2,986 research outputs found
Old open clusters in the outer Galactic disk
The outer parts of the Milky Way disk are believed to be one of the main
arenas where the accretion of external material in the form of dwarf galaxies
and subsequent formation of streams is taking place. The Monoceros stream and
the Canis Major and Argo over-densities are notorious examples. VLT high
resolution spectra have been acquired for five distant open clusters. We derive
accurate radial velocities to distinguish field interlopers and cluster
members. For the latter we perform a detailed abundance analysis and derive the
iron abundance [Fe/H] and the abundance ratios of several elements.
Our analysis confirms previous indications that the radial abundance gradient
in the outer Galactic disk does not follow the expectations extrapolated from
the solar vicinity, but exhibits a shallower slope. By combining the
metallicity of the five program clusters with eight more clusters for which
high resolution spectroscopy is available, we find that the mean metallicity in
the outer disk between 12 and 21 kpc from the Galactic center is [Fe/H]
, with only marginal indications for a radial variation. In
addition, all the program clusters exhibit solar scaled or slightly enhanced
elements, similar to open clusters in the solar vicinity and thin disk
stars. We investigate whether this outer disk cluster sample might belong to an
extra-galactic population, like the Monoceros ring. However, close scrutiny of
their properties - location, kinematics and chemistry - does not convincingly
favor this hypothesis. On the contrary, they appear more likely genuine
Galactic disk clusters. We finally stress the importance to obtain proper
motion measurements for these clusters to constrain their orbits.Comment: 19 pages, 9 eps figure, in press in A&A, abstract rephrased to fit i
Endogenous Minimum Participation in International Environmental Treaties
Many international treaties come into force only after a minimum number of countries have signed and ratified the treaty. Why do countries agree to introduce a minimum participation constraint among the rules characterising an international treaty? This question is particularly relevant in the case of environmental treaties dealing with global commons, where free-riding incentives are strong. Is a minimum participation rule a way to offset these free-riding incentives? Why do countries that know they have an incentive to free-ride accept to tie their hands through the introduction of a minimum participation constraint? This paper addresses the above questions by analysing a three-stage non-cooperative coalition formation game. In the first stage, countries set the minimum coalition size that is necessary for the treaty to come into force. In the second stage, countries decide whether to sign the treaty. In the third stage, the equilibrium values of the decision variables are set. At the equilibrium, both the minimum participation constraint and the number of signatories the coalition size are determined. This paper shows that a non-trivial partial coalition, sustained by a binding minimum participation constraint, forms at the equilibrium. This paper thus explains why in international negotiations all countries often agree on a minimum participation rule even when some of them do not intend to sign the treaty. The paper also analyses the optimal size of the minimum participation constraint
The Progeny of Stellar Dynamics and Stellar Evolution within an N-body model of NGC 188
We present a direct N-body simulation modeling the evolution of the old (7
Gyr) open cluster NGC 188. This is the first N-body open cluster simulation
whose initial binary population is directly defined by observations of a
specific open cluster: M35 (150 Myr). We compare the simulated color-magnitude
diagram at 7 Gyr to that of NGC 188, and discuss the blue stragglers produced
in the simulation. We compare the solar-type main sequence binary period and
eccentricity distributions of the simulation to detailed observations of
similar binaries in NGC 188. These results demonstrate the importance of
detailed observations in guiding N-body open cluster simulations. Finally, we
discuss the implications of a few discrepancies between the NGC 188 model and
observations and suggest a few methods for bringing N-body open cluster
simulations into better agreement with observations.Comment: Contributed talk at IAUS 266 'Star clusters: basic galactic building
blocks', R. de Grijs and J.R.D. Lepine, eds. 6 pages, 4 figure
Pinning and Tribology of Tethered Monolayers on Disordered Substrates
We study the statistical mechanics and dynamics of crystalline films with a
fixed internal connectivity on a random substrate. Defect free triangular
lattices exhibit a sharp transition to a low temperature glassy phase with
anomalous phonon fluctuations and a nonlinear force-displacement law with a
continuously variable exponent, similar to the vortex glass phase of directed
lines in 1+1 dimensions. The periodicity of the tethered monolayer acts like a
filter which amplifies particular Fourier components of the disorder. However,
the absence of annealed topological defects like dislocations is crucial: the
transition is destroyed when the constraint of fixed connectivity is relaxed
and dislocations are allowed to proliferate.Comment: revtex, preprint style, 27 pages. This submission is a revision of
cond-mat/9607184. The revisions affect only Appendix B, Appendix C, and Eqs.
2.27, 2.28, 2.3
A brown dwarf companion to the intermediate-mass star HR6037
In the course of an imaging survey we have detected a visual companion to the
intermediate-mass star HR 6037. In this letter, we present two epoch
observations of the binary with NACO/VLT, and near-IR spectroscopy of the
secondary with ISAAC/VLT. The NACO observations allow us to confirm HR 6037B as
a co-moving companion. Its J and H band ISAAC spectra suggest the object has an
spectral type of M9+-1, with a surface gravity intermediate between that of 10
Myr dwarfs and field dwarfs with identical spectral type. The comparison of its
Ks-band photometry with evolutionary tracks allows us to derive a mass,
effective temperature, and surface gravity of 62+-20 MJup, Teff = 2330+-200 K,
and log g = 5.1+-0.2, respectively. The small mass ratio of the binary, -0.03,
and its long orbital period, -5000 yr, makes HR 6037 a rare and uncommon binary
system.Comment: (5 pages, 4 figures, accepted for publication in A&A Letters
Open clusters towards the Galactic center: chemistry and dynamics. A VLT spectroscopic study of NGC6192, NGC6404, NGC6583
In the framework of the study of the Galactic metallicity gradient and its
time evolution, we present new high-resolution spectroscopic observations
obtained with FLAMES and the fiber link to UVES at VLT of three open clusters
(OCs) located within 7~kpc from the Galactic Center (GC): NGC~6192,
NGC~6404, NGC~6583. We also present new orbit determination for all OCs with
Galactocentric distances (R8~kpc and metallicity from
high-resolution spectroscopy. We aim to investigate the slope of the inner disk
metallicity gradient as traced by OCs and at discussing its implication on the
chemical evolution of our Galaxy. We have derived memberships of a group of
evolved stars for each clusters, obtaining a sample of 4, 4, and 2 member stars
in NGC~6192, NGC~6404, and NGC~6583, respectively. Using standard LTE analysis
we derived stellar parameters and abundance ratios for the iron-peak elements
Fe, Ni, Cr, and for the -elements Al, Mg, Si, Ti, Ca. We calculated the
orbits of the OCs currently located within 8~kpc from the GC, and discuss their
implication on the present-time radial location. {The average metallicities of
the three clusters are all oversolar: [Fe/H]= (NGC~6192),
(NGC 6404), (NGC 6583). They are in qualitative
agreement with their Galactocentric distances, being all internal OCs, and thus
expected to be metal richer than the solar neighborhood. The abundance ratios
of the other elements over iron [X/Fe] are consistent with solar values. The
clusters we have analysed, together with other OC and Cepheid data, confirm a
steep gradient in the inner disk, a signature of an evolutionary rate different
than in the outer disk.Comment: 17 pages, 13 figures, A&A accepted for publicatio
Anchors for the Cosmic Distance Scale: the Cepheids U Sgr, CF Cas and CEab Cas
New and existing X-ray, UBVJHKsW(1-4), and spectroscopic observations were
analyzed to constrain fundamental parameters for M25, NGC 7790, and dust along
their sight-lines. The star clusters are of particular importance given they
host the classical Cepheids U Sgr, CF Cas, and the visual binary Cepheids CEa
and CEb Cas. Precise results from the multiband analysis, in tandem with a
comprehensive determination of the Cepheids' period evolution (dP/dt) from ~140
years of observations, helped resolve concerns raised regarding the clusters
and their key Cepheid constituents. Specifically, distances derived for members
of M25 and NGC 7790 are 630+-25 pc and 3.40+-0.15 kpc, respectively.Comment: To appear in Astronomy and Astrophysic
Poly-chlorinated biphenyls (PCB) in European sea bass from different rearing systems
The chemical composition and the level of seven indicator congeners of PCB (BZ/IUPAC no. 28, 52, 101, 118, 138, 153, and 180) were determined in 133 specimens of farm-raised European sea bass (Dicentrarchus labrax). The fish were caught from different aquaculture rearing systems: extensive fish valley, semi-intensive ponds, sea-cages, and intensive concrete tanks. Fresh fillet chemical composition differed among the rearing systems (fat: 2.9, 7.5, 7.1, and 9.4%; P<0.001). Total concentrations of indicator congeners were below the EU limit (200ng/g fat) for meat, poultry and eggs, being the lowest in extensively-reared sea bass (75ng/g fat), intermediate in sea bass from semi-intensive ponds (119) and sea cages (116), and the highest in intensively-reared fish (133) (P<0.001). Similarly, PCB concentrations in fresh fillets were 2,438, 10,116, 8,491, and 12,952pg/g in the four systems (P<0.001). The congener 153 was the most represented in all rearing systems. TEQ concentrations for the dioxin-like congener no. 118 were 50 to 200 times lower than the maximum admitted value. Total concentration of indicator congeners of PCB was poorly correlated with fish slaughter weight (R2=0.17), while highly correlated with fat concentration of fish (R2=0.75)
On the mutual effect of ion temperature gradient instabilities and impurity peaking in the reversed field pinch
The presence of impurities is considered in gyrokinetic calculations of ion
temperature gradient (ITG) instabilities and turbulence in the reversed field
pinch device RFX-mod. This device usually exhibits hollow Carbon/Oxygen
profiles, peaked in the outer core region. We describe the role of the
impurities in ITG mode destabilization, and analyze whether ITG turbulence is
compatible with their experimental gradients.Comment: 19 pages, 9 figures, accepted for publication in Plasma Phys.
Control. Fusio
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