3,166 research outputs found
The Mass-to-Light Ratios of the Draco and Ursa Minor Dwarf Spheroidal Galaxies. II. The Binary Population and Its Effect in the Measured Velocity Dispersions of Dwarf Spheroidal Galaxies
We use a large set of radial velocities in the Ursa Minor and Draco dwarf
spheroidal galaxies to search for binary stars and to infer the binary
frequency. Of the 118 stars in our sample with multiple observations, six are
velocity variables with probabilities below 0.001. We use Monte Carlo
simulations that mimic our observations to determine the efficiency with which
our observations find binary stars. Our best, though significantly uncertain,
estimate of the binary frequency for stars near the turnoff in Draco and UMi is
0.2--0.3 per decade of period in the vicinity of periods of one year, which is
3--5 that found for the solar neighborhood. This frequency is high
enough that binary stars might significantly affect the measured velocity
dispersions of some dwarf spheroidal galaxies according to some previous
numerical experiments. However, in the course of performing our own
experiments, we discovered that this previous work had inadvertently
overestimated binary orbital velocities. Our first set of simulations of the
effects of binaries is based on the observed scatter in the individual velocity
measurements for the multiply-observed Draco and Ursa Minor stars. This scatter
is small compared to measured velocity dispersions and, so, the effect of
binaries on the dispersions is slight. This result is supported by our second
set of experiments, which are based on a model binary population normalized by
the observed binary frequency in Draco and Ursa Minor. We conclude that binary
stars have had no significant effect on the measured velocity dispersion and
inferred mass-to-light ratio of any dwarf spheroidal galaxy.Comment: 33 pages, 95kb uuencoded, gzipped postscript; Accepted by
Astronomical Journal; gzipped, tarred postscript of text, tables, figures
available at ftp://as.arizona.edu/pub/edo (binaries_in_dsph.tar.gz
Fabry-Perot Measurements of the Dynamics of Globular Cluster Cores: M15 (NGC~7078)
We report the first use of the Rutgers Imaging Fabry-Perot Spectrophotometer
to study the dynamics of the cores of globular clusters. We have obtained
velocities for cluster stars by tuning the Fabry-Perot to take a series of
narrow-band images at different wavelengths across one of the Na D (5890 AA)
absorption lines. Measuring the flux in every frame yields a short portion of
the spectrum for each star simultaneously. This proves to be a very efficient
method for obtaining accurate stellar velocities; in crowded regions we are
able to measure hundreds of velocities in 3-4 hours of observing time. We have
measured velocities with uncertainties of less than 5 km/s for 216 stars within
1.5' of the center of the globular cluster M15 (NGC 7078). The paper is a
uuencoded compressed postscript file
Automated Interpretation of the Mackay-Marg Tonograph by Digital Computer
journal articleBiomedical Informatic
Life cycle costing of diesel, natural gas, hybrid and hydrogen fuel cell bus systems: An Australian case study
The transit authority in Perth, Western Australia, has put several alternative fuel buses, including diesel-electric hybrid and hydrogen fuel cell buses, into revenue service over the years alongside conventional diesel and natural gas buses. Primary data from this fleet is used to construct a Life Cycle Cost (LCC) model, providing an empirical LCC result. The model is then used to forecast possible scenarios using cost estimates for next generation technologies. The methodology follows the Australian/New Zealand Standard for Life Cycle Costing, AS/NZS 4536:1999. The model outputs a dollar value in real terms that represents the LCC of each bus transportation technology. The study finds that Diesel buses deliver the lowest Total Cost of Ownership (TCO). The diesel-electric hybrid bus was found to have a TCO that is about 10% higher than conventional diesel. The premium to implement and operate a hydrogen bus, even if industry targets are attained, is still substantially greater than the TCO of a conventional diesel bus, unless a very large increase in the diesel fuel price occurs. However, the hybrid and hydrogen technologies are still very young in comparison to diesel and economies of scale are yet to be realised
Sharing MLM's: An Experiment between Columbia-Presbyterian and LDS Hospital
Conference PaperBiomedical Informatic
A MLM-Based Order Entry System: The Use of Knowledge in a Traditional HIS Application
Conference PaperBiomedical Informatic
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