The modelling of intermediate-age stellar populations: I- near-infrared properties

In this paper, we discuss how the integrated properties of intermediate-age single burst population, especially in the near-infrared, behave as a function of age and metallicity. Our models take into account all stellar evolutionary phases that affect the evolution of the integrated optical and near-infrared spectrum of such a population. Particular care was dedicated to the Asymptotic Giant Brach stars, which can be dominant at near-infrared wavelengths. First we present new synthetic model that takes into account the relevant physical processes that control the evolution through the thermally pulsing AGB. In the isochrone presented in this paper the lifetime and the nature of the AGB stars are established as consequences of the interplay between the physical processes that control the AGB star evolution. The contribution of these stars to the integrated light of the population is thus obtained in a consistent way. We optimize our models by using a new stellar library that explicitly takes into account the spectral features thar characterize only AGB stars in comparison to other cool and luminous stars (abriged)Comment: Accepted for publication in A&A, 19 page

Standardizing and destandardizing practices at a Flemish secondary school : a sociolinguistic ethnographic perspective on Flemish pupils’ speech practices

For a couple of decades now, in Flanders, the functional elaboration of what is generally called tussentaal, i.e. mesolectal language use situated in between (‘tussen’) acrolectal Standard Dutch and basilectal Flemish dialects, has caused increasing concern about the position of Standard Dutch relative to other recognized ways of speaking. This has provoked intense debate about the proper characterization of this evolution. This paper focuses on the daily language practices and overt attitudes of six girls at a Flemish secondary school to illustrate that it is relatively easy to find evidence that suggests this evolution is properly characterized as a type of destandardization. Yet by zooming in on the covert SLI-influenced language attitudes of the girls, I will argue that a close ethnographic study of daily language use is able to go beyond the surface appearances of larger-scale ideologies and can demonstrate the continuing influence of standardization. Sociolinguistic ethnography may therefore have a vital role to play in the ongoing debate about language variation in Flanders

A Spinor Model for Quantum Cosmology

The question of the interpretation of Wheeler-DeWitt solutions in the context of cosmological models is addressed by implementing the Hamiltonian constraint as a spinor wave equation in minisuperspace. We offer a relative probability interpretation based on a non-closed vector current in this space and a prescription for a parametrisation of classical solutions in terms of classical time. Such a prescription can accommodate classically degenerate metrics describing manifolds with signature change. The relative probability density, defined in terms of a Killing vector of the Dewitt metric on minisuperspace, should permit one to identify classical loci corresponding to geometries for a classical manifold. This interpretation is illustrated in the context of a quantum cosmology model for two-dimensional dilaton gravity.Comment: 17p, Lanc-Th-G940

Enhanced methods for local ancestry assignment in sequenced admixed individuals.

Inferring the ancestry at each locus in the genome of recently admixed individuals (e.g., Latino Americans) plays a major role in medical and population genetic inferences, ranging from finding disease-risk loci, to inferring recombination rates, to mapping missing contigs in the human genome. Although many methods for local ancestry inference have been proposed, most are designed for use with genotyping arrays and fail to make use of the full spectrum of data available from sequencing. In addition, current haplotype-based approaches are very computationally demanding, requiring large computational time for moderately large sample sizes. Here we present new methods for local ancestry inference that leverage continent-specific variants (CSVs) to attain increased performance over existing approaches in sequenced admixed genomes. A key feature of our approach is that it incorporates the admixed genomes themselves jointly with public datasets, such as 1000 Genomes, to improve the accuracy of CSV calling. We use simulations to show that our approach attains accuracy similar to widely used computationally intensive haplotype-based approaches with large decreases in runtime. Most importantly, we show that our method recovers comparable local ancestries, as the 1000 Genomes consensus local ancestry calls in the real admixed individuals from the 1000 Genomes Project. We extend our approach to account for low-coverage sequencing and show that accurate local ancestry inference can be attained at low sequencing coverage. Finally, we generalize CSVs to sub-continental population-specific variants (sCSVs) and show that in some cases it is possible to determine the sub-continental ancestry for short chromosomal segments on the basis of sCSVs

Analysing star cluster populations with stochastic models: the HST/WFC3 sample of clusters in M83

The majority of clusters in the Universe have masses well below 10^5 Msun. Hence their integrated fluxes and colors can be affected by the random presence of a few bright stars introduced by stochastic sampling of the stellar mass function. Specific methods are being developed to extend the analysis of cluster SEDs into the low-mass regime. In this paper, we apply such a method to observations of star clusters, in the nearby spiral galaxy M83. We reassess ages and masses of a sample of 1242 objects for which UBVIHalpha fluxes were obtained with the HST/WFC3 images. Synthetic clusters with known properties are used to characterize the limitations of the method. The ensemble of color predictions of the discrete cluster models are in good agreement with the distribution of observed colors. We emphasize the important role of the Halpha data in the assessment of the fraction of young objects, particularly in breaking the age-extinction degeneracy that hampers an analysis based on UBVI only. We find the mass distribution of the cluster sample to follow a power-law of index -2.1 +/-0.2, and the distribution of ages a power-law of index -1.0 +/-0.2 for M > 10^3.5 Msun and ages between 10^7 and 10^9 yr. An extension of our main method, that makes full use of the probability distributions of age and mass of the individual clusters, is explored. It produces similar power-law slopes and will deserve further investigation. Although the properties derived for individual clusters significantly differ from those obtained with traditional, non-stochastic models in ~30% of the objects, the first order aspect of the age and mass distributions are similar to those obtained previously for this M83 sample in the range of overlap of the studies. We extend the power-law description to lower masses with better mass and age resolution and without most of the artifacts produced by the classical method.Comment: accepted for publication in ApJ, 29 pages, 20 figure

Near-IR Spectra of Red Supergiants and Giants. I- Models with Solar and with Mixing-Induced Surface Abundance Ratios

We provide a grid of PHOENIX spectra of red giant and supergiant stars, that extend through optical and near-IR wavelengths. For the first time, models are also provided with modified surface abundances of C, N and O, as a step towards accounting for the changes known to occur due to convective dredge-up (and to be enhanced in the case of rotation). The aims are (i) to assess how well current models reproduce observed spectra, (ii) to quantify the effects of the abundance changes on the spectra, and (iii) to determine how these changes affect estimates of fundamental stellar parameters. Observed giant star spectra can be fitted very well at solar metallicity down to about 3400K. Modified surface abundances are preferred in only a minority of cases for luminosity class II, possibly indicating mixing in excess of standard first dredge-up. Supergiant stars show a larger variety of near-IR spectra, and good fits are currently obtained for about one third of the observations only. Modified surface abundances help reproducing strong CN bands, but do not suffice to resolve all the difficulties. The effect of the abundance changes on the estimated Teff depends on the wavelength range of observation and can amount several 100K. Reasons for the remaining discrepancies are discussed.Comment: To be published in A&A. 19 p., 35 postscript figures, uses aa.cls. Selected model spectra available through CD

New Near-Infrared Surface Brightness Fluctuation Models

We present new theoretical models for surface brightness fluctuations in the near-infrared. We show the time evolution of near-infrared brightness fluctuation properties over large age and metallicity ranges, i.e., from 12 Myr to 16 Gyr, and from Z/Zsun=1/50 to Z/Zsun=2.5, for single age, single metallicity stellar populations. All the stellar models are followed from the zero age main sequence to the central carbon ignition for massive stars, or to the end of the thermally pulsing regime of the asymptotic giant branch phase for low and intermediate mass stars. The new models are compared with observed near-infrared fluctuation absolute magnitudes and colours for a sample of Magellanic Cloud star clusters and Fornax Cluster galaxies. For star clusters younger than ~3 Gyr, the predicted near-infrared fluctuation properties are in a satisfactory agreement with observed ones over a wide range of stellar population metallicities. However, for older star clusters, the agreement between the observed and predicted near-IR brightness fluctuations depends on how the surface brightness absolute magnitudes are estimated. The computed set of models are not able to match the observed near-IR fluctuation absolute magnitudes and colours simultaneously. We argue that the observed discrepancies between the predicted and observed properties of old MC superclusters are more likely due to observational reasons.Comment: MNRAS, 2005, 362, 1208 (12 pages, 7 figures

Near-IR spectroscopic ages of massive star clusters in M82

Like other starburst galaxies, M82 hosts compact, massive young star clusters that are interesting both in their own right and as benchmarks for population synthesis models. Can spectral synthesis models at resolutions around 1000 adequately reproduce the near-IR spectral features and the energy distribution of these clusters between 0.8 and 2.4 microns? How do the derived cluster properties compare with previous results from optical studies? We analyse the spectra of 5 massive clusters in M82, using data acquired with the spectrograph SpeX on the InfraRed Telescope Facility (NASA/IRTF) and a new population synthesis tool with a highly improved near-IR extension, based on a recent collection of empirical and theoretical spectra of red supergiant stars. We obtain excellent fits across the near-IR with models at quasi-solar metallicity and a solar neighbourhood extinction law. Spectroscopy breaks a strong degeneracy between age and extinction in the near-IR colours in the red supergiant-dominated phase of evolution. The estimated near-IR ages cluster between 9 and 30 Myr, i.e. the ages at which the molecular bands due to luminous red supergiants are strongest in the current models. They do not always agree with optical spectroscopic ages. Adding optical data sometimes leads to the rejection of the solar neighbourhood extinction law. This is not surprising considering small-scale structure around the clusters, but it has no significant effect on the near-IR based spectroscopic ages. [abridged]Comment: 14 pages, 20 figures, uses aa.cl