10,106 research outputs found
Elemental abundances of mercury-manganese stars and the population 2 type star HD 109995
Ultraviolet and optical data for the Hg Mn stars Coronae Borealis and Cancri is being combined with data for the field horizontal branch population II star HD 109995 in order to derive the element abundances in their photospheres. Data collected by IUE is being utilized
The field horizontal-branch star HD 109995: New results with coadded ultraviolet and optical region spectra
A comprehensive ultraviolet and optical region abundance analysis of the field horizontal branch Population 2 A-type star HD 109995 is described. Coaddition of IUE high dispersion images and DAO 6.5 A/mm IIaO spectrograms improved the signal-to-noise ratio of the data. We have identified ultraviolet lines whose analysis will provide more complete and accurate elemental abundances than those obtained from optical region spectra alone. A preliminary elemental abundance analysis of the optical region shows that log Z/Z (solar) approx. = -2. A first attempt to synthesize two Fe 2 ultraviolet resonance lines yields an iron abundance a few tenths of a deg higher than the average obtained from optical region Fe 2 lines
Xenon in Mercury-Manganese Stars
Previous studies of elemental abundances in Mercury-Manganese (HgMn) stars
have occasionally reported the presence of lines of the ionized rare noble gas
Xe II, especially in a few of the hottest stars with Teff ~ 13000--15000 K. A
new study of this element has been undertaken using observations from Lick
Observatory's Hamilton Echelle Spectrograph. In this work, the spectrum
synthesis program UCLSYN has been used to undertake abundance analysis assuming
LTE. We find that in the Smith & Dworetsky sample of HgMn stars, Xe is vastly
over-abundant in 21 of 22 HgMn stars studied, by factors of 3.1--4.8 dex. There
does not appear to be a significant correlation of Xe abundance with Teff. A
comparison sample of normal late B stars shows no sign of Xe II lines that
could be detected, consistent with the expected weakness of lines at normal
abundance. The main reason for the previous lack of widespread detection in
HgMn stars is probably due to the strongest lines being at longer wavelengths
than the photographic blue. The lines used in this work were 4603.03A, 4844.33A
and 5292.22A.Comment: 8 pages, 4 figures. Accepted by Monthly Notices of the Royal
Astronomical Society, 8 January 200
The unexplained nature of reading.
The effects of properties of words on their reading aloud response times (RTs) are 1 major source of evidence about the reading process. The precision with which such RTs could potentially be predicted by word properties is critical to evaluate our understanding of reading but is often underestimated due to contamination from individual differences. We estimated this precision without such contamination individually for 4 people who each read 2,820 words 50 times each. These estimates were compared to the precision achieved by a 31-variable regression model that outperforms current cognitive models on variance-explained criteria. Most (around 2/3) of the meaningful (non-first-phoneme, non-noise) word-level variance remained unexplained by this model. Considerable empirical and theoretical-computational effort has been expended on this area of psychology, but the high level of systematic variance remaining unexplained suggests doubts regarding contemporary accounts of the details of the mechanisms of reading at the level of the word. Future assessment of models can take advantage of the availability of our precise participant-level database
Inclination-Independent Galaxy Classification
We present a new method to classify galaxies from large surveys like the
Sloan Digital Sky Survey using inclination-corrected concentration,
inclination-corrected location on the color-magnitude diagram, and apparent
axis ratio. Explicitly accounting for inclination tightens the distribution of
each of these parameters and enables simple boundaries to be drawn that
delineate three different galaxy populations: Early-type galaxies, which are
red, highly concentrated, and round; Late-type galaxies, which are blue, have
low concentrations, and are disk dominated; and Intermediate-type galaxies,
which are red, have intermediate concentrations, and have disks. We have
validated our method by comparing to visual classifications of high-quality
imaging data from the Millennium Galaxy Catalogue. The inclination correction
is crucial to unveiling the previously unrecognized Intermediate class.
Intermediate-type galaxies, roughly corresponding to lenticulars and early
spirals, lie on the red sequence. The red sequence is therefore composed of two
distinct morphological types, suggesting that there are two distinct mechanisms
for transiting to the red sequence. We propose that Intermediate-type galaxies
are those that have lost their cold gas via strangulation, while Early-type
galaxies are those that have experienced a major merger that either consumed
their cold gas, or whose merger progenitors were already devoid of cold gas
(the ``dry merger'' scenario).Comment: Accepted for publication in ApJ. 7 pages in emulateap
Lanthanides and other spectral oddities in a Centauri
Context: There is considerable interest in the helium variable a Cen as a
bridge between helium-weak and helium-strong CP stars. Aims: We investigate Ce
III and other possible lanthanides in the spectrum the of hottest chemically
peculiar (CP) star in which these elements have been found. A {Kr II line
appears within a broad absorption which we suggest may be due to a high-level
transition in C II. Methods: Wavelengths and equivalent widths are measured on
high-resolution UVES spectra, analyzed, and their phase-variations
investigated. Results: New, robust identifications of Ce III and Kr II are
demonstrated. Nd III is likely present. A broad absorption near 4619[A] is
present at all phases of a Cen, and in some other early B stars. Conclusions:
The presence of lanthanides in a Cen strengthens the view that this star is a
significant link between the cooler CP stars and the hotter helium-peculiar
stars. Broad absorptions in a Cen are not well explained.Comment: Research Note accepted by Astronomy and Astrophysics; 4 pages, 4
Figs. 2 Table
Instantaneous Pair Theory for High-Frequency Vibrational Energy Relaxation in Fluids
Notwithstanding the long and distinguished history of studies of vibrational
energy relaxation, exactly how it is that high frequency vibrations manage to
relax in a liquid remains somewhat of a mystery. Both experimental and
theoretical approaches seem to say that there is a natural frequency range
associated with intermolecular motions in liquids, typically spanning no more
than a few hundred cm^{-1}. Landau-Teller-like theories explain how a solvent
can absorb any vibrational energy within this "band", but how is it that
molecules can rid themselves of superfluous vibrational energies significantly
in excess of these values? We develop a theory for such processes based on the
idea that the crucial liquid motions are those that most rapidly modulate the
force on the vibrating coordinate -- and that by far the most important of
these motions are those involving what we have called the mutual nearest
neighbors of the vibrating solute. Specifically, we suggest that whenever there
is a single solvent molecule sufficiently close to the solute that the solvent
and solute are each other's nearest neighbors, then the instantaneous
scattering dynamics of the solute-solvent pair alone suffices to explain the
high frequency relaxation. The many-body features of the liquid only appear in
the guise of a purely equilibrium problem, that of finding the likelihood of
particularly effective solvent arrangements around the solute. These results
are tested numerically on model diatomic solutes dissolved in atomic fluids
(including the experimentally and theoretically interesting case of I_2 in Xe).
The instantaneous pair theory leads to results in quantitative agreement with
those obtained from far more laborious exact molecular dynamics simulations.Comment: 55 pages, 6 figures Scheduled to appear in J. Chem. Phys., Jan, 199
High resolution spectroscopic study of red clump stars in the Galaxy: iron group elements
The main atmospheric parameters and abundances of the iron group elements
(vanadium, chromium, iron, cobalt and nickel) are determined for 62 red giant
"clump" stars revealed in the Galactic field by the Hipparcos orbiting
observatory. The stars form a homogeneous sample with the mean value of
temperature T=4750 +- 160K, of surface gravity log g = 2.41 +- 0.26 and the
mean value of metallicity [Fe/H] = -0.04 +- 0.15 dex. A Gaussian fit to the
[Fe/H] distribution produces the mean [Fe/H] = -0.01 dex and dispersion of
[Fe/H] = 0.08 dex. The near-solar metallicity and small dispersion of [Fe/H] of
clump stars of the Galaxy obtained in this work confirm the theoretical model
of the Hipparcos clump by Girardi & Salaris (2001). This suggests that nearby
clump stars are (in the mean) relatively young objects, reflecting mainly the
near-solar metallicities developed in the local disk during the last few Gyrs
of its history. We find iron group element to iron abundance ratios in clump
giants to be close to solar.Comment: 9 pages, 7 figures, accepted for publication in MNRA
Vibration characteristics of ring-stiffened orthotropic shells of revolution
Computer program solves vibration modes and frequencies of thin shells of revolution having general meridional curvature and orthotropic elastic properties in order to evaluate the dynamic behavior of structures with thin shelled components
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