36 research outputs found
Abundance Ratios in the Galactic Bulge and Super Metal-Rich Type II Nucle osynthesis
We present abundance results from our Keck/HIRES observations of giants in
the Galactic Bulge. We confirm that the metallicity distribution of giants in
the low-reddening bulge field Baade's Window can be well-fit by a closed-box
enrichment model. We also confirm previous observations that find enhanced
[Mg/Fe], [Si/Fe] and [Ca/Fe] for all bulge giants, including those at
super-solar metallicities. However, we find that the [O/Fe] ratios of
metal-rich bulge dwarfs decrease with increasing metallicity, contrary to what
is expected if the enhancements of the other -elements is due to Type
II supernovae enrichment. We suggest that the decrease in oxygen production may
be due to mass loss in the pre-supernova evolution of metal-rich progenitors.Comment: Conference proceeding to Nuclei in the Cosmos VIII, Vancouver, BC,
July, 2004. Based on data obtained at the W. M. Keck Observatory, which is
operated as a scientific partnership among the California Institute of
Technology, the University of California, and NASA, and was made possible by
the generous financial support of the W. M. Keck Foundatio
Characterization and Performance of the Suomi-NPP VIIRS Solar Diffuser Stability Monitor
We describe the on-orbit characterization and performance of the Solar Diffuser Stability Monitor (SDSM) on-board Suomi-NPP/VIIRS. This description includes the observing procedure of each SDSM event, the algorithms used to generate the Solar Diffuser degradation corrective factors, and the results for the mission to date. We will also compare the performance of the VIIRS SDSM and SD to the similar components operating on the MODIS instrument on the EOS Terra and Aqua satellite
Alpha Enhancement and the Metallicity Distribution Function of Plaut's Window
We present Fe, Si, and Ca abundances for 61 giants in Plaut's Window
(l=-1,b=-8.5) and Fe abundances for an additional 31 giants in a second, nearby
field (l=0,b=-8) derived from high resolution (R~25,000) spectra obtained with
the Blanco 4m telescope and Hydra multifiber spectrograph. The median
metallicity of red giant branch (RGB) stars in the Plaut field is ~0.4 dex
lower than those in Baade's Window, and confirms the presence of an iron
abundance gradient along the bulge minor axis. The full metallicity range of
our (biased) RGB sample spans -1.5<[Fe/H]<+0.3, which is similar to that found
in other bulge fields. We also derive a photometric metallicity distribution
function for RGB stars in the (l=-1,b=-8.5) field and find very good agreement
with the spectroscopic metallicity distribution. The radial velocity and
dispersion data for the bulge RGB stars are in agreement with previous results
of the BRAVA survey, and we find evidence for a decreasing velocity dispersion
with increasing [Fe/H]. The [alpha/Fe] enhancement in Plaut field stars is
nearly identical to that observed in Baade's window, and suggests that an
[alpha/Fe] gradient does not exist between b=-4 and -8. Additionally, a subset
of our sample (23 stars) appear to be foreground red clump stars that are very
metal--rich, exhibit small metallicity and radial velocity dispersions, and are
enhanced in alpha elements. While these stars likely belong to the Galactic
inner disk population, they exhibit [alpha/Fe] ratios that are enhanced above
the thin and thick disk.Comment: Accepted for publication in ApJ. 38 pages, 11 figures, and 2 tables.
Requests for higher resolution figures and electronic versions of tables 1
and/or 2 in advance of publication may be sent to cijohnson[at]astro.ucla.ed
Abundances and Kinematics of Field Stars II: Kinematics and Abundance Relationships
As an investigation of the origin of ``-poor'' halo stars, we analyze
kinematic and abundance data for 73 intermediate metallicity stars (
[Fe/H] ) selected from Paper I of this series. We find evidence for a
connection between the kinematics and the enhancement of certain
element-to-iron ([X/Fe]) ratios in these stars. Statistically significant
correlations were found between [X/Fe] and galactic rest-frame velocities
(\vrf{}) for Na, Mg, Al, Si, Ca and Ni, with marginally significant
correlations existing for Ti and Y as well. We also find that the [X/Fe] ratios
for these elements all correlate with a similar level of significance with
[Na/Fe]. Finally, we compare the abundances of these halo stars against those
of stars in nearby dSph galaxies. We find significant differences between the
abundance ratios in the dSph stars and halo stars of similar metallicity. From
this result, it is unlikely that the halo stars in the solar neighborhood,
including even the ``-poor'' stars, were once members of disrupted dSph
galaxies similar to those studied to date.Comment: Accepted for publication in January 2002 AJ. 29 pages, 12 figures, 2
table
An Extremely Lithium-Rich Bright Red Giant in the Globular Cluster M3
We have serendipitously discovered an extremely lithium-rich star on the red
giant branch of the globular cluster M3 (NGC 5272). An echelle spectrum
obtained with the Keck I HIRES reveals a Li I 6707 Angstrom resonance doublet
of 520 milli-Angstrom equivalent width, and our analysis places the star among
the most Li-rich giants known: log[epsilon(Li)] ~= +3.0. We determine the
elemental abundances of this star, IV-101, and three other cluster members of
similar luminosity and color, and conclude that IV-101 has abundance ratios
typical of giants in M3 and M13 that have undergone significant mixing. We
discuss mechanisms by which a low-mass star may be so enriched in Li, focusing
on the mixing of material processed by the hydrogen-burning shell just below
the convective envelope. While such enrichment could conceivably only happen
rarely, it may in fact regularly occur during giant-branch evolution but be
rarely detected because of rapid subsequent Li depletion.Comment: 7-page LaTeX file, including 2 encapsulated ps figures + 1 table;
accepted for publication in the Astrophysical Journal Letter
A Large Sample Study of Red Giants in the Globular Cluster Omega Centauri (NGC 5139)
We present abundances of several light, alpha, Fe-peak, and neutron-capture
elements for 66 red giant branch (RGB) stars in the Galactic globular cluster
Omega Centauri. Our observations lie in the range 12.0<V<13.5 and focus on the
intermediate and metal-rich RGBs. We find that there are at least four peaks in
the metallicity distribution function at [Fe/H]=-1.75, -1.45, -1.05, and -0.75,
which correspond to about 55%, 30%, 10%, and 5% of our sample, respectively.
Additionally, the most metal-rich stars are the most centrally located. Na and
Al are correlated despite exhibiting star-to-star dispersions of more than a
factor of 10, but the distribution of those elements appears to be metallicity
dependent and are divided at [Fe/H]~-1.2. About 40-50% of stars with
[Fe/H]<-1.2 have Na and Al abundances consistent with production solely in Type
II supernovae and match observations of disk and halo stars at comparable
metallicity. The remaining metal-poor stars are enhanced in Na and Al compared
to their disk and halo counterparts and are mostly consistent with predicted
yields from >5 M_sun asymptotic giant branch (AGB) stars. At [Fe/H]>-1.2, more
than 75% of the stars are Na/Al enhanced and may have formed almost exclusively
from AGB ejecta. Most of these stars are enhanced in Na by at least 0.2 dex for
a given Al abundance than would be expected based on "normal" globular cluster
values. All stars in our sample are alpha-rich and have solar-scaled Fe-peak
abundances. Eu does not vary extensively as a function of metallicity; however,
[La/Fe] varies from about -0.4 to +2 and stars with [Fe/H]>-1.5 have [La/Eu]
values indicating domination by the s-process. A quarter of our sample have
[La/Eu]>+1 and may be the result of mass transfer in a binary system.Comment: ApJ Accepted; 90 pages, 16 Figures, 5 Table
Constraints on the Formation of the Galactic Bulge from Na, Al, and Heavy Element Abundances in Plaut's Field
We report chemical abundances of Na, Al, Zr, La, Nd, and Eu for 39 red giant
branch (RGB) stars and 23 potential inner disk red clump stars located in
Plaut-s low extinction window. We also measure lithium for a super Li-rich RGB
star. The abundances were determined by spectrum synthesis of high resolution
(R~25,000), high signal-to-noise (S/N~50-100 pixel-1) spectra obtained with the
Blanco 4m telescope and Hydra multifiber spectrograph. For the bulge RGB stars,
we find a general increase in the [Na/Fe] and [Na/Al] ratios with increasing
metallicity, and a similar decrease in [La/Fe] and [Nd/Fe]. Additionally, the
[Al/Fe] and [Eu/Fe] abundance trends almost identically follow those of the
{\alpha}-elements, and the [Zr/Fe] ratios exhibit relatively little change with
[Fe/H]. The consistently low [La/Eu] ratios of the RGB stars indicate that at
least a majority of bulge stars formed rapidly (<1 Gyr) and before the main
s-process could become a significant pollution source. In contrast, we find
that the potential inner disk clump stars exhibit abundance patterns more
similar to those of the thin and thick disks. Comparisons between the abundance
trends at different bulge locations suggest that the inner and outer bulge
formed on similar timescales. However, we find evidence of some abundance
differences between the most metal-poor and metal-rich stars in various bulge
fields. The data also indicate that the halo may have had a more significant
impact on the outer bulge initial composition than the inner bulge composition.
The [Na/Fe] and to a lesser extent [La/Fe] abundances further indicate that the
metal-poor bulge, at least at ~1 kpc from the Galactic center, and thick disk
may not share an identical chemistry.Comment: Accepted for publication in ApJ; 66 pages, 17 figures, 3 tables;
prior to publication, data tables in electronic form will be made available
upon reques