91 research outputs found
Constraints on Europa's water group torus from HST/COS observations
In-situ plasma measurements as well as remote mapping of energetic neutral
atoms around Jupiter provide indirect evidence that an enhancement of neutral
gas is present near the orbit of the moon Europa. Simulations suggest that such
a neutral gas torus can be sustained by escape from Europa's atmosphere and
consists primarily of molecular hydrogen, but the neutral gas torus has not yet
been measured directly through emissions or in-situ. Here we present
observations by the Cosmic Origins Spectrograph of the Hubble Space Telescope
(HST/COS) from 2020 and 2021, which scanned the equatorial plane between 8 and
10 planetary radii west of Jupiter. No neutral gas emissions are detected. We
derive upper limits on the emissions and compare these to modelled emissions
from electron impact and resonant scattering using a Europa torus Monte Carlo
model for the neutral gases. The comparison supports the previous findings that
the torus is dilute and primarily consists of molecular hydrogen. A detection
of sulfur ion emissions radially inward of the Europa orbit is consistent with
emissions from the extended Io torus and with sulfur ion fractional abundances
as previously detected
Communications Biophysics
Contains reports on six research projects.National Institutes of Health (Grant 2 P01 GM-14940-01)Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 28-043-AMC-02536(E)National Aeronautics and Space Administration (Grant NsG-496)National Institutes of Health (Grant 2 ROl NB-05462-03
Rosetta-Alice Observations of Exospheric Hydrogen and Oxygen on Mars
The European Space Agency's Rosetta spacecraft, en route to a 2014 encounter
with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on
25 February 2007, closest approach being at 01:54UT. The Alice instrument on
board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for
in situ cometary spectroscopy in the 750-2000 A spectral band, was used to
study the daytime Mars upper atmosphere including emissions from exospheric
hydrogen and oxygen. Offset pointing, obtained five hours before closest
approach, enabled us to detect and map the HI Lyman-alpha and Lyman-beta
emissions from exospheric hydrogen out beyond 30,000 km from the planet's
center. These data are fit with a Chamberlain exospheric model from which we
derive the hydrogen density at the 200 km exobase and the H escape flux. The
results are comparable to those found from the the Ultraviolet Spectrometer
experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen
emission at 1304 A is detected at altitudes of 400 to 1000 km above the limb
during limb scans shortly after closest approach. However, the derived oxygen
scale height is not consistent with recent models of oxygen escape based on the
production of suprathermal oxygen atoms by the dissociative recombination of
O2+.Comment: 17 pages, 8 figures, accepted for publication in Icaru
Submillimeter-resolution radiography of shielded structures with laser-accelerated electron beams
We investigate the use of energetic electron beams for high-resolution radiography of flaws embedded in thick solid objects. A bright, monoenergetic electron beam (with energy \u3e100 MeV) was generated by the process of laser-wakefield acceleration through the interaction of 50-TW, 30-fs laser pulses with a supersonic helium jet. The high energy, low divergence, and small source size of these beams make them ideal for high-resolution radiographic studies of cracks or voids embedded in dense materials that are placed at a large distance from the source. We report radiographic imaging of steel with submillimeter resolution
A Pulsed, Precessing Jet in Cepheus A
We present near-infrared H2, radio CO, and thermal infrared observations of
the nearby massive star-forming region Cepheus A (Cep A). From H2 bow shocks
arranged along four distinct jet axes, we infer that the massive protostellar
source HW2 drives a pulsed, precessing jet that has changed its orientation by
about 45 degrees in roughly 104 years. The current HW2 radio jet represents the
most recent event in this time series of eruptions. This scenario is consistent
with the recent discovery of a disk around HW2, perpendicular to the current
jet orientation, and with the presence of companions at projected distances
comparable to the disk radius. We propose that the Cep A system formed by the
disk-assisted capture of a sibling star by HW2. We present a numerical model of
a 15 M_sun star with a circumstellar disk, orbited by a companion in an
inclined, eccentric orbit. Close passages of the companion through or near the
disk result in periods of enhanced accretion and mass loss, as well as forced
precession of the disk and associated orientation changes in the jet. The
observations reveal a second powerful outflow that emerges from radio source
HW3c or HW3d. This flow is associated with blueshifted CO emission and a faint
H2 bow shock to the east, and with HH 168 to the west. A collision between the
flows from HW2 and HW3c/d may be responsible for X-ray and radio continuum
emission in Cep A West.Comment: 12 pages, to be published in Ap
Communications Biophysics
COntains reports on six research projects.National Institutes of Health (Grant 2 P01 MH-04737-06)National Institutes of Health (Grant 5 ROl NB-05462-02)Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 36-039-AMC-03200(E)National Science Foundation (Grant GK-835)National Aeronautics and Space Administration (Grant NsG-496
MCE domain proteins::conserved inner membrane lipid-binding proteins required for outer membrane homeostasis
AbstractBacterial proteins with MCE domains were first described as being important for Mammalian Cell Entry. More recent evidence suggests they are components of lipid ABC transporters. In Escherichia coli, the single-domain protein MlaD is known to be part of an inner membrane transporter that is important for maintenance of outer membrane lipid asymmetry. Here we describe two multi MCE domain-containing proteins in Escherichia coli, PqiB and YebT, the latter of which is an orthologue of MAM-7 that was previously reported to be an outer membrane protein. We show that all three MCE domain-containing proteins localise to the inner membrane. Bioinformatic analyses revealed that MCE domains are widely distributed across bacterial phyla but multi MCE domain-containing proteins evolved in Proteobacteria from single-domain proteins. Mutants defective in mlaD, pqiAB and yebST were shown to have distinct but partially overlapping phenotypes, but the primary functions of PqiB and YebT differ from MlaD. Complementing our previous findings that all three proteins bind phospholipids, results presented here indicate that multi-domain proteins evolved in Proteobacteria for specific functions in maintaining cell envelope homeostasis.</jats:p
- …