616 research outputs found
The intermediate age open cluster NGC 2660
We present CCD UBVI photometry of the intermediate old open cluster NGC2660,
covering from the red giants region to about seven magnitudes below the main
sequence turn-off. Using the synthetic Colour - Magnitude Diagram method, we
estimate in a self-consistent way values for distance modulus ((m-M)0 ~= 12.2),
reddening (E(B-V) ~= 0.40), metallicity ([Fe/H] about solar), and age (age ~ 1
Gyr). A 30% population of binary stars turns out to be probably present.Comment: 12 pages, 8 (encapsulated) figures, to be published on MNRA
Infrared photometry of Young Massive Clusters in the starburst galaxy NGC 4214
We present the results of an infrared photometric survey performed with
NICS@TNG in the nearby starburst galaxy NGC 4214. We derived accurate
integrated JK magnitudes of 10 young massive clusters and compared them with
the already available Hubble Space Telescope ultraviolet colors. These clusters
are located in the combined ultraviolet-infrared colors planes on well defined
sequences, whose shapes allow a precise determination of their age. By means of
the comparison with suitable stellar evolution models we estimated ages,
metallicities, reddening and masses of these clusters. All the analyzed
clusters appear to be younger than log(t/yr)<8.4, moderately metal-rich and
slightly less massive than present-day Galactic globular clusters. The derived
ages for clusters belonging to the secondary HII star forming complex are
significantly larger than those previously estimated in the literature. We also
discuss the possibility of using the ultraviolet-infrared color-color diagram
to select candidate young massive clusters hosting multiple stellar
populations.Comment: 12 pages, 9 figures, accepted for publication by MNRA
Spectroscopy of Red Giants in the globular cluster Terzan 8: kinematics and evidence for the surrounding Sagittarius stream
We present the results of a spectroscopic survey of Red Giants in the
globular cluster Terzan 8 with the aim of studying its kinematics. We derived
accurate radial velocities for 82 stars located in the innermost 7 arcmin from
the cluster center identifying 48 bona fide cluster members. The kinematics of
the cluster have been compared with a set of dynamical models accounting for
the effect of mass segregation and a variable fraction of binaries. The derived
velocity dispersion appears to be larger than that predicted for
mass-segregated stellar systems without binaries, indicating that either the
cluster is dynamically young or it contains a large fraction of binaries
(>30%). We detected 7 stars with a radial velocity compatible with the cluster
systemic velocity but with chemical patterns which stray from those of both the
cluster and the Galactic field. These stars are likely members of the
Sagittarius stream surrounding this stellar system.Comment: 10 pages, 8 figures, accepted for publication by MNRA
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
The incidence of binaries in Globular Cluster stellar populations
Binary fraction and orbital characteristics provide indications on the
conditions of star formation, as they shed light on the environment they were
born in. Multiple systems are more common in low density environments rather
than in higher density ones. In the current debate about the formation of
Globular Clusters and their multiple populations, studying the binary incidence
in the populations they host offers a crucial piece of information on the
environment of their birth and their subsequent dynamical evolution.
Through a multi-year observational campaign using FLAMES at VLT, we monitored
the radial velocity of 968 Red-Giant Branch stars located around the half-light
radii in a sample of 10 Galactic Globular Clusters. We found a total of 21
radial velocity variables identified as {\it bona fide} binary stars, for a
binary fraction of 2.2%0.5%. When separating the sample into first
generation and second generation stars, we find a binary fraction of
4.9%1.3% and 1.2%0.4% respectively. Through simulations that take
into account possible sources of bias in detecting radial velocity variations
in the two populations, we show that the difference is significant and only
marginally affected by such effects.
Such a different binary fraction strongly suggests different conditions in
the environment of formation and evolution of first and second generations
stars, with the latter being born in a much denser environment. Our result
hence strongly supports the idea that the second generation forms in a dense
subsystem at the center of the loosely distributed first generation, where
(loose) binaries are efficiently destroyed.Comment: A&A, Accepte
Element abundances in the metal rich open cluster NGC6253
We have carried out a big FLAMES survey of 10 Galactic open clusters aiming
at different goals. One of them is the determination of chemical abundances, in
order to put constraints on the radial metallicity gradient in the disk and its
evolution. One of the sample clusters is the very metal rich NGC 6253. We have
obtained UVES high resolution spectra of seven candidate cluster members (from
the turn off up to the red clump) with the goal of determining the chemical
composition of NGC 6253 and to investigate its origin and role in the
interpretation of the radial metallicity gradient in the disk. Equivalent width
analysis and spectral synthesis were performed using MOOG and Kurucz model
atmospheres. We derived abundances of Fe, alpha- and Fe-peak elements, the
light element Na and the s-process element Ba. Excluding two likely non-members
and the clump giant, whose metallicity from equivalent widths is overestimated,
we find an average [Fe/H]=+0.36+/-0.07 (rms) for the cluster. For most of the
other elements we derive solar abundance ratios.Comment: accepted by A&A (02/01/2007), 21 pages, 11 ps figure
Na-O anticorrelation and HB. IX. Kinematics of the program clusters. A link between systemic rotation and HB morphology?
We use accurate radial velocities for 1981 member stars in 20 Galactic
globular clusters, collected within our large survey aimed at the analysis of
the Na-O anti-correlation, to study the internal kinematics of the clusters. We
performed the first systematic exploration of the possible connections between
cluster kinematics and the multiple populations phenomenon in GCs. We did not
find any significant correlation between Na abundance and either velocity
dispersion or systemic rotation. We searched for systemic rotation in the eight
clusters of our sample that lack such analysis from previous works in the
literature (NGC2808, NGC5904, NGC6171, NGC6254, NGC6397, NGC6388, NGC6441, and
NGC6838). These clusters are found to span a large range of rotational
amplitudes, from ~0.0 km/s (NGC6397) to ~13.0 km/s (NGC6441). We found a
significant correlation between the ratio of rotational velocity to central
velocity dispersion (V_{rot}/sigma_0) and the Horizontal Branch Morphology
parameter (B-R)/(B+R+V). V_{rot}/sigma_0 is found to correlate also with
metallicity, possibly hinting to a significant role of dissipation in the
process of formation of globular clusters. V_{rot} is found to correlate well
with (B-R)/(B+R+V), M_V, sigma_0 and [Fe/H]. All these correlations strongly
suggest that systemic rotation may be intimately linked with the processes that
led to the formation of globular clusters and the stellar populations they
host.Comment: Accepted for publication on Astronomy & Astrophysics. Pdflatex, 16
pages, 16 pdf figures. The position angles of the rotation axes have been
corrected, since the values reported in the previous version were erroneous.
The results of the analysis are unchanged. The manuscript has also been
processed by a language edito
Open clusters as key tracers of Galactic chemical evolution. III. Element abundances in Berkeley 20, Berkeley 29, Collinder 261, and Melotte 66
Galactic open clusters are since long recognized as one of the best tools for
investigating the radial distribution of iron and other metals. We employed
FLAMES at VLT to collect UVES spectra of bright giant stars in a large sample
of open clusters, spanning a wide range of Galactocentric distances, ages, and
metallicities. We present here the results for four clusters: Berkeley 20 and
Berkeley 29, the two most distant clusters in the sample; Collinder 261, the
oldest and the one with the minimum Galactocentric distance; Melotte 66.
Equivalent width analysis was carried out using the spectral code MOOG and
Kurucz model atmospheres to derive abundances of Fe, Al, Mg, Si, Ca, Ti, Cr,
Ni, Ba; non-LTE Na abundances were derived by direct line-profile fitting. We
obtain subsolar metallicities for the two anticenter clusters Be 20
([Fe/H]=-0.30, rms=0.02) and Be 29 ([Fe/H]=-0.31, rms=0.03), and for Mel 66
([Fe/H]=-0.33, rms=0.03), located in the third Galactic quadrant, while Cr 261,
located toward the Galactic center, has higher metallicity ([Fe/H]=+0.13,
rms=0.05 dex). The alpha-elements Si, Ca and Ti, and the Fe-peak elements Cr
and Ni are in general close to solar; the s-process element Ba is enhanced.
Non-LTE computations of Na abundances indicate solar scaled values, suggesting
that the enhancement in Na previously determined in giants in open clusters
could be due to neglected non-LTE effects. Our results support the presence of
a steep negative slope of the Fe radial gradient up to about 10-11 kpc from the
Galactic center, while in the outer disk the [Fe/H] distribution seems flat.
All the elemental ratios measured are in very good agreement with those found
for disk stars of similar metallicity and no trend with Galactocentric distance
seems to be present.Comment: Accepted for publication on A&
NGC 6535: the lowest mass Milky Way globular cluster with a Na-O anti-correlation? Cluster mass and age in the multiple population context
To understand globular clusters (GCs) we need to comprehend how their
formation process was able to produce their abundance distribution of light
elements. In particular, we seek to figure out which stars imprinted the
peculiar chemical signature of GCs. One of the best ways is to study the
light-element anti-correlations in a large sample of GCs that are analysed
homogeneously. As part of our spectroscopic survey of GCs with FLAMES, we
present here the results of our study of about 30 red giant member stars in the
low-mass, low-metallicity Milky Way cluster NGC 6535. We measured the
metallicity (finding [Fe/H]=-1.95, rms=0.04 dex in our homogeneous scale) and
other elements of the cluster and, in particular, we concentrate here on O and
Na abundances. These elements define the normal Na-O anti-correlation of
classical GCs, making NGC 6535 perhaps the lowest mass cluster with a confirmed
presence of multiple populations. We updated the census of Galactic and
extragalactic GCs for which a statement on the presence or absence of multiple
populations can be made on the basis of high-resolution spectroscopy
preferentially, or photometry and low-resolution spectroscopy otherwise; we
also discuss the importance of mass and age of the clusters as factors for
multiple populations.Comment: In press on A&A. Table 2 available at CD
- …