460 research outputs found
Desorption of CO and O2 interstellar ice analogs
Solid O2 has been proposed as a possible reservoir for oxygen in dense clouds
through freeze-out processes. The aim of this work is to characterize
quantitatively the physical processes that are involved in the desorption
kinetics of CO-O2 ices by interpreting laboratory temperature programmed
desorption (TPD) data. This information is used to simulate the behavior of
CO-O2 ices under astrophysical conditions. The TPD spectra have been recorded
under ultra high vacuum conditions for pure, layered and mixed morphologies for
different thicknesses, temperatures and mixing ratios. An empirical kinetic
model is used to interpret the results and to provide input parameters for
astrophysical models. Binding energies are determined for different ice
morphologies. Independent of the ice morphology, the desorption of O2 is found
to follow 0th-order kinetics. Binding energies and temperature-dependent
sticking probabilities for CO-CO, O2-O2 and CO-O2 are determined. O2 is
slightly less volatile than CO, with binding energies of 912+-15 versus 858+-15
K for pure ices. In mixed and layered ices, CO does not co-desorb with O2 but
its binding energies are slightly increased compared with pure ice whereas
those for O2 are slightly decreased. Lower limits to the sticking probabilities
of CO and O2 are 0.9 and 0.85, respectively, at temperatures below 20K. The
balance between accretion and desorption is studied for O2 and CO in
astrophysically relevant scenarios. Only minor differences are found between
the two species, i.e., both desorb between 16 and 18K in typical environments
around young stars. Thus, clouds with significant abundances of gaseous CO are
unlikely to have large amounts of solid O2.Comment: 8 pages + 2 pages online material, 8 figures (1 online), accepted by
A&
Fully Sampled Maps of Ices and Silicates in Front of Cepheus A East with Spitzer
We report the first fully sampled maps of the distribution of interstellar
CO2 ices, H2O ices and total hydrogen nuclei, as inferred from the 9.7 micron
silicate feature, toward the star-forming region Cepheus A East with the IRS
instrument onboard the Spitzer Space Telescope. We find that the column density
distributions for these solid state features all peak at, and are distributed
around, the location of HW2, the protostar believed to power one of the
outflows observed in this star-forming region. A correlation between the column
density distributions of CO2 and water ice with that of total hydrogen
indicates that the solid state features we mapped mostly arise from the same
molecular clumps along the probed sight lines. We therefore derive average CO2
ice and water ice abundances with respect to the total hydrogen column density
of X(CO2)_ice~1.9x10^-5 and X(H2O)_ice~7.5x10^-5. Within errors, the abundances
for both ices are relatively constant over the mapped region exhibiting both
ice absorptions. The fraction of CO2 ice with respect to H2O ice is also
relatively constant at a value of 22% over that mapped region. A clear
triple-peaked structure is seen in the CO2 ice profiles. Fits to those profiles
using current laboratory ice analogs suggest the presence of both a
low-temperature polar ice mixture and a high-temperature methanol-rich ice
mixture along the probed sightlines. Our results further indicate that thermal
processing of these ices occurred throughout the sampled region.Comment: 26 pages, 8 figures, accepted for publication in Ap
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&
Semiconductor quantum dot - a quantum light source of multicolor photons with tunable statistics
We investigate the intensity correlation properties of single photons emitted
from an optically excited single semiconductor quantum dot. The second order
temporal coherence function of the photons emitted at various wavelengths is
measured as a function of the excitation power. We show experimentally and
theoretically, for the first time, that a quantum dot is not only a source of
correlated non-classical monochromatic photons but is also a source of
correlated non-classical \emph{multicolor} photons with tunable correlation
properties. We found that the emitted photon statistics can be varied by the
excitation rate from a sub-Poissonian one, where the photons are temporally
antibunched, to super-Poissonian, where they are temporally bunched.Comment: 4 pages, 2 figure
Spitzer Observations of CO2 Ice Towards Field Stars in the Taurus Molecular Cloud
We present the first Spitzer Infrared Spectrograph observations of the 15.2
micron bending mode of CO2 ice towards field stars behind a quiescent dark
cloud. CO2 ice is detected towards 2 field stars (Elias 16, Elias 3) and a
single protostar (HL Tau) with anabundance of ~15-20% relative to water ice.
CO2 ice is not detected towards the source with lowest extinction in our
sample, Tamura 17 (A_V = 3.9m). A comparison of the Elias 16 spectrum with
laboratory data demonstrates that the majority of CO2 ice is embedded in a
polar H2O-rich ice component, with ~15% of CO2 residing in an apolar H2O-poor
mantle. This is the first detection of apolar CO2 towards a field star. We find
that the CO2 extinction threshold is A_V = 4m +/- 1m, comparable to the
threshold for water ice, but significantly less than the threshold for CO ice,
the likely precursor of CO2. Our results confirm CO2 ice forms in tandem with
H2O ice along quiescent lines of sight. This argues for CO2 ice formation via a
mechanism similar to that responsible for H2O ice formation, viz. simple
catalytic reactions on grain surfaces.Comment: Accepted by Astrophysical Journal Letter
Optical spectroscopy of single quantum dots at tunable positive, neutral and negative charge states
We report on the observation of photoluminescence from positive, neutral and
negative charge states of single semiconductor quantum dots. For this purpose
we designed a structure enabling optical injection of a controlled unequal
number of negative electrons and positive holes into an isolated InGaAs quantum
dot embedded in a GaAs matrix. Thereby, we optically produced the charge states
-3, -2, -1, 0, +1 and +2. The injected carriers form confined collective
'artificial atoms and molecules' states in the quantum dot. We resolve
spectrally and temporally the photoluminescence from an optically excited
quantum dot and use it to identify collective states, which contain charge of
one type, coupled to few charges of the other type. These states can be viewed
as the artificial analog of charged atoms such as H, H, H,
and charged molecules such as H and H. Unlike higher
dimensionality systems, where negative or positive charging always results in
reduction of the emission energy due to electron-hole pair recombination, in
our dots, negative charging reduces the emission energy, relative to the
charge-neutral case, while positive charging increases it. Pseudopotential
model calculations reveal that the enhanced spatial localization of the
hole-wavefunction, relative to that of the electron in these dots, is the
reason for this effect.Comment: 5 figure
Negative capacitance in organic semiconductor devices: bipolar injection and charge recombination mechanism
We report negative capacitance at low frequencies in organic semiconductor
based diodes and show that it appears only under bipolar injection conditions.
We account quantitatively for this phenomenon by the recombination current due
to electron-hole annihilation. Simple addition of the recombination current to
the well established model of space charge limited current in the presence of
traps, yields excellent fits to the experimentally measured admittance data.
The dependence of the extracted characteristic recombination time on the bias
voltage is indicative of a recombination process which is mediated by localized
traps.Comment: 3 pages, 3 figures, accepted for publication in Applied Physics
Letter
Ices in the edge-on disk CRBR 2422.8-3423: Spitzer spectroscopy and Monte Carlo radiative transfer modeling
We present 5.2-37.2 micron spectroscopy of the edge-on circumstellar disk
CRBR 2422.8-3423 obtained using the InfraRed Spectrograph (IRS) of the Spitzer
Space Telescope. The IRS spectrum is combined with ground-based 3-5 micron
spectroscopy to obtain a complete inventory of solid state material present
along the line of sight toward the source. We model the object with a 2D
axisymmetric (effectively 3D) Monte Carlo radiative transfer code. It is found
that the model disk, assuming a standard flaring structure, is too warm to
contain the very large observed column density of pure CO ice, but is possibly
responsible for up to 50% of the water, CO2 and minor ice species. In
particular the 6.85 micron band, tentatively due to NH4+, exhibits a prominent
red wing, indicating a significant contribution from warm ice in the disk. It
is argued that the pure CO ice is located in the dense core Oph-F in front of
the source seen in the submillimeter imaging, with the CO gas in the core
highly depleted. The model is used to predict which circumstances are most
favourable for direct observations of ices in edge-on circumstellar disks. Ice
bands will in general be deepest for inclinations similar to the disk opening
angle, i.e. ~70 degrees. Due to the high optical depths of typical disk
mid-planes, ice absorption bands will often probe warmer ice located in the
upper layers of nearly edge-on disks. The ratios between different ice bands
are found to vary by up to an order of magnitude depending on disk inclination
due to radiative transfer effects caused by the 2D structure of the disk.
Ratios between ice bands of the same species can therefore be used to constrain
the location of the ices in a circumstellar disk. [Abstract abridged]Comment: 49 pages, accepted for publication in Ap
Apparent phonon side band modes in pi-conjugated systems: polymers, oligomers and crystals
The emission spectra of many pi-conjugated polymers and oligomers contain
side-band replicas with apparent frequencies that do not match the Raman active
mode frequencies. Using a time dependent model we show that in such many mode
systems, the increased damping of the time dependent transition dipole moment
correlation function results in an effective elimination of the vibrational
modes from the emission spectrum; subsequently causing the appearance of a
regularly spaced progression at a new apparent frequency. We use this damping
dependent vibrational reshaping to quantitatively account for the vibronic
structure in the emission spectra of pi-conjugated systems in the form of
films, dilute solutions and single crystals. In particular, we show that by
using the experimentally measured Raman spectrum we can account in detail for
the apparent progression frequencies and their relative intensities in the
emission spectrum.Comment: Presented in "Optical Probes 2005", Bangalore, Indi
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