25,834 research outputs found
Isochrones for Old (> 5 Gyr) Stars and Stellar Populations. I. Models for [Fe/H] , , and [\Fe]
Canonical grids of stellar evolutionary sequences have been computed for the
helium mass-fraction abundances , , and , and for iron
abundances that vary from to (in 0.2 dex increments) when
[/Fe] , or for the ranges [Fe/H] , [Fe/H] when [/Fe] and , respectively. The
grids, which consist of tracks for masses from to
- (depending on the metallicity) are based on
up-to-date physics, including the gravitational settling of helium (but not
metals diffusion). Interpolation software is provided to generate isochrones
for arbitrary ages between and Gyr and any values of ,
[/Fe], and [Fe/H] within the aforementioned ranges. Comparisons of
isochrones with published color-magnitude diagrams (CMDs) for the open clusters
M67 ([Fe/H] ) and NGC 6791 ([Fe/H] ) and for four of
the metal-poor globular clusters (47 Tuc, M3, M5, and M92) indicate that the
models for the observed metallicities do a reasonably good job of reproducing
the locations and slopes of the cluster main sequences and giant branches. The
same conclusion is reached from a consideration of plots of nearby subdwarfs
that have accurate Hipparcos parallaxes and metallicities in the range [Fe/H] on various CMDs and on the ()-diagram. A relatively hot temperature scale similar to that
derived in recent calibrations of the infrared flux method is favored by both
the isochrones and the adopted color transformations, which are based on the
latest MARCS model atmospheres.Comment: 55 pages, including 1 table and 18 figures; accepted for publication
in the Astrophysical Journa
Stress and large-scale spatial structures in dense, driven granular flows
We study the appearance of large-scale dynamical heterogeneities in a
simplified model of a driven, dissipative granular system. Simulations of
steady-state gravity-driven flows of inelastically colliding hard disks show
the formation of large-scale linear structures of particles with a high
collision frequency. These chains can be shown to carry much of the collisional
stress in the system due to a dynamical correlation that develops between the
momentum transfer and time between collisions in these "frequently-colliding"
particles. The lifetime of these dynamical stress heterogeneities is seen to
grow as the flow velocity decreases towards jamming, leading to slowly decaying
stress correlations reminiscent of the slow dynamics observed in supercooled
liquids.Comment: 8 pages, 6 figure
Locally Optimally-emitting Clouds and the Narrow Emission Lines in Seyfert Galaxies
The narrow emission line spectra of active galactic nuclei are not accurately
described by simple photoionization models of single clouds. Recent Hubble
Space Telescope images of Seyfert 2 galaxies show that these objects are rich
with ionization cones, knots, filaments, and strands of ionized gas. Here we
extend to the narrow line region the ``locally optimally emitting cloud'' (LOC)
model, in which the observed spectra are predominantly determined by powerful
selection effects. We present a large grid of photoionization models covering a
wide range of physical conditions and show the optimal conditions for producing
many of the strongest emission lines. We show that the integrated narrow line
spectrum can be predicted by an integration of an ensemble of clouds, and we
present these results in the form of diagnostic line ratio diagrams making
comparisons with observations. We also predict key diagnostic line ratios as a
function of distance from the ionizing source, and compare these to
observations. The predicted radial dependence of the [O III]/[O II] ratio may
be matched to the observed one in NGC4151, if the narrow line clouds see a more
intense continuum than we see. The LOC scenario when coupled with a simple
Keplerian gravitational velocity field will quite naturally predict the
observed line width versus critical density relationship. The influence of dust
within the ionized portion of the clouds is discussed and we show that the more
neutral gas is likely to be dusty, although a high ionization dust-free region
is most likely present too. This argues for a variety of NLR cloud origins.Comment: 29 pages plus 16 figures, accepted for publication in Ap
Far-UV Emission from Elliptical Galaxies at z=0.55
The restframe UV-to-optical flux ratio, characterizing the ``UV upturn''
phenomenon, is potentially the most sensitive tracer of age in elliptical
galaxies; models predict that it may change by orders of magnitude over the
course of a few Gyr. In order to trace the evolution of the UV upturn as a
function of redshift, we have used the far-UV camera on the Space Telescope
Imaging Spectrograph to image the galaxy cluster CL0016+16 at z=0.55. Our
25''x25'' field includes four bright elliptical galaxies, spectroscopically
confirmed to be passively evolving cluster members. The weak UV emission from
the galaxies in our image demonstrates that the UV upturn is weaker at a
lookback time 5.6 Gyr earlier than our own, as compared to measurements of the
UV upturn in cluster E and S0 galaxies at z=0 and z=0.375. These images are the
first with sufficient depth to demonstrate the fading of the UV upturn expected
at moderate redshifts. We discuss these observations and the implications for
the formation history of galaxies.Comment: 4 pages, Latex. 2 figures. Uses corrected version of emulateapj.sty
and apjfonts.sty (included). Accepted for publication in ApJ Letter
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