328 research outputs found
Searching for multiple stellar populations in the massive, old open cluster Berkeley 39
The most massive star clusters include several generations of stars with a
different chemical composition (mainly revealed by an Na-O anti-correlation)
while low-mass star clusters appear to be chemically homogeneous. We are
investigating the chemical composition of several clusters with masses of a few
10^4 Msun to establish the lower mass limit for the multiple stellar population
phenomenon. Using FLAMES@VLT spectra we determine abundances of Fe, O, Na, and
several other elements (alpha, Fe-peak, and neutron-capture elements) in the
old open cluster Berkeley 39. This is a massive open cluster: M~10^4 Msun,
approximately at the border between small globular clusters and large open
clusters. Our sample size of about 30 stars is one of the largest studied for
abundances in any open cluster to date, and will be useful to determine
improved cluster parameters, such as age, distance, and reddening when coupled
with precise, well-calibrated photometry. We find that Berkeley 39 is slightly
metal-poor, =-0.20, in agreement with previous studies of this cluster.
More importantly, we do not detect any star-to-star variation in the abundances
of Fe, O, and Na within quite stringent upper limits. The r.m.s. scatter is
0.04, 0.10, and 0.05 dex for Fe, O, and Na, respectively. This small spread can
be entirely explained by the noise in the spectra and by uncertainties in the
atmospheric parameters. We conclude that Berkeley 39 is a single-population
cluster.Comment: A&A in press, 10 pages, tables 2 & 3 available only on-lin
NGC 5694: another foster son of the Galactic Halo
We present the results of the analysis of high-resolution spectra obtained
with UVES-FLAMES@VLT for six red giant branch stars in the outer-halo
metal-poor ([Fe/H]I=-1.98 and [Fe/H]II=-1.83) Galactic globular cluster NGC
5694, which has been suggested as a possible incomer by Lee et al. (2006) based
on the anomalous chemical composition of a single cluster giant. We obtain
accurate abundances for a large number of elements and we find that: (a) the
six target stars have the same chemical composition within the uncertainties,
except for Na and Al; (b) the average cluster abundance of \alpha\ elements
(with the only exception of Si) is nearly solar, at odds with typical halo
stars and globular clusters of similar metallicity; (c) Y, Ba, La and Eu
abundances are also significantly lower than in Galactic field stars and star
clusters of similar metallicity. Hence we confirm the Lee et al. classification
of NGC 5694 as a cluster of extra-galactic origin. We provide the first insight
on the Na-O and Mg-Al anti-correlations in this cluster: all the considered
stars have very similar abundance ratios for these elements, except one that
has significantly lower [Na/Fe] and [Al/Fe] ratios, suggesting that some degree
of early self-enrichment has occurred also in this cluster.Comment: Accepted for publication by MNRAS. 15 pages, 8 figures, 4 table
Search for Associations Containing Young stars (SACY): Chemical tagging IC 2391 & the Argus association
We explore the possible connection between the open cluster IC 2391 and the
unbound Argus association identified by the SACY survey. In addition to common
kinematics and ages between these two systems, here we explore their chemical
abundance patterns to confirm if the two substructures shared a common origin.
We carry out a homogenous high-resolution elemental abundance study of eight
confirmed members of IC 2391 as well as six members of the Argus association
using UVES spectra. We derive spectroscopic stellar parameters and abundances
for Fe, Na, Mg, Al, Si, Ca, Ti, Cr, Ni and Ba.
All stars in the open cluster and Argus association were found to share
similar abundances with the scatter well within the uncertainties, where [Fe/H]
= -0.04 +/-0.03 for cluster stars and [Fe/H] = -0.06 +/-0.05 for Argus stars.
Effects of over-ionisation/excitation were seen for stars cooler than roughly
5200K as previously noted in the literature. Also, enhanced Ba abundances of
around 0.6 dex were observed in both systems. The common ages, kinematics and
chemical abundances strongly support that the Argus association stars
originated from the open cluster IC 2391. Simple modeling of this system find
this dissolution to be consistent with two-body interactions.Comment: 17 pages, 7 figs, accepted for publication in MNRA
NGC 6139: a normal massive globular cluster or a first-generation dominated cluster? Clues from the light elements
Information on globular clusters (GC) formation mechanisms can be gathered by
studying the chemical signature of the multiple populations that compose these
stellar systems. In particular, we are investigating the anticorrelations among
O, Na, Al, and Mg to explore the influence of cluster mass and environment on
GCs in the Milky Way and in extragalactic systems. We present here the results
obtained on NGC 6139 which, on the basis of its horizontal branch morphology,
had been proposed to be dominated by first-generation stars. In our extensive
study based on high resolution spectroscopy, the first for this cluster, we
found a metallicity of [Fe/H]= -1.579 +/- 0.015 +/- 0.058 (rms=0.040 dex, 45
bona fide member stars) on the UVES scale defined by our group. The stars in
NGC 6139 show a chemical pattern normal for GCs, with a rather extended Na-O
(and Mg-Al) anticorrelation. NGC 6139 behaves like expected from its mass and
contains a large fraction (about two thirds) of second-generation stars.Comment: Accepted for publication on A&
Lithium abundances in globular cluster giants: NGC 6218 (M12) and NGC 5904 (M5)
Convergent lines of evidence suggest that globular clusters host multiple
stellar populations. It appears that they experience at least two episodes of
star formation whereby a fraction of first-generation stars contribute astrated
ejecta to form the second generation(s). To identify the polluting progenitors
we require distinguishing chemical signatures such as that provided by lithium.
Theoretical models predict that lithium can be synthesised in AGB stars,
whereas no net Li production is expected from other candidates. It has been
shown that in order to reproduce the abundance pattern found in M4, Li
production must occur within the polluters, favouring the AGB scenario. Here we
present Li and Al abundances for a large sample of RGB stars in M12 and M5.
These clusters have a very similar metallicity, whilst demonstrating
differences in several cluster properties. Our results indicate that the
first-generation and second-generation stars share the same Li content in M12;
we recover an abundance pattern similar to that observed in M4. In M5 we find a
higher degree of complexity and a simple dilution model fails in reproducing
the majority of the stellar population. In both clusters we require Li
production across the different stellar generations, but production seems to
have occurred to different extents. We suggest that such a difference might be
related to the cluster mass with the Li production being more efficient in
less-massive clusters. This is the first time a statistically significant
correlation between the Li spread within a GC and its luminosity has been
demonstrated. Finally, although Li-producing polluters are required to account
for the observed pattern, other mechanisms, such as MS depletion, might have
played a role in contributing to the Li internal variation, though at
relatively low level.Comment: Accepted for publication in The Astrophysical Journal. 15 pages, 14
figure
On the serendipitous discovery of a Li-rich giant in the globular cluster NGC 362
We have serendipitously identified the first lithium-rich giant star located
close to the red giant branch bump in a globular cluster. Through
intermediate-resolution FLAMES spectra we derived a lithium abundance of
A(Li)=2.55 (assuming local thermodynamical equilibrium), which is extremely
high considering the star's evolutionary stage. Kinematic and photometric
analysis confirm the object as a member of the globular cluster NGC 362. This
is the fourth Li-rich giant discovered in a globular cluster but the only one
known to exist at a luminosity close to the bump magnitude. The three previous
detections are clearly more evolved, located close to, or beyond the tip of
their red giant branch. Our observations are able to discard the accretion of
planets/brown dwarfs, as well as an enhanced mass-loss mechanism as a formation
channel for this rare object. Whilst the star sits just above the cluster bump
luminosity, its temperature places it towards the blue side of the giant branch
in the colour-magnitude diagram. We require further dedicated observations to
unambiguously identify the star as a red giant: we are currently unable to
confirm whether Li production has occurred at the bump of the luminosity
function or if the star is on the pre zero-age horizontal branch. The latter
scenario provides the opportunity for the star to have synthesised Li rapidly
during the core helium flash or gradually during its red giant branch ascent
via some extra mixing process.Comment: Accepted for publication in The Astrophysical Journal Letter
Kinematics of a globular cluster with an extended profile: NGC5694
We present a study of the kinematics of the remote globular cluster NGC5694
based on GIRAFFE@VLT medium resolution spectra. A sample of 165 individual
stars selected to lie on the Red Giant Branch in the cluster Color Magnitude
Diagram was considered. Using radial velocity and metallicity from Calcium
triplet, we were able to select 83 bona-fide cluster members. The addition of
six previously known members leads to a total sample of 89 cluster giants with
typical uncertainties <1.0 km/s in their radial velocity estimates. The sample
covers a wide range of projected distances from the cluster center, from ~0.2
arcmin to 6.5 arcmin = 23 half-light radii (r_h). We find only very weak
rotation, as typical of metal-poor globular clusters. The velocity dispersion
gently declines from a central value of sigma=6.1 km/s to sigma = 2.5 km/s at
~2 arcmin = 7.1= r_h, then it remainins flat out to the next (and last)
measured point of the dispersion profile, at ~4 arcmin = 14.0 r_h, at odds with
the predictions of isotropic King models. We show that both isotropic
single-mass non-collisional models and multi-mass anisotropic models can
reproduce the observed surface brightness and velocity dispersion profiles.Comment: Accepted for publication by MNRAS. Pdflatex, 10 pages, 10 figure
Lithium abundances in globular cluster giants: NGC 1904, NGC 2808, and NGC 362
The presence of multiple populations in globular clusters has been well
established thanks to high-resolution spectroscopy. It is widely accepted that
distinct populations are a consequence of different stellar generations:
intra-cluster pollution episodes are required to produce the peculiar chemistry
observed in almost all clusters. Unfortunately, the progenitors responsible
have left an ambiguous signature and their nature remains unresolved. To
constrain the candidate polluters, we have measured lithium and aluminium
abundances in more than 180 giants across three systems: NGC~1904, NGC~2808,
and NGC~362. The present investigation along with our previous analysis of M12
and M5 affords us the largest database of simultaneous determinations of Li and
Al abundances. Our results indicate that Li production has occurred in each of
the three clusters. In NGC~362 we detected an M12-like behaviour, with first
and second-generation stars sharing very similar Li abundances favouring a
progenitor that is able to produce Li, such as AGB stars. Multiple progenitor
types are possible in NGC~1904 and NGC~2808, as they possess both an
intermediate population comparable in lithium to the first generation stars and
also an extreme population, that is enriched in Al but depleted in Li. A simple
dilution model fails in reproducing this complex pattern. Finally, the internal
Li variation seems to suggest that the production efficiency of this element is
a function of the cluster's mass and metallicity - low-mass or relatively
metal-rich clusters are more adept at producing Li.Comment: Accepted for publication in MNRAS. 10 pages, 8 figure
Sodium content as a predictor of the advanced evolution of globular cluster stars
The asymptotic giant branch (AGB) phase is the final stage of nuclear burning
for low-mass stars. Although Milky Way globular clusters are now known to
harbour (at least) two generations of stars they still provide relatively
homogeneous samples of stars that are used to constrain stellar evolution
theory. It is predicted by stellar models that the majority of cluster stars
with masses around the current turn-off mass (that is, the mass of the stars
that are currently leaving the main sequence phase) will evolve through the AGB
phase. Here we report that all of the second-generation stars in the globular
cluster NGC 6752 -- 70 per cent of the cluster population -- fail to reach the
AGB phase. Through spectroscopic abundance measurements, we found that every
AGB star in our sample has a low sodium abundance, indicating that they are
exclusively first-generation stars. This implies that many clusters cannot
reliably be used for star counts to test stellar evolution timescales if the
AGB population is included. We have no clear explanation for this observation.Comment: Published in Nature (online 29 May 2013, hard copy 13 June), 12
pages, 3 figures + supplementary information sectio
The Asymptotic Giant Branches of GCs: Selective Entry Only
The handful of available observations of AGB stars in Galactic Globular
Clusters suggest that the GC AGB populations are dominated by cyanogen-weak
stars. This contrasts strongly with the distributions in the RGB (and other)
populations, which generally show a 50:50 bimodality in CN band strength. If it
is true that the AGB populations show very different distributions then it
presents a serious problem for low mass stellar evolution theory, since such a
surface abundance change going from the RGB to AGB is not predicted by stellar
models. However this is only a tentative conclusion, since it is based on very
small AGB sample sizes. To test whether this problem really exists we have
carried out an observational campaign specifically targeting AGB stars in GCs.
We have obtained medium resolution spectra for about 250 AGB stars across 9
Galactic GCs using the multi-object spectrograph on the AAT (2df/AAOmega). We
present some of the preliminary findings of the study for the second parameter
trio of GCs: NGC 288, NGC 362 and NGC 1851. The results indeed show that there
is a deficiency of stars with strong CN bands on the AGB. To confirm that this
phenomenon is robust and not just confined to CN band strengths and their
vagaries, we have made observations using FLAMES/VLT to measure elemental
abundances for NGC 6752.We present some initial results from this study also.
Our sodium abundance results show conclusively that only a subset of stars in
GCs experience the AGB phase of evolution. This is the first direct, concrete
confirmation of the phenomenon.Comment: 4 pages, to appear in conference proceedings of "Reading the book of
globular clusters with the lens of stellar evolution", Rome, 26-28 November
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