69,136 research outputs found
Can the James Webb Space Telescope detect isolated population III stars?
Isolated population III stars are postulated to exist at approximately
z=10-30 and may attain masses up to a few hundred solar masses. The James Webb
Space telescope (JWST) is the next large space based infrared telescope and is
scheduled for launch in 2014. Using a 6.5 meter primary mirror, it will
probably be able to detect some of the first galaxies forming in the early
Universe. A natural question is whether it will also be able to see any
isolated population III stars. Here, we calculate the apparent broadband
AB-magnitudes for 300 solar masses population III stars in JWST filters at
z=10-20. Our calculations are based on realistic stellar atmospheres and take
into account the potential flux contribution from the surrounding HII region.
The gravitational magnification boost achieved when pointing JWST through a
foreground galaxy cluster is also considered. Using this machinery, we derive
the conditions required for JWST to be able to detect population III stars in
isolation. We find that a detection of individual population III stars with
JWST is unlikely at these redshifts. However, the main problem is not
necessarily that these stars are too faint, once gravitational lensing is taken
into account, but that their surface number densities are too low.Comment: 6 pages, 3 figures, proceedings of CRF2010, DESY Hamburg, Nov 9-12
201
Constituent Loads and Trends in the Upper Illinois River Watershed and Upper White River Basin
Water chemistry can greatly influence the quality of surface waters and affect the ability for streams and rivers to meet their designated use(s). In Arkansas, many streams and rivers were placed on the 2008 303(d) list of impaired water bodies due to excess levels of nutrients, chlorides, sulfates, and sediments (ADEQ, 2008). These constituents continue to be listed as the potential cause for waterâquality impairments through the most recent draft 303(d) list (ADEQ, 2014). The Arkansas NonâPoint Source (NPS) Management Program wants to reduce pollâ utant loading from the landscape and improve water quality, where funding for projects is targeted to priority watersheds throughout the State
Constituent Load Estimation in the Lower Ouachita-Smackover Watershed
Water quality was monitored at 21 sites in the Lower OuachitaâSmackover Watershed from 2013 November through 2014 September. The U.S. Geological Survey maintains discharge monitoring stations at two of these sites, Moro Creek (USGS 07362500) and Smackover Creek (USGS 07362100), which were sampled during base flow and storm event conditions, whereas the other sites were only sampled during baseflow. The Arkansas Water Resources Center (AWRC) estimated constituent loads for nitrateâN (NOâââN), total nitrogen (TN), soluble reactive phosphorus (SRP), total phosphorus (TP) and total suspended solids (TSS) using the U.S. Geological Survey LOADEST software. LOADEST creates regression models between constituent concentrations and discharge, as well as time. The resulting models were applied to daily discharge throughout calendar years 2013 and 2014 to estimate loads. Annual and monthly loads and flow volumes for each site are summarized in this report
Simultaneous polymerization and molding of Pyrrone polymers
Simultaneous polymerization and hot pressing of Pyrrone polymer
A catch-free stock assessment model with application to goliath grouper (Epinephelus itajara) off southern Florida
Many modern stock assessment methods provide the machinery for determining the status of a stock in relation to certain reference points and for estimating how quickly a stock can be rebuilt. However, these methods typically require catch data, which are not always available. We introduce a model-based framework for estimating reference points, stock status, and recovery times in situations where catch data and other measures of absolute abundance are unavailable. The specif ic estimator developed is essentially an age-structured production model recast in terms relative to pre-exploitation levels. A Bayesian estimation scheme is adopted to allow the incorporation of pertinent auxiliary information such as might be obtained from meta-analyses of similar stocks or anecdotal observations. The approach is applied to the population of goliath grouper (Epinephelus itajara) off southern Florida, for which there are three indices of relative abundance but no reliable catch data. The results confirm anecdotal accounts of a marked decline in abundance during the 1980s followed by a substantial increase after the harvest of goliath grouper was banned in 1990. The ban appears to have reduced fishing pressure to between 10% and 50% of the levels observed during the 1980s. Nevertheless, the predicted fishing mortality rate under the ban appears to remain substantial, perhaps owing to illegal harvest and depth-related release mortality. As a result, the base model predicts that there is less than a 40% chance that the spawning biomass will recover to a level that would produce a 50% spawning potential ratio
Delayed Recombination
Under the standard model for recombination of the primeval plasma, and the
cold dark matter model for structure formation, recent measurements of the
first peak in the angular power spectrum of the cosmic microwave background
temperature indicate the spatial geometry of the universe is nearly flat. If
sources of Lya resonance radiation, such as stars or active galactic nuclei,
were present at z ~ 1000 they would delay recombination, shifting the first
peak to larger angular scales, and producing a positive bias in this measure of
space curvature. It can be distinguished from space curvature by its
suppression of the secondary peaks in the spectrum.Comment: submitted to ApJ
Contingent Claims Valuation of Corporate Liabilities: Theory and Empirical Tests
Although the Contingent Claims Analysis model has become the premier theory of how value is allocated among claimants on firms,its empirical validity remains an open question. In addition to being of academic interest, a test of the model would have significant practical implications. If it can be established that the model predicts actual market prices, then the model can be used to price new and untraded claims, to infer firm values from prices of traded claims like equity and to price covenants separately. In this paper evidence is presented on how well a model which makes the usual assumptions in the literature does in predicting market prices for claims in standard capital structures. The results suggest that the usual assumption list requires modification before it can serve as a basis for valuing corporate claims.
Gamma rays from ultracompact primordial dark matter minihalos
Ultracompact minihalos have recently been proposed as a new class of dark
matter structure. These minihalos would be produced by phase transitions in the
early Universe or features in the inflaton potential, and constitute
non-baryonic massive compact halo objects (MACHOs) today. We examine the
prospect of detecting ultracompact minihalos in gamma-rays if dark matter
consists of self-annihilating particles. We compute present-day fluxes from
minihalos produced in the electron-positron annihilation epoch, and the QCD and
electroweak phase transitions in the early Universe. Even at a distance of 100
pc, minihalos produced during the electron-positron annihilation epoch should
be eminently detectable today, either by the Fermi satellite, current Air
Cherenkov telescopes, or even in archival EGRET data. Within ~1 pc, minihalos
formed in the QCD phase transition would have similar predicted fluxes to the
dwarf spheroidal galaxies targeted by current indirect dark matter searches, so
might also be detectable by present or upcoming experiments.Comment: 5 pages, 3 figures. Minor update to match published version of
erratu
- âŠ