263 research outputs found
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
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
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
Testing Newtonian gravity with distant globular clusters: NGC1851 and NGC1904
Globular clusters are useful to test the validity of Newtonian dynamics in
the low acceleration regime typical of galaxies, without the complications of
non-baryonic dark matter. Specifically, in absence of disturbing effects, e.g.
tidal heating, their velocity dispersion is expected to vanish at large radii.
If such behaviour is not observed, and in particular if, as observed in
elliptical galaxies, the dispersion is found constant at large radii below a
certain threshold acceleration, this might indicate a break down of Newtonian
dynamics.
To minimise the effects of tidal heating in this paper we study the velocity
dispersion profile of two distant globular clusters, NGC 1851 and NGC 1904.
The velocity dispersion profile is derived from accurate radial velocities
measurements, obtained at the ESO 8m VLT telescope. Reliable data for 184 and
146 bona fide cluster star members, respectively for NGC 1851 and NGC 1904,
were obtained.
These data allow to trace the velocity dispersion profile up to ~2r0, where
r0 is the radius at which the cluster internal acceleration of gravity is a0 =
10e-8 cm/s/s. It is found that in both clusters the velocity dispersion becomes
constant beyond ~r0. These new results are fully in agreement with those found
for other five globular clusters previously investigated as part of this
project. Taken all together, these 7 clusters support the claim that the
velocity dispersion is constant beyond r0, irrespectively of the specific
physical properties of the clusters: mass, size, dynamical history, and
distance from the Milky Way. The strong similarly with the constant velocity
dispersion observed in elliptical galaxies beyond r0 is suggestive of a common
origin for this phenomenon in the two class of objects, and might indicate a
breakdown of Newtonian dynamics below a0.Comment: Accepted for publication by A&A main journal. 12 pages, 12 figure
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&
The extended structure of the dwarf irregular galaxy Sagittarius
We present a detailed study of the stellar and HI structure of the dwarf
irregular galaxy Sagittarius. We use new deep and wide field photometry to
trace the surface brightness profile of the galaxy out to ~5.0' (corresponding
to ~1600 pc) and down to mag/arcsec, thus showing that
the stellar body of the galaxy is much more extended than previously believed,
and it is similarly (or more) extended than the overall HI distribution. The
whole major-axis profile is consistent with a pure exponential, with a scale
radius of pc. The surface density maps reveal that the
distribution of old and intermediate-age stars is smooth and remarkably
flattened out to its edges, while the associated HI has a much rounder shape,
is off-centred and presents multiple density maxima and a significant hole. No
clear sign of systemic rotation is detectable in the complex HI velocity field.
No metallicity gradient is detected in the old and intermediate age population
of the galaxy, and we confirm that this population has a much more extended
distribution than young stars (age Gyr).Comment: 11 pages, 14 figures, accepted for publication on A&A. arXiv admin
note: text overlap with arXiv:1404.169
deterministic safety technology for rbmk reactors
The present paper deals with the description of the technical activities conducted within the TACIS Project R2.03/97, 2 EC Contract no. 30303, related to RBMK. The project activities are focused toward the setting-up of a chain of computational tools suitable for the analysis of transients expected in the RBMK nuclear power plant (NPP). The accident leading to the rupture of one pressure channel, with fuel melting or high temperature damage, creep and brittle failure of the pressure tube and of graphite bricks with possibility of rupture propagation, constitutes the reference scenario for the project. However, a series of expected scenarios has been selected to prove the capability of the individual codes or chains of code in simulating the envisaged phenomenology. The paper summarizes the activities performed at NIKIET in Moscow and at University of Pisa (UNIPI) in Pisa. A top-down approach is pursued in structuring the executive summary that includes the following sections: (i) the safety needed for the RBMK NPP, (ii) the roadmap, (iii) the adopted computational tools, (iv) key findings, (v) Emphasis is given to the multiple pressure tube rupture (MPTR) issue and the individual channel monitoring (ICM) proposal
New HST WFC3/UVIS observations augment the stellar-population complexity of omega Centauri
We used archival multi-band Hubble Space Telescope observations obtained with
the Wide-Field Camera 3 in the UV-optical channel to present new important
observational findings on the color-magnitude diagram (CMD) of the Galactic
globular cluster omega Centauri. The ultraviolet WFC3 data have been coupled
with available WFC/ACS optical-band data. The new CMDs, obtained from the
combination of colors coming from eight different bands, disclose an even more
complex stellar population than previously identified. This paper discusses the
detailed morphology of the CMDs.Comment: 17 pages, 14 figures (11 in low res), 3 tables. Accepted for
publication in AJ on June 19, 201
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