143 research outputs found
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An atomic physics viewpoint of stellar abundance analysis
textElement abundance trends with overall metallicity contain vital clues to the formation and evolution of the Galaxy. Abundances may be used to elucidate nucleosynthesis mechanisms and to ascertain rates of Galactic enrichment. To obtain accurate abundances, several crucial inputs such as high-quality spectroscopic observations, rigorous calculations of line transfer, and precise atomic data (e.g. transition probabilities) are necessary. The current work endeavors to improve abundance values for key elements with a four-fold approach: accumulation of hundreds of high-resolution stellar spectra in order to commence a systematic and thorough Manganese abundance derivation in cluster and halo field stars; re-determination of the neutral chromium oscillator strengths and application of this data to stellar abundance analyses; modification of a radiative line transfer code in order to yield accurate abundances from evolved stars; and semi-empirical derivation of transition probabilities to allow for the utilization of spectral features in the red visible and infrared wavelength ranges for abundance determinations. The first comprehensive investigation of manganese in globular clusters is done in this work. A subsolar Mn abundance trend for both halo globular cluster and field stars is found. The analysis shows that for the metallicity range -0.7>(Fe/H)>-2.7 stars of 19 globular clusters have a a mean relative abundance of = -0.37±0.01 (σ=0.10), a value in agreement with that of the field stars: = -0.36± 0.01 (σ=0.08). Remarkably, the ratio remains constant in both stellar populations over a 2 orders of magnitude span in metallicity. Next, the present study employed branching fraction measurements from Fourier transform spectra in conjunction with published radiative lifetimes to determine transition probabilities for 263 lines of neutral chromium. These laboratory values are used to derive a new photospheric abundance for the Sun: log [element of](Cr I)⊙= 5.64±0.01 (σ=0:07). In addition, oscillator strengths for singly-ionized chromium recently reported by the FERRUM Project are employed to determine: log [element of](Cr II)⊙ = 5.77±0.03 (σ= 0.13). No indications of departures from LTE are found in the neutral chromium abundances. The current work then takes advantage of the fact that transition metals exhibit relatively pure LS coupling and employs standard formulae to yield semi-empirical oscillator strengths. These data were then compared to experimental gf values in order to assess accuracy. Finally, this study undertakes a new abundance investigation of the RGB and RHB stars of the M15 globular cluster. A detailed examination of the both the metallicity and n capture elements is performed. This work appears to confirm that star-to-star abundance variations do occur among the M15 giants (which was initially observed by Sneden et al. 1997, 2000).Physic
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
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
Carbon-enhanced Metal-poor Stars in SDSS/SEGUE. I. Carbon Abundance Estimation and Frequency of CEMP Stars
We describe a method for the determination of stellar [C/Fe] abundance ratios
using low-resolution (R = 2000) stellar spectra from the SDSS and SEGUE. By
means of a star-by-star comparison with a set of SDSS/SEGUE spectra with
available estimates of [C/Fe] based on published high-resolution analyses, we
demonstrate that we can measure [C/Fe] from SDSS/SEGUE spectra with S/N > 15 to
a precision better than 0.35 dex. Using the measured carbon-to-iron abundance
ratios obtained by this technique, we derive the frequency of carbon-enhanced
stars ([C/Fe] > +0.7) as a function of [Fe/H], for both the SDSS/SEGUE stars
and other samples from the literature. We find that the differential frequency
slowly rises from almost zero to about 14% at [Fe/H] ~ -2.4, followed by a
sudden increase, by about a factor of three, to 39% from [Fe/H] ~ -2.4 to
[Fe/H] ~ -3.7. We also examine how the cumulative frequency of CEMP stars
varies across different luminosity classes. The giant sample exhibits a
cumulative CEMP frequency of 32% for [Fe/H] < -2.5, 31% for [Fe/H] < -3.0, and
33% for [Fe/H] < -3.5. For the main-sequence turnoff stars, we obtain a lower
cumulative CEMP frequency, around 10% for [Fe/H] < -2.5. The dwarf population
displays a large change in the cumulative frequency for CEMP stars below [Fe/H]
= -2.5, jumping from 15% for [Fe/H] < -2.5 to about 75% for [Fe/H] < -3.0. When
we impose a restriction with respect to distance from the Galactic mid-plane
(|Z| < 5 kpc), the frequency of the CEMP giants does not increase at low
metallicity ([Fe/H] < -2.5), but rather, decreases, due to the dilution of
C-rich material in stars that have undergone mixing with CNO-processed material
from their interiors. The frequency of CEMP stars near the main-sequence
turnoff, which are not expected to have experienced mixing, increases for
[Fe/H] < -3.0. [abridged]Comment: 19 pages, 10 figures, 6 tables, accepted for publication in AJ on
August 20, 201
Hubble Space Telescope Near-Ultraviolet Spectroscopy of the Bright CEMP-no Star BD+44 493
We present an elemental-abundance analysis, in the near-ultraviolet (NUV)
spectral range, for the extremely metal-poor star BD+44 493, a 9th magnitude
sub-giant with [Fe/H] = -3.8 and enhanced carbon, based on data acquired with
the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. This
star is the brightest example of a class of objects that, unlike the great
majority of carbon-enhanced metal-poor (CEMP) stars, does not exhibit
over-abundances of heavy neutron-capture elements (CEMP-no). In this paper, we
validate the abundance determinations for a number of species that were
previously studied in the optical region, and obtain strong upper limits for
beryllium and boron, as well as for neutron-capture elements from zirconium to
platinum, many of which are not accessible from ground-based spectra. The boron
upper limit we obtain for BD+44 493, logeps(B) < -0.70, the first such
measurement for a CEMP star, is the lowest yet found for very and extremely
metal-poor stars. In addition, we obtain even lower upper limits on the
abundances of beryllium, logeps(Be) < -2.3, and lead, logeps(Pb) < -0.23
([Pb/Fe] < +1.90), than those reported by previous analyses in the optical
range. Taken together with the previously measured low abundance of lithium,
the very low upper limits on Be and B suggest that BD+44 493 was formed at a
very early time, and that it could well be a bona-fide second-generation star.
Finally, the Pb upper limit strengthens the argument for non-s-process
production of the heavy-element abundance patterns in CEMP-no stars.Comment: 18 pages, 12 figures; accepted for publication in Ap
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