1,633 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
Prompt Iron Enrichment, Two r-Process Components, and Abundances in Very Metal-Poor Stars
We present a model to explain the wide range of abundances for heavy
r-process elements (mass number A > 130) at low [Fe/H]. This model requires
rapid star formation and/or an initial population of supermassive stars in the
earliest condensed clots of matter to provide a prompt or initial Fe inventory.
Subsequent Fe and r-process enrichment was provided by two types of supernovae:
one producing heavy r-elements with no Fe on a rather short timescale and the
other producing light r-elements (A < or = 130) with Fe on a much longer
timescale.Comment: 5 pages, 2 postscript figures, to appear in ApJ
New Lithium Measurements in Metal-Poor Stars
We provide *lambda*6708 Li 1 measurements in 37 metal-poor stars, most of
which are poorly-studied or have no previous measurements, from high-resolution
and high-S/N spectroscopy obtained with the McDonald Observatory 2.1m and 2.7m
telescopes. The typical line strength and abundance uncertainties, confirmed by
the thinness of the Spite plateau manifested by our data and by comparison with
previous measurements, are <=4 mAng and <=0.07-0.10 dex respectively. Two rare
moderately metal-poor solar-Teff dwarfs, HIP 36491 and 40613, with
significantly depleted but still detectable Li are identified; future light
element determinations in the more heavily depeleted HIP 40613 may provide
constraints on the Li depletion mechanism acting in this star. We note two
moderately metal-poor and slightly evolved stars, HIP 105888 and G265-39, that
appear to be analogs of the low-Li moderately metal-poor subgiant HD 201889.
Preliminary abundance analysis of G 265-39 finds no abnormalities that suggest
the low Li content is associated with AGB mass-transfer or deep mixing and
p-capture. We also detect line doubling in HIP 4754, heretofore classified as
SB1.Comment: Accepted for publication in PASP, volume 912 (Feb 2012) 15 pages, 3
figures, 2 table
Rubidium and lead abundances in giant stars of the globular clusters M4 and M5
We present measurements of the neutron-capture elements Rb and Pb for bright
giants in the globular clusters M4 and M5. The clusters are of similar
metallicity ([Fe/H] = -1.2) but M4 is decidedly s-process enriched relative to
M5: [Ba/Fe] = +0.6 for M4 but 0.0 for M5. The Rb and Pb abundances were derived
by comparing synthetic spectra with high-resolution, high signal-to-noise ratio
spectra obtained with MIKE on the Magellan telescope. Abundances of Y, Zr, La,
and Eu were also obtained. In M4, the mean abundances from 12 giants are
[Rb/Fe] = 0.39 +/- 0.02 (sigma = 0.07), [Rb/Zr] = 0.17 +/- 0.03 (sigma = 0.08),
and [Pb/Fe] = 0.30 +/- 0.02 (sigma = 0.07). In M5, the mean abundances from two
giants are [Rb/Fe] = 0.00 +/- 0.05 (sigma = 0.06), [Rb/Zr] = 0.08 +/- 0.08
(sigma = 0.11), and [Pb/Fe] = -0.35 +/- 0.02 (sigma = 0.04). Within the
measurement uncertainties, the abundance ratios [Rb/Fe], [Pb/Fe] and [Rb/X] for
X = Y, Zr, La are constant from star-to-star in each cluster and none of these
ratios are correlated with O or Na abundances. While M4 has a higher Rb
abundance than M5, the ratios [Rb/X] are similar in both clusters indicating
that the nature of the s-products are very similar for each cluster but the gas
from which M4's stars formed had a higher concentration of these products.Comment: Accepted for publication in Ap
An abundance analysis for four Red Horizontal Branch Stars in the extremely metal rich globular cluster NGC 6528
We present the results of the first analysis of high dispersion spectra of
four red HB stars in the metal rich globular cluster NGC 6528, located in
Baade's Window. We find that the mean [Fe/H] for NGC 6528 is +0.07+-0.01 dex
(error of the mean), with a star-to-star scatter of sigma = 0.02 dex (4 stars),
although the total error is likely to be larger (~0.1 dex) due to systematic
errors related to the effective temperature scale and to model atmospheres.
This metallicity is somewhat larger than both the mean abundance in the
galactic bulge found by McWilliam & Rich (1994) and that found in our previous
paper for NGC 6553. However, we find that the spectra of clump stars in NGC
6528 and NGC 6553 are very similar each other, the slightly different metal
abundances found being possibly due to the different atmospheric parameters
adopted in the two analyses. For NGC 6528 we find excesses for the
alpha-process elements Si and Ca ([Si/Fe]=+0.4 and [Ca/Fe]=+0.2), whereas Mn is
found to be underabundant ([Mn/Fe]=-0.4). We find a solar abundance of O;
however this is somewhat uncertain due to the dependence of the O abundance on
the adopted atmospheric parameters and to coupling between C and O abundances
in these cool, metal-rich stars. Finally, we find large Na excesses ([Na/Fe]~
+0.4) in all stars examined. Since the present analysis is based on higher
quality material, we propose to revise our previous published metal abundance
for NGC 6553 to [Fe/H]=$0.06+-0.15.Comment: accepted for publication in the September 2001 issue of The
Astronomical Journal; 3 new figures and updated results and calibration
Heavy element abundances in giant stars of the globular clusters M4 and M5
We present a comprehensive abundance analysis of 27 heavy elements in bright
giant stars of the globular clusters M4 and M5 based on high resolution, high
signal-to-noise ratio spectra obtained with the Magellan Clay Telescope. We
confirm and expand upon previous results for these clusters by showing that (1)
all elements heavier than, and including, Si have constant abundances within
each cluster, (2) the elements from Ca to Ni have indistinguishable
compositions in M4 and M5, (3) Si, Cu, Zn, and all s-process elements are
approximately 0.3 dex overabundant in M4 relative to M5, and (4) the r-process
elements Sm, Eu, Gd, and Th are slightly overabundant in M5 relative to M4. The
cluster-to-cluster abundance differences for Cu and Zn are intriguing,
especially in light of their uncertain nucleosynthetic origins. We confirm that
stars other than Type Ia supernovae must produce significant amounts of Cu and
Zn at or below the clusters' metallicities. If intermediate-mass AGB stars or
massive stars are responsible for the Cu and Zn enhancements in M4, the similar
[Rb/Zr] ratios and (preliminary) Mg isotope ratios in both clusters may be
problematic for either scenario. For the elements from Ba to Hf, we assume that
the s- and r-process contributions are scaled versions of the solar s- and
r-process abundances. We quantify the relative fractions of s- and r-process
material for each cluster and show that they provide an excellent fit to the
observed abundances.Comment: Accepted for publication in Ap
The Infrared Ca II triplet as metallicity indicator
From observations of almost 500 RGB stars in 29 Galactic open and globular
clusters, we have investigated the behaviour of the infrared Ca II triplet
(8498, 8542 and 8662 \AA) in the age range 13Age/Gyr0.25 and the
metallicity range [Fe/H] +0.47. These are the widest ranges of
ages and metallicities in which the behaviour of the Ca II triplet lines has
been investigated in a homogeneous way. We report the first empirical study of
the variation of the CaII triplet lines strength, for given metallicities, with
respect to luminosity. We find that the sequence defined by each cluster in the
Luminosity-Ca plane is not exactly linear. However, when only stars in
a small magnitude interval are observed, the sequences can be considered as
linear. We have studied the the Ca II triplet lines on three metallicities
scales. While a linear correlation between the reduced equivalent width (
or ) versus metallicity is found in the \citet{cg97} and \citet{ki03}
scales, a second order term needs to be added when the \citet{zw84} scale is
adopted. We investigate the role of age from the wide range of ages covered by
our sample. We find that age has a weak influence on the final relationship.
Finally, the relationship derived here is used to estimate the metallicities of
three poorly studied open clusters: Berkeley 39, Trumpler 5 and Collinder 110.
For the latter, the metallicity derived here is the first spectroscopic
estimate available.Comment: 52 pages, 16 figures, accepted for publication in Astronomical
Journa
Abundance Patterns in the Draco, Sextans and Ursa Minor Dwarf Spheroidal Galaxies
The Keck I telescope has been used to obtain HIRES spectra for red giants
belonging to the Draco, Sextans and Ursa Minor dwarf spheroidal (dSph)
galaxies. An analysis of these spectra is presented, along with abundance
ratios for more than 20 elements. The resulting database of element abundances
for 17 stars is the most extensive yet assembled for stars in dSph
environments. Our main findings are summarized as follows: (1) There is
unambiguous evidence for a large internal spread in metallicity in all three
galaxies: our program stars span a range of [Fe/H] = 1.53, 1.40 and 0.73 dex in
Draco, Sextans and Ursa Minor, respectively. (2) The abundance patterns among
the dSph stars are remarkably uniform, suggesting that all three galaxies have
similar nucleosynthetic histories. (3) A comparison of the measured abundance
ratios for our sample of dSph stars with published values for Galactic halo and
disk field stars suggests that the dSph galaxies have 0.02 < [alpha/Fe] < 0.13
dex, whereas the halo field star sample has [alpha/Fe] ~ 0.28 dex over the same
range in metallicity. (4) The most metal-rich dSph stars in our sample have
[Y/Fe] abundances which are significantly lower than those measured for halo
field stars of similar metallicity, while the measured [Ba/Eu] ratios for the
dSph stars suggest that the early chemical evolution of these galaxies was
dominated by the r-process. Taken together, these results suggest that the
Galactic halo is unlikely to have assembled, in its entirety, through the
disruption of dwarf galaxies similar to the low-luminosity dSphs studied here.
(ABRIDGED).Comment: 24 pages, 8 postscript figures (including 6 color figures). Accepted
for publication in the Astrophysical Journa
Line Broadening in Field Metal-poor Red Giant and Red Horizontal Branch Stars
We report 349 radial velocities for 45 metal-poor field red giant and red
horizontal branch stars. We have have identified one new spectroscopic binary,
HD 4306, and one possible such system, HD 184711. We also report 57 radial
velocities for 11 of the 91 stars reported on previously by Carney et al.
(2003). As was found in the previous study, radial velocity "jitter" is present
in many of the most luminous stars. Excluding stars showing spectroscopic
binary orbital motion, all 7 of the red giants with M(V) <= -2.0 display
jitter, as well as 3 of the 14 stars with -2.0 <= M(V) <= -1.4. We have also
measured line broadening in all of the new spectra, using synthetic spectra as
templates. The most luminous red giants show significant line broadening, as do
many of the red horizontal branch stars, and we discuss briefly possible
causes.Comment: To appear in the Astronomical Journa
Rubidium in Metal-Deficient Disk and Halo Stars
We report the first extensive study of stellar Rb abundances. High-resolution
spectra have been used to determine, or set upper limits on, the abundances of
this heavy element and the associated elements Y, Zr, and Ba in 44 dwarfs and
giants with metallicities spanning the range -2.0 <[Fe/H] < 0.0. In
metal-deficient stars Rb is systematically overabundant relative to Fe; we find
an average [Rb/Fe] of +0.21 for the 32 stars with [Fe/H] < -0.5 and measured
Rb. This behavior contrasts with that of Y, Zr, and Ba, which, with the
exception of three new CH stars (HD 23439A and B and BD +5 3640), are
consistently slightly deficient relative to Fe in the same stars; excluding the
three CH stars, we find the stars with [Fe/H] < -0.5 have average [Y/Fe],
[Zr/Fe], and [Ba/Fe] of --0.19 (24 stars), --0.12 (28 stars), and --0.06 (29
stars), respectively. The different behavior of Rb on the one hand and Y, Zr,
and Ba on the other can be attributed in part to the fact that in the Sun and
in these stars Rb has a large r-process component while Y, Zr, and Ba are
mostly s-process elements with only small r-process components. In addition,
the Rb s-process abundance is dependent on the neutron density at the
s-processing site. Published observations of Rb in s-process enriched red
giants indicate a higher neutron density in the metal-poor giants. These
observations imply a higher s-process abundance for Rb in metal-poor stars. The
calculated combination of the Rb r-process abundance, as estimated for the
stellar Eu abundances, and the s-process abundance as estimated for red giants
accounts satisfactorily for the observed run of [Rb/Fe] with [Fe/H].Comment: 23 pages, 5 tables, 7 figure
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