9,682 research outputs found
One Loop Effects in Various Dimensions and D-Branes
We calculate some one loop corrections to the effective action of theories in
dimensions that arise on the dimensional reduction of a Weyl fermion in
dimensions. The terms that we are interested in are of a topological nature.
Special attention is given to the effective actions of the super Yang Mills
theories that arise on dimensional reduction of the N=1 theory in six
dimensions or on the dimensional reduction of the N=1 theory in ten dimensions.
In the latter case we suggest an interpretation of the quantum effect as a
coupling of the gauge field on the brane to a relative background gauge field.Comment: 18 pages, LaTe
Neutron-Capture Nucleosynthesis in the First Stars
Recent studies suggest that metal-poor stars enhanced in carbon but
containing low levels of neutron-capture elements may have been among the first
to incorporate the nucleosynthesis products of the first generation of stars.
We have observed 16 stars with enhanced carbon or nitrogen using the MIKE
Spectrograph on the Magellan Telescopes at Las Campanas Observatory and the
Tull Spectrograph on the Smith Telescope at McDonald Observatory. We present
radial velocities, stellar parameters, and detailed abundance patterns for
these stars. Strontium, yttrium, zirconium, barium, europium, ytterbium, and
other heavy elements are detected. In four stars, these heavy elements appear
to have originated in some form of r-process nucleosynthesis. In one star, a
partial s-process origin is possible. The origin of the heavy elements in the
rest of the sample cannot be determined unambiguously. The presence of elements
heavier than the iron group offers further evidence that zero-metallicity
rapidly-rotating massive stars and pair instability supernovae did not
contribute substantial amounts of neutron-capture elements to the regions where
the stars in our sample formed. If the carbon- or nitrogen-enhanced metal-poor
stars with low levels of neutron-capture elements were enriched by products of
zero-metallicity supernovae only, then the presence of these heavy elements
indicates that at least one form of neutron-capture reaction operated in some
of the first stars.Comment: Accepted for publication in the Astrophysical Journal (36 pages, 26
figures
Detection of Bending Stresses in Buried Pipelines
When gas or oil pipelines must be laid through sand or permafrost or other unstable soils, the pipe walls can he subjected to large bending loads if the soil shifts. In order to detect this condition and correct it, it would be useful to monitor the state of stress along the pipeline\u27at regular time intervals using a vehicle that is moved through the line by the fluid or gas in it. The experiments described here demonstrate that such a vehicle, using EMATs to excite and detect ultrasonic waves in the pipe wall, would be feasible because the transduction efficiency of the EMATs and thus the insertion loss of such a system can be related quantitatively to the stress level in the pipe wall
A Detailed Study of Giants and Horizontal Branch Stars in M68: Atmospheric Parameters and Chemical Abundances
In this paper, we present a detailed high-resolution spectroscopic study of
post main sequence stars in the Globular Cluster M68. Our sample, which covers
a range of 4000 K in , and 3.5 dex in , is comprised of
members from the red giant, red horizontal, and blue horizontal branch, making
this the first high-resolution globular cluster study covering such a large
evolutionary and parameter space. Initially, atmospheric parameters were
determined using photometric as well as spectroscopic methods, both of which
resulted in unphysical and unexpected , , , and
[Fe/H] combinations. We therefore developed a hybrid approach that addresses
most of these problems, and yields atmospheric parameters that agree well with
other measurements in the literature. Furthermore, our derived stellar
metallicities are consistent across all evolutionary stages, with
[Fe/H] = 2.42 ( = 0.14) from 25 stars. Chemical
abundances obtained using our methodology also agree with previous studies and
bear all the hallmarks of globular clusters, such as a Na-O anti-correlation,
constant Ca abundances, and mild -process enrichment.Comment: Accepted to the Astronomical Journa
A Search for Stars of Very Low Metal Abundance. VI. Detailed Abundances of 313 Metal-Poor Stars
We present radial velocities, equivalent widths, model atmosphere parameters,
and abundances or upper limits for 53 species of 48 elements derived from high
resolution optical spectroscopy of 313 metal-poor stars. A majority of these
stars were selected from the metal-poor candidates of the HK Survey of Beers,
Preston, and Shectman. We derive detailed abundances for 61% of these stars for
the first time. Spectra were obtained during a 10-year observing campaign using
the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Telescopes at
Las Campanas Observatory, the Robert G. Tull Coude Spectrograph on the Harlan
J. Smith Telescope at McDonald Observatory, and the High Resolution
Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory. We perform
a standard LTE abundance analysis using MARCS model atmospheres, and we apply
line-by-line statistical corrections to minimize systematic abundance
differences arising when different sets of lines are available for analysis. We
identify several abundance correlations with effective temperature. A
comparison with previous abundance analyses reveals significant differences in
stellar parameters, which we investigate in detail. Our metallicities are, on
average, lower by approx. 0.25 dex for red giants and approx. 0.04 dex for
subgiants. Our sample contains 19 stars with [Fe/H] < -3.5, 84 stars with
[Fe/H] < -3.0, and 210 stars with [Fe/H] < -2.5. Detailed abundances are
presented here or elsewhere for 91% of the 209 stars with [Fe/H] < -2.5 as
estimated from medium resolution spectroscopy by Beers, Preston, and Shectman.
We will discuss the interpretation of these abundances in subsequent papers.Comment: Accepted for publication in the Astronomical Journal. 60 pages, 59
figures, 18 tables. Machine-readable versions of the long tables can be found
in the ancillary data file
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