby reducing circulating bioactive glucagon. In summary, octreotide induced a remission of clinical symptoms by inhibiting posttranslational conversion of preproglucagon to glucagon but did not prevent tumor growth. Therefore, octreotide is a valuable therapy for rapid relief of clinical symptoms, thereby improving the possibilities for other tumor-reducing therapies
Formation of COMs through CO hydrogenation on interstellar grains
Glycoaldehyde, ethylene glycol, and methyl formate are complex organic
molecules that have been observed in dark molecular clouds. Because there is no
efficient gas-phase route to produce these species, it is expected that a
low-temperature surface route existst that does not require energetic
processing. CO hydrogenation experiments at low temperatures showed that this
is indeed the case. Glyoxal can form through recombination of two HCO radicals
and is then further hydrogenated. Here we aim to constrain the methyl formate,
glycolaldehyde, and ethylene glycol formation on the surface of interstellar
dust grains through this cold and dark formation route. We also probe the
dependence of the grain mantle composition on the initial gas-phase composition
and the dust temperature. A full CO hydrogenation reaction network was built
based on quantum chemical calculations for the rate constants and branching
ratios. This network was used in combination with a microscopic kinetic Monte
Carlo simulation to simulate ice chemistry, taking into account all positional
information. After benchmarking the model against CO-hydrogenation experiments,
simulations under molecular cloud conditions were performed. COMs are formed in
all interstellar conditions we studied, even at temperatures as low as 8 K.
This is because the HCO + HCO reaction can occur when HCO radicals are formed
close to each other and do not require to diffuse. Relatively low abundances of
methyl formate are formed. The final COM abundances depend more on the H-to-CO
ratio and less on temperature. Only above 16 K, where CO build-up is less
efficient, does temperature start to play a role. Molecular hydrogen is
predominantly formed through abstraction reactions on the surface. Our
simulations are in agreement with observed COM ratios for mantles that have
been formed at low temperatures
SURFRESIDE2: An ultrahigh vacuum system for the investigation of surface reaction routes of interstellar interest
A new ultrahigh vacuum experiment is described to study atom and radical addition reactions in interstellar ice analogues for astronomically relevant temperatures. The new setup – SURFace REaction SImulation DEvice (SURFRESIDE2) – allows a systematic investigation of solid state pathways resulting in the formation of molecules of astrophysical interest. The implementation of a double beam line makes it possible to expose deposited ice molecules to different atoms and/or radicals sequentially or at the same time. Special efforts are made to perform experiments under fully controlled laboratory conditions, including precise atom flux determinations, in order to characterize reaction channels quantitatively. In this way, we can compare and combine different surface reaction channels with the aim to unravel the solid state processes at play in space. Results are constrained in situ by means of a Fourier transform infrared spectrometer and a quadrupole mass spectrometer using reflection absorption infrared spectroscopy and temperature programmed desorption, respectively. The performance of the new setup is demonstrated on the example of carbon dioxide formation by comparing the efficiency through two different solid state channels (CO + OH → CO_2 + H and CO + O → CO_2) for which different addition products are needed. The potential of SURFRESIDE2 to study complex molecule formation, including nitrogen containing (prebiotic) compounds, is discussed
Revisiting the reactivity between HCO and CH on interstellar grain surfaces
Formation of interstellar complex organic molecules is currently thought to
be dominated by the barrierless coupling between radicals on the interstellar
icy grain surfaces. Previous standard DFT results on the reactivity between
CH and HCO on amorphous water surfaces, showed that formation of CH +
CO by H transfer from HCO to CH assisted by water molecules of the ice was
the dominant channel. However, the adopted description of the electronic
structure of the biradical (i.e., CH/HCO) system was inadequate (without
the broken-symmetry (BS) approach). In this work, we revisit the original
results by means of BS-DFT both in gas phase and with one water molecule
simulating the role of the ice. Results indicate that adoption of BS-DFT is
mandatory to describe properly biradical systems. In the presence of the single
water molecule, the water-assisted H transfer exhibits a high energy barrier.
In contrast, CHCHO formation is found to be barrierless. However, direct H
transfer from HCO to CH to give CO and CH presents a very low energy
barrier, hence being a potential competitive channel to the radical coupling
and indicating, moreover, that the physical insights ofthe original work remain
valid.Comment: Submitted to MNRAS main journal. For associated supporting material
refer to the publication in MNRAS. Accepted 2020 February 14. Received 2020
February 1
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