677 research outputs found
Resonant Scattering of Emission Lines in Coronal Loops: Effects on Image Morphology and Line Ratios
We have investigated the effects of resonant scattering of emission lines on
the image morphology and intensity from coronal loop structures. It has
previously been shown that line of sight effects in optically thin line
emission can yield loop images that appear uniformly bright at one viewing
angle, but show ``looptop sources'' at other viewing angles. For optically
thick loops where multiple resonant scattering is important, we use a 3D Monte
Carlo radiation transfer code. Our simulations show that the intensity
variation across the image is more uniform than the optically thin simulation
and, depending on viewing angle, the intensity may be lower or higher than that
predicted from optically thin simulations due to scattering out of or into the
line of sight.Comment: Accepted for publication in Ap
Varying Calcium Abundances in Solar Flares seen by Solar Maximum Mission
We report on calcium abundance estimates during the decay
phases of 194 solar X-ray flares using archived data from the Bent Crystal
Spectrometer (BCS) on Solar Maximum Mission (operational 1980~--~1989). The
abundances are derived from the ratio of the total calcium X-ray line emission
in BCS channel~1 to that in neighboring continuum, with temperature from a
satellite-to-resonance line ratio. Generally the calcium abundance is found to
be about three times the photospheric abundance, as previously found,
indicating a ``FIP'' (first ionization potential) effect for calcium which has
a relatively low FIP value. The precision of the abundance estimates (referred
to hydrogen on a logarithmic scale with ), is typically , enabling any time variations of during the flare decay
to be examined. For a total of 270 short time segments with
determined to better than 2.3\% accuracy, many (106; 39\%) showed variations in
at the level. For the majority, 74 (70\%) of these 106
segments decreased with time, and for 32 (30\%)
increased with time. For 79 out of 270 (29\%) we observed constant or nearly
constant , and the remaining 85 (31\%) with irregular time
behavior. A common feature was the presence of discontinuities in the time
behavior of . Relating these results to the ponderomotive force
theory of Laming, we attribute the nature of varying to the
emergence of loop structures in addition to the initial main loop, each with
its characteristic calcium abundance.Comment: Astrophysical Journal (to be published). 14 pages with 8 figure
The Full-sky Astrometric Mapping Explorer -- Astrometry for the New Millennium
FAME is designed to perform an all-sky, astrometric survey with unprecedented
accuracy. It will create a rigid astrometric catalog of 4x10^7 stars with 5 <
m_V < 15. For bright stars, 5 < m_V < 9, FAME will determine positions and
parallaxes accurate to < 50 microarcseconds, with proper motion errors < 50
microarcseconds/year. For fainter stars, 9 < m_V < 15, FAME will determine
positions and parallaxes accurate to < 500 microarcseconds, with proper motion
errors < 500 microarcseconds/year. It will also collect photometric data on
these 4 x 10^7 stars in four Sloan DSS colors.Comment: 6 pages, 4 figures, to appear in "Working on the Fringe
Isotopic constraints on the genesis and evolution of basanitic lavas at Haleakala, Island of Maui, Hawaii
© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geochimica et Cosmochimica Acta 195 (2016): 201-225, doi:10.1016/j.gca.2016.08.017.To understand the dynamics of solid mantle upwelling and melting in the Hawaiian
plume, we present new major and trace element data, Nd, Sr, Hf, and Pb isotopic
compositions, and 238U-230Th-226Ra and 235U-231Pa-227Ac activities for 13 Haleakala
Crater nepheline normative basanites with ages ranging from ~900 to 4100 yr B.P..
These basanites of the Hana Volcanics exhibit an enrichment in incompatible trace
elements and a more depleted isotopic signature than similarly aged Hawaiian
shield lavas from Kilauea and Mauna Loa. Here we posit that as the Pacific
lithosphere beneath the active shield volcanoes moves away from the center of the
Hawaiian plume, increased incorporation of an intrinsic depleted component with
relatively low 206Pb/204Pb produces the source of the basanites of the Hana
Volcanics. Haleakala Crater basanites have average (230Th/238U) of 1.23 (n=13),
average age-corrected (226Ra/230Th) of 1.25 (n=13), and average (231Pa/235U) of 1.67
(n=4), significantly higher than Kilauea and Mauna Loa tholeiites. U-series modeling
shows that solid mantle upwelling velocity for Haleakala Crater basanites ranges
from ~0.7 to 1.0 cm/yr, compared to ~10 to 20 cm/yr for tholeiites and ~1 to 2
cm/yr for alkali basalts. These modeling results indicate that solid mantle upwelling
rates and porosity of the melting zone are lower for Hana Volcanics basanites than
for shield-stage tholeiites from Kilauea and Mauna Loa and alkali basalts from
Hualalai. The melting rate, which is directly proportional to both the solid mantle
upwelling rate and the degree of melting, is therefore greatest in the center of the
Hawaiian plume and lower on its periphery. Our results indicate that solid mantle
upwelling velocity is at least 10 times higher at the center of the plume than at its
periphery under Haleakala.Funding for this project was provided by NSF grants EAR-0001924 and EAR-9909473 to KWWS.2018-08-2
BVRI Light Curves for 29 Type Ia Supernovae
BVRI light curves are presented for 27 Type Ia supernovae discovered during
the course of the Calan/Tololo Survey and for two other SNe Ia observed during
the same period. Estimates of the maximum light magnitudes in the B, V, and I
bands and the initial decline rate parameter m15(B) are also given.Comment: 17 pages, figures and tables are not included (contact first author
if needed), to appear in the Astronomical Journa
Transcriptomic links to muscle mass loss and declines in cumulative muscle protein synthesis during short-term disuse in healthy younger humans
Muscle disuse leads to a rapid decline in muscle mass, with reduced muscle protein synthesis (MPS) considered the primary physiological mechanism. Here, we employed a systems biology approach to uncover molecular networks and key molecular candidates that quantitatively link to the degree of muscle atrophy and/or extent of decline in MPS during short-term disuse in humans. After consuming a bolus dose of deuterium oxide (D2O; 3 mL.kg−1), eight healthy males (22 ± 2 years) underwent 4 days of unilateral lower-limb immobilization. Bilateral muscle biopsies were obtained post-intervention for RNA sequencing and D2O-derived measurement of MPS, with thigh lean mass quantified using dual-energy X-ray absorptiometry. Application of weighted gene co-expression network analysis identified 15 distinct gene clusters (“modules”) with an expression profile regulated by disuse and/or quantitatively connected to disuse-induced muscle mass or MPS changes. Module scans for candidate targets established an experimentally tractable set of candidate regulatory molecules (242 hub genes, 31 transcriptional regulators) associated with disuse-induced maladaptation, many themselves potently tied to disuse-induced reductions in muscle mass and/or MPS and, therefore, strong physiologically relevant candidates. Notably, we implicate a putative role for muscle protein breakdown-related molecular networks in impairing MPS during short-term disuse, and further establish DEPTOR (a potent mTOR inhibitor) as a critical mechanistic candidate of disuse driven MPS suppression in humans. Overall, these findings offer a strong benchmark for accelerating mechanistic understanding of short-term muscle disuse atrophy that may help expedite development of therapeutic interventions
s-Process Abundances in Planetary Nebulae
The s-process should occur in all but the lower mass progenitor stars of
planetary nebulae, and this should be reflected in the chemical composition of
the gas which is expelled to create the current planetary nebula shell. Weak
forbidden emission lines are expected from several s-process elements in these
shells, and have been searched for and in some cases detected in previous
investigations. Here we extend these studies by combining very high
signal-to-noise echelle spectra of a sample of PNe with a critical analysis of
the identification of the emission lines of Z>30 ions. Emission lines of Br,
Kr, Xe, Rb, Ba, and Pb are detected with a reasonable degree of certainty in at
least some of the objects studied here, and we also tentatively identify lines
from Te and I, each in one object. The strengths of these lines indicate
enhancement of s-process elements in the central star progenitors, and we
determine the abundances of Br, Kr, and Xe, elements for which atomic data
relevant for abundance determination have recently become available. As
representative elements of the ``light'' and ``heavy'' s-process peaks Kr and
Xe exhibit similar enhancements over solar values, suggesting that PNe
progenitors experience substantial neutron exposure.Comment: 56 Pages, 6 figures, accepted for publication in ApJ This version
corrects missing captions in Figure 1-3 and minor typo
A meta-analysis of long-term effects of conservation agriculture on maize grain yield under rain-fed conditions
Conservation agriculture involves reduced tillage, permanent soil cover and crop rotations to enhance soil fertility and to supply food from a dwindling land resource. Recently, conservation agriculture has been promoted in Southern Africa, mainly for maize-based farming systems. However, maize yields under rain-fed conditions are often variable. There is therefore a need to identify factors that influence crop yield under conservation agriculture and rain-fed conditions. Here, we studied maize grain yield data from experiments lasting 5 years and more under rain-fed conditions. We assessed the effect of long-term tillage and residue retention on maize grain yield under contrasting soil textures, nitrogen input and climate. Yield variability was measured by stability analysis. Our results show an increase in maize yield over time with conservation agriculture practices that include rotation and high input use in low rainfall areas. But we observed no difference in system stability under those conditions. We observed a strong relationship between maize grain yield and annual rainfall. Our meta-analysis gave the following findings: (1) 92% of the data show that mulch cover in high rainfall areas leads to lower yields due to waterlogging; (2) 85% of data show that soil texture is important in the temporal development of conservation agriculture effects, improved yields are likely on well-drained soils; (3) 73% of the data show that conservation agriculture practices require high inputs especially N for improved yield; (4) 63% of data show that increased yields are obtained with rotation but calculations often do not include the variations in rainfall within and between seasons; (5) 56% of the data show that reduced tillage with no mulch cover leads to lower yields in semi-arid areas; and (6) when adequate fertiliser is available, rainfall is the most important determinant of yield in southern Africa. It is clear from our results that conservation agriculture needs to be targeted and adapted to specific biophysical conditions for improved impact
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