60 research outputs found
Reconstructing fossil sub-structures of the Galactic disk: clues from abundance patterns of old open clusters and moving groups
The long term goal of large-scale chemical tagging is to use stellar
elemental abundances as a tracer of dispersed substructures of the Galactic
disk. The identification of such lost stellar aggregates and the exploration of
their chemical properties will be key in understanding the formation and
evolution of the disk. Present day stellar structures such as open clusters and
moving groups are the ideal testing grounds for the viability of chemical
tagging, as they are believed to be the remnants of the original larger
starforming aggregates. Until recently, high accuracy elemental abundance
studies of open clusters and moving groups having been lacking in the
literature. In this paper we examine recent high resolution abundance studies
of open clusters to explore the various abundance trends and reasses the
prospects of large-scale chemical tagging.Comment: Accepted for publication in the Publications of the Astronomical
Society of Australi
A large stellar evolution database for population synthesis studies. IV. Integrated properties and spectra
This paper is the 4th in a series describing the latest additions to the
BaSTI stellar evolution database, which consists of a large set of homogeneous
models and tools for population synthesis studies. Here we present a new set of
low and high resolution synthetic spectra based on the BaSTI stellar models,
covering a large range of simple stellar populations (SSPs) for both scaled
solar and alpha-enhanced metal mixtures. This enables a completely consistent
study of the photometric and spectroscopic properties of both resolved and
unresolved stellar populations, and allows us to make detailed tests on their
integrated properties. Our low resolution spectra are suitable for deriving
broadband magnitudes and colors in any photometric system. These spectra cover
the full wavelength range (9-160000nm) and include all evolutionary stages up
to the end of AGB evolution. Our high resolution spectra are suitable for
studying the behaviour of line indices and we have tested them against a large
sample of Galactic globular clusters. We find that the range of ages, iron
abundances [Fe/H], and degree of alpha-enhancement predicted by the models
matches observed values very well. We have also tested the global consistency
of the BaSTI models by making detailed comparisons between ages and
metallicities derived from isochrone fitting to observed CMDs, and from line
index strengths, for the Galactic globular cluster 47Tuc and the open cluster
M67. For 47Tuc we find reasonable agreement between the 2 methods, within the
estimated errors. From the comparison with M67 we find non-negligible effects
on derived line indices caused by statistical fluctuations, which are a result
of the specific method used to populate an isochrone and assign appropriate
spectra to individual stars. (abridged)Comment: 21 pages including 13 figures. Accepted for publication in ApJ. Low
and high resolution integrated spectra, magnitudes, and mass-to-light ratios
will appear on the BaSTI website by 1st November 2008 - see
http://193.204.1.62/index.htm
Observing the products of stellar evolution in the old open cluster M67 with APOGEE
© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society Recent works have shown how the [C/N] ratio in stars after the first dredge-up (FDU) can be used as an age estimator in virtue of its dependence on stellar mass. For this purpose, precise predictions of the surface chemical composition before and after the mixing takes place in the convective envelope of subgiant stars are necessary. Stellar evolution models can provide us with such predictions, although a comparison with objects of known age is needed for calibration. Open clusters are excellent test cases, as they represent a single stellar population for which the age can be derived through, e.g. isochrone fitting. In this study, we present a detailed analysis of stars belonging to the well-known open cluster M67 observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey in the twelfth data release of the Sloan Digital Sky Survey and whose chemical properties were derived with the APOGEE Stellar Parameters and Chemical Abundances Pipeline. We find that the [C/N] abundance of subgiant branch stars is overestimated by âŒ0.2 dex due to an offset in the determination of the [N/Fe] abundance. Stars on the red giant branch and red clump are shown not to be affected by this offset. We derive [C/N]FDU = â0.46 ± 0.03 dex, which poses a strong constraint on calibrations of [C/N]FDU as age indicator. We also do not find any clear signature of additional chemical mixing processes that set in after the red giant branch bump. The results obtained for M67 indicate the importance of conducting high-resolution spectroscopic studies of open clusters of different ages in order to establish an accurate age-dating method for field stars
An Examination of Recent Transformations to the BV(RI)_C Photometric System from the Perspective of Stellar Models for Old Stars
Isochrones for ages > 4 Gyr and metallicities in the range -2.5 < [Fe/H] <
+0.3 that take the diffusion of helium and recent advances in stellar physics
into account are compared with observations in the Johnson-Cousins BV(RI)_C
photometric system for several open and globular star clusters. The adopted
color-Teff relations include those which we have derived from the latest MARCS
model atmospheres and empirical transformations for dwarf and subgiant stars
given by Casagrande et al (2010, A&A, 512, 54; CRMBA). Those reported by
VandenBerg & Clem (2003, AJ, 126, 778) have also been considered, mainly to
resolve some outstanding questions concerning them. Remarkably, when the
subdwarfs in the CRMBA data set that have sigma_pi/pi < 0.15 are superimposed
on a set of 12 Gyr isochrones spanning a wide range in [Fe/H], the inferred
metallicities and effective temperatures agree, in the mean, with those given
by CRMBA to within +/- 0.05 dex and +/- 10 K, respectively. Thus the hot Teff
scale derived by CRMBA is nearly identical with that predicted by stellar
models and consequently, there is excellent consistency between theory and
observations on the H-R diagram and the different color-magnitude diagrams
considered in this investigation. To obtain similar consistency, the colors
obtained from the MARCS and VandenBerg & Clem B-V vs. Teff relations for
metal-poor dwarf stars should be adjusted to the red by 0.02-0.03 mag. In
general, isochrones that employ the CRMBA transformations provide reasonably
good fits to our BV(RI)_C photometry for main-sequence stars in the globular
clusters 47 Tuc, M3, M5, M92 and NGC 1851 - but not the cluster giants (when
adopting the synthetic MARCS colors). We speculate that differences between the
actual heavy-element mixtures and those assumed in the theoretical models may
be the primary cause of this difficulty.Comment: To appear in 2010, AJ, 140, 102
Stellar Nucleosynthesis in the Hyades Open Cluster
We report a comprehensive light element (Li, C, N, O, Na, Mg, and Al)
abundance analysis of three solar-type main sequence (MS) dwarfs and three red
giant branch (RGB) clump stars in the Hyades open cluster using high-resolution
and high signal-to-noise spectroscopy. For each group (MS or RGB), the CNO
abundances are found to be in excellent star-to-star agreement. Our results
confirm that the giants have undergone the first dredge-up and that material
processed by the CN cycle has been mixed to the surface layers. The observed
abundances are compared to predictions of a standard stellar model based on the
Clemson-American University of Beirut (CAUB) stellar evolution code. The model
reproduces the observed evolution of the N and O abundances, as well as the
previously derived 12C/13C ratio, but it fails to predict by a factor of 1.5
the observed level of 12C depletion. Li abundances are derived to determine if
non-canonical extra mixing has occurred in the Hyades giants. The Li abundance
of the giant gamma Tau is in good accord with the predicted level of surface Li
dilution, but a ~0.35 dex spread in the giant Li abundances is found and cannot
be explained by the stellar model. Possible sources of the spread are
discussed; however, it is apparent that the differential mechanism responsible
for the Li dispersion must be unrelated to the uniformly low 12C abundances of
the giants. Na, Mg, and Al abundances are derived as an additional test of our
stellar model. All three elements are found to be overabundant by 0.2-0.5 dex
in the giants relative to the dwarfs. Such large enhancements of these elements
are not predicted by the stellar model, and non-LTE effects significantly
larger (and, in some cases, of opposite sign) than those implied by extant
literature calculations are the most likely cause.Comment: 40 pages, 6 figures, 6 tables; accepted by Ap
M67-1194, an unusually Sun-like solar twin in M67
The rich open cluster M67 is known to have a chemical composition close to
solar, and an age around 4Gyr. It thus offers the opportunity to check our
understanding of the physics and the evolution of solar-type stars in a cluster
environment. We present the first spectroscopic study at high resolution,
R~50,000, of the potentially best solar twin, M67-1194, identified among
solar-like stars in M67. Based on a pre-selection of solar-twin candidates
performed at medium resolution by Pasquini et al. (2008), we explore the
chemical-abundance similarities and differences between M67-1194 and the Sun,
using VLT/FLAMES-UVES. Working with a solar twin in the framework of a
differential analysis, we minimize systematic model errors in the abundance
analysis compared to previous studies which utilized more evolved stars to
determine the metallicity of M67. We find M67-1194 to have stellar parameters
indistinguishable from the solar values, with the exception of the overall
metallicity which is slightly super-solar ([Fe/H]=0.023 +/- 0.015). An age
determination based on evolutionary tracks yields 4.2 +/- 1.6Gyr. Most
surprisingly, we find the chemical abundance pattern to closely resemble the
solar one, in contrast to most known solar twins in the solar neighbourhood. We
confirm the solar-twin nature of M67-1194, the first solar twin known to belong
to a stellar association. This fact allows us to put some constraints on the
physical reasons for the seemingly systematic departure of M67-1194 and the Sun
from most known solar twins regarding chemical composition. We find that
radiative dust cleansing by nearby luminous stars may be the explanation for
the peculiar composition of both the Sun and M67-1194, but alternative
explanations are also possible. The chemical similarity between the Sun and
M67-1194 also suggests that the Sun once formed in a cluster like M67
The Gaia-ESO Survey: Detailed Abundances in the Metal-poor Globular Cluster NGC 4372
We present the abundance analysis for a sample of 7 red giant branch stars in
the metal-poor globular cluster NGC 4372 based on UVES spectra acquired as part
of the Gaia-ESO Survey. This is the first extensive study of this cluster from
high resolution spectroscopy. We derive abundances of O, Na, Mg, Al, Si, Ca,
Sc, Ti, Fe, Cr, Ni, Y, Ba, and La. We find a metallicity of [Fe/H] = -2.19
0.03 and find no evidence for a metallicity spread. This metallicity
makes NGC 4372 one of the most metal-poor galactic globular clusters. We also
find an {\alpha}-enhancement typical of halo globular clusters at this
metallicity. Significant spreads are observed in the abundances of light
elements. In particular we find a Na-O anti-correlation. Abundances of O are
relatively high compared with other globular clusters. This could indicate that
NGC 4372 was formed in an environment with high O for its metallicity. A Mg-Al
spread is also present which spans a range of more than 0.5 dex in Al
abundances. Na is correlated with Al and Mg abundances at a lower significance
level. This pattern suggests that the Mg-Al burning cycle is active. This
behavior can also be seen in giant stars of other massive, metal-poor clusters.
A relation between light and heavy s-process elements has been identified.Comment: 14 pages, 14 figures, accepted for publication in A&
Chemical Inhomogeneities in the Milky Way Stellar Halo
We have compiled a sample of 699 stars from the recent literature with
detailed chemical abundance information (spanning -4.2 < [Fe/H] < +0.3), and we
compute their space velocities and Galactic orbital parameters. We identify
members of the inner and outer stellar halo populations in our sample based
only on their kinematic properties and then compare the abundance ratios of
these populations as a function of [Fe/H]. In the metallicity range where the
two populations overlap (-2.5 < [Fe/H] < -1.5), the mean [Mg/Fe] of the outer
halo is lower than the inner halo by ~0.1 dex. For [Ni/Fe] and [Ba/Fe], the
star-to-star abundance scatter of the inner halo is consistently smaller than
in the outer halo. The [Na/Fe], [Y/Fe], [Ca/Fe], and [Ti/Fe] ratios of both
populations show similar means and levels of scatter. Our inner halo population
is chemically homogeneous, suggesting that a significant fraction of the Milky
Way stellar halo originated from a well-mixed ISM. In contrast, our outer halo
population is chemically diverse, suggesting that another significant fraction
of the Milky Way stellar halo formed in remote regions where chemical
enrichment was dominated by local supernova events. We find no abundance trends
with maximum radial distance from the Galactic center or maximum vertical
distance from the Galactic disk. We also find no common kinematic signature for
groups of metal-poor stars with peculiar abundance patters, such as the
alpha-poor stars or stars showing unique neutron-capture enrichment patterns.
Several stars and dSph systems with unique abundance patterns spend the
majority of their time in the distant regions of the Milky Way stellar halo,
suggesting that the true outer halo of the Galaxy may have little resemblance
to the local stellar halo.Comment: Accepted for publication in AJ. Full tables available upon reques
Two distinct halo populations in the solar neighborhood. II. Evidence from stellar abundances of Mn, Cu, Zn, Y, and Ba
A previous study (Nissen & Schuster 2010) of 94 dwarf stars with -1.6 <
[Fe/H] < -0.4 has revealed the existence of two distinct halo populations with
a systematic difference in [alpha/Fe] at a given metallicity. In continuation
of that work, abundances of Mn, Cu, Zn, Y, and Ba are determined for the same
sample of stars. Equivalent widths of atomic lines are measured from high
resolution VLT/UVES and NOT/FIES spectra and used to derive precise abundance
ratios from an LTE analysis based on MARCS model atmospheres. Systematic
differences between the `high-alpha' and `low-alpha' halo populations are found
for [Cu/Fe], [Zn/Fe], and [Ba/Y], whereas there is no significant difference in
the case of [Mn/Fe]. At a given metallicity, [Cu/Fe] shows a large scatter that
is closely correlated with a corresponding scatter in [Na/Fe] and [Ni/Fe]. The
metallicity trends of [Cu/Fe], [Zn/Fe], and [Ba/Y] can be explained from
existing nucleosynthesis calculations if the high-alpha stars formed in regions
with such a high star formation rate that only massive stars and Type II
supernovae contributed to the chemical enrichment. The low-alpha stars, on the
other hand, most likely originate from systems with a slower chemical
evolution, characterized by additional enrichment from Type Ia supernovae and
low-mass AGB stars.Comment: Accepted for publication in A&
The Gaia-ESO Survey: the chemical structure of the Galactic discs from the first internal data release
Most high-resolution spectroscopic studies of the Galactic discs were mostly
confined to objects in the solar vicinity. Here we aim at enlarging the volume
in which individual chemical abundances are used to characterise both discs,
using the first internal data release of the Gaia-ESO survey. We derive and
discuss the abundances of eight elements (Mg, Al, Si, Ca, Ti, Fe, Cr, Ni, and
Y). The trends of these elemental abundances with iron are very similar to
those in the solar neighbourhood. We find a natural division between alpha-rich
and alpha-poor stars, best seen in the bimodality of the [Mg/M] distributions
in bins of metallicity, which we attribute to thick- and thin-disc sequences,
respectively. With the possible exception of Al, the observed dispersion around
the trends is well described by the expected errors, leaving little room for
astrophysical dispersion. Using previously derived distances from Recio-Blanco
et al. (2014b), we further find that the thick-disc is more extended vertically
and is more centrally concentrated towards the inner Galaxy than the thin-disc,
which indicates a shorter scale-length. We derive the radial and vertical
gradients in metallicity, iron, four alpha-element abundances, and Al for the
two populations, taking into account the identified correlation between R_GC
and |Z|. Radial metallicity gradient is found in the thin disc. The positive
radial individual [alpha/M] gradients found are at variance from the gradients
observed in the RAVE survey. The thin disc also hosts a negative vertical
metallicity gradient, accompanied by positive individual [alpha/M] and [Al/M]
gradients. The thick-disc, presents no radial metallicity gradient, a shallower
vertical metallicity gradient than the thin-disc, an alpha-elements-to-iron
radial gradient in the opposite sense than that of the thin disc, and positive
vertical individual [alpha/M] and [Al/M] gradients.Comment: 24 pages, 10 figure
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