515 research outputs found
Iron-Dependent Gene Expression in Actinomyces Oris
Actinomyces oris is a Gram-positive bacterium that has been associated with healthy and diseased sites in the human oral cavity. Most pathogenic bacteria require iron to survive, and in order to acquire iron in the relatively iron-scarce oral cavity A. oris has been shown to produce iron-binding molecules known as siderophores. The genes encoding these siderophores and transporters are thought to be regulated by the amount of iron in the growth medium and by the metal-dependent repressor, AmdR, which we showed previously binds to the promoter of proposed iron-regulated genes
Synthesis of 3-D coronal-solar wind energetic particle acceleration modules
1. Introduction Acute space radiation hazards pose one of the most serious risks to future human and robotic exploration. Large solar energetic particle (SEP) events are dangerous to astronauts and equipment. The ability to predict when and where large SEPs will occur is necessary in order to mitigate their hazards. The Coronal-Solar Wind Energetic Particle Acceleration (C-SWEPA) modeling effort in the NASA/NSF Space Weather Modeling Collaborative [Schunk, 2014] combines two successful Living With a Star (LWS) (http://lws. gsfc.nasa.gov/) strategic capabilities: the Earth-Moon-Mars Radiation Environment Modules (EMMREM) [Schwadron et al., 2010] that describe energetic particles and their effects, with the Next Generation Model for the Corona and Solar Wind developed by the Predictive Science, Inc. (PSI) group. The goal of the C-SWEPA effort is to develop a coupled model that describes the conditions of the corona, solar wind, coronal mass ejections (CMEs) and associated shocks, particle acceleration, and propagation via physics-based modules. Assessing the threat of SEPs is a difficult problem. The largest SEPs typically arise in conjunction with X class flares and very fast (\u3e1000 km/s) CMEs. These events are usually associated with complex sunspot groups (also known as active regions) that harbor strong, stressed magnetic fields. Highly energetic protons generated in these events travel near the speed of light and can arrive at Earth minutes after the eruptive event. The generation of these particles is, in turn, believed to be primarily associated with the shock wave formed very low in the corona by the passage of the CME (injection of particles from the flare site may also play a role). Whether these particles actually reach Earth (or any other point) depends on their transport in the interplanetary magnetic field and their magnetic connection to the shock
Modeling Accelerated Pick-up Ion Distributions at an Interplanetary Shock
The acceleration of interstellar pick-up ions as well as solar wind species
has been observed at a multitude of interplanetary (IP) shocks by different
spacecraft. The efficiency of injection of the pick-up ion component differs
from that of the solar wind, and is expected to be strongly enhanced at highly
oblique and quasi-perpendicular shock events, in accord with inferences from
{\it in situ} observations. This paper explores theoretical modeling of the
phase space distributions of accelerated ions obtained by the Ulysses mission
for the Day 292, 1991 shock associated with a corotating interaction region,
encountered before Ulysses' fly-by of Jupiter. A Monte Carlo simulation is used
to model the acceleration process, adapting a technique that has been
successfully tested on earlier IP shocks possessing minimal pick-up ion
presence. Phase space distributions from the simulation technique for various
low mass ions are compared with SWICS and HI-SCALE data to deduce values of a
``turbulence parameter'' that controls the efficiency of injection, and the
degree of cross-field diffusion. Acceptable fits are obtained for the and
populations using standard prescriptions for the pick-up ion
distribution; spectral data was only fit well for scenarios very
close to the Bohm diffusion limit. It is also found that the simulation
successfully accounts for the observation of energetic protons farther upstream
of the forward shock than lower energy pick-up protons, using the same
turbulence parameter that is required to achieve reasonable spectral fits.Comment: 15 pages, 2 embedded figures, Advances Space Research, in pres
On the Energy Dependence of Galactic Cosmic Ray Anisotropies in the Very Local Interstellar Medium
We report on the energy dependence of galactic cosmic rays (GCRs) in the very
local interstellar medium (VLISM) as measured by the Low Energy Charged
Particle (LECP) instrument on the Voyager 1 (V1) spacecraft. The LECP
instrument includes a dual-ended solid state detector particle telescope
mechanically scanning through 360 deg across eight equally-spaced angular
sectors. As reported previously, LECP measurements showed a dramatic increase
in GCR intensities for all sectors of the >=211 MeV count rate (CH31) at the V1
heliopause (HP) crossing in 2012, however, since then the count rate data have
demonstrated systematic episodes of intensity decrease for particles around
90{\deg} pitch angle. To shed light on the energy dependence of these GCR
anisotropies over a wide range of energies, we use V1 LECP count rate and pulse
height analyzer (PHA) data from >=211 MeV channel together with lower energy
LECP channels. Our analysis shows that while GCR anisotropies are present over
a wide range of energies, there is a decreasing trend in the amplitude of
second-order anisotropy with increasing energy during anisotropy episodes. A
stronger pitch-angle scattering at the higher velocities is argued as a
potential cause for this energy dependence. A possible cause for this velocity
dependence arising from weak rigidity dependence of the scattering mean free
path and resulting velocity-dominated scattering rate is discussed. This
interpretation is consistent with a recently reported lack of corresponding GCR
electron anisotropies
Imaging the initial condition of heavy-ion collisions and nuclear structure across the nuclide chart
A major goal of the hot QCD program, the extraction of the properties of the
quark gluon plasma (QGP), is currently limited by our poor knowledge of the
initial condition of the QGP, in particular how it is shaped from the colliding
nuclei. To attack this limitation, we propose to exploit collisions of selected
species to precisely assess how the initial condition changes under variations
of the structure of the colliding ions. This knowledge, combined with
event-by-event measures of particle correlations in the final state of
heavy-ion collisions, will provide in turn a new way to probe the collective
structure of nuclei, and to confront and exploit the predictions of
state-of-the-art ab initio nuclear structure theories. The US nuclear community
should capitalize on this interdisciplinary connection by pursuing collisions
of well-motivated species at high-energy colliders.Comment: 23 pages, 6 figure
Diffusive Acceleration of Particles at Oblique, Relativistic, Magnetohydrodynamic Shocks
Diffusive shock acceleration (DSA) at relativistic shocks is expected to be
an important acceleration mechanism in a variety of astrophysical objects
including extragalactic jets in active galactic nuclei and gamma ray bursts.
These sources remain good candidate sites for the generation of ultra-high
energy cosmic rays. In this paper, key predictions of DSA at relativistic
shocks that are germane to production of relativistic electrons and ions are
outlined. The technique employed to identify these characteristics is a Monte
Carlo simulation of such diffusive acceleration in test-particle, relativistic,
oblique, magnetohydrodynamic (MHD) shocks. Using a compact prescription for
diffusion of charges in MHD turbulence, this approach generates particle
angular and momentum distributions at any position upstream or downstream of
the shock. Simulation output is presented for both small angle and large angle
scattering scenarios, and a variety of shock obliquities including superluminal
regimes when the de Hoffmann-Teller frame does not exist. The distribution
function power-law indices compare favorably with results from other
techniques. They are found to depend sensitively on the mean magnetic field
orientation in the shock, and the nature of MHD turbulence that propagates
along fields in shock environs. An interesting regime of flat spectrum
generation is addressed; we provide evidence for it being due to shock drift
acceleration, a phenomenon well-known in heliospheric shock studies. The impact
of these theoretical results on blazar science is outlined. Specifically,
Fermi-LAT gamma-ray observations of these relativistic jet sources are
providing significant constraints on important environmental quantities for
relativistic shocks, namely the field obliquity, the frequency of scattering
and the level of field turbulence.Comment: 25 pages, 12 figures, accepted for publication in The Astrophysical
Journa
Acceleration of Solar Wind Ions by Nearby Interplanetary Shocks: Comparison of Monte Carlo Simulations with Ulysses Observations
The most stringent test of theoretical models of the first-order Fermi
mechanism at collisionless astrophysical shocks is a comparison of the
theoretical predictions with observational data on particle populations. Such
comparisons have yielded good agreement between observations at the
quasi-parallel portion of the Earth's bow shock and three theoretical
approaches, including Monte Carlo kinetic simulations. This paper extends such
model testing to the realm of oblique interplanetary shocks: here observations
of proton and alpha particle distributions made by the SWICS ion mass
spectrometer on Ulysses at nearby interplanetary shocks are compared with test
particle Monte Carlo simulation predictions of accelerated populations. The
plasma parameters used in the simulation are obtained from measurements of
solar wind particles and the magnetic field upstream of individual shocks. Good
agreement between downstream spectral measurements and the simulation
predictions are obtained for two shocks by allowing the the ratio of the
mean-free scattering length to the ionic gyroradius, to vary in an optimization
of the fit to the data. Generally small values of this ratio are obtained,
corresponding to the case of strong scattering. The acceleration process
appears to be roughly independent of the mass or charge of the species.Comment: 26 pages, 6 figures, AASTeX format, to appear in the Astrophysical
Journal, February 20, 199
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White paper on microbial anti-cancer therapy and prevention
In this White Paper, we discuss the current state of microbial cancer therapy. This paper resulted from a meeting (âMicrobial Based Cancer Therapyâ) at the US National Cancer Institute in the summer of 2017. Here, we define âMicrobial Therapyâ to include both oncolytic viral therapy and bacterial anticancer therapy. Both of these fields exploit tumor-specific infectious microbes to treat cancer, have similar mechanisms of action, and are facing similar challenges to commercialization. We designed this paper to nucleate this growing field of microbial therapeutics and increase interactions between researchers in it and related fields. The authors of this paper include many primary researchers in this field. In this paper, we discuss the potential, status and opportunities for microbial therapy as well as strategies attempted to date and important questions that need to be addressed. The main areas that we think will have the greatest impact are immune stimulation, control of efficacy, control of delivery, and safety. There is much excitement about the potential of this field to treat currently intractable cancer. Much of the potential exists because these therapies utilize unique mechanisms of action, difficult to achieve with other biological or small molecule drugs. By better understanding and controlling these mechanisms, we will create new therapies that will become integral components of cancer care
The TESS Grand Unified Hot Jupiter Survey. I. Ten TESS Planets
We report the discovery of ten short-period giant planets (TOI-2193A b,
TOI-2207 b, TOI-2236 b, TOI-2421 b, TOI-2567 b, TOI-2570 b, TOI-3331 b,
TOI-3540A b, TOI-3693 b, TOI-4137 b). All of the planets were identified as
planet candidates based on periodic flux dips observed by NASA's Transiting
Exoplanet Survey Satellite (TESS). The signals were confirmed to be from
transiting planets using ground-based time-series photometry, high angular
resolution imaging, and high-resolution spectroscopy coordinated with the TESS
Follow-up Observing Program. The ten newly discovered planets orbit relatively
bright F and G stars (,~ between 4800 and 6200 K).
The planets' orbital periods range from 2 to 10~days, and their masses range
from 0.2 to 2.2 Jupiter masses. TOI-2421 b is notable for being a Saturn-mass
planet and TOI-2567 b for being a ``sub-Saturn'', with masses of and Jupiter masses, respectively. In most cases, we
have little information about the orbital eccentricities. Two exceptions are
TOI-2207 b, which has an 8-day period and a detectably eccentric orbit (), and TOI-3693 b, a 9-day planet for which we can set an upper
limit of . The ten planets described here are the first new planets
resulting from an effort to use TESS data to unify and expand on the work of
previous ground-based transit surveys in order to create a large and
statistically useful sample of hot Jupiters.Comment: 44 pages, 15 tables, 21 figures; revised version submitted to A
A second planet transiting LTT 1445A and a determination of the masses of both worlds
K.H. acknowledges support from STFC grant ST/R000824/1.LTT 1445 is a hierarchical triple M-dwarf star system located at a distance of 6.86 pc. The primary star LTT 1445A (0.257 Mâ) is known to host the transiting planet LTT 1445Ab with an orbital period of 5.36 days, making it the second-closest known transiting exoplanet system, and the closest one for which the host is an M dwarf. Using Transiting Exoplanet Survey Satellite data, we present the discovery of a second planet in the LTT 1445 system, with an orbital period of 3.12 days. We combine radial-velocity measurements obtained from the five spectrographs, Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, High Accuracy Radial Velocity Planet Searcher, High-Resolution Echelle Spectrometer, MAROON-X, and Planet Finder Spectrograph to establish that the new world also orbits LTT 1445A. We determine the mass and radius of LTT 1445Ab to be 2.87 ± 0.25 Mâ and 1.304-0.060+0.067 Râ, consistent with an Earth-like composition. For the newly discovered LTT 1445Ac, we measure a mass of 1.54-0.19+0.20 Mâ and a minimum radius of 1.15 Râ, but we cannot determine the radius directly as the signal-to-noise ratio of our light curve permits both grazing and nongrazing configurations. Using MEarth photometry and ground-based spectroscopy, we establish that star C (0.161 Mâ) is likely the source of the 1.4 day rotation period, and star B (0.215 Mâ) has a likely rotation period of 6.7 days. We estimate a probable rotation period of 85 days for LTT 1445A. Thus, this triple M-dwarf system appears to be in a special evolutionary stage where the most massive M dwarf has spun down, the intermediate mass M dwarf is in the process of spinning down, while the least massive stellar component has not yet begun to spin down.Publisher PDFPeer reviewe
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