61 research outputs found
Detailed chemical abundance analysis of the thick disk star cluster Gaia 1
Star clusters, particularly those objects in the disk-bulge-halo interface
are as of yet poorly charted, albeit carrying important information about the
formation and the structure of the Milky Way. Here, we present a detailed
chemical abundance study of the recently discovered object Gaia 1. Photometry
has previously suggested it as an intermediate-age, moderately metal-rich
system, although the exact values for its age and metallicity remained
ambiguous in the literature. We measured detailed chemical abundances of 14
elements in four red giant members, from high-resolution (R=25000) spectra that
firmly establish Gaia 1 as an object associated with the thick disk. The
resulting mean Fe abundance is 0.03(stat.)0.10(sys.) dex, which
is more metal-poor than indicated by previous spectroscopy from the literature,
but it is fully in line with values from isochrone fitting. We find that Gaia 1
is moderately enhanced in the -elements, which allowed us to
consolidate its membership with the thick disk via chemical tagging. The
cluster's Fe-peak and neutron-capture elements are similar to those found
across the metal-rich disks, where the latter indicate some level of
-process activity. No significant spread in iron nor in other heavy elements
was detected, whereas we find evidence of light-element variations in Na, Mg,
and Al. Nonetheless, the traditional Na-O and Mg-Al (anti-)correlations,
typically seen in old globular clusters, are not seen in our data. This
confirms that Gaia 1 is rather a massive and luminous open cluster than a
low-mass globular cluster. Finally, orbital computations of the target stars
bolster our chemical findings of Gaia 1's present-day membership with the thick
disk, even though it remains unclear, which mechanisms put it in that place.Comment: 11 pages, 11 figures, accepted for publication in Astronomy &
Astrophysics. Some figure sizes reduce
Abundances and kinematics of carbon-enhanced metal-poor stars in the Galactic halo*; A new classification scheme based on Sr and Ba
Carbon-enhanced metal-poor (CEMP) stars span a wide range of stellar
populations, from bona fide second-generation stars to later forming stars that
provide excellent probes of, e.g., binary mass transfer. Here we analyse 11
metal-poor stars of which 10 are CEMP stars. Based on high signal-to-noise
(SNR) X-Shooter spectra, we derive abundances of 20 elements (C, N, O, Na, Mg,
Ca, Sc, Ti, Cr, Mn, Fe, Ni, Sr, Y, Ba, La, Ce, Pr, Nd, Eu). From the high SNR
spectra, we trace the chemical contribution of the rare earth elements (REE)
from various production sites, finding a preference for metal-poor low-mass AGB
stars of 1.5Mo in CEMP-s stars, while CEMP-r/s stars may indicate a more
massive AGB contribution (2-5Mo). A contribution from the r-process - possibly
from neutron star mergers (NSM), is also detectable in the REE abundances,
especially in the CEMP-r/s. Combining spectra with Gaia DR2 astrometric data
indicates that all but one star in our sample (and most literature stars)
belong to the Galactic halo. They exhibit a median orbital eccentricity of 0.7,
and are found on both pro- and retrograde orbits. The orbital parameters of
CEMP-no and CEMP4s stars are remarkably similar in the 98 stars we study. A
special CEMP-no star, with very low Sr and Ba content, possesses the most
eccentric orbit among the stars in our sample, passing close to the Galactic
centre. Finally, we propose an improved scheme to sub-classify the CEMP stars,
making use of the SrBa ratio, which can also be used to separate very
metal-poor stars from CEMP stars in 93 stars in the metallicity range
[Fe/H]. The Sr/Ba ratio can also be used for distinguishing
CEMP-s,-r/s and -no stars. The Sr/Ba ratio is also a powerful astro-nuclear
indicator, as AGB stars exhibit very different Sr/Ba ratios, compared to fast
rotating massive stars and NSM, and it is fairly unbiased by NLTE and 3D
corrections.(abridged)Comment: 15 pages, 4 pages appendix, 11 figures, accepted for publication in
A&
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
Observational Constraints on First-Star Nucleosynthesis. I. Evidence for Multiple Progenitors of CEMP-no Stars
We investigate anew the distribution of absolute carbon abundance, (C) (C), for carbon-enhanced metal-poor (CEMP) stars in the halo of
the Milky Way, based on high-resolution spectroscopic data for a total sample
of 305 CEMP stars. The sample includes 147 CEMP- (and CEMP-r/s) stars, 127
CEMP-no stars, and 31 CEMP stars that are unclassified, based on the currently
employed [Ba/Fe] criterion. We confirm previous claims that the distribution of
(C) for CEMP stars is (at least) bimodal, with newly determined peaks
centered on (C) (the high-C region) and (C) (the low-C
region). A very high fraction of CEMP- (and CEMP-r/s) stars belong to the
high-C region, while the great majority of CEMP-no stars reside in the low-C
region. However, there exists complexity in the morphology of the (C)-[Fe/H]
space for the CEMP-no stars, a first indication that more than one class of
first-generation stellar progenitors may be required to account for their
observed abundances. The two groups of CEMP-no stars we identify exhibit
clearly different locations in the (Na)-(C) and (Mg)-(C) spaces,
also suggesting multiple progenitors. The clear distinction in (C) between
the CEMP- (and CEMP-) stars and the CEMP-no stars appears to be $as\
successfullikely\ more\ astrophysically\ fundamental$, for the
separation of these sub-classes as the previously recommended criterion based
on [Ba/Fe] (and [Ba/Eu]) abundance ratios. This result opens the window for its
application to present and future large-scale low- and medium-resolution
spectroscopic surveys.Comment: 26pages, 7 figures, and 3 Tables ; Accepted for publication in ApJ;
added more data and corrected minor inconsistencies existed in the compiled
data of the previous studie
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
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
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