396 research outputs found
Dissecting Solvent Effects on Hydrogen Bonding
The experimental isolation of Hâbond energetics from the typically dominant influence of the solvent remains challenging. Here we use synthetic molecular balances to quantify amine/amide Hâbonds in competitive solvents. Over 200 conformational free energy differences were determined using 24 Hâbonding balances in 9 solvents spanning a wide polarity range. The correlations between experimental interaction energies and gasâphase computed energies exhibited wild solventâdependent variation. However, excellent correlations were found between the same computed energies and the experimental data following empirical dissection of solvent effects using Hunter's α/ÎČ solvation model. In addition to facilitating the direct comparison of experimental and computational data, changes in the fitted donor and acceptor constants reveal the energetics of secondary local interactions such as competing Hâbonds
Submerged carbonate banks aggregate pelagic megafauna in offshore tropical Australia
This research was undertaken for the Marine Biodiversity Hub, a collaborative partnership supported through funding from the Australian Governmentâs National Environmental Research Program (NERP). PB was the recipient of a scholarship for international fees during the course of this work. SN publishes with permission of the Chief Executive Officer, Geoscience Australia.The conservation of marine biodiversity is firmly embedded in national and international policy frameworks. However, the difficulties associated with conducting broad-scale surveys of oceanic environments restrict the evidence base available for applied management in pelagic waters. For example, the Oceanic Shoals Australian Marine Park (AMP) was established in 2012 in a part of Australiaâs continental shelf where unique topographic features are thought to support significant levels of biodiversity, yet where our understanding of ecological processes remains limited. We deployed mid-water baited remote underwater video systems (mid-water BRUVS) in the Oceanic Shoals AMP to provide the first non-extractive baseline assessment of pelagic wildlife communities in the area. We used these observations and high-resolution multibeam swaths of the seafloor to explore potential relationships between prominent geomorphological features and the (i) composition, (ii) richness, and (iii) relative abundance of pelagic communities. We documented 32 vertebrate species across three sampling areas, ranging from small baitfish to large sharks and rays, and estimated that up to nearly twice as many taxa may occur within the region as a whole. This highlights the Oceanic Shoals AMP as a reservoir of biodiversity comparable to other documented offshore oceanic hotspots. Our results also confirm the AMP as a possible distant foraging destination for IUCN red listed sea turtles, and a potential breeding and/or nursing ground for a number of charismatic cetaceans. Model outputs indicate that both species richness and abundance increase in proximity to raised geomorphic structures such as submerged banks and pinnacles, highlighting the influence of submarine topography on megafauna distribution. By providing a foundational understanding of spatial patterns in pelagic wildlife communities throughout a little studied region, our work demonstrates how a combination of non-destructive sampling techniques and predictive models can provide new opportunities to support decision-making under data shortage.Publisher PDFPeer reviewe
Lensing-Induced Structure of Submillimeter Sources: Implications for the Microwave Background
We consider the effect of lensing by galaxy clusters on the angular
distribution of submillimeter wavelength objects. While lensing does not change
the total flux and number counts of submillimeter sources, it can affect the
number counts and fluxes of flux-limited samples. Therefore imposing a flux cut
on point sources not only reduces the overall Poisson noise, but imprints the
correlations between lensing clusters on the unresolved flux distribution.
Using a simple model, we quantify the lensing anisotropy induced in
flux-limited samples and compare this to Poisson noise. We find that while the
level of induced anisotropies on the scale of the cluster angular correlation
length is comparable to Poisson noise for a slowly evolving cluster model, it
is negligible for more realistic models of cluster evolution. Thus the removal
of point sources is not expected to induce measurable structure in the
microwave or far-infrared backgrounds.Comment: 22 pages, 9 figures, accepted to Astrophysical Journa
Testing Cold Dark Matter Models At Moderate to High Redshift
The COBE microwave background temperature fluctuations and the abundance of
local rich clusters of galaxies provide the two most powerful constraints on
cosmological models. When all variants of the standard cold dark matter (CDM)
model are subject to the combined constraint, the power spectrum of any model
is fixed to accuracy in both the shape and overall amplitude. These
constrained models are not expected to differ dramatically in their local
large-scale structure properties. However, their evolutionary histories differ,
resulting in dramatic differences towards high redshift. We examine in detail
six standardized, COBE and cluster normalized CDM models with respect to a
large set of independent observations. The observations include correlation
function of rich clusters of galaxies, galaxy power spectrum, evolution of rich
cluster abundance, gravitational lensing by moderate -to-high redshift
clusters, \lya forest, damped \lya systems, high redshift galaxies,
reionization of the universe and future CMB experiments. It seems that each of
the independent observations examined is or potentially is capable of
distinguishing between at least some of the models. The combined power of
several or all of these observations is tremendous. Thus, we appear to be on
the verge of being able to make dramatic tests of all models in the near future
using a rapidly growing set of observations, mostly at moderate to high
redshift. Consistency or inconsistency between different observed phenomena on
different scales and/or at different epochs with respect to the models will
have profound implications for theory of growth of cosmic structure.Comment: ApJ in press (1998), 26 emulateapj page
Double-Peaked Low-Ionization Emission Lines in Active Galactic Nuclei
We present a new sample of 116 double-peaked Balmer line Active Galactic
Nuclei (AGN) selected from the Sloan Digital Sky Survey. Double-peaked emission
lines are believed to originate in the accretion disks of AGN, a few hundred
gravitational radii (Rg) from the supermassive black hole. We investigate the
properties of the candidate disk emitters with respect to the full sample of
AGN over the same redshifts, focusing on optical, radio and X-ray flux, broad
line shapes and narrow line equivalent widths and line flux-ratios. We find
that the disk-emitters have medium luminosities (~10^44erg/s) and FWHM on
average six times broader than the AGN in the parent sample. The double-peaked
AGN are 1.6 times more likely to be radio-sources and are predominantly (76%)
radio quiet, with about 12% of the objects classified as LINERs. Statistical
comparison of the observed double-peaked line profiles with those produced by
axisymmetric and non-axisymmetric accretion disk models allows us to impose
constraints on accretion disk parameters. The observed Halpha line profiles are
consistent with accretion disks with inclinations smaller than 50 deg, surface
emissivity slopes of 1.0-2.5, outer radii larger than ~2000 Rg, inner radii
between 200-800Rg, and local turbulent broadening of 780-1800 km/s. The
comparison suggests that 60% of accretion disks require some form of asymmetry
(e.g., elliptical disks, warps, spiral shocks or hot spots).Comment: 60 pages, 19 figures, accepted for publication in AJ. For high
quality figures and full tables, please see
http://astro.princeton.edu/~iskra/disks.htm
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