278 research outputs found
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
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
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
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
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
the SDSS-III APOGEE Spectral Line List for H-Band Spectroscopy
We present the H-band spectral line lists adopted by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). The APOGEE line lists comprise astrophysical, theoretical, and laboratory sources from the literature, as well as newly evaluated astrophysical oscillator strengths and damping parameters. We discuss the construction of the APOGEE line list, which is one of the critical inputs for the APOGEE Stellar Parameters and Chemical Abundances Pipeline, and present three different versions that have been used at various stages of the project. The methodology for the newly calculated astrophysical line lists is reviewed. The largest of these three line lists contains 134,457 molecular and atomic transitions. In addition to the format adopted to store the data, the line lists are available in MOOG, Synspec, and Turbospectrum formats. The limitations of the line lists along with guidance for its use on different spectral types are discussed. We also present a list of H-band spectral features that are either poorly represented or completely missing in our line list. This list is based on the average of a large number of spectral fit residuals for APOGEE observations spanning a wide range of stellar parameters.Alfred P. Sloan FoundationNational Science FoundationU.S. Department of Energy Office of ScienceJanos Bolyai Research Scholarship of the Hungarian Academy of SciencesSpanish Ministry of Economy and Competitiveness AYA-2011-27754, AYA-2014-58082-PRSF 14-50-00043McDonald Observator
Deep SDSS optical spectroscopy of distant halo stars I. Atmospheric parameters and stellar metallicity distribution
We analyze a sample of tens of thousands of spectra of halo turnoff stars,
obtained with the optical spectrographs of the Sloan Digital Sky Survey (SDSS),
to characterize the stellar halo population "in situ" out to a distance of a
few tens of kpc from the Sun. In this paper we describe the derivation of
atmospheric parameters. We also derive the overall stellar metallicity
distribution based on F-type stars observed as flux calibrators for the
Baryonic Oscillations Spectroscopic Survey (BOSS). Our analysis is based on an
automated method that determines the set of parameters of a model atmosphere
that reproduces each observed spectrum best. We used an optimization algorithm
and evaluate model fluxes by means of interpolation in a precomputed grid. In
our analysis, we account for the spectrograph's varying resolution as a
function of fiber and wavelength. Our results for early SDSS (pre-BOSS upgrade)
data compare well with those from the SEGUE Stellar Parameter Pipeline (SSPP),
except for stars with logg (cgs units) lower than 2.5. An analysis of stars in
the globular cluster M13 reveals a dependence of the inferred metallicity on
surface gravity for stars with logg < 2.5, confirming the systematics
identified in the comparison with the SSPP. We find that our metallicity
estimates are significantly more precise than the SSPP results. We obtain a
halo metallicity distribution that is narrower and more asymmetric than in
previous studies. The lowest gravity stars in our sample, at tens of kpc from
the Sun, indicate a shift of the metallicity distribution to lower abundances,
consistent with what is expected from a dual halo system in the Milky Way.Comment: 10 pages, 5 figures, Table 1 includes model ugriz magnitudes for
stars with different atmospheric parameters in electronic forma
Target Selection for the SDSS-IV APOGEE-2 Survey
APOGEE-2 is a high-resolution, near-infrared spectroscopic survey observing
roughly 300,000 stars across the entire sky. It is the successor to APOGEE and
is part of the Sloan Digital Sky Survey IV (SDSS-IV). APOGEE-2 is expanding
upon APOGEE's goals of addressing critical questions of stellar astrophysics,
stellar populations, and Galactic chemodynamical evolution using (1) an
enhanced set of target types and (2) a second spectrograph at Las Campanas
Observatory in Chile. APOGEE-2 is targeting red giant branch (RGB) and red
clump (RC) stars, RR Lyrae, low-mass dwarf stars, young stellar objects, and
numerous other Milky Way and Local Group sources across the entire sky from
both hemispheres. In this paper, we describe the APOGEE-2 observational design,
target selection catalogs and algorithms, and the targeting-related
documentation included in the SDSS data releases.Comment: 19 pages, 6 figures. Accepted to A
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