375 research outputs found
Deep multi-telescope photometry of NGC 5466. I. Blue Stragglers and binary systems
We present a detailed investigation of the radial distribution of blue
straggler star and binary populations in the Galactic globular cluster NGC
5466, over the entire extension of the system. We used a combination of data
acquired with the ACS on board the Hubble Space Telescope, the LBC-blue mounted
on the Large Binocular Telescope, and MEGACAM on the Canadian-France-Hawaii
Telescope. Blue straggler stars show a bimodal distribution with a mild central
peak and a quite internal minimum. This feature is interpreted in terms of a
relatively young dynamical age in the framework of the "dynamical clock"
concept proposed by Ferraro et al. (2012). The estimated fraction of binaries
is 6-7% in the central region (r<90") and slightly lower (5.5%) in the
outskirts, at r>200". Quite interestingly, the comparison with the results of
Milone et al. (2012) suggests that also binary systems may display a bimodal
radial distribution, with the position of the minimum consistent with that of
blue straggler stars. If confirmed, this feature would give additional support
to the scenario where the radial distribution of objects more massive than the
average cluster stars is primarily shaped by the effect of dynamical friction.
Moreover, this would also be consistent with the idea that the unperturbed
evolution of primordial binaries could be the dominant BSS formation process in
low-density environments.Comment: Accepted for publication on Ap
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
HII regions within a compact high velocity cloud. A nearly star-less dwarf galaxy?
Within the SECCO survey we identified a candidate stellar counterpart to the
Ultra Compact High Velocity Cloud (UCHVC) HVC274.68+74.70-123, that was
suggested by Adams et al. (2013) as a possible mini-halo within the Local Group
of galaxies. The spectroscopic follow-up of the brightest sources within the
candidate reveals the presence of two HII regions whose radial velocity is
compatible with physical association with the UVHVC. The available data does
not allow us to give a definite answer on the nature of the newly identified
system. A few alternative hypotheses are discussed. However, the most likely
possibility is that we have found a new faint dwarf galaxy residing in the
Virgo cluster of galaxies, which we name SECCO-1. Independently of its actual
distance, SECCO-1 displays a ratio of neutral hydrogen mass to V luminosity of
M_{HI}/L_V>= 20, by far the largest among local dwarfs. Hence, it appears as a
nearly star-less galaxy and it may be an example of the missing links between
normal dwarfs and the dark mini halos that are predicted to exist in large
numbers according to the currently accepted cosmological model.Comment: Accepted for publication in ApJ Letters. Pdflatex, emulateapj.cls. 6
pages, 3 figures, 2 table
Assessment of Natural Resources Use for Sustainable Development - DPSIR Framework for Case Studies in Portsmouth and Thames Gateway, U.K.
This chapter reports on the uses of the DPSIR framework to assess the sustainability of the intertidal environments within the two UK case study areas, Portsmouth and Thames Gateway. It focuses on statutory conservation areas dominated by intertidal habitats. Two are located in Portsmouth (Portsmouth and Langstone Harbours) and four in the Thames Gateway (Benfleet Marshes, South Thames Estuary, Medway Estuary and the Swale in the Thames Gateway). Based on the reduction of a number of pressures and impacts observed in recent decades and the improvement of overall environmental quality, all six SSSIs are considered to be sustainable in the short and medium term. In the future, it is possible that the impacts of climate change, especially sea-level rise, might result in further reduction in the area and/or quality of intertidal habitats. Further integration between conservation and planning objectives (both for urban development and management of flood risk) at local level is needed to support the long-term sustainability of intertidal habitats
Discovery of Carbon/Oxygen depleted Blue Straggler Stars in 47 Tucanae: the chemical signature of a mass-transfer formation process
We use high-resolution spectra obtained with the ESO Very Large Telescope to
measure surface abundance patterns of 43 Blue Stragglers stars (BSS) in 47 Tuc.
We discovered that a sub-population of BSS shows a significant depletion of
Carbon and Oxygen with respect to the dominant population. This evidence would
suggest the presence of CNO burning products on the BSS surface coming from a
deeply peeled parent star, as expected in the case of mass-transfer process.
This is the first detection of a chemical signature clearly pointing to a
specific BSS formation process in a globular cluster.Comment: Published on 2006, August 10, in ApJ 647, L5
The binary fraction in the globular cluster M10 (NGC 6254): comparing core and outer regions
We study the binary fraction of the globular cluster M10 (NGC 6254) as a
function of radius from the cluster core to the outskirts, by means of a quan-
titative analysis of the color distribution of stars relative to the fiducial
main sequence. By taking advantage of two data-sets, acquired with the Advanced
Camera for Survey and the Wide Field Planetary Camera 2 on board the Hubble
Space Telescope, we have studied both the core and the external regions of the
cluster. The binary fraction is found to decrease from 14% within the core, to
1.5% in a region between 1 and 2 half-mass radii from the cluster centre. Such
a trend and the derived values are in agreement with previous results ob-
tained in clusters of comparable total magnitude. The estimated binary fraction
is sufficient to account for the suppression of mass segregation observed in
M10, without any need to invoke the presence of an intermediate-mass black hole
in its centre.Comment: Accepted for publication in ApJ (22 pages, 7 figures, 3 tables
A Correlation between Blue Straggler and Binary Fractions in the core of Galactic Globular Clusters
Context. Blue Stragglers Stars (BSSs) are thought to form in globular
clusters by two main formation channels: i) mergers induced by stellar
collisions and ii) coalescence or mass-transfer between companions in binary
systems. The detailed study of the BSS properties is therefore crucial for
understanding the binary evolution mechanisms, and the complex interplay
between dynamics and stellar evolution in dense stellar systems. Aims. We
present the first comparison between the BSS specific frequency and the binary
fraction in the core of a sample of Galactic globular clusters, with the aim of
investigating the relative efficiency of the two proposed formation mechanisms.
Methods. We derived the frequency of BSSs in the core of thirteen low-density
Galactic globular clusters by using deep ACS@HST observations and investigated
its correlation with the binary fraction and various other cluster parameters.
Results. We observed a correlation between the BSS specific frequency and the
binary fraction. The significance of the correlation increases by including a
further dependence on the cluster central velocity dispersion. Conclusions. We
conclude that the unperturbed evolution of primordial binaries could be the
dominant BSS formation process, at least in low-density environments.Comment: 5 pages, 2 figures, accepted for publication by A&
The Blue Straggler population in the globular cluster M53 (NGC5024): a combined HST, LBT, CFHT study
We used a proper combination of multiband high-resolution and wide field
multi-wavelength observations collected at three different telescopes (HST, LBT
and CFHT) to probe Blue Straggler Star (BSS) populations in the globular
cluster M53. Almost 200 BSS have been identified over the entire cluster
extension. The radial distribution of these stars has been found to be bimodal
(similarly to that of several other clusters) with a prominent dip at ~60'' (~2
r_c) from the cluster center. This value turns out to be a factor of two
smaller than the radius of avoidance (r_avoid, the radius within which all the
stars of ~1.2 M_sun have sunk to the core because of dynamical friction effects
in an Hubble time). While in most of the clusters with a bimodal BSS radial
distribution, r_avoid has been found to be located in the region of the
observed minimum, this is the second case (after NGC6388) where this
discrepancy is noted. This evidence suggests that in a few clusters the
dynamical friction seems to be somehow less efficient than expected.
We have also used this data base to construct the radial star density profile
of the cluster: this is the most extended and accurate radial profile ever
published for this cluster, including detailed star counts in the very inner
region. The star density profile is reproduced by a standard King Model with an
extended core (~25'') and a modest value of the concentration parameter
(c=1.58). A deviation from the model is noted in the most external region of
the cluster (at r>6.5' from the center). This feature needs to be further
investigated in order to address the possible presence of a tidal tail in this
cluster.Comment: 25 pages, 9 figures, accepted for publication on Ap
- …