91 research outputs found
Boundary Conditions of the Heliosphere
Radiative transfer equilibrium models of nearby interstellar matter (ISM)
yield the boundary conditions of the heliosphere when constrained with
observations of ISM inside and outside of the heliosphere. Filtration factors
for interstellar neutrals crossing the heliosheath region, from charge exchange
with interstellar plasma, are given for H, He, N, O, Ar, and Ne. The best
models predict n(HI)~0.2 /cc, n(e)~0.1 /cc, however if the isotropic 2 kHz
emission observed by Voyager (Kurth & Gurnett 2003) is formed in the
surrounding ISM, an alternate model with lower electron densities is indicated.
Observations of nearby ISM, the radiative transfer models, and historical 10Be
records provide information on past variations in the galactic environment of
the Sun
The Ionization of the Local Interstellar Medium, as Revealed by FUSE Observations of N, O and Ar toward White Dwarf Stars
FUSE spectra of the white dwarf stars G191-B2B, GD 394, WD 2211-495 and WD
2331-475 cover the absorption features out of the ground electronic states of N
I, N II, N III, O I and Ar I in the far ultraviolet, providing new insights on
the origin of the partial ionization of the Local Interstellar Medium (LISM),
and for the case of G191-B2B, the interstellar cloud that immediately surrounds
the solar system. Toward these targets the interstellar abundances of Ar I, and
sometimes N I, are significantly below their cosmic abundances relative to H I.
In the diffuse interstellar medium, these elements are not likely to be
depleted onto dust grains. Generally, we expect that Ar should be more strongly
ionized than H (and also O and N whose ionizations are coupled to that of H via
charge exchange reactions) because the cross section for the photoionization of
Ar I is very high. Our finding that Ar I/H I is low may help to explain the
surprisingly high ionization of He in the LISM found by other investigators.
Our result favors the interpretation that the ionization of the local medium is
maintained by a strong EUV flux from nearby stars and hot gases, rather than an
incomplete recovery from a past, more highly ionized condition.Comment: 13 pages, 2 figures. To appear in a special issue of the
Astrophysical Journal Letters devoted to the first scientific results from
the FUSE missio
A noiseless kilohertz frame rate imaging detector based on microchannel plates read out with the Medipix2 CMOS pixel chip
A new hybrid optical imaging detector is described that is being developed for the next generation adaptive optics (AO) wavefront sensors (WFS) for ground-based telescopes. The detector consists of a photocathode and proximity focused microchannel plates (MCPs) read out by the Medipix2 CMOS pixel ASIC. Each pixel of the Medipix2 device measures 55x55 um2 and comprises pre-amplifier, a window discriminator and a 14-bit counter. The 256x256 Medipix2 array can be read out noiselessly in 287 us. The readout can be electronically shuttered down to a temporal window of a few us. The Medipix2 is buttable on 3 sides to produce 512x(n*256) pixel devices. Measurements with ultraviolet light yield a spatial resolution of the detector at the Nyquist limit. Sub-pixel resolution can be achieved using centroiding algorithms. For the AO application, very high continuous frame rates of the order of 1 kHz are required for a matrix of 512x512 pixels. The design concepts of a parallel readout board are presented that will allow this fast data throughput. The development status of the optical WFS tube is also explained
The Boundary Conditions of the Heliosphere: Photoionization Models Constrained by Interstellar and In Situ Data
The boundary conditions of the heliosphere are set by the ionization, density
and composition of inflowing interstellar matter. Constraining the properties
of the Local Interstellar Cloud (LIC) at the heliosphere requires radiative
transfer ionization models. We model the background interstellar radiation
field using observed stellar FUV and EUV emission and the diffuse soft X-ray
background. We also model the emission from the boundary between the LIC and
the hot Local Bubble (LB) plasma, assuming that the cloud is evaporating
because of thermal conduction. We create a grid of models covering a plausible
range of LIC and LB properties, and use the modeled radiation field as input to
radiative transfer/thermal equilibrium calculations using the Cloudy code. Data
from in situ observations of He^O, pickup ions and anomalous cosmic rays in the
heliosphere, and absorption line measurements towards epsilon CMa were used to
constrain the input parameters. A restricted range of assumed LIC HI column
densities and LB plasma temperatures produce models that match all the
observational constraints. The relative weakness of the constraints on N(HI)
and T_h contrast with the narrow limits predicted for the H^O and electron
density in the LIC at the Sun, n(H^0) = 0.19 - 0.20 cm^-3, and n(e) = 0.07 +/-
0.01 cm^-3. Derived abundances are mostly typical for low density gas, with
sub-solar Mg, Si and Fe, possibly subsolar O and N, and S about solar; however
C is supersolar. The interstellar gas at the Sun is warm, low density, and
partially ionized, with n(H) = 0.23 - 0.27 cm^-3, T = 6300 K, X(H^+) ~ 0.2, and
X(He^+) ~ 0.4. These results appear to be robust since acceptable models are
found for substantially different input radiation fields. Our results favor low
values for the reference solar abundances for the LIC composition.Comment: 14 pages, 4 figures, submitted to Astronomy & Astrophysics together
with papers from the International Space Sciences Institute workshop on
Interstellar Hydrogen in the Heliospher
Interstellar Matter and the Boundary Conditions of the Heliosphere
The interstellar cloud surrounding the solar system regulates the galactic
environment of the Sun, and determines the boundary conditions of the
heliosphere. Both the Sun and interstellar clouds move through space, so these
boundary conditions change with time. Data and theoretical models now support
densities in the cloud surrounding the solar system of n(HI)=0.22+/-0.06 cm^-3,
and n(e-)~0.1 cm-3, with larger values allowed for n(HI) by radiative transfer
considerations. Ulysses and Extreme Ultraviolet Explorer satellite HeI data
yield a cloud temperature of 6,400 K. Nearby interstellar gas appears to be
structured and inhomogeneous. The interstellar gas in the Local Fluff cloud
complex exhibits elemental abundance patterns in which refractory elements are
enhanced over the depleted abundances found in cold disk gas. Within a few
parsecs of the Sun, inconclusive evidence for factors of 2--5 variation in MgII
and FeII gas phase abundances is found, providing evidence for variable grain
destruction. Observations of the hydrogen pile-up at the nose of the
heliosphere are consistent with a barely subsonic motion of the heliosphere
with respect to the surrounding interstellar cloud. Uncertainties on the
velocity vector of the cloud that surrounds the solar system indicate that it
is uncertain as to whether the Sun and alpha Cen are or are not immersed in the
same interstellar cloud.Comment: 24 pages 3 figure
Is the Sun Embedded in a Typical Interstellar Cloud?
The physical properties and kinematics of the partially ionized interstellar
material near the Sun are typical of warm diffuse clouds in the solar vicinity.
The interstellar magnetic field at the heliosphere and the kinematics of nearby
clouds are naturally explained in terms of the S1 superbubble shell. The
interstellar radiation field at the Sun appears to be harder than the field
ionizing ambient diffuse gas, which may be a consequence of the low opacity of
the tiny cloud surrounding the heliosphere. The spatial context of the Local
Bubble is consistent with our location in the Orion spur.Comment: "From the Outer Heliosphere to the Local Bubble", held at
International Space Sciences Institute, October 200
Patient-derived xenografts and organoids model therapy response in prostate cancer.
Therapy resistance and metastatic processes in prostate cancer (PCa) remain undefined, due to lack of experimental models that mimic different disease stages. We describe an androgen-dependent PCa patient-derived xenograft (PDX) model from treatment-naïve, soft tissue metastasis (PNPCa). RNA and whole-exome sequencing of the PDX tissue and organoids confirmed transcriptomic and genomic similarity to primary tumor. PNPCa harbors BRCA2 and CHD1 somatic mutations, shows an SPOP/FOXA1-like transcriptomic signature and microsatellite instability, which occurs in 3% of advanced PCa and has never been modeled in vivo. Comparison of the treatment-naïve PNPCa with additional metastatic PDXs (BM18, LAPC9), in a medium-throughput organoid screen of FDA-approved compounds, revealed differential drug sensitivities. Multikinase inhibitors (ponatinib, sunitinib, sorafenib) were broadly effective on all PDX- and patient-derived organoids from advanced cases with acquired resistance to standard-of-care compounds. This proof-of-principle study may provide a preclinical tool to screen drug responses to standard-of-care and newly identified, repurposed compounds
A cost-effective algae-based biosensor for water quality analysis: Development and testing in collaboration with peasant communities
New anthropic potentially harmful compounds are released into the environment everyday. In this context, broad range bioassays have emerged providing economically viable and widely applicable alternatives due to their ability to detect the cumulative toxicity of mixtures of both known and unknown chemicals in a sample, thus allowing direct information about water quality. Here we present a low-cost, wide-range algae-based biosensor that is easy to assemble and operate by untrained users and provides direct readings. It was developed as a request of a peasant social movement organization to assess the toxicity of drinking water in rural communities affected by pesticide spraying. Two fresh water algae strains, Scenedesmus acutus and Pseudokirchneriella subcapitata, were immobilized in alginate beads and tested as bioindicators. After incubation with different pollutants for five days, naked eye analysis by several observers proved to be a successful method to survey algae’s growth and establish the detection limits. Best detection limits were 10 ppm for technical-grade acid glyphosate, 15 ppm for glyphosate-based formulation, 50 ppb for atrazine formulation, 7.5 ppm for copper and 250 ppb for chromium. Absorbance measurements upon algae resuspension validated these results. The developed device was successfully tested in participatory workshops conducted at rural communities. Children, adults and elders with no scientific training were able to build the sensor and interpret the results, thus evaluating the quality of rain and well water used in their communities.Universidad Nacional de Santiago del EsteroConsejo Nacional de Investigaciones Científicas y Técnica
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