63 research outputs found
LOTUS: A (non-)LTE Optimization Tool for Uniform derivation of Stellar atmospheric parameters
Precise fundamental atmospheric stellar parameters and abundance
determination of individual elements in stars are important for all stellar
population studies. Non-Local Thermodynamic Equilibrium (Non-LTE; hereafter
NLTE) models are often important for such high precision, however, can be
computationally complex and expensive, which renders the models less utilized
in spectroscopic analyses. To alleviate the computational burden of such
models, we developed a robust 1D, LTE and NLTE fundamental atmospheric stellar
parameter derivation tool, , to determine the effective
temperature , surface gravity , metallicity
\mbox{[Fe/H]} and microturbulent velocity for FGK type
stars, from equivalent width (EW) measurements of Fe I and Fe II lines. We
utilize a generalized curve of growth method to take into account the EW
dependencies of each Fe I and Fe II line on the corresponding atmospheric
stellar parameters. A global differential evolution optimization algorithm is
then used to derive the optimized fundamental parameters. Additionally,
can determine precise uncertainties for each stellar parameter
using a Markov Chain Monte Carlo (MCMC) algorithm. We test and apply
on a sample of benchmark stars, as well as stars with
available asteroseismic surface gravities from the K2 survey, and metal-poor
stars from -process Alliance (RPA) survey. We find very good agreement
between our NLTE-derived parameters in to non-spectroscopic
values within K and dex for
benchmark stars. We provide open access of our code, as well as of the
interpolated pre-computed NLTE EW grids available on Github, and documentation
with working examples on Readthedocs.Comment: 42 pages, 12 figures, 3 tables, accepted for publication in AJ. For
Code on Github, see: https://github.com/Li-Yangyang/LOTUS. For Code
Documentation, see: https://lotus-nlte.readthedocs.io/en/latest
Ultra-metal-poor Stars: Spectroscopic Determination of Stellar Atmospheric Parameters Using Iron Non-LTE Line Abundances
We present new ultra-metal-poor stars parameters with [Fe/H] < -4.0 based on line-by-line non-local thermodynamic equilibrium (NLTE) abundances using an up-to-date iron model atom with a new recipe for non-elastic hydrogen collision rates. We study the departures from LTE in their atmospheric parameters and show that they can grow up to ∼1.00 dex in [Fe/H], ∼150 K in T eff and ∼0.5 dex in log g toward the lowest metallicities. Accurate NLTE atmospheric stellar parameters, in particular [Fe/H] being significantly higher, are the first step to eventually providing full NLTE abundance patterns that can be compared with Population III supernova nucleosynthesis yields to derive properties of the first stars. Overall, this maximizes the potential of these likely second-generation stars to investigate the early universe and how the chemical elements were formed
CHEMICAL DIVERSITY IN THE ULTRA-FAINT DWARF GALAXY TUCANA II
We present the first detailed chemical abundance study of the ultra-faint dwarf galaxy Tucana II, based on high-resolution Magellan/MIKE spectra of four red giant stars. The metallicities of these stars range from [Fe/H] = −3.2 to −2.6, and all stars are low in neutron-capture abundances ([Sr/Fe] and [Ba/Fe] < −1). However, a number of anomalous chemical signatures are present. One star is relatively metal-rich ([Fe/H] = −2.6) and shows [Na, α, Sc/Fe] < 0, suggesting an extended star formation history with contributions from AGB stars and SNe Ia. Two stars with [Fe/H] < −3 are mildly carbon-enhanced ([C/Fe] ~ 0.7) and may be consistent with enrichment by faint supernovae, if such supernovae can produce neutron-capture elements. A fourth star with [Fe/H] = −3 is carbon-normal, and exhibits distinct light element abundance ratios from the carbon-enhanced stars. This carbon-normal star implies that at least two distinct nucleosynthesis sources, both possibly associated with Population III stars, contributed to the early chemical enrichment of this galaxy. Despite its very low luminosity, Tucana II shows a diversity of chemical signatures that preclude it from being a simple "one-shot" first galaxy yet still provide a window into star and galaxy formation in the early universe.National Science Foundation (U.S.) (AST- 1255160)National Science Foundation (U.S.) (PHY-1430152
The R-Process Alliance: A Comprehensive Abundance Analysis of HD 222925, a Metal-Poor Star with an Extreme R-Process Enhancement of [Eu/H] = -0.14
We present a detailed abundance analysis of the bright (V = 9.02), metal-poor
([Fe/H] = -1.47 +/- 0.08) field red horizontal-branch star HD 222925, which was
observed as part of an ongoing survey by the R-Process Alliance. We calculate
stellar parameters and derive abundances for 46 elements based on 901 lines
examined in a high-resolution optical spectrum obtained using the Magellan
Inamori Kyocera Echelle spectrograph. We detect 28 elements with 38 <= Z <= 90;
their abundance pattern is a close match to the Solar r-process component. The
distinguishing characteristic of HD 222925 is an extreme enhancement of
r-process elements ([Eu/Fe] = +1.33 +/- 0.08, [Ba/Eu] = -0.78 +/- 0.10) in a
moderately metal-poor star, so the abundance of r-process elements is the
highest ([Eu/H] = -0.14 +/- 0.09) in any known r-process-enhanced star. The
abundance ratios among lighter (Z <= 30) elements are typical for metal-poor
stars, indicating that production of these elements was dominated by normal
Type II supernovae, with no discernible contributions from Type Ia supernovae
or asymptotic giant branch stars. The chemical and kinematic properties of HD
222925 suggest it formed in a low-mass dwarf galaxy, which was enriched by a
high-yield r-process event before being disrupted by interaction with the Milky
Way.Comment: Accepted for publication in the Astrophysical Journal (17 pages, 4
figures, 3 tables
The R-Process Alliance: Chemical Abundances for a Trio of R-Process-Enhanced Stars -- One Strong, One Moderate, One Mild
We present detailed chemical abundances of three new bright (V ~ 11),
extremely metal-poor ([Fe/H] ~ -3.0), r-process-enhanced halo red giants based
on high-resolution, high-S/N Magellan/MIKE spectra. We measured abundances for
20-25 neutron-capture elements in each of our stars. J1432-4125 is among the
most r-process rich r-II stars, with [Eu/Fe]= +1.44+-0.11. J2005-3057 is an r-I
star with [Eu/Fe] = +0.94+-0.07. J0858-0809 has [Eu/Fe] = +0.23+-0.05 and
exhibits a carbon abundance corrected for evolutionary status of [C/Fe]_corr =
+0.76, thus adding to the small number of known carbon-enhanced r-process
stars. All three stars show remarkable agreement with the scaled solar
r-process pattern for elements above Ba, consistent with enrichment of the
birth gas cloud by a neutron star merger. The abundances for Sr, Y, and Zr,
however, deviate from the scaled solar pattern. This indicates that more than
one distinct r-process site might be responsible for the observed
neutron-capture element abundance pattern. Thorium was detected in J1432-4125
and J2005-3057. Age estimates for J1432-4125 and J2005-3057 were adopted from
one of two sets of initial production ratios each by assuming the stars are
old. This yielded individual ages of 12+-6 Gyr and 10+-6 Gyr, respectively.Comment: 30 pages, includes a long table, 5 figure
Uranium Abundances and Ages of -process Enhanced Stars with Novel U II Lines
The ages of the oldest stars shed light on the birth, chemical enrichment,
and chemical evolution of the Universe. Nucleocosmochronometry provides an
avenue to determining the ages of these stars independent from stellar
evolution models. The uranium abundance, which can be determined for metal-poor
-process enhanced (RPE) stars, has been known to constitute one of the most
robust chronometers known. So far, U abundance determination has used a
U II line at \r{A}. Consequently, U abundance has been
reliably determined for only five RPE stars. Here, we present the first
homogeneous U abundance analysis of four RPE stars using two novel U II lines
at \r{A} and \r{A}, in addition to the canonical
\r{A} line. We find that the U II lines at \r{A}
and \r{A} are reliable and render U abundances in agreement with
the U abundance, for all the stars. We, thus, determine revised U
abundances for RPE stars, 2MASS J09544277+5246414, RAVE J203843.2-002333, HE
1523-0901, and CS 31082-001, using multiple U II lines. We also provide
nucleocosmochronometric ages of these stars based on the newly derived U, Th,
and Eu abundances. The results of this study open up a new avenue to reliably
and homogeneously determine U abundance for a significantly larger number of
RPE stars. This will, in turn, enable robust constraints on the
nucleocosmochronometric ages of RPE stars, which can be applied to understand
the chemical enrichment and evolution in the early Universe, especially of
-process elements.Comment: Resubmitted to Ap
Spectroscopy of the Young Stellar Association Price-Whelan 1: Origin in the Magellanic Leading Arm and Constraints on the Milky Way Hot Halo
We report spectroscopic measurements of stars in the recently discovered
young stellar association Price-Whelan 1 (PW 1), which was found in the
vicinity of the Leading Arm (LA) of the Magellanic Stream. We obtained
Magellan+MIKE high-resolution spectra of the 28 brightest stars in PW 1 and
used The Cannon to determine their stellar parameters. We find that the mean
metallicity of PW 1 is [Fe/H]=-1.23 with a small scatter of 0.06 dex and the
mean radial velocity is Vhelio=276.7 km/s with a dispersion of 11.0 km/s. Our
results are consistent in Teff, logg, and [Fe/H] with the young and metal-poor
characteristics (116 Myr and [Fe/H]=-1.1) determined for PW 1 from our
discovery paper. We find a strong correlation between the spatial pattern of
the PW 1 stars and the LA II gas with an offset of -10.15 deg in L_MS and +1.55
deg in B_MS. The similarity in metallicity, velocity, and spatial patterns
indicates that PW 1 likely originated in LA II. We find that the spatial and
kinematic separation between LA II and PW 1 can be explained by ram pressure
from Milky Way gas. Using orbit integrations that account for the LMC and MW
halo and outer disk gas, we constrain the halo gas density at the orbital
pericenter of PW 1 to be n_halo (17 kpc) = 2.7 (3) x 10^-3 atoms/cm^3 and the
disk gas density at the midplane at 20 kpc to be n_disk (20 kpc,0) = 6.0 (1.8)
x 10^-2 atoms/cm^3. We, therefore, conclude that PW 1 formed from the LA II of
the Magellanic Stream, making it a powerful constraint on the Milky
Way-Magellanic interaction.Comment: 18 pages, 13 figures, 1 table, submitted to Ap
The R-process Alliance: First Magellan/MIKE Release from the Southern Search for R-Process-enhanced Stars
Extensive progress has been recently made into our understanding of heavy
element production via the -process in the Universe, specifically with the
first observed neutron star binary merger (NSBM) event associated with the
gravitational wave signal detected by LIGO, GW170817. The chemical abundance
patterns of metal-poor -process-enhanced stars provides key evidence into
the dominant site(s) of the -process, and whether NSBMs are sufficiently
frequent or prolific -process sources to be responsible for the majority of
-process material in the Universe. We present atmospheric stellar parameters
(using a Non-Local Thermodynamic Equilibrium analysis) and abundances from a
detailed analysis of 141 metal-poor stars, carried out as part of the
-Process Alliance (RPA) effort. We obtained high-resolution "snapshot"
spectroscopy of the stars using the MIKE spectrograph on the 6.5m Magellan Clay
telescope at Las Campanas Observatory in Chile. We find 10 new highly enhanced
-II (with [Eu/Fe] ), 62 new moderately enhanced -I (
[Eu/Fe] ) and 17 new limited- ([Eu/Fe] ) stars. Among
those, we find 17 new carbon-enhanced metal-poor (CEMP) stars, of which five
are CEMP-no. We also identify one new -process-enhanced ([Ba/Eu ]),
and five new ( [Ba/Eu] ) stars. In the process, we
discover a new ultra metal-poor (UMP) star at [Fe/H]=4.02. One of the -II
stars shows a deficit in and Fe-peak elements, typical of dwarf galaxy
stars. Our search for -process-enhanced stars by RPA efforts, has already
roughly doubled the known -process sample.Comment: 17 pages, 9 figures, 6 tables, Accepted for publication in Ap
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