3,347 research outputs found
Constraining the third dredge-up via carbon stars in the Magellanic Clouds
We use the available data for Magellanic Cloud carbon stars to constrain the
efficiency of the third dredge-up process in TP-AGB models. We show that star
counts in LMC clusters provide quite stringent limits to the lifetime of the
C-star phase, with a duration between 2 and 3 Myr for stars in the mass range
from 1.5 to 2.8 Msun. Together with the luminosity functions of field C stars,
this information allows us to re-calibrate the third dredge-up parameters log
T_b_dred and lambda in TP-AGB models that include variable molecular opacities
(Marigo 2002). Preliminary results are presented here.Comment: in proceedings of the St. Luc conference ``CNO in the Universe'',
eds. C. Charbonnel, D. Schaerer, & G. Meynet, ASP Conference Series, in pres
Quoted spreads and trade imbalance dynamics in the European treasury bond market
Using high-frequency transaction data for the three largest European markets (France, Germany and Italy), this paper documents the existence of an asymmetric relationship between market liquidity and trading imbalances: when quoted spreads rise (fall) and liquidity falls (increases) buy (sell) rders tend to prevail. Risk-averse market-makers, with inventory-depletion risk being their main concern, tend to quote wider narrower) spreads when they think bond appreciation is more (less) likely to occur. It is also found that the probability of being in a specific regime is related to observable bond market characteristics, tock market volatility, macroeconomic
releases and liquidity management operations of the monetary authorities
The WIYN Open Cluster Study Photometric Binary Survey: Initial Findings for NGC 188
The WIYN open cluster study (WOCS) has been working to yield precise
magnitudes in the Johnson-Kron-Cousins UBVRI system for all stars in the field
of a selection of ``prototypical'' open clusters. Additionally, WOCS is using
radial velocities to obtain orbit solutions for all cluster binary stars with
periods of less than 1000 days. Recently, WOCS is being expanded to include the
near-infrared JHK_s (deep ground-based plus 2MASS) and mid-infrared ([3.6],
[4.5], [5.8], [8.0]) photometry from Spitzer/IRAC observations. This
multi-wavelength data (0.3--8.0 microns) allows us photometrically to identify
binaries, with mass ratios from 1.0--0.3, across a wide range of primary
masses. The spectral energy distribution (SED) fitter by Robitaille et al.
(2007) is used to fit the fluxes of 10--12 bands, converted from the observed
magnitudes, to Kurucz stellar models. Using this photometric technique, we find
that NGC 188 has a binary fraction of 36--49% and provide a star-by-star
comparison to the WOCS radial velocity-based binary study.Comment: 2 pages, 2 figures, Conference Proceedings from "Dynamical Evolution
of Dense Stellar Systems'', IAU Symposium 246, Eds. E. Vesperini, M. Giersz,
& A. Sill
Evolution of zero-metallicity massive stars
We discuss the evolutionary properties of primordial massive and very massive
stars, supposed to have formed from metal-free gas. Stellar models are
presented over a large range of initial masses (8 Msun <= Mi <= 1000 Msun),
covering the hydrogen- and helium-burning phases up to the onset of carbon
burning. In most cases the evolution is followed at constant mass. To estimate
the possible effect of mass loss via stellar winds, recent analytic formalisms
for the mass-loss rates are applied to the very massive models (Mi >= 120
Msun).Comment: Invited talk at IAU Symp. 212, ``A Massive Star Odyssey, from Main
Sequence to Supernova'', K.A. van der Hucht, A. Herrero, C. Esteban (eds.), 7
pages, 5 postscript figure
Structure and Evolution of Galaxy Clusters: Internal Dynamics of ABCG 209 at z~0.21
We study the internal dynamics of the rich galaxy cluster ABGC 209 on the
basis of new spectroscopic and photometric data. The distribution in redshift
shows that ABCG 209 is a well isolated peak of 112 detected member galaxies at
z=0.209, characterised by a high value of the line-of-sight velocity
dispersion, sigma_v=1250-1400 Km/s, on the whole observed area (1 Mpc/h from
the cluster center), that leads to a virial mass of M=1.6-2.2x10^15 M_sun
within the virial radius, assuming the dynamical equilibrium. The presence of a
velocity gradient in the velocity field, the elongation in the spatial
distribution of the colour-selected likely cluster members, the elongation of
the X-ray contour levels in the Chandra image, and the elongation of cD galaxy
show that ABCG 209 is characterised by a preferential NW-SE direction. We also
find a significant deviation of the velocity distribution from a Gaussian, and
relevant evidence of substructure and dynamical segregation. All these facts
show that ABCG 209 is a strongly evolving cluster, possibly in an advanced
phase of merging.Comment: 26 pages, 14 figures. A&A in pres
Coupling emitted light and chemical yields from stars: a basic constraint to population synthesis models of galaxies
In this paper we emphasize the close connection between the chemical and
spectrophotometric evolution of stellar systems: Chemical yields from stars
correspond to a precise fraction of their emitted light. We translate this
concept quantitatively. Starting from simple stellar populations, we derive
useful analytical relations to calculate the stellar fuel consumption (emitted
light) as a function of basic quantities predicted by stellar models, i.e. the
mass of the core and the chemical composition of the envelope. The final
formulas explicate the relation between integrated light contribution (total or
limited to particular evolutionary phases), chemical yields and stellar
remnants. We test their accuracy in the case of low- and intermediate-mass
stars, and indicate the way to extend the analysis to massive stars. This
formalism provides an easy tool to check the internal consistency between the
different stellar inputs adopted in galaxy models: The fuel computed by means
of the analytical formulas (corresponding to a given set of chemical yields)
should be compared to the exact values given by the luminosity integration
along the stellar evolutionary tracks or isochrones (corresponding to a given
set of spectrophotometric models). Only if both estimates of the fuel are
similar, the stellar inputs can be considered self-consistent in terms of their
energetics. This sets an important requirement to galaxy models, also in
consideration of the fact that different sources of input stellar data are
frequently used to model their spectro-photometric and chemical evolution.Comment: 17 pages, 7 figures, 3 tables, accepted for publication in A&
Clump stars in the Solar Neighbourhood
Hipparcos data has allowed the identification of a large number of clump
stars in the Solar Neighbourhood. We discuss our present knowledge about their
distributions of masses, ages, colours, magnitudes, and metallicities. We point
out that the age distribution of clump stars is ``biased'' towards
intermediate-ages. Therefore, the metallicity information they contain is
different from that provided by the local G dwarfs. Since accurate abundance
determinations are about to become available, these may provide useful
constraints to chemical evolution models of the local disc.Comment: 6 pages, proc. of the Sept. 20-24, 1999 Vulcano Workshop "The
chemical evolution of the Milky Way: stars vs. clusters", eds. F. Matteucci,
F. Giovanell
Integrated parameters of star clusters: A comparison of theory and observations
(Abridged) This paper presents integrated magnitude and colours for synthetic
clusters. The integrated parameters have been obtained for the whole cluster
population as well as for the main-sequence (MS) population of star clusters.
We have also estimated observed integrated magnitudes and colours of MS
population of galactic open clusters, LMC and SMC star clusters. It is found
that the colour evolution of MS population of star clusters is not affected by
the stochastic fluctuations, however these fluctuations significantly affect
the colour evolution of the whole cluster population. The fluctuations are
maximum in colour in the age range 6.7 log (age) 7.5. Evolution
of integrated colours of MS population of the clusters in the Milky Way, LMC
and SMC, obtained in the present study are well explained by the present
synthetic cluster model. The observed integrated colours of MS
population of LMC star clusters having age 500 Myr seem to be
distributed around 0.004 model, whereas colours are found to be
more bluer than those predicted by the 0.004 model. vs
two-colour diagram for the MS population of the Milky Way star clusters shows a
fair agreement between the observations and present model, however the diagrams
for LMC and SMC clusters indicate that observed colours are relatively
bluer. Possible reasons for this anomaly have been discussed.Comment: 35 pages, 20 figs, accepted for publication in MNRA
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