15,910 research outputs found
Multiple Time Scales in Diffraction Measurements of Diffusive Surface Relaxation
We grew SrTiO3 on SrTiO3 (001) by pulsed laser deposition, using x-ray
scattering to monitor the growth in real time. The time-resolved small angle
scattering exhibits a well-defined length scale associated with the spacing
between unit cell high surface features. This length scale imposes a discrete
spectrum of Fourier components and rate constants upon the diffusion equation
solution, evident in multiple exponential relaxation of the "anti-Bragg"
diffracted intensity. An Arrhenius analysis of measured rate constants confirms
that they originate from a single activation energy.Comment: 4 pages, 3 figure
Bowen ratio estimates of evapotranspiration for stands on the Virgin River in Southern Nevada
A Bowen ratio energy balance was conducted over a Tamarix ramosissima (saltcedar) stand growing in a riparian corridor along the Virgin River in southern Nevada. Measurements in two separate years were compared and contrasted on the basis of changes in growing conditions. In 1994, a drought year, record high temperatures, dry winds, and a falling water table caused partial wilt of outer smaller twigs in the canopy of many trees in the stand around the Bowen tower. Subsequently, evapotranspiration (ET) estimates declined dramatically over a 60‐day period (11 mm d−1 tod−1). In 1995, the Virgin River at the Bowen tower area changed its course, hydrologically isolating the Tamarix stand in the vicinity of the tower. In 1996, a 25% canopy loss was visually estimated for the Tamarix growing in the area of the tower. Higher soil temperatures relative to air temperatures were recorded in 1996 in response to this loss in canopy. With a more open canopy, thermally induced turbulence was observed in 1996. On day 160 of 1996, a 28°C rise over a 9‐hour period was correlated with increased wind speeds of greater than 4 m s−1. Subsequently, higher ET estimates were made in 1996 compared to 1994 (145 cm versus 75 cm). However, the energy balance was dominated by advection in 1996, with latent energy flux exceeding net radiation 65% of the measurement days compared to only 11% in 1994. We believe this advection was on a scale of the floodplain (hundreds of meters) as opposed to regional advection, since the majority of wind (90%) was in a N–S direction along the course of the river, and that a more open canopy allowed the horizontal transfer of energy into the Tamarix stand at the Bowen tower. Our results suggest that Tamarix has the potential to be both a low water user and a high water user, depending on moisture availability, canopy development, and atmospheric demand, and that advection can dominate energy balances and ET in aridland riparian zones such as the Virgin River
Particulate Matter Exposure Impairs Systemic Microvascular Endothelium-Dependent Dilation
Acute exposure to airborne pollutants, such as solid particulate matter (PM), increases the risk of cardiovascular dysfunction, but the mechanisms by which PM evokes systemic effects remain to be identified. The purpose of this study was to determine if pulmonary exposure to a PM surrogate, such as residual oil fly ash (ROFA), affects endothelium-dependent dilation in the systemic microcirculation. Rats were intratracheally instilled with ROFA at 0.1, 0.25, 1 or 2 mg/rat 24 hr before experimental measurements. Rats intratracheally instilled with saline or titanium dioxide (0.25 mg/rat) served as vehicle or particle control groups, respectively. In vivo microscopy of the spinotrapezius muscle was used to study systemic arteriolar dilator responses to the Ca(2+) ionophore A23187, administered by ejection via pressurized micropipette into the arteriolar lumen. We used analysis of bronchoalveolar lavage (BAL) samples to monitor identified pulmonary inflammation and damage. To determine if ROFA exposure affected arteriolar nitric oxide sensitivity, sodium nitroprusside was iontophoretically applied to arterioles of rats exposed to ROFA. In saline-treated rats, A23187 dilated arterioles up to 72 ± 7% of maximum. In ROFA- and TiO(2)-exposed rats, A23187-induced dilation was significantly attenuated. BAL fluid analysis revealed measurable pulmonary inflammation and damage after exposure to 1 and 2 mg ROFA (but not TiO(2) or < 1 mg ROFA), as evidenced by significantly higher polymorphonuclear leukocyte cell counts, enhanced BAL albumin levels, and increased lactate dehydrogenase activity in BAL fluid. The sensitivity of arteriolar smooth muscle to NO was similar in saline-treated and ROFA-exposed rats, suggesting that pulmonary exposure to ROFA affected endothelial rather than smooth muscle function. A significant increase in venular leukocyte adhesion and rolling was observed in ROFA-exposed rats, suggesting local inflammation at the systemic microvascular level. These results indicate that pulmonary PM exposure impairs systemic endothelium-dependent arteriolar dilation. Moreover, because rats exposed to < 1 mg ROFA or TiO(2) did not exhibit BAL signs of pulmonary damage or inflammation, it appears that PM exposure can impair systemic microvascular function independently of detectable pulmonary inflammation
Do OB Runaway Stars Have Pulsar Companions?
We have conducted a VLA search for radio pulsars at the positions of 44
nearby OB runaway stars. The observations involved both searching images for
point sources of continuum emission and a time series analysis. Our mean flux
sensitivity to pulsars slower than 50 ms was 0.2 mJy. No new pulsars were found
in the survey. The size of the survey, combined with the high sensitivity of
the observations, sets a significant constraint on the probability, , of a
runaway OB star having an observable pulsar companion. We find \%
with 95\% confidence, if the general pulsar luminosity function is applicable
to OB star pulsar companions. If a pulsar beaming fraction of \onethird\ is
assumed, then we estimate that fewer than 20\% of runaway OB stars have neutron
star companions, unless pulsed radio emission is frequently obscured by the OB
stellar wind. Our result is consistent with the dynamical (or cluster) ejection
model for the formation of OB runaways. The supernova ejection model is not
ruled out, but is constrained by these observations to allow only a small
binary survival fraction, which may be accommodated if neutron stars acquire
significant natal kicks. According to Leonard, Hills and Dewey (1994), a 20\%
survival fraction corresponds to a 3-d kick velocity of 420 km s. This
value is in close agreement with recent revisions of the pulsar velocity
distribution.Comment: Submitted to the Astronomical Journal. 16 pages. Latex uses
aaspp4.sty. 3 postscript figures. Address correspondence to Colin Philp
([email protected]). Revision was to replace .ps file with latex fil
Signatures of Interstellar-Intracluster Medium Interactions: Spiral Galaxy Rotation Curves in Abell 2029
We investigate the rich cluster Abell 2029 (z~0.08) using optical imaging and
long-slit spectral observations of 52 disk galaxies distributed throughout the
cluster field. No strong emission-line galaxies are present within ~400 kpc of
the cluster center, a region largely dominated by the similarly-shaped X-ray
and low surface brightness optical envelopes centered on the giant cD galaxy.
However, two-thirds of the galaxies observed outside the cluster core exhibit
line emission. H-alpha rotation curves of 14 cluster members are used in
conjunction with a deep I band image to study the environmental dependence of
the Tully-Fisher relation. The Tully-Fisher zero-point of Abell 2029 matches
that of clusters at lower redshifts, although we do observe a relatively larger
scatter about the Tully-Fisher relation. We do not observe any systematic
variation in the data with projected distance to the cluster center: we see no
environmental dependence of Tully-Fisher residuals, R-I color, H-alpha
equivalent width, and the shape and extent of the rotation curves.Comment: 22 pages, 6 figures, 3 tables; to appear in the August 2000
Astronomical Journa
Measurements of Surface Diffusivity and Coarsening During Pulsed Laser Deposition
Pulsed Laser Deposition (PLD) of homoepitaxial SrTiO3 was studied with
in-situ x-ray specular reflectivity and surface diffuse x-ray scattering.
Unlike prior reflectivity-based studies, these measurements access both the
time- and the length-scales of the evolution of the surface morphology during
growth. In particular, we show that this technique allows direct measurements
of the diffusivity for both inter- and intra-layer transport. Our results
explicitly limit the possible role of island break-up, demonstrate the key
roles played by nucleation and coarsening in PLD, and place an upper bound on
the Ehrlich-Schwoebel (ES) barrier for downhill diffusion
CC273 Farm Energy Tips - Use Energy Wisely - Stretching Fertilizers
Campaign Circular 273 is about how to make fertilizer last and how much fertilizer is actually needed to help your crops
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