905 research outputs found
Ba & Eu Abundances in M15 giant stars
To investigate the Ba and Eu abundances for a sample of 63 giant stars in the
globular cluster M15. This is the largest sample of M15 giants stars for which
Ba abundances have been determined and, due to the target selection of the
original research programme, the Ba abundances are complete along the red giant
branch. Stellar parameters were taken from the previous key study and a
microturbulence-surface gravity relation was determined for precise measurement
of the Ba line at 6496.898 Angstroms, which has a high sensitivity to
microturbulence. Element abundances for Ba, La, Eu, Ca, Ni and Fe were
calculated using spectrum synthesis and equivalent widths techniques. A bimodal
distribution in Ba, Eu and La abundances was found within the sample. The low
Ba,Eu,La mode had mean abundances of =-2.41+/-0.16,
=-1.80+/-0.08 and =-2.19+/-0.13 while the high Ba,Eu,La mode
had mean abundances of =-2.00+/-0.16, =-1.65+/-0.13 and
=-1.95+/-0.11. Both modes are indicative of a pollution scenario
dominated by the r-process, hence contributions from explosive nucleosynthesis
of massive stars. There may be evidence of further enhancement by another heavy
element process and of potential anticorrelations in Na-O for both modes
indicating a complex formation and evolution history for M15.Comment: 20 pages, 15 figure
Magnetic field induced non-Fermi liquid to Fermi liquid crossover at the quantum critical point of YbCuAu
The temperature (T) dependence of the muon and Cu nuclear spin-lattice
relaxation rates in YbCu4.4Au0.6 is reported over nearly four decades.
It is shown that for diverges following the behaviour
predicted by the self-consistent renormalization (SCR) theory developed by
Moriya for a ferromagnetic quantum critical point. On the other hand, the
static uniform susceptibility is observed to diverge as and
, a behaviour which is not accounted for by SCR theory.
The application of a magnetic field is observed to induce a crossover to a
Fermi liquid behaviour and for is found to obey the scaling
law .Comment: 4 pages, 4 figure
Multiple populations in Omega Centauri: a cluster analysis of spectroscopic data
Omega Cen is composed of several stellar populations. Their history might
allow us to reconstruct the evolution of this complex object. We performed a
statistical cluster analysis on the large data set provided by Johnson and
Pilachowski (2010). Stars in Omega Cen divide into three main groups. The
metal-poor group includes about a third of the total. It shows a moderate O-Na
anticorrelation, and similarly to other clusters, the O-poor second generation
stars are more centrally concentrated than the O-rich first generation ones.
This whole population is La-poor, with a pattern of abundances for n-capture
elements which is very close to a scaled r-process one. The metal-intermediate
group includes the majority of the cluster stars. This is a much more complex
population, with an internal spread in the abundances of most elements. It
shows an extreme O-Na anticorrelation, with a very numerous population of
extremely O-poor and He-rich second generation stars. This second generation is
very centrally concentrated. This whole population is La-rich, with a pattern
of the abundances of n-capture elements that shows a strong contribution by the
s-process. The spread in metallicity within this metal-intermediate population
is not very large, and we might attribute it either to non uniformities of an
originally very extended star forming region, or to some ability to retain a
fraction of the ejecta of the core collapse SNe that exploded first, or both.
As previously noticed, the metal-rich group has an Na-O correlation, rather
than anticorrelation. There is evidence for the contribution of both massive
stars ending their life as core-collapse SNe, and intermediate/low mass stars,
producing the s-capture elements. Kinematics of this population suggests that
it formed within the cluster rather than being accreted.Comment: Accepted for publication in Astronomy and Astrophysic
Origin of the abundance patterns in Galactic globular clusters: constraints on dynamical and chemical properties of globular clusters
Aims. We analyse the effects of a first generation of fast rotating massive
stars on the dynamical and chemical properties of globular clusters. Methods.
We use stellar models of fast rotating massive stars, losing mass through a
slow mechanical equatorial winds to produce material rich in H-burning
products. We propose that stars with high Na and low O abundances (hereafter
anomalous stars) are formed from matter made of slow winds of individual
massive stars and of interstellar matter. The proportion of slow wind and of
interstellar material is fixed in order to reproduce the observed Li-Na
anticorrelation in NGC 6752. Results. In the case that globular clusters,
during their lifetime, did not lose any stars, we found that to reproduce the
observed ratio of normal to anomalous stars, a flat initial mass function (IMF)
is needed, with typically a slope x=0.55 (a Salpeter's IMF has x=1.35). In the
case that globular clusters suffer from an evaporation of normal stars, the IMF
slope can be steeper: to have x=1.35, about 96% of the normal stars would be
lost. We make predictions for the distribution of stars as a function of their
[O/Na] and obtain quite reasonable agreement with that one observed for NGC
6752. Predictions for the number fraction of stars with different values of
helium, of the 12C/13C and 16O/17O ratios are discussed, as well as the
expected relations between values of [O/Na] and those of helium, of [C/N], of
12C/13C and of 16O/17O. Future observations might test these predictions. We
also provide predictions for the present day mass of the clusters expressed in
units of mass of the gas used to form stars, and for the way the present day
mass is distributed between the first and second generation of stars and the
stellar remnants.Comment: 15 pages, 14 figures, accepted in A&
RR Lyrae variables in the globular cluster M3 (NGC5272). I. BVI CCD photometry
New BVI CCD photometry is presented for 60 RR Lyrae variables in the globular
cluster M3. Light curves have been constructed and ephemerides have been
(re)-derived for all of them. Four stars (i.e. V29, V136, V155 and V209),
although recognized as variables, had no previous period determinations. Also,
the period derived for V129 is significantly different from the one published
by Sawyer-Hogg (1973). Light curve parameters, i.e. mean magnitudes, amplitudes
and rise-times, have been derived. The discussion of these results in the
framework of the stellar evolution and pulsation theories will be presented in
a forthcoming paper.Comment: 19 pages, latex, uses mn.sty, 12 encapsulated figures, to be
published in MNRAS, text and figures also available at
http://www.bo.astro.it/bap/BAPhome.html or via anonymous ftp at
ftp://boas3.bo.astro.it/bap/files (bap98-12-textfig.ps
Mining SDSS in search of Multiple Populations in Globular Clusters
Several recent studies have reported the detection of an anomalous color
spread along the red giant branch (RGB) of some globular clusters (GC) that
appears only when color indices including a near ultraviolet band (such as
Johnson U or Stromgren u) are considered. This anomalous spread in color
indexes such as U-B or c_{y} has been shown to correlate with variations in the
abundances of light elements such as C, N, O, Na, etc., which, in turn, are
generally believed to be associated with subsequent star formation episodes
that occurred in the earliest few 10^{8} yr of the cluster's life. Here we use
publicly available u, g, r Sloan Digital Sky Survey photometry to search for
anomalous u-g spreads in the RGBs of nine Galactic GCs. In seven of them (M 2,
M 3, M 5, M 13, M 15, M 92 and M 53), we find evidence of a statistically
significant spread in the u-g color, not seen in g-r and not accounted for by
observational effects. In the case of M 5, we demonstrate that the observed u-g
color spread correlates with the observed abundances of Na, the redder stars
being richer in Na than the bluer ones. In all the seven clusters displaying a
significant u-g color spread, we find that the stars on the red and blue sides
of the RGB, in (g, u-g) color magnitude diagrams, have significantly different
radial distributions. In particular, the red stars (generally identified with
the second generation of cluster stars, in the current scenario) are always
more centrally concentrated than blue stars (generally identified with the
first generation) over the range sampled by the data (0.5r_{h} < r < 5r_{h}),
in qualitative agreement with the predictions of some recent models of the
formation and chemical evolution of GCs. Our results suggest that the
difference in the radial distribution between first and second generation stars
may be a general characteristic of GCs.Comment: 11 pages, 5 figures, typos adde
Frustration driven structural distortion in VOMoO4
Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR),
magnetization measurements and electronic structure calculations in VOMoO4 are
presented. It is found that VOMoO4 is a frustrated two-dimensional
antiferromagnet on a square lattice with competing exchange interactions along
the side J1 and the diagonal J2 of the square. From magnetization measurements
J1+J2 is estimated around 155 K, in satisfactory agreement with the values
derived from electronic structure calculations. Around 100 K a structural
distortion, possibly driven by the frustration, is evidenced. This distortion
induces significant modifications in the NMR and EPR spectra which can be
accounted for by valence fluctuations. The analysis of the spectra suggests
that the size of the domains where the lattice is distorted progressively grows
as the temperature approaches the transition to the magnetic ground state at
Tc=42 K
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
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