185 research outputs found
Abundances and Kinematics of Field Halo and Disk Stars I: Observational Data and Abundance Analysis
We describe observations and abundance analysis of a high-resolution,
high-S/N survey of 168 stars, most of which are metal-poor dwarfs. We follow a
self-consistent LTE analysis technique to determine the stellar parameters and
abundances, and estimate the effects of random and systematic uncertainties on
the resulting abundances. Element-to-iron ratios are derived for key alpha,
odd, Fe-peak, r- and s-process elements. Effects of Non-LTE on the analysis of
Fe I lines are shown to be very small on the average. Spectroscopically
determined surface gravities are derived that are generally close to those
obtained from Hipparcos parallaxes.Comment: 41 pages, 7 Postscript figures. Accepted for publication in the A
Quantum Smoluchowski equation: Escape from a metastable state
We develop a quantum Smoluchowski equation in terms of a true probability
distribution function to describe quantum Brownian motion in configuration
space in large friction limit at arbitrary temperature and derive the rate of
barrier crossing and tunneling within an unified scheme. The present treatment
is independent of path integral formalism and is based on canonical
quantization procedure.Comment: 10 pages, To appear in the Proceedings of Statphys - Kolkata I
Improved Color-Temperature Relations and Bolometric Corrections for Cool Stars
We present new grids of colors and bolometric corrections for F-K stars
having 4000 K < Teff < 6500 K, 0.0 < log g < 4.5 and -3.0 < [Fe/H] < 0.0. A
companion paper extends these calculations into the M giant regime. Colors are
tabulated for Johnson U-V and B-V; Cousins V-R and V-I; Johnson-Glass V-K, J-K
and H-K; and CIT/CTIO V-K, J-K, H-K and CO. We have developed these
color-temperature (CT) relations by convolving synthetic spectra with
photometric filter-transmission-profiles. The synthetic spectra have been
computed with the SSG spectral synthesis code using MARCS stellar atmosphere
models as input. Both of these codes have been improved substantially,
especially at low temperatures, through the incorporation of new opacity data.
The resulting synthetic colors have been put onto the observational systems by
applying color calibrations derived from models and photometry of field stars
which have Teffs determined by the infrared-flux method. The color calibrations
have zero points and slopes which change most of the original synthetic colors
by less than 0.02 mag and 5%, respectively. The adopted Teff scale (Bell &
Gustafsson 1989) is confirmed by the extraordinary agreement between the
predicted and observed angular diameters of the field stars. We have also
derived empirical CT relations from the field-star photometry. Except for the
coolest dwarfs (Teff < 5000 K), our calibrated, solar-metallicity model colors
are found to match these and other empirical relations quite well. Our
calibrated, 4 Gyr, solar-metallicity isochrone also provides a good match to
color-magnitude diagrams of M67. We regard this as evidence that our calibrated
colors can be applied to many astrophysical problems, including modelling the
integrated light of galaxies. (abridged)Comment: To appear in the March 2000 issue of the Astronomical Journal. 72
pages including 16 embedded postscript figures (one page each) and 6 embedded
postscript tables (18 pages total
Oxygen Abundances in Metal-Poor Stars
We present oxygen abundances derived from both the permitted and forbidden
oxygen lines for 55 subgiants and giants with [Fe/H] values between -2.7 and
solar with the goal of understanding the discrepancy in the derived abundances.
A first attempt, using \teff{} values from photometric calibrations and surface
gravities from luminosities, obtained agreement between the indicators for
turn-off stars, but the disagreement was large for evolved stars. We find that
the difference in the oxygen abundances derived from the permitted and
forbidden lines is most strongly affected by \teff{}, and we derive a new
\teff{} scale based on forcing the two sets of lines to give the same oxygen
abundances. These new parameters, however, do not agree with other observables,
such as theoretical isochrones or Balmer-line profile based \teff{}
determinations. Our analysis finds that one-dimensional, LTE analyses (with
published NLTE corrections for the permitted lines) cannot fully resolve the
disagreement in the two indicators without adopting a temperature scale
incompatible with other temperature indicators. We also find no evidence of
circumstellar emission in the forbidden lines, removing such emission as a
possible cause for the discrepancy.Comment: 41 pages, 12 tables, 17 figures. Accepted for publication in Oct.
2003 Ap
Modeling Mid-Ultraviolet Spectra. I. Temperatures of Metal-Poor Stars
Determining the properties of old stellar systems using evolutionary
population synthesis requires a library of reliable model stellar fluxes.
Empirical libraries are limited to spectra of stars in the solar neighborhood,
with nearly solar abundances and abundance ratios. We report here a first step
towards providing a flux library that includes nonsolar abundances, based on
calculations from first principles that are calibrated empirically. We have
started with main-sequence stars, whose light dominates the mid-ultraviolet
spectrum of an old stellar system. We have calculated mid-ultraviolet spectra
for the Sun and nine nearby, near-main-sequence stars spanning metallicities
from less than 1/100 solar to greater than solar, encompassing a range of
light-element abundance enhancements. We first determined temperatures of eight
of the stars by analyzing optical echelle spectra together with the
mid-ultraviolet. Both could be matched at the same time only when models with
no convective overshoot were adopted, and only when an approximate chromosphere
was incorporated near the surface of relatively metal-rich models. Extensive
modifications to mid-UV line parameters were also required, notably the manual
assignment of approximate identifications for mid-UV lines missing from
laboratory linelists. Without recourse to additional missing opacity, these
measures suffice to reproduce in detail almost the entire mid-UV spectrum of
solar-temperature stars up to one-tenth solar metallicity, and the region from
2900A to 3100A throughout the entire metallicity range. Ramifications for
abundance determinations in individual metal-poor stars and for age-metallicity
determinations of old stellar systems are briefly discussed, with emphasis on
the predictive power of the calculations.Comment: Proof revision -Minor changes to revised version submitted to
Astrophysical Journal May 1, 2001. 29 pages, 4 figures (Fig. 3 with 5 panels
and Fig. 4 with 6 panels). Figures 1 and 2 are .gif; postscript versions of
Figures 1 and 2 are available from
http://www.astro.virginia.edu/~rtr/uv/index.htm
Stellar Archaeology: a Keck Pilot Program on Extremely Metal- Poor Stars From the Hamburg/ESO Survey. III. The Lead (Pb) Star HE 0024-2523
We present a detailed abundance analysis, including spectral syntheses, of a
very metal-poor ([Fe/H]= -2.7), peculiar main sequence star, HE0024-2523
detected during the course of the Keck Pilot Program. Radial velocities of this
star were obtained during four different observing runs over a time span of 1.1
years, and demonstrate that it is clearly a short period spectroscopic binary.
An orbital solution was obtained, and orbital parameters were determined with
high precision. The rotational velocity was also measured (vsin i=9.71.5
kms); rotation appears likely to be synchronous with the orbit. The abundance
analysis and spectral syntheses indicate that the object is a CH star
characterized by extreme s-process enrichment, likely due to mass accretion
from an evolved companion which has now probably become a white dwarf. The lead
(Pb) abundance of HE0024-2523 is very high, the same as that of the recently
discovered lead-rich metal-poor star CS 29526-110, [Pb/Fe]=+3.3. The abundance
ratio of the heavy-s to light-s elements, as characterized by Pb and Ba,
[Pb/Ba]=+1.9, is the highest yet found for any metal-poor star, and is about
0.7 dex higher than that of CS29526-110. On the basis of the measured isotopic
ratio of carbon (12C/13C about 6) we argue that the mass donor must have had an
original mass of at least 3 Msun. The unusually short period of this CH star
suggests that it underwent a past common-envelope phase with its evolved
companion. Our results are compared to the latest available models for AGB
yields and s-process nucleosynthesis. We also discuss the possible connection
between HE0024-2523 the lithium depletion of halo stars, and halo blue
straggler formation.Comment: 55 pages, 17 color figures included; Accepted for publication in the
February 2003 issue of the Astronomical Journa
Regulation of Transgene Expression in Tumor Cells by Exploiting Endogenous Intracellular Signals
Recently, we have proposed a novel strategy for a cell-specific gene therapy system based on responses to intracellular signals. In this system, an intracellular signal that is specifically and abnormally activated in the diseased cells is used for the activation of transgene expression. In this study, we used protein kinase C (PKC)α as a trigger to activate transgene expression. We prepared a PKCα-responsive polymer conjugate [PPC(S)] and a negative control conjugate [PPC(A)], in which the phosphorylation site serine (Ser) was replaced with alanine (Ala). The phosphorylation for polymer/DNA complexes was determined with a radiolabel assay using [γ-32P]ATP. PPC(S)/DNA complexes were phosphorylated by the addition of PKCα, but no phosphorylation of the PPC(A)/DNA complex was observed. Moreover, after microinjection of polymer/GFP-encoding DNA complexes into HepG2 cells at cation/anion (C/A) ratios of 0.5 to 2.0, significant expression of GFP was observed in all cases using PPC(S)/DNA complexes, but no GFP expression was observed in the negative control PPC(A)/DNA complex-microinjected cells at C/A ratios of 1.0 and 2.0. On the other hand, GFP expression from PPC(S)/DNA complexes was completely suppressed in cells pretreated with PKCα inhibitor (Ro31-7549). These results suggest that our gene regulation system can be used for tumor cell-specific expression of a transgene in response to PKCα activity
Spectroscopic Signatures of Convection in the Spectrum of Procyon. Fundamental Parameters and Iron Abundance
We have observed the spectrum of Procyon A (F5IV) from 4559 to 5780 A with a
S/N of ~ 1e3 and a resolving power of 2e5. We have measured the line bisectors
and relative line shifts of a large number of Fe I and Fe II lines, comparing
them to those found in the Sun. A three-dimensional(3D) hydrodynamical model
atmosphere has been computed and is tested against observations. The model
reproduces in detail most of the features observed, although we identify some
room for improvement. At all levels, the comparison of the 3D time-dependent
calculations with the observed spectral lines shows a much better agreement
than for classical homogeneous models, making it possible to refine previous
estimates of the iron abundance, the projected rotational velocity, the
limb-darkening, and the systemic velocity of the Procyon binary system.
The difference between the iron abundance determined with the 3D model and
its
1D counterpart is <~ 0.05 dex. We find consistency between the iron abundance
derived from Fe I and Fe II lines, suggesting that departures from LTE in the
formation of the studied lines are relatively small. The scatter in the iron
abundance determined from different lines still exceeds the expectations from
the uncertainties in the atomic data, pointing out that one or more components
in the modeling can be refined further.Comment: 30 pages, 19 figures; uses emulateapj.sty (included); to appear in
ApJ (Feb 2002
The Chemical Composition of Carbon-Rich, Very Metal-Poor Stars: A New Class of Mildly Carbon-Rich Objects Without Excess of Neutron-Capture Elements
We report on an analysis of the chemical composition of five carbon-rich,
very metal-poor stars based on high-resolution spectra. One star, CS22948-027,
exhibits very large overabundances of carbon, nitrogen, and the neutron-capture
elements, as found in the previous study of Hill et al.. This result may be
interpreted as a consequence of mass transfer from a binary companion that
previously evolved through the asymptotic giant branch stage. By way of
contrast, the other four stars we investigate exhibit no overabundances of
barium ([Ba/Fe]<0), while three of them have mildly enhanced carbon and/or
nitrogen ([C+N]+1). We have been unable to determine accurate carbon and
nitrogen abundances for the remaining star (CS30312-100). These stars are
rather similar to the carbon-rich, neutron-capture-element-poor star
CS22957-027 discussed previously by Norris et al., though the carbon
overabundance in this object is significantly larger ([C/Fe]=+2.2). Our results
imply that these carbon-rich objects with ``normal'' neutron-capture element
abundances are not rare among very metal-deficient stars. One possible process
to explain this phenomenon is as a result of helium shell flashes near the base
of the AGB in very low-metallicity, low-mass (M~< 1M_sun) stars, as recently
proposed by Fujimoto et al..
The moderate carbon enhancements reported herein ([C/Fe]+1) are similar to
those reported in the famous r-process-enhanced star CS22892-052. We discuss
the possibility that the same process might be responsible for this similarity,
as well as the implication that a completely independent phenomenon was
responsible for the large r-process enhancement in CS22892-052.Comment: 53 pages, 8 figures, to appear in Ap
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