98 research outputs found
Evolution of the Dust Coma in Comet 67P/Churyumov-Gerasimenko Before 2009 Perihelion
Comet 67P/Churyumov-Gerasimenko is the main target of ESA's Rosetta mission
and will be encountered in May 2014. As the spacecraft shall be in orbit the
comet nucleus before and after release of the lander {\it Philae}, it is
necessary necessary to know the conditions in the coma. Study the dust
environment, including the dust production rate and its variations along its
preperihelion orbit. The comet was observed during its approach to the Sun on
four epochs between early-June 2008 and mid-January 2009, over a large range of
heliocentric distances that will be covered by the mission in 2014. An
anomalous enhancement of the coma dust density was measured towards the comet
nucleus. The scalelength of this enhancement increased with decreasing
heliocentric distance of the comet. This is interpreted as a result of an
unusually slow expansion of the dust coma. Assuming a spherical symmetric coma,
the average amount of dust as well as its ejection velocity have been derived.
The latter increases exponentially with decreasing heliocentric distance (\rh),
ranging from about 1 m/s at 3 AU to about 25-35 m/s at 1.4 AU. Based on these
results we describe the dust environment at those nucleocentric distances at
which the spacecraft will presumably be in orbit.
Astronomy and Astrophysics, in pressComment: 5 pages, 4 figure
Acidic Extracellular pH Promotes Activation of Integrin αvβ3
Acidic extracellular pH is characteristic of the cell microenvironment in several important physiological and pathological contexts. Although it is well established that acidic extracellular pH can have profound effects on processes such as cell adhesion and migration, the underlying molecular mechanisms are largely unknown. Integrin receptors physically connect cells to the extracellular matrix, and are thus likely to modulate cell responses to extracellular conditions. Here, we examine the role of acidic extracellular pH in regulating activation of integrin [alpha]v[beta]3. Through computational molecular dynamics simulations, we find that acidic extracellular pH promotes opening of the [alpha]v[beta]3 headpiece, indicating that acidic pH can thereby facilitate integrin activation. This prediction is consistent with our flow cytometry and atomic force microscope-mediated force spectroscopy assays of integrin [alpha]v[beta]3 on live cells, which both demonstrate that acidic pH promotes activation at the intact cell surface. Finally, quantification of cell morphology and migration measurements shows that acidic extracellular pH affects cell behavior in a manner that is consistent with increased integrin activation. Taken together, these computational and experimental results suggest a new and complementary mechanism of integrin activation regulation, with associated implications for cell adhesion and migration in regions of altered pH that are relevant to wound healing and cancer.National Institute of Biomedical Imaging and Bioengineering (U.S.) (Award Number T32EB006348)Massachusetts Institute of Technology (Collamore-Rogers Fellowship)National Institutes of Health (U.S.) (NIH Cell Migration Consortium Grant U54-GM069668)National Science Foundation (U.S.) (CAREER Award)Singapore-MIT Alliance for Research and Technology (BioSystem and Micromechanics (BioSyM) Interdisciplinary Research Group
Characterization of stardust target comet 81P/Wild 2 from 1996 to 1998
We present the results of our monitoring of
comet 81P/Wild 2, the target of the STARDUST
mission. Broad-band filter images and medium
resolution spectra of this comet obtained along
its pre- and postperihelion orbit between 1996
and 1998 were analysed in terms of coma
morphology and activity. The morphological coma
investigation revealed the presence of
long-lasting fan structures, which remained
essentially unchanged for at least three months.
The (B-V) and (V-R) colour indices were
determined at three different positions along the
orbit. The two-dimensional analysis showed a
uniform colour over the entire coma. Hence, the
material composing the structures is not
different from that of the underlying coma. The
distribution of coma brightness as a function of
projected distance from the nucleus shows no
peculiarities. The evolution of the comet's
activity during the pre- and postperihelion phase
appears to be asymmetric in that activity
increases faster preperihelion than it decreases
postperihelion. The production rates (or their
upper limits in the case on a non-detection) of
CN, C2, C3, and
NH2 were determined from the
spectrophotometric observations and compared to
other values published in the literature. based
on observations obtained at ESO La Silla within
ESO programmes No. 57.F-0290, 58.F-0413,
58.F-0431 and 60.F-0266
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