882 research outputs found
The composition of HB stars : RR Lyrae variables
We used moderately high-resolution, high S/N spectra to study the chemical
composition of 10 field ab-type RR Lyrae stars. A new temperature scale was
determined from literature Infrared Flux Method measures of subdwarfs and the
Kurucz (1992) model atmospheres, and used to calibrate colors for both dwarfs
and RR Lyraes. The applicability of Kurucz (1992) model atmospheres in the
analysis of RR Lyraes at minimum light was analyzed: we found that they are
able to reproduce colors, excitation and ionization equilibria as well as the
wings of Halpha. We derived abundances for 21 species. The metal abundances of
the program stars span the range -2.50<[Fe/H]<+0.17$. Lines of most elements
are found to form in LTE conditions. Fe lines satisfy very well the excitation
and ionization equilibria. RR Lyraes share the typical abundance pattern of
other stars of similar [Fe/H]: alpha-elements are overabundant by about 0.4dex
and Mn is underabundant by about 0.6dex in stars with [Fe/H]<-1. Significant
departures from LTE are found only for a few species. We used our new [Fe/H]
abundances, as well as values from Butler and coworkers (corrected to our
system), and from high resolution spectroscopy of globular clusters giants, to
obtain a new calibration of the DeltaS index: [Fe/H]= -0.194(\pm 0.011)DeltaS
-0.08(\pm 0.18) and to update the metallicity calibration of the Ca II K line
index: [Fe/H]= 0.65(\pm 0.17)W'(K) -3.49(\pm 0.39). Finally, our new
metallicity scale was used to revise the [Fe/H] dependence of the absolute
magnitude of RR Lyrae stars, Mv: Mv = 0.20(\pm 0.03)[Fe/H] + 1.06(\pm 0.04).Comment: 59 pages, Latex using aaspp.sty, ps-files of text, tables (21) and
figures (23) available from ftp://boas3.bo.astro.it/pub/gisella To appear in
October 1995 Astronomical Journa
A Proper Motion Survey for White Dwarfs with the Wide Field Planetary Camera 2
We have performed a search for halo white dwarfs as high proper motion
objects in a second epoch WFPC2 image of the Groth-Westphal strip. We identify
24 high proper motion objects with mu > 0.014 ''/yr. Five of these high proper
motion objects are identified as strong white dwarf candidates on the basis of
their position in a reduced proper motion diagram. We create a model of the
Milky Way thin disk, thick disk and stellar halo and find that this sample of
white dwarfs is clearly an excess above the < 2 detections expected from these
known stellar populations. The origin of the excess signal is less clear.
Possibly, the excess cannot be explained without invoking a fourth galactic
component: a white dwarf dark halo. We present a statistical separation of our
sample into the four components and estimate the corresponding local white
dwarf densities using only the directly observable variables, V, V-I, and mu.
For all Galactic models explored, our sample separates into about 3 disk white
dwarfs and 2 halo white dwarfs. However, the further subdivision into the thin
and thick disk and the stellar and dark halo, and the subsequent calculation of
the local densities are sensitive to the input parameters of our model for each
Galactic component. Using the lowest mean mass model for the dark halo we find
a 7% white dwarf halo and six times the canonical value for the thin disk white
dwarf density (at marginal statistical significance), but possible systematic
errors due to uncertainty in the model parameters likely dominate these
statistical error bars. The white dwarf halo can be reduced to around 1.5% of
the halo dark matter by changing the initial mass function slightly. The local
thin disk white dwarf density in our solution can be made consistent with the
canonical value by assuming a larger thin disk scaleheight of 500 pc.Comment: revised version, accepted by ApJ, results unchanged, discussion
expande
The intermediate-age globular cluster NGC 1783 in the Large Magellanic Cloud
We present Hubble Space Telescope ACS deep photometry of the intermediate-age
globular cluster NGC 1783 in the Large Magellanic Cloud. By using this
photometric dataset, we have determined the degree of ellipticity of the
cluster (=0.140.03) and the radial density profile. This profile
is well reproduced by a standard King model with an extended core (r_c=24.5'')
and a low concentration (c=1.16), indicating that the cluster has not
experienced the collapse of the core.
We also derived the cluster age, by using the Pisa Evolutionary Library (PEL)
isochrones, with three different amount of overshooting (namely,
=0.0, 0.10 and 0.25). From the comparison of the observed
Color-Magnitude Diagram (CMD) and Main Sequence (MS) Luminosity Function (LF)
with the theoretical isochrones and LFs, we find that only models with the
inclusion of some overshooting (=0.10-0.25) are able to reproduce
the observables. By using the magnitude difference between the mean level of the He-clump and the flat
region of the SGB, we derive an age =1.40.2 Gyr.Comment: Accepted to publication by A
Optical and Radio Observations of the Afterglow from GRB990510: Evidence for a Jet
We present multi-color optical and two-frequency radio observations of the
bright SAX event, GRB 990510. The well-sampled optical decay, together with the
radio observations are inconsistent with simple spherical afterglow models. The
achromatic optical steepening and the decay of the radio afterglow both
occuring at day are evidence for hydrodynamical evolution of the
source, and can be most easily interpreted by models where the GRB ejecta are
collimated in a jet. Employing a simple jet model to interpret the
observations, we derive a jet opening angle of , reducing the
isotropic gamma-ray emission of erg by a factor . If the jet interpretation is correct, we conclude that GRB observations
to-date are consistent with an energy for the central source of E \lsim
10^{52} erg.Comment: 12 pages, 2 figures. Version accepted for publication in ApJ Letter
The disruption of nearby galaxies by the Milky Way
Interactions between galaxies are common and are an important factor in
determining their physical properties such as position along the Hubble
sequence and star-formation rate. There are many possible galaxy interaction
mechanisms, including merging, ram-pressure stripping, gas compression,
gravitational interaction and cluster tides. The relative importance of these
mechanisms is often not clear, as their strength depends on poorly known
parameters such as the density, extent and nature of the massive dark halos
that surround galaxies. A nearby example of a galaxy interaction where the
mechanism is controversial is that between our own Galaxy and two of its
neighbours -- the Large and Small Magellanic Clouds. Here we present the first
results of a new HI survey which provides a spectacular view of this
interaction. In addition to the previously known Magellanic Stream, which
trails 100 degrees behind the Clouds, the new data reveal a counter-stream
which lies in the opposite direction and leads the motion of the Clouds. This
result supports the gravitational model in which leading and trailing streams
are tidally torn from the body of the Magellanic Clouds.Comment: 17 pages with 5 figures in gif format, scheduled for publication in
the August 20th, 1998 issue of Natur
The Protean Challenge of Game Collections at Academic Libraries
The rise of game development and game studies on university campuses prompts academic libraries to consider how to support teaching and research in this area. This article examines current issues and challenges in the development of game collections at academic libraries. The gaming ecosystem has become more complex and libraries may need to move beyond collections largely based on console video games. This article will advance the discussion by considering emerging issues to support access to the full range of games. The article will use examples from Carleton University Library, Ottawa, which has been developing a game collection since 2008
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