arXiv subjects

Ariane Lançon

Publications and source records attributed to Ariane Lançon.

42 records · Page 3Linked to original sources

arXiv2010

Integrated Properties of AGB Stars in Unresolved Stellar Populations: Simple Stellar Populations and Star Clusters

The evolution of AGB stars is notoriously complex. The confrontation of AGB population models with observed stellar populations is a useful alternative to the detailed study of individual stars in efforts to converge towards a reliable evolution theory. I review here the impact of studies of star clusters on AGB models and AGB population synthesis, deliberately leaving out any more complex stellar populations. Over the last 10 years, despite much effort, the absolute uncertainties in the predictions of the light emitted by intermediate age populations have not been reduced to a satisfactory level. Observational sample definitions, as well as the combination of the natural variance in AGB properties with small number statistics, are largely responsible for this situation. There is hope that the constraints may soon become strong enough, thanks to large unbiased surveys of star clusters, resolved colour-magnitude diagrams, and new analysis methods that can account for the stochastic nature of AGB populations in clusters.

astro-ph.GA↗

arXiv2010

Accounting for Stochastic Fluctuations when Analysing Integrated Light of Star Clusters. I: First Systematics

Star clusters are studied widely both as benchmarks for stellar evolution models and in their own right. Cluster age and mass distributions within galaxies are probes of star formation histories, and of cluster formation and disruption processes. The vast majority of clusters in the Universe is small, and it is well known that the integrated fluxes and colors have broad probability distributions, due to small numbers of bright stars. This paper goes beyond the description of predicted probability distributions, and presents results of the analysis of cluster energy distributions in an explicitly stochastic context. The method developed is Bayesian. It provides posterior probability distributions in the age-mass-extinction space, using multi-wavelength photometric observations and a large collection of Monte-Carlo simulations of clusters of finite stellar masses. Both UBVI and UBVIK datasets are considered, and the study conducted in this paper is restricted to the solar metallicity. We first reassess and explain errors arising from the use of standard analysis methods, which are based on continuous population synthesis models: systematic errors on ages and random errors on masses are large, while systematic errors on masses tend to be smaller. The age-mass distributions obtained after analysis of a synthetic sample are very similar to those found for real galaxies in the literature. The Bayesian approach on the other hand, is very successful in recovering the input ages and masses. Taking stochastic effects into account is important, more important for instance than the choice of adding or removing near-IR data in many cases. We found no immediately obvious reason to reject priors inspired by previous (standard) analyses of cluster populations in galaxies, i.e. cluster distributions that scale with mass as M^-2 and are uniform on a logarithmic age scale.

astro-ph.SR↗

arXiv2009

Intermediate Mass Stars <--> Massive Stars. A workshop around causes and consequences of differing evolutionary paths

The post-main sequence evolution of stars of intermediate or large masses is notoriously complex. In the recent past, a number of workshops and meetings have focused on either the Asymptotic Giant Branch of intermediate mass stars, or the evolution of massive stars. But how well defined is the boundary between these categories of objects defined? How would an observer proceed to classify stars into one or the other category? How do objects near the boundary evolve, die, and contribute to the chemical evolution of their environment? During this 3-day international workshop, 26 high quality presentations were given by specialists in the relevant fields of astrophysics, and stimulating discussions followed. It is technically impossible to provide an exhaustive census of the results and ideas that emerged. In this brief article, we choose to point to key elements of the workshop, some of which are now the topic of new collaborations and will lead to publications elsewhere. For the sake of brevity, we deliberately cite only the contributors to the workshop and no external references. Many bibliographic references can be found in the original presentations, which can be retrieved through: http://astro.u-strasbg.fr/observatoire/obs/stars2009/stars2009.html The programme workshop, which includes the titles of the individual contributions, is provided as an appendix.

astro-ph.SR↗

arXiv2009

The mass-to-light ratio of rich star clusters

We point out a strong time-evolution of the mass-to-light conversion factor eta commonly used to estimate masses of unresolved star clusters from observed cluster spectro-photometric measures. We present a series of gas-dynamical models coupled with the Cambridge stellar evolution tracks to compute line-of-sight velocity dispersions and half-light radii weighted by the luminosity. We explore a range of initial conditions, varying in turn the cluster mass and/or density, and the stellar population's IMF. We find that eta, and hence the estimated cluster mass, may increase by factors as large as 3 over time-scales of 50 million years. We apply these results to an hypothetic cluster mass distribution function (d.f.) and show that the d.f. shape may be strongly affected at the low-mass end by this effect. Fitting truncated isothermal (Michie-King) models to the projected light profile leads to over-estimates of the concentration parameter c of delta c ~ 0.3 compared to the same functional fit applied to the projected mass density.

astro-ph.CO↗

arXiv2009

Constraining the evolution of red supergiants with the integrated light of star clusters

The integrated properties of young star clusters are subject to large cluster-to-cluster variations because they depend directly on small numbers of bright stars. At ages at which red supergiants are expected to exist, these luminous but rare stars can be blamed for most of the variations. If unresolved clusters are to be used to constrain red supergiant models, methods must be developed that take these stochastic fluctuations into account. We discuss prospects and open issues in this field, based on recent work on high mass clusters in M82, and on first experiments towards a Bayesian study of cluster populations.

astro-ph.SR↗

arXiv2001

Stochastic Fluctuations in the Spectrophotometric Properties of Star Clusters

Integrated spectrophotometric properties of stellar systems are intrinsically dispersed due to the stochastic nature of the small numbers of bright stars they contain. Among clusters, only the most massive ones can be used individually for comparison with the mean properties predicted by population synthesis calculations. The appropriate minimal masses depend, among others, on the waveband or colour index studied and on age. Selected indices (near-IR CO and H2O, EW(Halpha)) are used to illustrate the asymmetric nature of the probability distribution of observable properties and their dependence on cluster mass.

astro-ph↗