1,788 research outputs found
Extended Red Objects and Stellar Wind Bow Shocks in the Carina Nebula
We report the results of infrared photometry on 39 extended red objects
(EROs) in the Carina Nebula, observed with the Spitzer Space Telescope. Most
EROs are identified by bright, extended 8.0 um emission, which ranges from 10''
to 40'' in size, but our sample also includes 4 EROs identified by extended 24
um emission. Of particular interest are nine EROs associated with late O or
early B-type stars and characterized by arc-shaped morphology, suggesting
dusty, stellar-wind bow shocks. These objects are preferentially oriented
towards the central regions of the Carina Nebula, suggesting that these bow
shocks are generally produced by the interactions of OB winds with the bulk
expansion of the H II region rather than high proper motion. We identify
preferred regions of mid-infrared color space occupied by our bow shock
candidates, which also contain bow shock candidates in M17 and RCW 49 but are
well-separated from polycyclic aromatic hydrocarbon emission or circumstellar
discs. Color cuts identify an additional 12 marginally-resolved bow shock
candidates, 10 of which are also associated with known late O or early B stars.
H II region expansion velocities derived from bow shock candidate standoff
distances are ~10 km/s, assuming typical H II region gas densities, comparable
to expansion velocities derived from bow shocks in M17 and RCW 49. One
candidate bow shock provides direct evidence of physical interaction between
the massive stellar winds originating in the Trumpler 15 and Trumpler 14
clusters, supporting the conclusion that both clusters are at similar
heliocentric distances.Comment: 15 pages, 7 figures, MNRAS accepted (2014 October 10
Computational Study of Fluorinated Diglyoxime-Iron Complexes: Tuning the Electrocatalytic Pathways for Hydrogen Evolution
The ability to tune the properties of hydrogen-evolving molecular electrocatalysts is important for developing alternative energy sources. Fluorinated diglyoxime-iron complexes have been shown to evolve hydrogen at moderate overpotentials. Herein two such complexes, [(dAr^FgBF_2)_2Fe(py)_2], denoted A, and [(dAr^Fg_2H-BF_2)Fe(py)_2], denoted B [dAr^Fg = bis(pentafluorophenyl-glyoximato); py = pyridine], are investigated with density functional theory calculations. B differs from A in that one BF_2 bridge is replaced by a proton bridge of the form O–H–O. According to the calculations, the catalytic pathway for A involves two consecutive reduction steps, followed by protonation of an Fe^0 species to generate the active Fe^(II)-hydride species. B is found to proceed via two parallel pathways, where one pathway is similar to that for A, and the additional pathway arises from protonation of the O–H–O bridge, followed by spontaneous reduction to an Fe^0 intermediate and intramolecular proton transfer from the ligand to the metal center or protonation by external acid to form the same active Fe^(II)-hydride species. Simulated cyclic voltammograms (CVs) based on these mechanisms are in qualitative agreement with experimental CVs. The two parallel pathways identified for B arise from an equilibrium between the protonated and unprotonated ligand and result in two catalytic peaks in the CVs. The calculations predict that the relative probabilities for the two pathways, and therefore the relative magnitudes of the catalytic peaks, could be tuned by altering the pK_a of the acid or the substituents on the ligands of the electrocatalyst. The ability to control the catalytic pathways through acid strength or ligand substituents is critical for designing more effective catalysts for energy conversion processes
Orbiter Boundary Layer Transition Prediction Tool Enhancements
Updates to an analytic tool developed for Shuttle support to predict the onset of boundary layer transition resulting from thermal protection system damage or repair are presented. The boundary layer transition tool is part of a suite of tools that analyze the local aerothermodynamic environment to enable informed disposition of damage for making recommendations to fly as is or to repair. Using mission specific trajectory information and details of each d agmea site or repair, the expected time (and thus Mach number) of transition onset is predicted to help define proper environments for use in subsequent thermal and stress analysis of the thermal protection system and structure. The boundary layer transition criteria utilized within the tool were updated based on new local boundary layer properties obtained from high fidelity computational solutions. Also, new ground-based measurements were obtained to allow for a wider parametric variation with both protuberances and cavities and then the resulting correlations were calibrated against updated flight data. The end result is to provide correlations that allow increased confidence with the resulting transition predictions. Recently, a new approach was adopted to remove conservatism in terms of sustained turbulence along the wing leading edge. Finally, some of the newer flight data are also discussed in terms of how these results reflect back on the updated correlations
Finite Element Modeling of the Posterior Eye in Microgravity
Microgravity experienced during spaceflight affects astronauts in various ways, including weakened muscles and loss of bone density. Recently, visual impairment and intracranial pressure (VIIP) syndrome has become a major concern for space missions lasting longer than 30 days. Astronauts suffering from VIIP syndrome have changes in ocular anatomical and visual impairment that persist after returning to earth. It is hypothesized that a cephalad fluid shift in microgravity may increase the intracranial pressure (ICP), which leads to an altered biomechanical environment of the posterior globe and optic nerve sheath (ONS).Currently, there is a lack of knowledge of how elevated ICP may lead to vision impairment and connective tissue changes in VIIP. Our goal was to develop a finite element model to simulate the acute effects of elevated ICP on the posterior eye and optic nerve sheath. We used a finite element (FE) analysis approach to understand the response of the lamina cribrosa and optic nerve to the elevations in ICP thought to occur in microgravity and to identify which tissue components have the greatest impact on strain experienced by optic nerve head tissues
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PARP inhibitors trap PARP2 and alter the mode of recruitment of PARP2 at DNA damage sites
Dual-inhibitors of PARP1 and PARP2 are promising anti-cancer drugs. In addition to blocking PARP1&2 enzymatic activity, PARP inhibitors also extend the lifetime of DNA damage-induced PARP1&2 foci, termed trapping. Trapping is important for the therapeutic effects of PARP inhibitors. Using live-cell imaging, we found that PARP inhibitors cause persistent PARP2 foci by switching the mode of PARP2 recruitment from a predominantly PARP1- and PAR-dependent rapid exchange to a WGR domain-mediated stalling of PARP2 on DNA. Specifically, PARP1-deletion markedly reduces but does not abolish PARP2 foci. The residual PARP2 foci in PARP1-deficient cells are DNA-dependent and abrogated by the R140A mutation in the WGR domain. Yet, PARP2-R140A forms normal foci in PARP1-proficient cells. In PARP1-deficient cells, PARP inhibitors - niraparib, talazoparib, and, to a lesser extent, olaparib - enhance PARP2 foci by preventing PARP2 exchange. This trapping of PARP2 is independent of auto-PARylation and is abolished by the R140A mutation in the WGR domain and the H415A mutation in the catalytic domain. Taken together, we found that PARP inhibitors trap PARP2 by physically stalling PARP2 on DNA via the WGR-DNA interaction while suppressing the PARP1- and PAR-dependent rapid exchange of PARP2.
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Cavalieri integration
We use Cavalieri’s principle to develop a novel integration technique
which we call Cavalieri integration. Cavalieri integrals differ from Riemann integrals
in that non-rectangular integration strips are used. In this way we can use single
Cavalieri integrals to find the areas of some interesting regions for which it is difficult
to construct single Riemann integrals.
We also present two methods of evaluating a Cavalieri integral by first transforming
it to either an equivalent Riemann or Riemann-Stieltjes integral by using special transformation functions h(x) and its inverse g(x), respectively. Interestingly enough
it is often very difficult to find the transformation function h(x), whereas it is very
simple to obtain its inverse g(x).http://www.tandfonline.com/loi/tqma20hb201
An inductive approach to simulating multispectral MODIS surface reflectance time series
In this paper, a first order MODIS time series
simulator, which uses a Colored Simple Harmonic Oscillator, is
proposed. The simulated data can be used to augment data sets so
that data intensive classification and change detection algorithms
can be applied without enlarging the available ground truth data
sets. The simulator’s validity is tested by simulating data sets of
natural vegetation and human settlement areas and comparing it
to the ground truth data in the Gauteng province located in South
Africa. The difference found between the real and simulated
data sets, which is reported in the experiments is negligent. The
simulated and real world data sets are compared by using a wide
selection of class and pixel metrics. In particular the average
temporal Hellinger distance between the real and simulated data
sets is 0.2364 and 0.2269 for the vegetation and settlement class
respectively, while the average parameter Hellinger distance is
0.1835 and 0.2554 respectively.http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=8859hb2013ai201
Using Page's cumulative sum test on MODIS time series to detect land-cover changes
Human settlement expansion is one of the most
pervasive forms of land cover change in South Africa. The use
of Page’s Cumulative Sum Test is proposed as a method to
detect new settlement developments in areas that were previously
covered by natural vegetation using 500 m MODIS time series
satellite data. The method is a sequential per pixel change alarm
algorithm that can take into account positive detection delay,
probability of detection and false alarm probability to construct
a threshold. Simulated change data was generated to determine a
threshold during a preliminary off-line optimization phase. After
optimization the method was evaluated on examples of known
land cover change in the Gauteng and Limpopo provinces of
South Africa. The experimental results indicated that CUSUM
performs better than band differencing in the before mentioned
study areas.http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=8859hb2013ai201
UBVRI Light Curves of 44 Type Ia Supernovae
We present UBVRI photometry of 44 type-Ia supernovae (SN Ia) observed from
1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence
Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The
data set comprises 2190 observations and is the largest homogeneously observed
and reduced sample of SN Ia to date, nearly doubling the number of
well-observed, nearby SN Ia with published multicolor CCD light curves. The
large sample of U-band photometry is a unique addition, with important
connections to SN Ia observed at high redshift. The decline rate of SN Ia
U-band light curves correlates well with the decline rate in other bands, as
does the U-B color at maximum light. However, the U-band peak magnitudes show
an increased dispersion relative to other bands even after accounting for
extinction and decline rate, amounting to an additional ~40% intrinsic scatter
compared to B-band.Comment: 84 authors, 71 pages, 51 tables, 10 figures. Accepted for publication
in the Astronomical Journal. Version with high-res figures and electronic
data at http://astron.berkeley.edu/~saurabh/cfa2snIa
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