547 research outputs found
The Unimodal Distribution Of Blue Straggler Stars in M75 (NGC 6864)
We have used a combination of multiband high-resolution and wide-field
ground-based observations to image the Galactic globular cluster M75 (NGC
6864). The extensive photometric sample covers the entire cluster extension,
from the very central regions out to the tidal radius, allowing us to determine
the center of gravity and to construct the most extended star density profile
ever published for this cluster. We also present the first detailed star counts
in the very inner regions. The star density profile is well re-produced by a
standard King model with core radius r_c ~ 5.4" and intermediate-high
concentration c ~ 1.75. The present paper presents a detailed study of the BSS
population and its radial distribution. A total number of 62 bright BSSs (with
m_F255W < 21, corresponding to m_F555W < 20) has been identified, and they have
been found to be highly segregated in the cluster core. No significant upturn
in the BSS frequency has been observed in the outskirts of M75, in contrast to
several other clusters studied with the same technique. This observational fact
is quite similar to what has been found in M79 (NGC 1904) by Lanzoni et al.
(2007a). Indeed the BSS radial distributions in the two clusters is
qualitatively very similar, even if in M75 the relative BSS frequency seems to
decrease significantly faster than in M79: indeed it decreases by a factor of 5
(from 3.4 to 0.7) within 1 r_c. Such evidence indicate that the vast majority
of the cluster heavy stars (binaries) have already sunk to the core.Comment: ApJ accepted, 10 pages, 11 figures, 2 table
Another brick in understanding chemical and kinematical properties of BSSs: NGC 6752
We used high-resolution spectra acquired with the multifiber facility FLAMES
at the Very Large Telescope of the European Southern Observatory to investigate
the chemical and kinematical properties of a sample of 22 Blue Straggler Stars
(BSSs) and 26 red giant branch stars in the nearby globular cluster NGC 6752.
We measured radial and rotational velocities and Fe, O and C abundances.
According to radial velocities, metallicity and proper motions we identified 18
BSSs as likely cluster members. We found that all the BSSs rotate slowly (less
than 40 km/s), similar to the findings in 47 Tucanae, NGC 6397 and M30. The Fe
abundance analysis reveals the presence of 3 BSSs affected by radiative
levitation (showing [Fe/H] significantly higher than that measured in "normal"
cluster stars), confirming that element transport mechanisms occur in the
photosphere of BSSs hotter than 8000 K. Finally, BSS C and O abundances are
consistent with those measured in dwarf stars. No C and O depletion ascribable
to mass transfer processes has been found on the atmospheres of the studied
BSSs (at odds with previous results for 47 Tucanae and M30), suggesting the
collisional origin for BSSs in NGC 6752 or that the CO-depletion is a transient
phenomenon.Comment: ApJ accepte
No evidence of chemical anomalies in the bimodal turnoff cluster NGC 1806 in the LMC
We have studied the chemical composition of NGC 1806, a massive,
intermediate-age globular cluster that shows a double main sequence turnoff. We
analyzed a sample of high-resolution spectra (secured with FLAMES at the Very
Large Telescope) for 8 giant stars, members of the cluster, finding an average
iron content of [Fe/H]=--0.60 +- 0.01 dex and no evidence of intrinsic
star-to-star variations in the abundances of light elements (Na, O, Mg, Al).
Also, the (m_(F814W); m_(F336W)-m_(F814W)) color-magnitude diagram obtained by
combining optical and near-UV Hubble Space Telescope photometry exhibits a
narrow red giant branch, thus ruling out intrinsic variations of C and N
abundances in the cluster. These findings demonstrate that NGC 1806 does not
harbor chemically distinct sub-populations, at variance with what was found in
old globular clusters. In turn, this indicates that the double main sequence
turnoff phenomenon cannot be explained in the context of the self-enrichment
processes usually invoked to explain the chemical anomalies observed in old
globulars. Other solutions (i.e., stellar rotation, merging between clusters or
collisions with giant molecular clouds) should be envisaged to explain this
class of globulars.Comment: Accepted for publication by ApJ Letters; 15 pages, 4 figures, 1 tabl
The optical counterpart to the X-ray transient IGR J18245-2452 in the globular cluster M28
We report on the identification of the optical counterpart to the recently
detected INTEGRAL transient IGR J18245-2452 in the Galactic globular cluster
M28. From the analysis of a multi epoch HST dataset we have identified a
strongly variable star positionally coincident with the radio and Chandra X-ray
sources associated to the INTEGRAL transient. The star has been detected during
both a quiescent and an outburst state. In the former case it appears as a
faint, unperturbed main sequence star, while in the latter state it is about
two magnitudes brighter and slightly bluer than main sequence stars. We also
detected Halpha excess during the outburst state, suggestive of active
accretion processes by the neutron star.Comment: Accepted for publication by ApJ; 15 pages, 4 figures, 1 tabl
The Horizontal Branch in the UV Colour Magnitude Diagrams. II. The case of M3, M13 and M79
We present a detailed comparison between far-UV/optical colour Magnitude
Diagrams obtained with high-resolution Hubble Space Telescope data and suitable
theoretical models for three Galactic Globular Clusters: M3, M13 and M79. These
systems represents a classical example of clusters in the intermediate
metallicity regime that, even sharing similar metal content and age, show
remarkably different Horizontal Branch morphologies. As a consequence, the
observed differences in the colour distributions of Horizontal Branch stars
cannot be interpreted in terms of either first (metallicity) or a second
parameter such as age. We investigate here the possible role of variations of
initial Helium abundance (Y). Thanks to the use of a proper setup of far-UV
filters, we are able to put strong constraints on the maximum Y (Y_{max})
values compatible with the data. We find differences Delta Y_{max} ~ 0.02-0.04
between the clusters with M13 showing the largest value (Y_{max} ~ 0.30) and M3
the smallest (Y_{max} ~ 0.27). In general we observe that these values are
correlated with the colour extensions of their Horizontal Branches and with the
range of the observed Na-O anti-correlations.Comment: Accepted for publication by MNRAS. 15 pages, 15 figures, 1 tabl
UV observations of the globular cluster M10 from HST and GALEX. The BSS population
We present a combination of high-resolution Hubble Space Telescope and
wide-field ground-based and Galaxy Evolution Explorer data of the Galactic
Globular Cluster M10 (NGC6254). By using this large data-set we determined the
center of gravity of the cluster and we built its density profile from star
counts over its entire radial extension. We find that the density profile is
well reproduced by a single-mass King model with structural parameters c=1.41
and r_c=41". We also studied the Blue Straggler Star population and its radial
distribution. We count a total number of 120 BSS within the tidal radius. Their
radial distribution is bimodal: highly peaked in the cluster center, decreasing
at intermediate distances and rising again outwards. We discuss these results
in the context of the dynamical clock scheme presented by Ferraro et al. (2012)
and of recent results about the radial distribution of binary systems in this
cluster.Comment: Accepted for publication by ApJ; 26 pages, 11 figures, 1 tabl
FLAMES and XSHOOTER spectroscopy along the two BSS sequences of M30
We present spectroscopic observations acquired with FLAMES and XSHOOTER at
the Very Large Telescope for a sample of 15 Blue Straggler Stars (BSSs) in the
globular cluster (GC) M30. The targets have been selected to sample the two BSS
sequences discovered, with 7 BSSs along the blue sequence and 8 along the red
one. No difference in the kinematical properties of the two groups of BSSs has
been found. In particular, almost all the observed BSSs have projected
rotational velocity lower than ~30 km/s, with only one (blue) fast rotating BSS
(>90 km/s), identified as a W UMa binary. This rotational velocity distribution
is similar to those obtained in 47 Tucanae and NGC 6397, while M4 remains the
only GC studied so far harboring a large fraction of fast rotating BSSs. All
stars hotter than ~7800 K (regardless of the parent BSS sequence) show iron
abundances larger than those measured from normal cluster stars, with a
clearcut trend with the effective temperature. This behaviour suggests that
particle trasport mechanisms driven by radiative levitation occur in the
photosphere of these stars, as already observed for the BSSs in NGC 6397.
Finally, 4 BSSs belonging to the red sequence (not affected by radiative
levitation) show a strong depletion of [O/Fe], with respect to the abundance
measured in Red Giant Branch and Horizontal Branch stars. This O-depletion is
compatible with the chemical signature expected in BSSs formed by mass transfer
processes in binary systems, in agreement with the mechanism proposed for the
formation of BSSs in the red sequence.Comment: Accepted for publication in Ap
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