253 research outputs found
The largest oxigen bearing organic molecule repository
We present the first detection of complex aldehydes and isomers in three
typical molecular clouds located within 200pc of the center of our Galaxy.
We find very large abundances of these complex organic molecules (COMs) in
the central molecular zone (CMZ), which we attribute to the ejection of COMs
from grain mantles by shocks. The relative abundances of the different COMs
with respect to that of CH3OH are strikingly similar for the three sources,
located in very different environments in the CMZ. The similar relative
abundances point toward a unique grain mantle composition in the CMZ. Studying
the Galactic center clouds and objects in the Galactic disk having large
abundances of COMs, we find that more saturated molecules are more abundant
than the non-saturated ones. We also find differences between the relative
abundance between COMs in the CMZ and the Galactic disk, suggesting different
chemical histories of the grain mantles between the two regions in the Galaxy
for the complex aldehydes. Different possibilities for the grain chemistry on
the icy mantles in the GC clouds are briefly discussed. Cosmic rays can play an
important role in the grain chemistry. With these new detections, the molecular
clouds in the Galactic center appear to be one of the best laboratories for
studying the formation of COMs in the Galaxy.Comment: 20 pages, 4 figures, accepted in Ap
A search for ortho-benzyne (o-C6H4) in CRL 618
Polycyclic aromatic hydrocarbons (PAHs) have been proposed as potential
carriers of the unidentified infrared bands (UIRs) and the diffuse interstellar
bands (DIBs). PAHs are not likely to form by gas-phase or solid-state
interstellar chemistry, but rather might be produced in the outflows of
carbon-rich evolved stars. PAHs could form from acetylene addition to the
phenyl radical (C6H5), which is closely chemically related to benzene (C6H6)
and ortho-benzyne (o-C6H4). To date, circumstellar chemical models have been
limited to only a partial treatment of benzene-related chemistry, and so the
expected abundances of these species are unclear. A detection of benzene has
been reported in the envelope of the proto-planetary nebula (PPN) CRL 618, but
no other benzene-related species has been detected in this or any other source.
The spectrum of o-C6H4 is significantly simpler and stronger than that of C6H5,
and so we conducted deep Ku-, K- and Q-band searches for o-C6H4 with the Green
Bank Telescope. No transitions were detected, but an upper limit on the column
density of 8.4x10^13 cm^-2 has been determined. This limit can be used to
constrain chemical models of PPNe, and this study illustrates the need for
complete revision of these models to include the full set of benzene-related
chemistry.Comment: 13 pages, 4 figures, to be published in The Astrophysical Journal
Letter
Effects of a hydropower plant on Coleopteran diversity and abundance in the Udzungwa Mountains, Tanzania
The effects of river flow diversion on biodiversity were assessed using Coleoptera as an
indicator group in three habitats of the Kihansi Gorge (Udzungwa Mountains, Tanzania), before and after
commissioning of a hydropower plant. Data collected using sweep netting and pitfall traps showed that
the effect of diversion of the river flow was site-specific, affecting particularly the spray habitat.
Rarefaction analysis of both sweep netting and pitfall samples indicated that the expected richness of
Coleoptera declined significantly in all habitats after commissioning of the power plant. Sweep netting
and pitfall samples showed that the highest Shannon-Wiener diversity index value before the diversion
of the river flow was in the spray zone, but the index value decreased after diversion. Changes in the
other two habitats were less prominent. Analysis of variance using diversity index values from five pitfall
samples in each habitat type before and after commissioning indicated that there were no statistically
significant differences in the diversity index between the two sampling periods or among the three habitat
types. Renkonen's similarity index between habitats showed that pitfall samples had higher similarity
(87%) than did samples from sweep netting (69%). It is suggested that for mitigation purposes,
artificial spray systems, which have been installed in other wetlands of the Kihansi Gorge, also be
installed to cover the whole Lower Wetland in which this study was undertaken. In order to maintain
overall biodiversity in the Kihansi Gorge, it is suggested that the ecosystem conservation approach be
prioritised
A Search for Hydroxylamine (NH2OH) toward Select Astronomical Sources
Observations of 14 rotational transitions of hydroxylamine (NH2OH) using the
NRAO 12 m Telescope on Kitt Peak are reported towards IRC+10216, Orion KL,
Orion S, Sgr B2(N), Sgr B2(OH), W3IRS5, and W51M. Although recent models
suggest the presence of NH2OH in high abundance, these observations resulted in
non-detection. Upper limits are calculated to be as much as six orders of
magnitude lower than predicted by models. Possible explanations for the lower
than expected abundance are explored.Comment: 18 pages, 3 figures, 3 table
Large scale grain mantle disruption in the Galactic Center
We present observations of C2H5OH toward molecular clouds in Sgr A, Sgr B2
and associated with thermal and non-thermal features in the Galactic center.
C2H5OH emission in Sgr A and Sgr B2 is widespread, but not uniform. C2H5OH
emission is much weaker or it is not detected in some molecular clouds in both
complexes, in particular those with radial velocities between 70 and 120 km/s.
While most of the clouds associated with the thermal features do not show
C2H5OH emission, that associated with the Non-Thermal Radio Arc shows emission.
The fractional abundance of C2H5OH in most of the clouds with radial velocities
between 0 and 70 km/s in Sgr A and Sgr B2 is relatively high, of few 1e-8. The
C2H5OH abundance decreases by more than one order of magnitude (aprox. 1e-9) in
the clouds associated with the thermal features. The large abundance of C2H5OH
in the gas-phase indicates that C2H5OH has formed in grains and released to
gas-phase by shocks in the last aprox. 1e5 years.Comment: In press in Astrophysical Journal Letters 7 pages, 1 table, 1 figur
Infrared spectroscopy of solid CO-CO2 mixtures and layers
The spectra of pure, mixed and layered CO and CO2 ices have been studied
systematically under laboratory conditions using infrared spectroscopy. This
work provides improved resolution spectra (0.5 cm-1) of the CO2 bending and
asymmetric stretching mode, as well as the CO stretching mode, extending the
existing Leiden database of laboratory spectra to match the spectral resolution
reached by modern telescopes and to support the interpretation of the most
recent data from Spitzer. It is shown that mixed and layered CO and CO2 ices
exhibit very different spectral characteristics, which depend critically on
thermal annealing and can be used to distinguish between mixed, layered and
thermally annealed CO-CO2 ices. CO only affects the CO2 bending mode spectra in
mixed ices below 50K under the current experimental conditions, where it
exhibits a single asymmetric band profile in intimate mixtures. In all other
ice morphologies the CO2 bending mode shows a double peaked profile, similar to
that observed for pure solid CO2. Conversely, CO2 induces a blue-shift in the
peak-position of the CO stretching vibration, to a maximum of 2142 cm-1 in
mixed ices, and 2140-2146 cm-1 in layered ices. As such, the CO2 bending mode
puts clear constraints on the ice morphology below 50K, whereas beyond this
temperature the CO2 stretching vibration can distinguish between initially
mixed and layered ices. This is illustrated for the low-mass YSO HH46, where
the laboratory spectra are used to analyse the observed CO and CO2 band
profiles and try to constrain the formation scenarios of CO2.Comment: Accepted in A&
Rotational spectra of isotopic species of methyl cyanide, CHCN, in their ground vibrational states up to terahertz frequencies
Methyl cyanide is an important trace molecule in star-forming regions. It is
one of the more common molecules used to derive kinetic temperatures in such
sources. As preparatory work for Herschel, SOFIA, and in particular ALMA we
want to improve the rest frequencies of the main as well as minor isotopologs
of methyl cyanide. The laboratory rotational spectrum of methyl cyanide in
natural isotopic composition has been recorded up to 1.63 THz. Transitions with
good signal-to-noise ratio could be identified for CHCN, CHCN,
CHCN, CHCN, CHDCN, and CHCN in their
ground vibrational states up to about 1.2 THz. The main isotopic species could
be identified even in the highest frequency spectral recordings around 1.6 THz.
The highest quantum numbers included in the fit are 64 for
CHCN and 89 for the main isotopic species. Greatly improved
spectroscopic parameters have been obtained by fitting the present data
together with previously reported transition frequencies. The present data will
be helpful to identify isotopologs of methyl cyanide in the higher frequency
bands of instruments such as the recently launched Herschel satellite, the
upcoming airplane mission SOFIA or the radio telescope array ALMA.Comment: 13 pages, 2 figures, article appeared; CDMS links update
CLTs and asymptotic variance of time-sampled Markov chains
For a Markov transition kernel P and a probability distribution
Ό on nonnegative integers, a time-sampled Markov chain evolves according
to the transition kernel PÎŒ = ÎŁkÎŒ(k)Pk. In this note we obtain CLT
conditions for time-sampled Markov chains and derive a spectral formula
for the asymptotic variance. Using these results we compare efficiency of
Barker's and Metropolis algorithms in terms of asymptotic variance
A Brownian particle in a microscopic periodic potential
We study a model for a massive test particle in a microscopic periodic
potential and interacting with a reservoir of light particles. In the regime
considered, the fluctuations in the test particle's momentum resulting from
collisions typically outweigh the shifts in momentum generated by the periodic
force, and so the force is effectively a perturbative contribution. The
mathematical starting point is an idealized reduced dynamics for the test
particle given by a linear Boltzmann equation. In the limit that the mass ratio
of a single reservoir particle to the test particle tends to zero, we show that
there is convergence to the Ornstein-Uhlenbeck process under the standard
normalizations for the test particle variables. Our analysis is primarily
directed towards bounding the perturbative effect of the periodic potential on
the particle's momentum.Comment: 60 pages. We reorganized the article and made a few simplifications
of the conten
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