1,808 research outputs found
Photometry of 40 LMC Cepheids
We present V and I_c CCD photometry for 40 LMC Cepheids at 1 to 3 epochs.
This represents a significant increase in the number of LMC Cepheids with
-band data, and, as we show, is a useful addition to the sample which can be
used to calibrate the period--luminosity relations in these important bands
GRBs as Probes of the IGM
Gamma-ray Bursts (GRBs) are the most powerful explosions known, capable of
outshining the rest of gamma-ray sky during their short-lived prompt emission.
Their cosmological nature makes them the best tool to explore the final stages
in the lives of very massive stars up to the highest redshifts. Furthermore,
studying the emission from their low-energy counterparts (optical and infrared)
via rapid spectroscopy, we have been able to pin down the exact location of the
most distant galaxies as well as placing stringent constraints on their host
galaxies and intervening systems at low and high-redshift (e.g. metallicity and
neutral hydrogen fraction). In fact, each GRB spectrum contains absorption
features imprinted by metals in the host interstellar medium (ISM) as well as
the intervening intergalactic medium (IGM) along the line of sight. In this
chapter we summarize the progress made using a large dataset of GRB spectra in
understanding the nature of both these absorbers and how GRBs can be used to
study the early Universe, in particular to measure the neutral hydrogen
fraction and the escape fraction of UV photons before and during the epoch of
re-ionization.Comment: 18 pages; 5 Figures. Accepted for publication in Space Science
Review
Forecasting seasonal hydrologic response in major river basins.
Seasonal precipitation variation due to natural climate variation influences stream flow and the apparent frequency and severity of extreme hydrological conditions such as flood and drought. To study hydrologic response and understand the occurrence of extreme hydrological events, the relevant forcing variables must be identified. This study attempts to assess and quantify the historical occurrence and context of extreme hydrologic flow events and quantify the relation between relevant climate variables. Once identified, the flow data and climate variables are evaluated to identify the primary relationship indicators of hydrologic extreme event occurrence. Existing studies focus on developing basin-scale forecasting techniques based on climate anomalies in El Nino/La Nina episodes linked to global climate. Building on earlier work, the goal of this research is to quantify variations in historical river flows at seasonal temporal-scale, and regional to continental spatial-scale. The work identifies and quantifies runoff variability of major river basins and correlates flow with environmental forcing variables such as El Nino, La Nina, sunspot cycle. These variables are expected to be the primary external natural indicators of inter-annual and inter-seasonal patterns of regional precipitation and river flow. Relations between continental-scale hydrologic flows and external climate variables are evaluated through direct correlations in a seasonal context with environmental phenomenon such as sun spot numbers (SSN), Southern Oscillation Index (SOI), and Pacific Decadal Oscillation (PDO). Methods including stochastic time series analysis and artificial neural networks are developed to represent the seasonal variability evident in the historical records of river flows. River flows are categorized into low, average and high flow levels to evaluate and simulate flow variations under associated climate variable variations. Results demonstrated not any particular method is suited to represent scenarios leading to extreme flow conditions. For selected flow scenarios, the persistence model performance may be comparable to more complex multivariate approaches, and complex methods did not always improve flow estimation. Overall model performance indicates inclusion of river flows and forcing variables on average improve model extreme event forecasting skills. As a means to further refine the flow estimation, an ensemble forecast method is implemented to provide a likelihood-based indication of expected river flow magnitude and variability. Results indicate seasonal flow variations are well-captured in the ensemble range, therefore the ensemble approach can often prove efficient in estimating extreme river flow conditions. The discriminant prediction approach, a probabilistic measure to forecast streamflow, is also adopted to derive model performance. Results show the efficiency of the method in terms of representing uncertainties in the forecasts
On the accretion origin of a vast extended stellar disk around the Andromeda galaxy
We present the discovery of an inhomogenous, low-surface brightness, extended
disk-like structure around the Andromeda galaxy (M31) based on a large
kinematic survey of more than 2800 stars with the Keck/DEIMOS spectrograph. The
stellar structure spans radii from 15 kpc out to ~40 kpc, with detections out
to R ~ 70 kpc. The constituent stars lag the expected velocity of circular
orbits in the plane of the M31 disk by ~40 kms and have a velocity dispersion
of ~30 kms. The color range on the upper RGB shows a large spread indicative of
a population with a significant range of metallicity. The mean metallicity of
the population, measured from Ca II equivalent widths, is [Fe/H] = -0.9 +/-
0.2. The morphology of the structure is irregular at large radii, and shows a
wealth of substructures which must be transitory in nature, and are almost
certainly tidal debris. The presence of these substructures indicates that the
global entity was formed by accretion. This extended disk follows smoothly on
from the central parts of M31 disk with an exponential density law of
scale-length of 5.1 +/- 0.1 kpc, similar to that of the bright inner disk. The
population possesses similar kinematic and abundance properties over the entire
region where it is detected in the survey. We estimate that the structure
accounts for approximately 10% of the total luminosity of the M31 disk, and
given the huge scale, contains ~30% of the total disk angular momentum. This
finding indicates that at least some galactic stellar disks are vastly larger
than previously thought and are formed, at least in their outer regions,
primarily by accretion. [abridged]Comment: 20 pages, 30 figures, ApJ submitte
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Metal-Coated Defect-Core Photonic Crystal Fiber for THz Propagation
Modal solutions for metal-coated defect-core photonic crystal fiber (PCF) with a central air-hole have been obtained by using a full-vectorial finite element method to model the guidance of THz waves. It has been shown that the surface plasmon modes can couple with the defect-core PCF mode to form supermodes, with potential for sensing applications
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