444 research outputs found
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
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&
Coherent Optimal Control of Multiphoton Molecular Excitation
We give a framework for molecular multiphoton excitation process induced by
an optimally designed electric field. The molecule is initially prepared in a
coherent superposition state of two of its eigenfunctions. The relative phase
of the two superposed eigenfunctions has been shown to control the optimally
designed electric field which triggers the multiphoton excitation in the
molecule. This brings forth flexibility in desiging the optimal field in the
laboratory by suitably tuning the molecular phase and hence by choosing the
most favorable interfering routes that the system follows to reach the target.
We follow the quantum fluid dynamical formulation for desiging the electric
field with application to HBr molecule.Comment: 5 figure
Infrared spectroscopy of HCOOH in interstellar ice analogues
Context: HCOOH is one of the more common species in interstellar ices with
abundances of 1-5% with respect to solid H2O. Aims: This study aims at
characterizing the HCOOH spectral features in astrophysically relevant ice
mixtures in order to interpret astronomical data. Methods: The ices are grown
under high vacuum conditions and spectra are recorded in transmission using a
Fourier transform infrared spectrometer. Pure HCOOH ices deposited at 15 K and
145 K are studied, as well as binary and tertiary mixtures containing H2O, CO,
CO2 and CH3OH. The mixture concentrations are varied from 50:50% to ~10:90% for
HCOOH:H2O. Binary mixtures of HCOOH:X and tertiary mixtures of HCOOH:H2O:X with
X = CO, CO2, and CH3OH, are studied for concentrations of ~10:90% and
~7:67:26%, respectively. Results: Pure HCOOH ice spectra show broad bands which
split around 120 K due to the conversion of a dimer to a chain-structure. Broad
single component bands are found for mixtures with H2O. Additional spectral
components are present in mixtures with CO, CO2 and CH3OH. The resulting peak
position, full width at half maximum and band strength depend strongly on ice
structure, temperature, matrix constituents and the HCOOH concentration.
Comparison of the solid HCOOH 5.9, 7.2, and 8.1 micron features with
astronomical data toward the low mass source HH 46 and high mass source W 33A
shows that spectra of binary mixtures do not reproduce the observed ice
features. However, our tertiary mixtures especially with CH3OH match the
astronomical data very well. Thus interstellar HCOOH is most likely present in
tertiary or more complex mixtures with H2O, CH3OH and potentially also CO or
CO2, providing constraints on its formation.Comment: 11 pages, 10 figures, accepted by A&
Search for Interstellar Water in the Translucent Molecular Cloud toward HD 154368
We report an upper limit of 9 x 10^{12} cm-2 on the column density of water
in the translucent cloud along the line of sight toward HD 154368. This result
is based upon a search for the C-X band of water near 1240 \AA carried out
using the Goddard High Resolution Spectrograph of the Hubble Space Telescope.
Our observational limit on the water abundance together with detailed chemical
models of translucent clouds and previous measurements of OH along the line of
sight constrain the branching ratio in the dissociative recombination of H_3O+
to form water. We find at the level that no more than 30% of
dissociative recombinations of H_3O+ can lead to H_2O. The observed spectrum
also yielded high-resolution observations of the Mg II doublet at 1239.9 \AA
and 1240.4 \AA, allowing the velocity structure of the dominant ionization
state of magnesium to be studied along the line of sight. The Mg II spectrum is
consistent with GHRS observations at lower spectral resolution that were
obtained previously but allow an additional velocity component to be
identified.Comment: Accepted by ApJ, uses aasp
XMM-Newton observations of the supernova remnant IC443: I. soft X-ray emission from shocked interstellar medium
The shocked interstellar medium around IC443 produces strong X-ray emission
in the soft energy band (E<1.5 keV). We present an analysis of such emission as
observed with the EPIC MOS cameras on board the XMM-Newotn observatory, with
the purpose to find clear signatures of the interactions with the interstellar
medium (ISM) in the X-ray band, which may complement results obtained in other
wavelenghts. We found that the giant molecular cloud mapped in CO emission is
located in the foreground and gives an evident signature in the absorption of
X-rays. This cloud may have a torus shape and the part of torus interacting
with the IC443 shock gives rise to 2MASS-K emission in the southeast. The
measured density of emitting X-ray shocked plasma increases toward the
northeastern limb, where the remnant is interacting with an atomic cloud. We
found an excellent correlation between emission in the 0.3-0.5 keV band and
bright optical/radio filament on large spatial scales. The partial shell
structure seen in this band therefore traces the encounter with the atomic
cloud.Comment: 10 pages, 10 figures, accepted for publication in ApJ (20 September
2006, v649). For hi-res figures, see
http://www.astropa.unipa.it/Library/OAPA_preprints/ic443ele1.ps.g
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
VLT-ISAAC 3-5 micron spectroscopy of embedded young low-mass stars. III. Intermediate-mass sources in Vela
We performed a spectroscopic survey toward five intermediate-mass class I
YSOs located in the Southern Vela molecular cloud in the L and M bands at
resolving powers 600-800 up to 10,000, using the Infrared Spectrometer and
Array Camera mounted on the VLT-ANTU. Lower mass companion objects were
observed simultaneously in both bands. Solid H2O at 3 micron is detected in all
sources, including the companion objects. CO ice at 4.67 micron is detected in
a few main targets and one companion object. One object (LLN 19) shows little
CO ice but strong gas-phase CO ro-vibrational lines in absorption. The CO ice
profiles are different from source to source. The amount of water ice and CO
ice trapped in a water-rich mantle may correlate with the flux ratio at 12 and
25 micron. The abundance of H2O-rich CO likely correlates with that of water
ice. A weak feature at 3.54 mu attributed to solid CH3OH and a broad feature
near 4.62 mu are observed toward LLN17, but not toward the other sources. The
derived abundances of solid CH3OH and OCN- are ~10% and ~1% of the H2O ice
abundance respectively. The H2O optical depths do not show an increase with
envelope mass, nor do they show lower values for the companion objects compared
with the main protostar. The line-of-sight CO ice abundance does not correlate
with the source bolometric luminosity. Comparison of the solid CO profile
toward LLN17, which shows an extremely broad CO ice feature, and that of its
lower mass companion at a few thousand AU, which exhibits a narrow profile,
together with the detection of OCN- toward LLN17 provide direct evidences for
local thermal processing of the ice.Comment: Replace wrong files. Accepted by A&A, 22 pages, 18 figure
On the Influence of Uncertainties in Chemical Reaction Rates on Results of the Astrochemical Modelling
With the chemical reaction rate database UMIST95 (Millar et al. 1997) we
analyze how uncertainties in rate constants of gas-phase chemical reactions
influence the modelling of molecular abundances in the interstellar medium.
Random variations are introduced into the rate constants to estimate the
scatter in theoretical abundances. Calculations are performed for dark and
translucent molecular clouds where gas phase chemistry is adequate. Similar
approach was used by Pineau des Forets & Roueff (2000) for the study of
chemical bistability. All the species are divided into 6 sensitivity groups
according to the value of the scatter in their model abundances computed with
varied rate constants. It is shown that the distribution of species within
these groups depends on the number of atoms in a molecule and on the adopted
physical conditions. The simple method is suggested which allows to single out
reactions that are most important for the evolution of a given species.Comment: 4 pages. To appear in the proceedings of the 4th Cologne-Bonn Zermatt
Symposiu
Shocked Molecular Gas in the Supernova Remnant HB 21
We report the discovery of the shocked molecular gas in the supernova remnant
HB 21. We derive the physical parameters of the shocked gas from CO J=1-0 and
J=2-1 line observations. We discuss the correlation of the shocked molecular
gas with the previously detected, shocked atomic gas and the associated
infrared emission.Comment: 24 pages, 10 figures, To appear in the ApJ, scheduled for the April
10, 2001 issue (v551
- …