853 research outputs found
Fixed boundary conditions analysis of the 3d Gonihedric Ising model with
The Gonihedric Ising model is a particular case of the class of models
defined by Savvidy and Wegner intended as discrete versions of string theories
on cubic lattices. In this paper we perform a high statistics analysis of the
phase transition exhibited by the 3d Gonihedric Ising model with in the
light of a set of recently stated scaling laws applicable to first order phase
transitions with fixed boundary conditions. Even though qualitative evidence
was presented in a previous paper to support the existence of a first order
phase transition at , only now are we capable of pinpointing the
transition inverse temperature at and of checking the
scaling of standard observables.Comment: 14 pages, 5 tables, 2 figures, uses elsart.cls packag
The helium content of globular clusters: NGC6121 (M4)
He has been proposed as a key element to interpret the observed multiple MS,
SGB, and RGB, as well as the complex horizontal branch (HB) morphology. Stars
belonging to the bluer part of the HB, are thought to be more He rich (\Delta
Y=0.03 or more) and more Na-rich/O-poor than those located in the redder part.
This hypothesis was only partially confirmed in NGC 6752, where stars of the
redder zero-age HB showed a He content of Y=0.25+-0.01, fully compatible with
the primordial He content of the Universe, and were all Na-poor/O-rich. Here we
study hot blue HB (BHB) stars in the GC NGC 6121 (M4) to measure their He plus
O/Na content. We observed 6 BHB stars using the UVES@VLT2 spectroscopic
facility. In addition to He, O, Na, and Fe abundances were estimated. Stars
turned out to be all Na-rich and O-poor and to have a homogeneous enhanced He
content with a mean value of Y=0.29+-0.01(random)+-0.01(systematic). The high
He content of blue HB stars in M4 is also confirmed by the fact that they are
brighter than red HB stars (RHB). Theoretical models suggest the BHB stars are
He-enhanced by \Delta Y=0.02-0.03 with respect to the RHB stars. The whole
sample of stars has a metallicity of [Fe/H]=-1.06+-0.02 (internal error). This
is a rare direct measurement of the (primordial) He abundance for stars
belonging to the Na-rich/O-poor population of GC stars in a temperature regime
where the He content is not altered by sedimentation or extreme mixing as
suggested for the hottest, late helium flash HB stars. Our results support
theoretical predictions that the Na-rich/O-poor population is also more He-rich
than the Na-poor/O-rich generation and that a leading contender for the 2^{nd}
parameter is the He abundance.Comment: 18 pages, 8 figures, accepted for publication on Ap
Absolute motions of globular clusters. II. [HST astrometry and VLT radial velocities in NGC6397]
In this paper we present a new, accurate determination of the three
components of the absolute space velocity of the Galactic globular cluster
NGC6397 (l 338d, b -12d). We used three HST/WFPC2 fields with multi-epoch
observations to obtain astrometric measurements of objects in three different
fields in this cluster. The identification of 33 background galaxies with sharp
nuclei allowed us to determine an absolute reference point and measure the
absolute proper motion of the cluster. The third component has been obtained
from radial velocities measured on spectra from the multi-fiber spectrograph
FLAMES at UT2-VLT. We find [mu_alpha cos(delta), mu_delta](J2000.0) = [+3.39
+/- 0.15, -17.55 +/- 0.15] mas/yr, and V_rad = +18.36 +/- 0.09 (+/-0.10) km/s.
Assuming a Galactic potential, we calculate the cluster orbit for various
assumed distances, and briefly discuss the implications.Comment: 7 pages, 5 figures, 4 tables. Accepted for publication in A&A, on
April 27 200
The peculiar Na-O anticorrelation of the bulge globular cluster NGC 6440
Context. Galactic Globular Clusters (GCs) are essential tools to understand
the earliest epoch of the Milky Way, since they are among the oldest objects in
the Universe and can be used to trace its formation and evolution. Current
studies using high resolution spectroscopy for many stars in each of a large
sample of GCs allow us to develop a detailed observational picture about their
formation and their relation with the Galaxy. However, it is necessary to
complete this picture by including GCs that belong to all major Galactic
components, including the Bulge. Aims. Our aim is to perform a detailed
chemical analyses of the bulge GC NGC 6440 in order to determine if this object
has Multiple Populations (MPs) and investigate its relation with the Bulge of
the Milky Way and with the other Galactic GCs, especially those associated with
the Bulge, which are largely poorly studied. Methods. We determined the stellar
parameters and the chemical abundances of light elements (Na, Al), iron-peak
elements (Fe, Sc, Mn, Co, Ni), -elements (O, Mg, Si, Ca, Ti) and heavy
elements (Ba, Eu) in seven red giant members of NGC 6440 using high resolution
spectroscopy from FLAMES@UVES. Results. We found a mean iron content of
[Fe/H]=-0.500.03 dex in agreement with other studies. We found no internal
iron spread. On the other hand, Na and Al show a significant intrinsic spread,
but the cluster has no significant O-Na anticorrelation nor exhibits a Mg-Al
anticorrelation. The -elements show good agreement with the Bulge field
star trend, although they are at the high alpha end and are also higher than
those of other GCs of comparable metallicity. The heavy elements are dominated
by the r-process, indicating a strong contribution by SNeII. The chemical
analysis suggests an origin similar to that of the Bulge field stars.Comment: 12 pages, 13 figures, Accepted for publication in A&
NGC 5385, NGC 2664 and Collinder 21: three candidate Open Cluster Remnants
We present CCD UBVI photometric and medium/high resolution spectroscopic
observations obtained in the field of the previously unstudied dissolving open
cluster candidates NGC 5385, NGC 2664 and Collinder~21. Our analysis stands on
the discussion of star counts, photometry, radial velocity distribution, and
proper motions available from the Tycho~2 catalogue. All the three aggregates
clearly emerge from the mean Galactic field, but, regrettably, the close
scrutiny of proper motions and radial velocities reveals that we are not facing
any physical group. Instead, what we are looking at are just chance alignments
of a few bright unrelated stars. Our analysis casts some doubt on the Bica et
al. (2001) criterion to look for Possible Open Cluster Remnants. It seems
mandatory to define a better criterion to adopt for further investigations.Comment: 12 pages, 12 eps figures (7, 9 and 11 degraded in resolution),
accepted for publication in Astronomy & Astrophysic
A double stellar generation in the Globular Cluster NGC6656 (M 22). Two stellar groups with different iron and s-process element abundance
AIMS. In this paper we present the chemical abundance analysis from high
resolution UVES spectra of seventeen bright giant stars of the Globular Cluster
M~22. RESULTS. We obtained an average iron abundance of [Fe/H]=-1.76\pm0.02
(internal errors only) and an \alpha enhancement of 0.36\pm0.04 (internal
errors only). Na and O, and Al and O follow the well known anti-correlation
found in many other GCs. We identified two groups of stars with significantly
different abundances of the s-process elements Y, Zr and Ba. The relative
numbers of the two group members are very similar to the ratio of the stars in
the two SGBs of M22 recently found by Piotto (2009). Y and Ba abundances do not
correlate with Na, O and Al. The s-element rich stars are also richer in iron
and have higher Ca abundances. The results from high resolution spectra have
been further confirmed by lower resolution GIRAFFE spectra of fourteen
additional M22 stars. GIRAFFE spectra show also that the Eu -- a pure r-process
element -- abundance is not related to the iron content. We discuss the
chemical abundance pattern of M22 stars in the context of the multiple stellar
populations in GC scenario.Comment: 17 Pages, 21 figures, Accepted for publication in A&
Metal abundances in extremely distant Galactic old open clusters. II. Berkeley 22 and Berkeley 66
We report on high resolution spectroscopy of four giant stars in the Galactic
old open clusters Berkeley~22 and Berkeley~66 obtained with HIRES at the Keck
telescope. We find that and for
Berkeley~22 and Berkeley~66, respectively. Based on these data, we first revise
the fundamental parameters of the clusters, and then discuss them in the
context of the Galactic disk radial abundance gradient. We found that both
clusters nicely obey the most updated estimate of the slope of the gradient
from \citet{fri02} and are genuine Galactic disk objects.Comment: 20 pages, 6 eps figures, accepted for publication in the Astronomical
Journa
Chemical composition of stellar populations in Omega Centauri
We derive abundances of Fe, Na, O, and s-elements from GIRAFFE@VLT spectra
for more than 200 red giant stars in the Milky Way satellite Omega Centauri.
Our preliminary results are that: (i) we confirm that Omega Centauri exibiths
large star-to-star metallicity variations ( 1.4 dex); (ii) the
metallicity distribution reveals the presence of at least five stellar
populations with different [Fe/H]; (iii) a clear Na-O anticorrelation is
clearly observed for the metal-poor and metal-intermediate populations while
apparently the anticorrelation disappears for the most metal-rich populations.
Interestingly the Na level grows with iron.Comment: 2 pages, 2 figures. To appear in the proceedings of IAU Symp. 268
"Light elements in the Universe" (C. Charbonnel, M. Tosi, F. Primas, C.
Chiappini, eds., Cambridge Univ. Press
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