335 research outputs found
Empirically Derived Integrated Stellar Yields of Fe-Peak Elements
We present here the initial results of a new study of massive star yields of
Fe-peak elements. We have compiled from the literature a database of carefully
determined solar neighborhood stellar abundances of seven iron-peak elements,
Ti, V, Cr, Mn, Fe, Co, and Ni and then plotted [X/Fe] versus [Fe/H] to study
the trends as functions of metallicity. Chemical evolution models were then
employed to force a fit to the observed trends by adjusting the input massive
star metallicity-sensitive yields of Kobayashi et al. Our results suggest that
yields of Ti, V, and Co are generally larger as well as anticorrelated with
metallicity, in contrast to the Kobayashi et al. predictions. We also find the
yields of Cr and Mn to be generally smaller and directly correlated with
metallicity compared to the theoretical results. Our results for Ni are
consistent with theory, although our model suggests that all Ni yields should
be scaled up slightly. The outcome of this exercise is the computation of a set
of integrated yields, i.e., stellar yields weighted by a slightly flattened
time-independent Salpeter initial mass function and integrated over stellar
mass, for each of the above elements at several metallicity points spanned by
the broad range of observations. These results are designed to be used as
empirical constraints on future iron-peak yield predictions by stellar
evolution modelers. Special attention is paid to the interesting behavior of
[Cr/Co] with metallicity -- these two elements have opposite slopes -- as well
as the indirect correlation of [Ti/Fe] with [Fe/H]. These particular trends, as
well as those exhibited by the inferred integrated yields of all iron-peak
elements with metallicity, are discussed in terms of both supernova
nucleosynthesis and atomic physics.Comment: 27 pages, 6 figures; Accepted for Publication in the Astrophysical
Journa
Ba & Eu Abundances in M15 giant stars
To investigate the Ba and Eu abundances for a sample of 63 giant stars in the
globular cluster M15. This is the largest sample of M15 giants stars for which
Ba abundances have been determined and, due to the target selection of the
original research programme, the Ba abundances are complete along the red giant
branch. Stellar parameters were taken from the previous key study and a
microturbulence-surface gravity relation was determined for precise measurement
of the Ba line at 6496.898 Angstroms, which has a high sensitivity to
microturbulence. Element abundances for Ba, La, Eu, Ca, Ni and Fe were
calculated using spectrum synthesis and equivalent widths techniques. A bimodal
distribution in Ba, Eu and La abundances was found within the sample. The low
Ba,Eu,La mode had mean abundances of =-2.41+/-0.16,
=-1.80+/-0.08 and =-2.19+/-0.13 while the high Ba,Eu,La mode
had mean abundances of =-2.00+/-0.16, =-1.65+/-0.13 and
=-1.95+/-0.11. Both modes are indicative of a pollution scenario
dominated by the r-process, hence contributions from explosive nucleosynthesis
of massive stars. There may be evidence of further enhancement by another heavy
element process and of potential anticorrelations in Na-O for both modes
indicating a complex formation and evolution history for M15.Comment: 20 pages, 15 figure
Europium, Samarium, and Neodymium Isotopic Fractions in Metal-Poor Stars
We have derived isotopic fractions of europium, samarium, and neodymium in
two metal-poor giants with differing neutron-capture nucleosynthetic histories.
These isotopic fractions were measured from new high resolution (R ~ 120,000),
high signal-to-noise (S/N ~ 160-1000) spectra obtained with the 2dCoude
spectrograph of McDonald Observatory's 2.7m Smith telescope. Synthetic spectra
were generated using recent high-precision laboratory measurements of hyperfine
and isotopic subcomponents of several transitions of these elements and matched
quantitatively to the observed spectra. We interpret our isotopic fractions by
the nucleosynthesis predictions of the stellar model, which reproduces
s-process nucleosynthesis from the physical conditions expected in low-mass,
thermally-pulsing stars on the AGB, and the classical method, which
approximates s-process nucleosynthesis by a steady neutron flux impinging upon
Fe-peak seed nuclei. Our Eu isotopic fraction in HD 175305 is consistent with
an r-process origin by the classical method and is consistent with either an r-
or an s-process origin by the stellar model. Our Sm isotopic fraction in HD
175305 suggests a predominantly r-process origin, and our Sm isotopic fraction
in HD 196944 is consistent with an s-process origin. The Nd isotopic fractions,
while consistent with either r-process or s-process origins, have very little
ability to distinguish between any physical values for the isotopic fraction in
either star. This study for the first time extends the n-capture origin of
multiple rare earths in metal-poor stars from elemental abundances to the
isotopic level, strengthening the r-process interpretation for HD 175305 and
the s-process interpretation for HD196944.Comment: 40 pages, 16 figures. Accepted for publication in ApJ. Full versions
of tables 4 and 5 are available from the first author upon reques
New Detections of Arsenic, Selenium, and Other Heavy Elements in Two Metal-Poor Stars
We use the Space Telescope Imaging Spectrograph on board the Hubble Space
Telescope to obtain new high-quality spectra covering the 1900 to 2360 Angstrom
wavelength range for two metal-poor stars, HD 108317 and HD 128279. We derive
abundances of Cu II, Zn II, As I, Se I, Mo II, and Cd II, which have not been
detected previously in either star. Abundances derived for Ge I, Te I, Os II,
and Pt I confirm those derived from lines at longer wavelengths. We also derive
upper limits from the non-detection of W II, Hg II, Pb II, and Bi I. The mean
[As/Fe] ratio derived from these two stars and five others in the literature is
unchanged over the metallicity range -2.8 = +0.28
+/- 0.14 (std. dev. = 0.36 dex). The mean [Se/Fe] ratio derived from these two
stars and six others in the literature is also constant, = +0.16 +/-
0.09 (std. dev. = 0.26 dex). The As and Se abundances are enhanced relative to
a simple extrapolation of the iron-peak abundances to higher masses, suggesting
that this mass region (75 < A < 82) may be the point at which a different
nucleosynthetic mechanism begins to dominate the quasi-equilibrium alpha-rich
freezeout of the iron peak. = +0.56 +/- 0.23 in HD 108317 and HD
128279, and we infer that lines of Cu I may not be formed in local
thermodynamic equilibrium in these stars. The [Zn/Fe], [Mo/Fe], [Cd/Fe], and
[Os/Fe] ratios are also derived from neutral and ionized species, and each
ratio pair agrees within the mutual uncertainties, which range from 0.15 to
0.52 dex.Comment: Accepted for publication in the Astrophysical Journal. 13 pages, 10
figure
Detection of the Second r-process Peak Element Tellurium in Metal-Poor Stars
Using near-ultraviolet spectra obtained with the Space Telescope Imaging
Spectrograph onboard the Hubble Space Telescope, we detect neutral tellurium in
three metal-poor stars enriched by products of r-process nucleosynthesis, BD+17
3248, HD 108317, and HD 128279. Tellurium (Te, Z=52) is found at the second
r-process peak (A=130) associated with the N=82 neutron shell closure, and it
has not been detected previously in Galactic halo stars. The derived tellurium
abundances match the scaled solar system r-process distribution within the
uncertainties, confirming the predicted second peak r-process residuals. These
results suggest that tellurium is predominantly produced in the main component
of the r-process, along with the rare earth elements.Comment: Accepted for publication in the Astrophysical Journal Letters (5
pages, 2 figures
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The Abundances Of Neutron-Capture Species In The Very Metal-Poor Globular Cluster M15: A Uniform Analysis Of Red Giant Branch And Red Horizontal Branch Stars
The globular cluster M15 is unique in its display of star-to-star variations in the neutron-capture elements. Comprehensive abundance surveys have been previously conducted for handfuls of M15 red giant branch (RGB) and red horizontal branch (RHB) stars. No attempt has been made to perform a single, self-consistent analysis of these stars, which exhibit a wide range in atmospheric parameters. In the current effort, a new comparative abundance derivation is presented for three RGB and six RHB members of the cluster. The analysis employs an updated version of the line transfer code MOOG, which now appropriately treats coherent, isotropic scattering. The apparent discrepancy in the previously reported values for the metallicity of M15 RGB and RHB stars is addressed and a resolute disparity of Delta(RHB-RGB) approximate to 0.1 dex in the iron abundance was found. The anti-correlative behavior of the light neutron-capture elements (Sr, Y, Zr) is clearly demonstrated with both Ba and Eu, standard markers of the s- and r-process, respectively. No conclusive detection of Pb was made in the RGB targets. Consequently for the M15 cluster, this suggests that the main component of the s-process has made a negligible contribution to those elements normally dominated by this process in solar system material. Additionally for the M15 sample, a large Eu abundance spread is confirmed, which is comparable to that of the halo field at the same metallicity. These abundance results are considered in the discussion of the chemical inhomogeneity and nucleosynthetic history of M15.National Science Foundation AST 07-07447, AST 09-08978Astronom
APOGEE Kinematics I: Overview of the Kinematics of the Galactic Bulge as Mapped by APOGEE
We present the stellar kinematics across the Galactic bulge and into the disk
at positive longitudes from the SDSS-III APOGEE spectroscopic survey of the
Milky Way. APOGEE includes extensive coverage of the stellar populations of the
bulge along the mid-plane and near-plane regions. From these data, we have
produced kinematic maps of 10,000 stars across longitudes 0 deg < l < 65 deg,
and primarily across latitudes of |b| < 5 deg in the bulge region. The APOGEE
data reveal that the bulge is cylindrically rotating across all latitudes and
is kinematically hottest at the very centre of the bulge, with the smallest
gradients in both kinematic and chemical space inside the inner-most region
(l,|b|) < (5,5) deg. The results from APOGEE show good agreement with data from
other surveys at higher latitudes and a remarkable similarity to the rotation
and dispersion maps of barred galaxies viewed edge on. The thin bar that is
reported to be present in the inner disk within a narrow latitude range of |b|
< 2 deg appears to have a corresponding signature in [Fe/H] and [alpha/Fe].
Stars with [Fe/H] > -0.5 have dispersion and rotation profiles that are similar
to that of N-body models of boxy/peanut bulges. There is a smooth kinematic
transition from the thin bar and boxy bulge (l,|b|) < (15,12) deg out into the
disk for stars with [Fe/H] > -1.0, and the chemodynamics across (l,b) suggests
the stars in the inner Galaxy with [Fe/H] > -1.0 have an origin in the disk.Comment: Accepted by ApJ 15 December 201
APOGEE DR14/DR15 Abundances in the Inner Milky Way
We present an overview of the distributions of 11 elemental abundances in the
Milky Way's inner regions, as traced by APOGEE stars released as part of SDSS
Data Release 14/15 (DR14/DR15), including O, Mg, Si, Ca, Cr, Mn, Co, Ni, Na,
Al, and K. This sample spans ~4000 stars with R_GC<4 kpc, enabling the most
comprehensive study to date of these abundances and their variations within the
innermost few kiloparsecs of the Milky Way. We describe the observed abundance
patterns ([X/Fe]-[Fe/H]), compare to previous literature results and to
patterns in stars at the solar Galactic radius, and discuss possible trends
with DR14/DR15 effective temperatures. We find that the position of the
[Mg/Fe]-[Fe/H] "knee" is nearly constant with R_GC, indicating a well-mixed
star-forming medium or high levels of radial migration in the early inner
Galaxy. We quantify the linear correlation between pairs of elements in
different subsamples of stars and find that these relationships vary; some
abundance correlations are very similar between the alpha-rich and alpha-poor
stars, but others differ significantly, suggesting variations in the
metallicity dependencies of certain supernova yields. These empirical trends
will form the basis for more detailed future explorations and for the
refinement of model comparison metrics. That the inner Milky Way abundances
appear dominated by a single chemical evolutionary track and that they extend
to such high metallicities underscore the unique importance of this part of the
Galaxy for constraining the ingredients of chemical evolution modeling and for
improving our understanding of the evolution of the Galaxy as a whole.Comment: Submitted to AAS Journals; revised after referee repor
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