666 research outputs found
Photodesorption of CO ice
At the high densities and low temperatures found in star forming regions, all
molecules other than H2 should stick on dust grains on timescales shorter than
the cloud lifetimes. Yet these clouds are detected in the millimeter lines of
gaseous CO. At these temperatures, thermal desorption is negligible and hence a
non-thermal desorption mechanism is necessary to maintain molecules in the gas
phase. Here, the first laboratory study of the photodesorption of pure CO ice
under ultra high vacuum is presented, which gives a desorption rate of 3E-3 CO
molecules per UV (7-10.5 eV) photon at 15 K. This rate is factors of 1E2-1E5
larger than previously estimated and is comparable to estimates of other
non-thermal desorption rates. The experiments constrains the mechanism to a
single photon desorption process of ice surface molecules. The measured
efficiency of this process shows that the role of CO photodesorption in
preventing total removal of molecules in the gas has been underestimated.Comment: 5 pages, 4 figures, accepted by ApJ
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&
A data mining approach to the SAR values over large MR image repositories
Purpose: In magnetic resonance imaging, the radiofrequency energy
absorption arises as one of the main safety concerns, being mainly related with
increased body temperature. Monitoring radiofrequency absorption is achieved
by the estimation of specific absorption rate (SAR), whose implementation lies
on equipment manufacturers, which in turn are not totally enlightening about its
calculus. This work presents an exploratory approach of whole-body SAR
values stored in DICOM metadata aiming to find correlation with body weight,
body mass index (BMI), gender and pulse sequences for abdominal/pelvic
(17.812 series) and head (29.907 series) studies.
Methods and Materials: All studies were acquired in a 3 Tesla scanner with
high-performance gradients. Data were extracted using Dicoogle, a DICOM
metadata mining tool. Several DICOM tags were analysed (e.g. patient weight,
height, gender, sequence name). For each study type, specifically weighted
pulse sequences were related with weight, BMI and gender through boxplot
diagrams, statistical and effect size analysis.
Results: SAR limits were never exceeded. Generally, SAR values tended to
decrease with increasing body weight and BMI values for abdominal/pelvic
studies. On the other hand, head studies showed different trends regarding
distinct pulse sequences. SAR values tend to be higher in male individuals
(p<0,05). As expected, turbo spin echo sequences present the highest SAR
values. The values found for echo gradient spoiled sequence (FLASH) were
also high.
Conclusion: It is confirmed that SAR estimates are related with the analysed
variables. An individual examination of pulse sequences is recommended to
observe trends regarding weight, BMI or gender.publishe
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Laboratory experiments on interstellar ice analogs: The sticking and desorption of small physisorbed molecules
Molecular oxygen and nitrogen are difficult to observe since they are infrared inactive and radio quiet. The low O2 abundances found so far combined with general considerations of dense cloud conditions suggest molecular oxygen is frozen out at low temperatures (< 20 K) in the shielded inner regions of cloud cores. In solid form O2 and N2 can only be observed as adjuncts within other ice constituents, like CO. In this work we focus on fundamental properties of N2 and O2 in CO ice-gas systems, e.g. desorption characteristics and sticking probabilities at low temperatures for different ice morphologies
Rotation-tunneling spectrum and astrochemical modeling of dimethylamine, CHNHCH, and searches for it in space
Methylamine has been the only simple alkylamine detected in the interstellar
medium for a long time. With the recent secure and tentative detections of
vinylamine and ethylamine, respectively, dimethylamine has become a promising
target for searches in space. Its rotational spectrum, however, has been known
only up to 45 GHz until now. Here we investigate the rotation-tunneling
spectrum of dimethylamine in selected regions between 76 and 1091 GHz using
three different spectrometers in order to facilitate its detection in space.
The quantum number range is extended to and , yielding an
extensive set of accurate spectroscopic parameters. To search for
dimethylamine, we refer to the spectral line survey ReMoCA carried out with the
Atacama Large Millimeter/submillimeter Array toward the high-mass star-forming
region Sagittarius B2(N) and a spectral line survey of the molecular cloud
G+0.6930.027 employing the IRAM 30 m and Yebes 40 m radio telescopes. We
report nondetections of dimethylamine toward the hot molecular cores Sgr
B2(N1S) and Sgr B2(N2b) as well as G+0.6930.027 which imply that
dimethylamine is at least 14, 4.5 and 39 times less abundant than methylamine
toward these sources, respectively. The observational results are compared to
computational results from a gas-grain astrochemical model. The modeled
methylamine to dimethylamine ratios are compatible with the observational
ratios. However, the model produces too much ethylamine compared with
methylamine which could mean that the already fairly low levels of
dimethylamine in the models may also be too high.Comment: Mon. Not. R. Astron. Soc., accepted. 33 pages including tables,
figures, and appendi
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