118 research outputs found
An Abundance Analysis of Bright Giants in the Globular Cluster NGC 1851
We present the chemical compositions for eight bright giants in the globular cluster NGC 1851. Our analysis reveals large star-to-star abundance variations and correlations of the light elements O, Na, and Al, a feature found in every well-studied globular cluster. However, NGC 1851 also exhibits large star-to-star abundance variations of the s-process elements Zr and La. These s-process elements are correlated with Al and anticorrelated with O. Furthermore, the Zr and La abundances appear to cluster around two distinct values. A recent study revealed a double subgiant branch in NGC 1851. Our data reinforce the notion that there are two stellar populations in NGC 1851 and indicate that this cluster has experienced a complicated formation history with similarities to ω Centauri
Nitrogen abundances in giant stars of the globular cluster NGC 6752
We present N abundances for 21 bright giants in the globular cluster NGC 6752
based on high-resolution UVES spectra of the 3360A NH lines. We confirm that
the Stromgren c1 index traces the N abundance and find that the star-to-star N
abundance variation is 1.95 dex, at the sample's luminosity. We find
statistically significant correlations, but small amplitude variations, between
the abundances of N and alpha-, Fe-peak, and s-process elements. Analyses using
model atmospheres with appropriate N, O, Na, and Al abundances would
strengthen, rather than mute, these correlations. If the small variations of
heavy elements are real, then the synthesis of the N anomalies must take place
in stars which also synthesize alpha-, Fe-peak, and s-process elements. These
correlations offer support for contributions from both AGB and massive stars to
the globular cluster abundance anomalies.Comment: Accepted for publication in Ap
Exploiting the open clusters in the Kepler and CoRoT fields
The open clusters in the Kepler and CoRoT fields potentially provide tight
constraints for tests of stellar models and observational methods because they
allow a combination of complementary methods. We are in the process of identi-
fying and measuring parameters for detached eclipsing binaries (dEBs) in the
open clusters in the Kepler and CoRoT fields. We make use of measurements of
dEBs in the clusters to test the accuracy of asteroseismic scaling relations
for mass. We are able to provide strong indications that the asteroseismic
scaling relations over- estimate the stellar mass, but we are not yet able to
distinguish between different proposed corrections from the literature. We
argue how our ongoing measurements of more dEBs in more clusters, complemented
by dEBs in the field, should be able to break the degeneracy. We also briefly
describe how we can identify cluster stars that have evolved through
non-standard evolution by making use of ensemble asteroseismology.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
An abundance analysis of bright giants in the globular cluster NGC 1851
We present the chemical compositions for eight bright giants in the globular
cluster NGC 1851. Our analysis reveals large star-to-star abundance variations
and correlations of the light elements O, Na, and Al, a feature found in every
well studied globular cluster. However, NGC 1851 also exhibits large
star-to-star abundance variations of the s-process elements Zr and La. These
s-process elements are correlated with Al, and anticorrelated with O.
Furthermore, the Zr and La abundances appear to cluster around two distinct
values. A recent study revealed a double subgiant branch in NGC 1851. Our data
reinforce the notion that there are two stellar populations in NGC 1851 and
indicate that this cluster has experienced a complicated formation history with
similarities to omega Centauri.Comment: Accepted for publication in ApJ Letter
CNO abundances in the globular clusters NGC 1851 and NGC 6752
We measure the C+N+O abundance sum in red giant stars in two Galactic globular clusters,
NGC 1851 and NGC 6752. NGC 1851 has a split subgiant branch which could be due to different
ages or C+N+O content while NGC 6752 is representative of the least complex globular
clusters. For NGC 1851 and NGC 6752, we obtain average values of A(C+N+O) = 8.16 ±
0.10 (σ = 0.34) and 7.62 ± 0.02 (σ = 0.06), respectively. When taking into account the
measurement errors, we find a constant C+N+O abundance sum in NGC 6752. The C+N+O
abundance dispersion is only 0.06 dex, and such a result requires that the source of the light
element abundance variations does not increase the C+N+O sum in this cluster. For NGC
1851, we confirm a large spread in C+N+O. In this cluster, the anomalous RGB has a higher
C+N+O content than the canonical RGB by a factor of 4 (∼0.6 dex). This result lends further
support to the idea that the two subgiant branches in NGC 1851 are roughly coeval, but with
different CNO abundances
Radial distributions of sub-populations in the globular cluster M15: a more centrally concentrated primordial population
We examine the radial distributions of stellar populations in the globular
cluster (GC) M15, using HST/WFC3 photometry of red giants in the
nitrogen-sensitive F343N-F555W color. Surprisingly, we find that giants with
"primordial" composition (i.e., N abundances similar to those in field stars)
are the most centrally concentrated within the WFC3 field. We then combine our
WFC3 data with SDSS u, g photometry and find that the trend reverses for radii
>1' (3 pc) where the ratio of primordial to N-enhanced giants increases
outwards, as already found by Lardo et al. The ratio of primordial to enriched
stars thus has a U-shaped dependency on radius with a minimum near the
half-light radius. N-body simulations show that mass segregation might produce
a trend resembling the observed one, but only if the N-enhanced giants are
~0.25 Mo less massive than the primordial giants, which requires extreme He
enhancement (Y~0.40). However, such a large difference in Y is incompatible
with the negligible optical color differences between primordial and enriched
giants which suggest Delta Y < 0.03 and thus a difference in turn-off mass of
Delta M < 0.04 Mo between the different populations. The radial trends in M15
are thus unlikely to be of dynamical origin and presumably reflect initial
conditions, a result that challenges all current GC formation scenarios. We
note that population gradients in the central regions of GCs remain poorly
investigated and may show a more diverse behavior than hitherto thought.Comment: 16 pages, accepted for Ap
Spectroscopic Study of the Open Cluster NGC 6811
The NASA space telescope Kepler has provided unprecedented time-series
observations which have revolutionised the field of asteroseismology, i.e. the
use of stellar oscillations to probe the interior of stars. The Kepler-data
include observations of stars in open clusters, which are particularly
interesting for asteroseismology. One of the clusters observed with Kepler is
NGC 6811, which is the target of the present paper. However, apart from
high-precision time-series observations, sounding the interiors of stars in
open clusters by means of asteroseismology also requires accurate and precise
atmospheric parameters as well as cluster membership indicators for the
individual stars. We use medium-resolution (R~25,000) spectroscopic
observations, and three independent analysis methods, to derive effective
temperatures, surface gravities, metallicities, projected rotational velocities
and radial velocities, for 15 stars in the field of the open cluster NGC 6811.
We discover two double-lined and three single-lined spectroscopic binaries.
Eight stars are classified as either certain or very probable cluster members,
and three stars are classified as non-members. For four stars, cluster
membership could not been assessed. Five of the observed stars are G-type
giants which are located in the colour-magnitude diagram in the region of the
red clump of the cluster. Two of these stars are surely identified as red clump
stars for the first time. For those five stars, we provide chemical abundances
of 31 elements. The mean radial-velocity of NGC 6811 is found to be
+6.680.08 km s and the mean metallicity and overall abundance
pattern are shown to be very close to solar with an exception of Ba which we
find to be overabundant.Comment: 18 pages, 11 tables, 7 figures, accepted for MNRA
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