34,432 research outputs found
Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories
(abridged) We introduce and discuss the properties of a theoretical
(B-K)-(J-K) integrated colour diagram for single-age, single-metallicity
stellar populations. This combination of integrated colours is able to largely
disentangle the well known age-metallicity degeneracy when the age of the
population is greater than ~300 Myr. We discuss in detail the effect on this
colour-colour diagram of alpha-enhanced metal abundance ratios, the presence of
blue horizontal branch stars unaccounted for in the theoretical calibration,
and of statistical colour fluctuations in low mass stellar systems. In the case
of populations with multiple stellar generations, the luminosity-weighted mean
age obtained from this diagram is shown to be heavily biased towards the
youngest stellar components. We apply this method to several datasets for which
optical and near-IR photometry are available in the literature. For the two
Local Group dwarf galaxies NGC185 and NGC6822, the mean ages derived from the
integrated colours are consistent with the star formation histories inferred
independently from photometric observations of their resolved stellar
populations. A sample of bright field and Virgo cluster elliptical galaxies is
found to exhibit a range of luminosity-weighted mean ages from 3 to 14 Gyr,
with a mean of 8 Gyr, independent of environment, and mean metallicities at or
just above the solar value. Colour gradients are found in all of the galaxies
studied, in the sense that central regions are redder. Aperture data for five
Virgo early-type dwarf galaxies show that these galaxies appear to be shifted
to lower mean metallicities and lower mean ages (range 1 to 6 Gyr) than their
higher luminosity counterparts.Comment: (1) Liverpool John Moores University, UK; (2) University of Cardiff,
UK; (3) University of Bristol, UK; (4) INAF-Osservatorio Astronomico di
Collurania, I; 12 pages, 9 figures, MNRAS in pres
Linking Cellular and Mechanical Processes in Articular Cartilage Lesion Formation: A Mathematical Model
A severe application of stress on articular cartilage can initiate a cascade
of biochemical reactions that can lead to the development of osteoarthritis. We
constructed a multiscale mathematical model of the process with three
components: cellular, chemical, and mechanical. The cellular component
describes the different chondrocyte states according to the chemicals these
cells release. The chemical component models the change in concentrations of
those chemicals. The mechanical component contains a simulation of pressure
application onto a cartilage explant and the resulting strains that initiate
the biochemical processes. The model creates a framework for incorporating
explicit mechanics, simulated by finite element analysis, into a theoretical
biology framework
Economic, Neurobiological and Behavioral Perspectives on Building America's Future Workforce
A growing proportion of the U.S. workforce will have been raised in disadvantaged environments that are associated with relatively high proportions of individuals with diminished cognitive and social skills. A cross-disciplinary examination of research in economics, developmental psychology, and neurobiology reveals a striking convergence on a set of common principles that account for the potent effects of early environment on the capacity for human skill development. Central to these principles are the findings that early experiences have a uniquely powerful influence on the development of cognitive and social skills, as well as on brain architecture and neurochemistry; that both skill development and brain maturation are hierarchical processes in which higher level functions depend on, and build on, lower level functions; and that the capacity for change in the foundations of human skill development and neural circuitry is highest earlier in life and decreases over time. These findings lead to the conclusion that the most efficient strategy for strengthening the future workforce, both economically and neurobiologically, and for improving its quality of life is to invest in the environments of disadvantaged children during the early childhood years.
Diffusion in a Granular Fluid - Simulation
The linear response description for impurity diffusion in a granular fluid
undergoing homogeneous cooling is developed in the preceeding paper. The
formally exact Einstein and Green-Kubo expressions for the self-diffusion
coefficient are evaluated there from an approximation to the velocity
autocorrelation function. These results are compared here to those from
molecular dynamics simulations over a wide range of density and inelasticity,
for the particular case of self-diffusion. It is found that the approximate
theory is in good agreement with simulation data up to moderate densities and
degrees of inelasticity. At higher density, the effects of inelasticity are
stronger, leading to a significant enhancement of the diffusion coefficient
over its value for elastic collisions. Possible explanations associated with an
unstable long wavelength shear mode are explored, including the effects of
strong fluctuations and mode coupling
Seasonal Movements, Migratory Behavior, and Site Fidelity of West Indian Manatees along the Atlantic Coast of the United States as Determined by Radio-telemetry
The study area encompassed the eastern coasts of Florida, Georgia, and South Carolina,
including inland waterways such as the St. Johns River (Fig. 1). Manatees inhabited the
relatively narrow band of water that lies between the barrier beaches and the mainland,
occasionally venturing into the ocean close to shore. Between Miami and Fernandina Beach,
Florida, 19 inlets provided manatees with corridors between the intracoastal waters and the
Atlantic Ocean; the distance between adjacent inlets averaged 32 km(SD = 24 km) and varied
from 3 to 88 km. Habitats used by manatees along this 900-km stretch ofcoastline varied
widely and included estuaries, lagoons, rivers and creeks, shallow bays and sounds, and ocean
inlets. Salinities in most areas were brackish, but ranged from completely fresh to completely
marine. The predominant communities of aquatic vegetation also varied geographically and
with salinity: seagrass meadows and mangrove swamps in brackish and marine waters along the
southern half of peninsular Florida; salt marshes in northeastern Florida and Georgia; benthic
macroalgae in estuarine and marine habitats; and a variety of submerged, floating, and emergent
vegetation in freshwater rivers, canals, and streams throughout the region.
Radio-telemetry has been used successfully to track manatees in other regions ofFlorida
(Bengtson 1981, Powell and Rathbun 1984, Lefebvre and Frohlich 1986, Rathbun et al. 1990)
and Georgia (Zoodsma 1991), but these early studies relied primarily on conventional VHF (very
high frequency) transmitters and were limited in their spatial and temporal scope (see O'Shea
and Kochman 1990 for overview). Typically, manatees were tagged at a thermal refuge in the
winter and then tracked until the tag detached, usually sometime between the spring and fall of
the same year. Our study differs from previous research on manatee movements in several
important respects. First, we relied heavily on data from satellite-monitored transmitters using
the Argos system, which yielded a substantially greater number of locations and more systematic
collection of data compared to previous VHF tracking studies (Deutsch et al. 1998). Second, our
tagging and tracking efforts encompassed the entire range of manatees along the Atlantic coast,
from the Florida Keys to South Carolina, so inferences were not limited to a small geographic
area. Third, we often used freshwater to lure manatees to capture sites, which allowed tagging
in all months of the year; this provided more information about summer movement patterns than
had previous studies which emphasized capture and tracking at winter aggregations. Finally, the
study spanned a decade, and success in retagging animals and in replacing transmitters allowed
long-term tracking ofmany individuals. This provided the opportunity to investigate variation in
seasonal movements, migratory behavior, and site fidelity across years for individual manatees.
(254 page document.
Transport properties of dense dissipitive hard-sphere fluids for arbitrary energy loss models
The revised Enskog approximation for a fluid of hard spheres which lose
energy upon collision is discussed for the case that the energy is lost from
the normal component of the velocity at collision but is otherwise arbitrary.
Granular fluids with a velocity-dependent coefficient of restitution are an
important special case covered by this model. A normal solution to the Enskog
equation is developed using the Chapman-Enskog expansion. The lowest order
solution describes the general homogeneous cooling state and a generating
function formalism is introduced for the determination of the distribution
function. The first order solution, evaluated in the lowest Sonine
approximation, provides estimates for the transport coefficients for the
Navier-Stokes hydrodynamic description. All calculations are performed in an
arbitrary number of dimensions.Comment: 27 pages + 1 figur
Discovery of an extended debris disk around the F2V star HD 15745
Using the Advanced Camera for Surveys aboard the Hubble Space Telescope, we
have discovered dust-scattered light from the debris disk surrounding the F2V
star HD 15745. The circumstellar disk is detected between 2.0" and 7.5" radius,
corresponding to 128 - 480 AU radius. The circumstellar disk morphology is
asymmetric about the star, resembling a fan, and consistent with forward
scattering grains in an optically thin disk with an inclination of ~67 degrees
to our line of sight. The spectral energy distribution and scattered light
morphology can be approximated with a model disk composed of silicate grains
between 60 and 450 AU radius, with a total dust mass of 10E-7 M_sun (0.03
M_earth) representing a narrow grain size distribution (1 - 10 micron).
Galactic space motions are similar to the Castor Moving Group with an age of
~10E+8 yr, although future work is required to determine the age of HD 15745
using other indicators.Comment: 7 pages, 4 figures, ApJ Letters, in pres
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Lithium and carbon isotopic fractionations between the alteration assemblages of Nakhla and Lafayette
Nakhla and Lafayette delta 7Li values for samples and extracts (4.1-14.2�) are consistent with brine evaporation. Relatively 13C-poor siderite in Lafayette suggests more than one carbon source was sampled
Seeking the Ultraviolet Ionizing Background at z~3 with the Keck Telescope
We describe the initial results of a deep long-slit emission line search for
redshifted (2.7<z<4.1) Lyman-alpha. These observations are used to constrain
the fluorescent Ly-alpha emission from the population of clouds whose
absorption produces the higher-column-density component of the Ly-alpha forest
in quasar spectra. We use the results to set an upper limit on the ultraviolet
ionizing background. Our spectroscopic data obtained with the Keck II telescope
at lambda/(Delta lambda FWHM)~2000 reveals no candidate Ly-alpha emission over
the wavelength range of 4500-6200 Ang along a 3 arcmin slit in a 5400 s
integration. Our 3 sigma upper bound on the mean intensity of the ionizing
background at the Lyman limit is J(nu 0) < 2E-21 erg/s/cm**2/Hz/sr for
2.7<z<3.1 (where we are most sensitive), assuming Lyman limit systems have
typical radii of 70 kpc (q_0=0.5, H_0=50 km/s/Mpc). This constraint is more
than an order of magnitude more stringent than any previously published direct
limit. However, it is still a factor of three above the ultraviolet background
level expected due to the integrated light of known quasars at z~3. This pilot
study confirms the conclusion of Gould \& Weinberg (1996) that integrations of
several hours on a 10-m class telescope should be capable of measuring J(nu 0)
at high redshift.Comment: 22 pages, 2 postscipt figures. Latex requires aaspp4.sty and epsf.sty
(included). Accepted for publication in the Astronomical Journal (Nov 1998
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