353 research outputs found
On the Correlation Between CO Absorption and Far-Ultraviolet Non-Linear Extinction Toward Galactic OB Stars
A sample of 59 sight lines to reddened Galactic OB stars was examined for
correlations of the strength of the CO Fourth Positive (A - X) absorption band
system with the ultraviolet interstellar extinction curve parameters. We used
archival high-dispersion NEWSIPS IUE spectra to measure the CO absorption for
comparison to parametric fits of the extinction curves from the literature. A
strong correlation with the non-linear far-UV curvature term was found with
greater absorption, normalized to E(B-V), being associated with more curvature.
A weaker trend with the linear extinction term was also found. Mechanisms for
enhancing CO in dust environments exhibiting high non-linear curvature are
discussed.Comment: 10 pages, including 6 figures. LaTeX2e (emulateapj5.sty). To appear
in ApJ, Sep 20, 200
The structural changes of water ice I during warmup
The polymorph transitions of vapor deposited water ice I during warmup from 15 K to 210 K was mapped by means of selected area electron diffraction. The polymorph transitions account for many phenomena observed in laboratory analog studies of cometary outgassing and radial diffusion in UV photolyzed interstellar ices
High-Density Amorphous Ice, the Frost on Interstellar Grains
Most water ice in the universe is in a form which does not occur naturally on Earth and of which only minimal amounts have been made in the laboratory. We have encountered this 'high-density amorphous ice' in electron diffraction experiments of low-temperature (T less than 30 K) vapor-deposited water and have subsequently modeled its structure using molecular dynamics simulations. The characteristic feature of high-density amorphous ice is the presence of 'interstitial' oxygen pair distances between 3 and 4 A. However, we find that the structure is best described as a collapsed lattice of the more familiar low-density amorphous form. These distortions are frozen in at temperatures below 38 K because, we propose, it requires the breaking of one hydrogen bond, on average, per molecule to relieve the strain and to restructure the lattice to that of low-density amorphous ice. Several features of astrophysical ice analogs studied in laboratory experiments are readily explained by the structural transition from high-density amorphous ice into low-density amorphous ice. Changes in the shape of the 3.07 gm water band, trapping efficiency of CO, CO loss, changes in the CO band structure, and the recombination of radicals induced by low-temperature UV photolysis all covary with structural changes that occur in the ice during this amorphous to amorphous transition. While the 3.07 micrometers ice band in various astronomical environments can be modeled with spectra of simple mixtures of amorphous and crystalline forms, the contribution of the high-density amorphous form nearly always dominates
Can neutral and ionized PAHs be carriers of the UV extinction bump and the diffuse interstellar bands?
Up to now, no laboratory-based study has investigated polycyclic aromatic
hydrocarbon (PAH) species as potential carriers of both the diffuse
interstellar bands (DIBs) and the 2175 A UV bump. We examined the proposed
correlation between these two features by applying experimental and theoretical
techniques on two specific medium-sized/large PAHs (dibenzorubicene C30H14 and
hexabenzocoronene C42H18) in their neutral and cationic states. It was already
shown that mixtures of sufficiently large, neutral PAHs can partly or even
completely account for the UV bump. We investigated how the absorption bands
are altered upon ionization of these molecules by interstellar UV photons. The
experimental studies presented here were realized by performing matrix
isolation spectroscopy with subsequent far-UV irradiation. The main effects
were found to be a broadening of the absorption bands in the UV combined with
slight red shifts. The position of the complete pi - pi* absorption structure
around 217.5 nm, however, remains more or less unchanged which could explain
the observed position invariance of the interstellar bump for different lines
of sight. This favors the assignment of this feature to the interstellar PAH
population. As far as the DIBs are concerned, neither our investigations nor
the laboratory studies carried out by other research groups support a possible
connection with this class of molecules. Instead, there are reasonable
arguments that neutral and singly ionized cationic PAHs cannot be made
responsible for the DIBs.Comment: 11 pages, 7 figures, 1 tabl
Improved Understanding of the Inflammatory Response in Synovial Fluid and Serum after Traumatic Knee Injury, Excluding Fractures of the Knee:A Systematic Review
Background: Traumatic knee injury results in a 4- to 10-fold increased risk of post-traumatic osteoarthritis (PTOA). Currently, there are no successful interventions for preventing PTOA after knee injury. The aim of this study is to identify inflammatory proteins that are increased in serum and synovial fluid after acute knee injury, excluding intra-articular fractures. Methods: A literature search was done according to the PRISMA guidelines. Articles reporting about inflammatory proteins after knee injury, except fractures, up to December 8, 2021 were collected. Inclusion criteria were as follows: patients younger than 45 years, no radiographic signs of knee osteoarthritis at baseline, and inflammatory protein measurement within 1 year after trauma. Risk of bias was assessed of the included studies. The level of evidence was determined by the Strength of Recommendation Taxonomy. Results: Ten studies were included. All included studies used a healthy control group or the contralateral knee as healthy control. Strong evidence for interleukin 6 (IL-6) and limited evidence for CCL4 show elevated concentrations of these proteins in synovial fluid (SF) after acute knee injury; no upregulation in SF for IL-2, IL-10, CCL3, CCL5, CCL11, granulocyte colony-stimulating factor (G-CSF), and granulocyte-macrophage colony-stimulating factor (GM-CSF) was found. Limited evidence was found for no difference in serum concentration of IL-1β, IL-6, IL-10, CCL2, and tumor necrosis factor alpha (TNF-α) after knee injury. Conclusion: Interleukin 6 and CCL4 are elevated in SF after acute knee injury. Included studies failed to demonstrate increased concentration of inflammatory proteins in SF samples taken 6 weeks after trauma. Future research should focus on SF inflammatory protein measurements taken less than 6 weeks after injury.</p
Dynamical Model for the Zodiacal Cloud and Sporadic Meteors
The solar system is dusty, and would become dustier over time as asteroids
collide and comets disintegrate, except that small debris particles in
interplanetary space do not last long. They can be ejected from the solar
system by Jupiter, thermally destroyed near the Sun, or physically disrupted by
collisions. Also, some are swept by the Earth (and other planets), producing
meteors. Here we develop a dynamical model for the solar system meteoroids and
use it to explain meteor radar observations. We find that the Jupiter Family
Comets (JFCs) are the main source of the prominent concentrations of meteors
arriving to the Earth from the helion and antihelion directions. To match the
radiant and orbit distributions, as measured by the Canadian Meteor Orbit Radar
(CMOR) and Advanced Meteor Orbit Radar (AMOR), our model implies that comets,
and JFCs in particular, must frequently disintegrate when reaching orbits with
low perihelion distance. Also, the collisional lifetimes of millimeter
particles may be longer (>10^5 yr at 1 AU) than postulated in the standard
collisional models (10^4 yr at 1 AU), perhaps because these chondrule-sized
meteoroids are stronger than thought before. Using observations of the Infrared
Astronomical Satellite (IRAS) to calibrate the model, we find that the total
cross section and mass of small meteoroids in the inner solar system are
(1.7-3.5)x10^11 km^2 and 4x10^19 g, respectively, in a good agreement with
previous studies. The mass input required to keep the Zodiacal Cloud (ZC) in a
steady state is estimated to be 10^4-10^5 kg/s. The input is up to 10 times
larger than found previously, mainly because particles released closer to the
Sun have shorter collisional lifetimes, and need to be supplied at a faster
rate
The optical spectrum of a large isolated polycyclic aromatic hydrocarbon: hexa-peri-hexabenzocoronene, C42H18
The first optical spectrum of an isolated polycyclic aromatic hydrocarbon
large enough to survive the photophysical conditions of the interstellar medium
is reported. Vibronic bands of the first electronic transition of the all
benzenoid polycyclic aromatic hydrocarbon hexa-peri-hexabenzocoronene were
observed in the 4080-4530 Angstrom range by resonant 2-color 2-photon
ionization spectroscopy. The strongest feature at 4264 Angstrom is estimated to
have an oscillator strength of f=1.4x10^-3, placing an upper limit on the
interstellar abundance of this polycyclic aromatic hydrocarbon at 4x10^12
cm^-2, accounting for a maximum of ~0.02% of interstellar carbon. This study
opens up the possibility to rigorously test neutral polycyclic aromatic
hydrocarbons as carriers of the diffuse interstellar bands in the near future.Comment: 9 pages, 1 figure. Fixed a typo on the frequency of the 'b' ban
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