3,488 research outputs found
Quasar Outflow Deceleration or Acceleration: Predictions and a Search
Quasar winds can shock and sweep up ambient interstellar medium (ISM) gas,
contributing to galactic quenching. We combine and extend past models of
energy-conserving shock bubbles around quasars, investigate model implications
from an observational standpoint, and test model predictions using new
high-resolution spectroscopic observations of the broad absorption line quasar
SDSS J030000.56+004828.0 (J0300). Even with constant energy input from the
wind, a bubble's expansion decelerates over time as more ISM gas is swept up.
Our new observations enable a direct search for this deceleration. We obtain
the tightest reported 3-sigma limit on the average rest-frame deceleration (or
acceleration) of a quasar outflow: |a|0.1 km s yr ( cm s) in the relatively low-velocity Ca II outflow of J0300
over 9.65 rest-frame years. We can satisfy these limits with certain parameter
choices in our model, but the large velocity range of the Ca II absorption in
J0300 rules out the hypothesis that such gas shares the velocity of the
swept-up ISM gas in a self-similar shock bubble. We investigate the possibility
of ram-pressure acceleration of preexisting ISM clouds and conclude that the
velocity range seen in Ca II in J0300 is potentially consistent with such an
explanation. The Ca II-absorbing gas clouds in J0300 have been inferred to have
high densities by Choi et al., in which case they can only have been
accelerated to their current speeds if they were originally at least an order
of magnitude less dense than they are today.Comment: MNRAS, in pres
Blooms of Dinoflagellate Mixotrophs in a Lower Chesapeake Bay Tributary: Carbon and Nitrogen Uptake over Diurnal, Seasonal, and Interannual Timescales
A multi-year study was conducted in the eutrophic Lafayette River, a sub-tributary of the lower Chesapeake Bay during which uptake of inorganic and organic nitrogen (N) and C compounds was measured during multiple seasons and years when different dinoflagellate species were dominant. Seasonal dinoflagellate blooms included a variety of mixotrophic dinoflagellates including Heterocapsa triquetra in the late winter, Prorocentrum minimum in the spring, Akashiwo sanguinea in the early summer, and Scrippsiella trochoidea and Cochlodinium polykrikoides in late summer and fall. Results showed that no single N source fueled algal growth, rather rates of N and C uptake varied on seasonal and diurnal timescales, and within blooms as they initiated and developed. Rates of photosynthetic C uptake were low yielding low assimilation numbers during much of the study period and the ability to assimilate dissolved organic carbon augmented photosynthetic C uptake during bloom and non-bloom periods. The ability to use dissolved organic C during the day and night may allow mixotrophic bloom organisms a competitive advantage over co-occurring phytoplankton that are restricted to photoautotrophic growth, obtaining N and C during the day and in well-lit surface waters
Enhanced Transport at High Plasma Pressure and Subthreshold Kinetic Ballooning Modes in Wendelstein 7-X
High-performance fusion plasmas, requiring high pressure Ī², are not well understood in stellarator-type experiments. Here, the effect of Ī² on ion-temperature-gradient-driven (ITG) turbulence is studied in Wendelstein 7-X (W7-X), showing that subdominant kinetic ballooning modes (KBMs) are unstable well below the ideal MHD threshold and get strongly excited in the turbulence. By zonal-flow erosion, these subthreshold KBMs (stKBMs) affect ITG saturation and enable higher heat fluxes. Controlling stKBMs will be essential to allow W7-X and future stellarators to achieve maximum performance.</p
Enhanced Transport at High Plasma Pressure and Subthreshold Kinetic Ballooning Modes in Wendelstein 7-X
High-performance fusion plasmas, requiring high pressure Ī², are not well understood in stellarator-type experiments. Here, the effect of Ī² on ion-temperature-gradient-driven (ITG) turbulence is studied in Wendelstein 7-X (W7-X), showing that subdominant kinetic ballooning modes (KBMs) are unstable well below the ideal MHD threshold and get strongly excited in the turbulence. By zonal-flow erosion, these subthreshold KBMs (stKBMs) affect ITG saturation and enable higher heat fluxes. Controlling stKBMs will be essential to allow W7-X and future stellarators to achieve maximum performance.</p
Enhanced transport at high plasma and sub-threshold kinetic ballooning modes in Wendelstein 7-X
The effect of plasma pressure on ion-temperature-gradient-driven
(ITG) turbulence is studied in the Wendelstein 7-X (W7-X) stellarator, showing
that subdominant kinetic ballooning modes (KBMs) are unstable well below the
ideal MHD threshold and get strongly excited in the quasi-stationary state. By
zonal-flow erosion, these highly non-ideal KBMs affect ITG saturation and
thereby enable higher heat fluxes. Controlling these KBMs will be essential in
order to allow W7-X and future stellarators to achieve maximum performance.Comment: 16 pages, 5 figure
Assessing phytoplankton nutritional status and potential impact of wet deposition in seasonally oligotrophic waters of the MidāAtlantic Bight
Author Posting. Ā© American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 3203-3211, doi:10.1002/2017GL075361.To assess phytoplankton nutritional status in seasonally oligotrophic waters of the southern MidāAtlantic Bight, and the potential for rain to stimulate primary production in this region during summer, shipboard bioassay experiments were performed using natural seawater and phytoplankton collected north and south of the Gulf Stream. Bioassay treatments comprised iron, nitrate, iron + nitrate, iron + nitrate + phosphate, and rainwater. Phytoplankton growth was inferred from changes in chlorophyll a, inorganic nitrogen, and carbonā13 uptake, relative to unamended control treatments. Results indicated the greatest growth stimulation by iron + nitrate + phosphate, intermediate growth stimulation by rainwater, modest growth stimulation by nitrate and iron + nitrate, and no growth stimulation by iron. Based on these data and analysis of seawater and atmospheric samples, nitrogen was the proximate limiting nutrient, with a secondary limitation imposed by phosphorus. Our results imply that summer rain events increase new production in these waters by contributing nitrogen and phosphorus, with the availability of the latter setting the upper limit on raināstimulated new production.US National Science Foundation Grant Numbers: OCEā1260454, OCEā1260454, OCEā12605742018-09-1
Laser transit-time measurements between earth and moon with a transportable system
A high radiance, pulsed laser system with a transportable transmitting unit was used to measure the transit times of 25 ns, 10 joule, and 530 nm pulses from earth to the Apollo 15 retroreflector on the moon and back
Etiology of severe childhood pneumonia in the Gambia, West Africa, determined by conventional and molecular microbiological analyses of lung and pleural aspirate samples.
Molecular analyses of lung aspirates from Gambian children with severe pneumonia detected pathogens more frequently than did culture and showed a predominance of bacteria, principally Streptococcus pneumoniae, >75% being of serotypes covered by current pneumococcal conjugate vaccines. Multiple pathogens were detected frequently, notably Haemophilus influenzae (mostly nontypeable) together with S. pneumoniae
LDEF Interplanetary Dust Experiment (IDE) results
The Interplanetary Dust Experiment (IDE) provided high time resolution detection of microparticle impacts on the Long Duration Exposure Facility satellite. Particles, in the diameter range from 0.2 microns to several hundred microns, were detected impacting on six orthogonal surfaces of the gravity-gradient stabilized LDEF spacecraft. The total sensitive surface area was about one square meter, distributed between LDEF rows 3 (Wake or West), 6 (South), 9 (Ram or East), 12 (North), as well as the Space and Earth ends of LDEF. The time of each impact is known to an accuracy that corresponds to better than one degree in orbital longitude. Because LDEF was gravity-gradient stabilized and magnetically damped, the direction of the normal to each detector panel is precisely known for each impact. The 11 1/2 month tape-recorded data set represents the most extensive record gathered of the number, orbital location, and incidence direction for microparticle impacts in low Earth orbit. Perhaps the most striking result from IDE was the discovery that microparticle impacts, especially on the Ram, South, and North surfaces, were highly episodic. Most such impacts occurred in localized regions of the orbit for dozens or even hundreds of orbits in what we have termed Multiple Orbit Event Sequences (MOES). In addition, more than a dozen intense and short-lived 'spikes' were seen in which impact fluxes exceeded the background by several orders of magnitude. These events were distributed in a highly non-uniform fashion in time and terrestrial longitude and latitude
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