3,439 research outputs found
Hubble Space Telescope Constraints on the Winds and Astrospheres of Red Giant Stars
We report on an ultraviolet spectroscopic survey of red giants observed by
the Hubble Space Telescope, focusing on spectra of the Mg II h & k lines near
2800 A in order to study stellar chromospheric emission, winds, and
astrospheric absorption. We focus on spectral types between K2 III and M5 III,
a spectral type range with stars that are noncoronal, but possessing strong,
chromospheric winds. We find a very tight relation between Mg II surface flux
and photospheric temperature, supporting the notion that all K2-M5 III stars
are emitting at a basal flux level. Wind velocities (V_w) are generally found
to decrease with spectral type, with V_w decreasing from ~40 km/s at K2 III to
~20 km/s at M5 III. We find two new detections of astrospheric absorption, for
Sigma Pup (K5 III) and Gamma Eri (M1 III). This absorption signature had
previously only been detected for Alpha Tau (K5 III). For the three
astrospheric detections the temperature of the wind after the termination shock
correlates with V_w, but is lower than predicted by the Rankine-Hugoniot shock
jump conditions, consistent with the idea that red giant termination shocks are
radiative shocks rather than simple hydrodynamic shocks. A full hydrodynamic
simulation of the Gamma Eri astrosphere is provided to explore this further.Comment: 16 pages, 8 figures, to appear in The Astrophysical Journa
Energetically Optimal Travel across Terrain: Visualizations and a New Metric of Geographic Distance with Archaeological Applications
We present a visualization and computation tool for modeling the caloric cost of pedestrian travel across three dimensional terrains. This tool is being used in ongoing archaeological research that analyzes how costs of locomotion affect the spatial distribution of trails and artifacts across archaeological landscapes. Throughout human history, traveling by foot has been the most common form of transportation, and therefore analyses of pedestrian travel costs are important for understanding prehistoric patterns of resource acquisition, migration, trade, and political interaction. Traditionally, archaeologists have measured geographic proximity based on as the crow flies distance. We propose new methods for terrain visualization and analysis based on measuring paths of least caloric expense, calculated using well established metabolic equations. Our approach provides a human centered metric of geographic closeness, and overcomes significant limitations of available Geographic Information System (GIS) software. We demonstrate such path computations and visualizations applied to archaeological research questions. Our system includes tools to visualize: energetic cost surfaces, comparisons of the elevation profiles of shortest paths versus least cost paths, and the display of paths of least caloric effort on Digital Elevation Models (DEMs). These analysis tools can be applied to calculate and visualize 1) likely locations of prehistoric trails and 2) expected ratios of raw material types to be recovered at archaeological sites
Explicit physics-informed neural networks for non-linear upscaling closure: the case of transport in tissues
In this work, we use a combination of formal upscaling and data-driven
machine learning for explicitly closing a nonlinear transport and reaction
process in a multiscale tissue. The classical effectiveness factor model is
used to formulate the macroscale reaction kinetics. We train a multilayer
perceptron network using training data generated by direct numerical
simulations over microscale examples. Once trained, the network is used for
numerically solving the upscaled (coarse-grained) differential equation
describing mass transport and reaction in two example tissues. The network is
described as being explicit in the sense that the network is trained using
macroscale concentrations and gradients of concentration as components of the
feature space.
Network training and solutions to the macroscale transport equations were
computed for two different tissues. The two tissue types (brain and liver)
exhibit markedly different geometrical complexity and spatial scale (cell size
and sample size). The upscaled solutions for the average concentration are
compared with numerical solutions derived from the microscale concentration
fields by a posteriori averaging. There are two outcomes of this work of
particular note: 1) we find that the trained network exhibits good
generalizability, and it is able to predict the effectiveness factor with high
fidelity for realistically-structured tissues despite the significantly
different scale and geometry of the two example tissue types; and 2) the
approach results in an upscaled PDE with an effectiveness factor that is
predicted (implicitly) via the trained neural network. This latter result
emphasizes our purposeful connection between conventional averaging methods
with the use of machine learning for closure; this contrasts with some machine
learning methods for upscaling where the exact form of the macroscale equation
remains unknown
Synthetic Applications and Methodological Developments of Donor-Acceptor Cyclopropanes and Related Compounds
Donor-acceptor cyclopropanes are convenient precursors to reactive and versatile 1,3-dipoles, and have found application in the synthesis of a variety of carbo- and heterocyclic scaffolds. This perspective review details our laboratory’s use of donor-acceptor cyclopropanes as intermediates toward the total synthesis of various natural products. We also discuss our work in the development of novel cycloadditions and rearrangements of donor-acceptor cyclopropanes and aziridines, as well as an example of an aryne insertion proceeding via fragmentation of a transient donor-acceptor cyclobutane
Hubble Space Telescopeconstraints on the Winds and Astrospheres of Red Giant Stars
We report on an ultraviolet spectroscopic survey of red giants observed by the Hubble Space Telescope, focusing on spectra of the Mg ii h and k lines near 2800 Å in order to study stellar chromospheric emission, winds, and astrospheric absorption. We focus on spectral types between K2 III and M5 III, a spectral type range with stars that are noncoronal, but possessing strong, chromospheric winds. We find a very tight relation between Mg ii surface flux and photospheric temperature, supporting the notion that all K2-M5 III stars are emitting at a basal flux level. Wind velocities (V w ) are generally found to decrease with spectral type, with V w decreasing from ~40 km s−1 at K2 III to ~20 km s−1 at M5 III. We find two new detections of astrospheric absorption, for σ Pup (K5 III) and γ Eri (M1 III). This absorption signature had previously only been detected for α Tau (K5 III). For the three astrospheric detections, the temperature of the wind after the termination shock (TS) correlates with V w , but is lower than predicted by the Rankine–Hugoniot shock jump conditions, consistent with the idea that red giant TSs are radiative shocks rather than simple hydrodynamic shocks. A full hydrodynamic simulation of the γ Eri astrosphere is provided to explore this further
Live Imaging Of Drosophila melanogaster Embryonic Hemocyte Migrations
Many studies address cell migration using in vitro methods, whereas the physiologically relevant environment is that of the organism itself. Here we present a protocol for the mounting of Drosophila melanogaster embryos and subsequent live imaging of fluorescently labeled hemocytes, the embryonic macrophages of this organism. Using the Gal4-uas system1 we drive the expression of a variety of genetically encoded, fluorescently tagged markers in hemocytes to follow their developmental dispersal throughout the embryo. Following collection of embryos at the desired stage of development, the outer chorion is removed and the embryos are then mounted in halocarbon oil between a hydrophobic, gas-permeable membrane and a glass coverslip for live imaging. In addition to gross migratory parameters such as speed and directionality, higher resolution imaging coupled with the use of fluorescent reporters of F-actin and microtubules can provide more detailed information concerning the dynamics of these cytoskeletal components
Exploring the Possibility of O And Ne Contamination in Ulysses Observations of Interstellar Helium
We explore the possibility that interstellar O and Ne may be contributing to the particle signal from the GAS instrument on Ulysses, which is generally assumed to be entirely He. Motivating this study is the recognition that an interstellar temperature higher than any previously estimated from Ulysses data could potentially resolve a discrepancy between Ulysses He measurements and those from the Interstellar Boundary Explorer (IBEX). Contamination by O and Ne could lead to Ulysses temperature measurements that are too low. We estimate the degree of O and Ne contamination necessary to increase the inferred Ulysses temperature to 8500 K, which would be consistent with both the Ulysses and IBEX data given the same interstellar flow speed. We find that producing the desired effect requires a heavy element contamination level of ~9% of the total Ulysses/GAS signal. However, this degree of heavy element contribution is about an order of magnitude higher than expected based on our best estimates of detection efficiencies, ISM abundances, and heliospheric survival probabilities, making it unlikely that heavy element contamination is significantly affecting temperatures derived from Ulysses data
Coronal Emission Measures and Abundances for Moderately Active K Dwarfs Observed by Chandra
We have used Chandra to resolve the nearby 70 Oph (K0 V+K5 V) and 36 Oph (K1
V+K1 V) binary systems for the first time in X-rays. The LETG/HRC-S spectra of
all four of these stars are presented and compared with an archival LETG
spectrum of another moderately active K dwarf, Epsilon Eri. Coronal densities
are estimated from O VII line ratios and emission measure distributions are
computed for all five of these stars. We see no substantial differences in
coronal density or temperature among these stars, which is not surprising
considering that they are all early K dwarfs with similar activity levels.
However, we do see significant differences in coronal abundance patterns.
Coronal abundance anomalies are generally associated with the first ionization
potential (FIP) of the elements. On the Sun, low-FIP elements are enhanced in
the corona relative to high-FIP elements, the so-called "FIP effect." Different
levels of FIP effect are seen for our stellar sample, ranging from 70 Oph A,
which shows a prominent solar-like FIP effect, to 70 Oph B, which has no FIP
bias at all or possibly even a weak inverse FIP effect. The strong abundance
difference exhibited by the two 70 Oph stars is unexpected considering how
similar these stars are in all other respects (spectral type, age, rotation
period, X-ray flux). It will be difficult for any theoretical explanation for
the FIP effect to explain how two stars so similar in all other respects can
have coronae with different degrees of FIP bias. Finally, for the stars in our
sample exhibiting a FIP effect, a curious difference from the solar version of
the phenomenon is that the data seem to be more consistent with the high-FIP
elements being depleted in the corona rather than a with a low-FIP enhancementComment: 35 pages, 8 figures, AASTEX v5.0 plus EPSF extensions in mkfig.sty;
accepted by Ap
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