137 research outputs found
Open clusters: probes of galaxy evolution and bench tests of stellar models
Open clusters are the only example of single-age, single initial chemical
composition populations in the Galaxy, and they play an important role in the
study of the formation and evolution of the Galactic disk. In addition, they
have been traditionally employed to test theoretical stellar evolution models.
A brief review of constraints/tests of white dwarf models/progenitors, and
rotating star models based on Galactic open clusters' observations is
presented, introducing also recent contributions of asteroseismic analyses.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
The main sequences of NGC2808: constraints on the early disc accretion scenario
[Abridged] A new scenario --early disc accretion-- has been proposed very
recently to explain the origin of the multiple population phenomenon in
Galactic globular clusters. It envisages the possibility that a fraction of
low- and very low-mass cluster stars may accrete the ejecta of interacting
massive binary (and possibly also fast rotating massive) stars during the fully
convective, pre-main sequence stage, to reproduce the CN and ONa
anticorrelations observed among stars in individual clusters. This scenario is
assumed to be able to explain the presence (and properties) of the multiple
populations in the majority of globular clusters in the Milky Way. Here we have
considered the well studied cluster NGC 2808, which displays a triple main
sequence with well defined and separate He abundances. Knowledge of these
abundances allowed us to put strong constraints on the He mass fraction and
amount of matter to be accreted by low-mass pre-main sequence stars. We find
that the minimum He mass fraction in the accreted gas has to be to
produce the observed sequences and that at fixed initial mass of the accreting
star, different efficiencies for the accretion are required to produce stars
placed onto the multiple main sequences. This may be explained by differences
in the orbital properties of the progenitors and/or different spatial
distribution of intracluster gas with varying He abundances. Both O-Na and C-N
anticorrelations appear naturally along the main sequences, once considering
the predicted relationship between He and CNONa abundances in the ejecta of the
polluters. As a consequence of the accretion, we predict no discontinuity
between the abundance ranges covered by intermediate and blue main sequence
stars, but we find a sizeable (several 0.1 dex) discontinuity of the N and Na
abundances between objects on the intermediate and red main sequences.Comment: 8 pages, 9 figures, Astronomy & Astrophysics in pres
Chemical element transport in stellar evolution models
Stellar evolution computations provide the foundation of several methods
applied to study the evolutionary properties of stars and stellar populations,
both Galactic and extragalactic. The accuracy of the results obtained with
these techniques is linked to the accuracy of the stellar models, and in this
context the correct treatment of the transport of chemical elements is crucial.
Unfortunately, in many respects calculations of the evolution of the chemical
abundance profiles in stars are still affected by sometime sizable
uncertainties. Here, we review the various mechanisms of element transport
included in the current generation of stellar evolution calculations, how they
are implemented, the free parameters and uncertainties involved, the impact on
the models, and the observational constraints.Comment: 72 pages, 33 figures, invited review paper to be published in Royal
Society Open Science Journa
On the evolution of intra-cluster gas within Galactic globular clusters
It has been known since the 1950's that the observed gas content of Galactic
globular clusters (GCs) is 2-3 orders of magnitude less than the mass lost by
stars between Galactic disk crossings. In this work we address the question:
What happens to this stellar gas? Using an Eulerian nested grid code, we
present 3D simulations to determine how stellar wind material evolves within
the GC environment. We expand upon work done in the 70's and move a single-mass
King-model GC through the Galactic halo medium, stripping a 10^5 Msun GC of its
intra-cluster medium but predicting a detectable medium for a 10^6 Msun
cluster. We find from new multi-mass King model simulations, the first to
incorporate empirical mass-loss formulae, that the single-mass King model
underestimates the retention of intra-cluster gas in the cluster. Lastly, we
present a simple discretised multi-mass GC model, which yields lower levels of
intra-cluster medium compared to the continuous single- and multi-mass King
models. Our results show that there is still an issue with the predicted
intra-cluster gas content of massive GCs. We conclude that by modelling GC
systems more accurately, in particular the stellar structure and description of
mass loss, we will be able to work towards resolving this issue and begin to
fill in some of the gaps in our understanding of the evolution of globular
clusters.Comment: 19 pages, 19 pdf figures. Accepted for publication in Monthly Notices
of the Royal Astronomical Societ
The red giant branch phase transition: Implications for the RGB luminosity function bump and detections of Li-rich red clump stars
We performed a detailed study of the evolution of the luminosity of
He-ignition stage and of the red giant branch bump luminosity during the red
giant branch phase transition for various metallicities. To this purpose we
calculated a grid of stellar models that sample the mass range of the
transition with a fine mass step equal to . We find that for
a stellar population with a given initial chemical composition, there is a
critical age (of 1.1-1.2~Gyr) around which a decrease in age of just 20-30
million years causes a drastic drop in the red giant branch tip brightness. We
also find a narrow age range (a few yr) around the transition,
characterized by the luminosity of the red giant branch bump being brighter
than the luminosity of He ignition. We discuss a possible link between this
occurrence and observations of Li-rich core He-burning stars.Comment: 5 pages, 4 figures, Astronomy & Astrophysics in pres
The age of the oldest Open Clusters
We determine ages of 71 old Open Clusters by a two-step method: we use
main-squence fitting to 10 selected clusters, in order to obtain their
distances, and derive their ages from comparison with our own isochrones used
before for Globular Clusters. We then calibrate the morphological age indicator
delta(V), which can be obtained for all remaining clusters, in terms of age and
metallicity. Particular care is taken to ensure consistency in the whole
procedure. The resulting Open Cluster ages connect well to our previous
Globular Cluster results. From the Open Cluster sample, as well as from the
combined sample, questions regarding the formation process of Galactic
components are addressed. The age of the oldest open clusters (NGC6791 and
Be17) is of the order of 10 Gyr. We determine a delay by 2.0+-1.5 Gyr between
the start of the halo and thin disk formation, whereas thin and thick disk
started to form approximately at the same time. We do not find any significant
age-metallicity relationship for the open cluster sample. The cumulative age
distribution of the whole open cluster sample shows a moderately significant
(~2sigma level) departure from the predictions for an exponentially declining
dissolution rate with timescale of 2.5 Gyr. The cumulative age distribution
does not show any trend with galactocentric distance, but the clusters with
larger height to the Galactic plane have an excess of objects between 2-4 and 6
Gyr with respect to their counterpart closer to the plane of the Galaxy.Comment: Astronomy & Astrophysics, in pres
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