An inverse method to interpret colour-magnitude diagrams

An inverse method is developed to determine the star formation history, the age-metallicity relation, and the IMF slope from a colour-magnitude diagram. The method is applied to the Hipparcos HR diagram. We found that the thin disk of our Galaxy shows a peak of stellar formation 1.6 Gyr ago. The stars close to the Sun have a solar metallicity and a mean IMF index equal to 3.2. However, the model and the evolutionary tracks do not correctly reproduce the horizontal giant branch.Comment: 14 pages, 11 figures. To be published in Astronomy & Astrophysic

The Age of the Solar Neighbourhood

High-quality Hipparcos data for a complete sample of nearly 12000 main-sequence and subgiant stars, together with Padua isochrones, are used to constrain the star-formation history of the solar neigbourhood and the processes that stochastically accelerate disk stars. The velocity dispersion of a coeval group of stars is found to increase with time from ~8 kms at birth as t^{0.33}. In the fits, the slope of the IMF near 1 Msun proves to be degenerate with the rate at which the star-formation rate declines. If the slope of the IMF is to lie near Salpeter's value, -2.35, the star-formation rate has to be very nearly constant. The age of the solar neighbourhood is found to be 11.2+/-0.75 Gyr with remarkably little sensitivity to variations in the assumed metallicity distribution of old disk stars. This age is only a Gyr younger than the age of the oldest globular clusters when the same isochrones and distance scale are employed. It is compatible with current indications of the redshift of luminous galaxy formation only if there is a large cosmological constant. A younger age is formally excluded because it provides a poor fit to the number density of red stars. Since this density is subject to a significantly uncertain selection function, ages as low as 9 Gyr are plausible even though they lie outside our formal error bars.Comment: 7 pages; typographical corrections onl

The Geneva-Copenhagen Survey of the Solar neighbourhood II. New uvby calibrations and rediscussion of stellar ages, the G dwarf problem, age-metallicity diagram, and heating mechanisms of the disk

Ages, metallicities, space velocities, and Galactic orbits of stars in the Solar neighbourhood are fundamental observational constraints on models of galactic disk evolution. We aim to consolidate the calibrations of uvby photometry into Te, [Fe/H], distance, and age for F and G stars and rediscuss the results of the Geneva-Copenhagen Survey (Nordstrom et al. 2004; GCS) in terms of the evolution of the disk. We substantially improve the Te and [Fe/H] calibrations for early F stars, where spectroscopic temperatures have large systematic errors. Our recomputed ages are in excellent agreement with the independent determinations by Takeda et al. (2007), indicating that isochrone ages can now be reliably determined. The revised G-dwarf metallicity distribution remains incompatible with closed-box models, and the age-metallicity relation for the thin disk remains almost flat, with large and real scatter at all ages (sigma intrinsic = 0.20 dex). Dynamical heating of the thin disk continues throughout its life; specific in-plane dynamical effects dominate the evolution of the U and V velocities, while the W velocities remain random at all ages. When assigning thick and thin-disk membership for stars from kinematic criteria, parameters for the oldest stars should be used to characterise the thin disk.Comment: Accepted for publication in A&A on June 20, 200

The red tail of carbon stars in the LMC: Models meet 2MASS and DENIS observations

Carbon stars are known to exhibit systematically redder near-infrared colours with respect to M-type stars. In the near-infrared colour-magnitude diagrams provided by the 2MASS and DENIS surveys, the LMC C-type stars draw a striking red tail, well separated from the sequences of O-rich giants. So far, this conspicuous feature has been absent from any set of available isochrones, even the few existing ones that include the TP-AGB evolution of low- and intermediate-mass stars. To investigate such issue we simulate the complete 2MASS Ks vs.(J-Ks) data towards the LMC by means of a population synthesis approach, that relies on extended libraries of published stellar evolutionary tracks, including the TP-AGB phase. The simulations provide quite a detailed description of the several vertical fingers and inclined sequences seen in 2MASS data, due to both Galactic foreground and LMC O-rich stars. Instead, as mentioned, the red tail of C-stars sets a major difficulty: we find that TP-AGB models with solar-scaled molecular opacities, the usual assumption of existing AGB calculations, do not succeed in reproducing this feature. Our tests indicate that the main reason for this failure should not be ascribed to empirical Teff - (J-K) transformations for C-type stars. Instead, the discrepancy is simply removed by adopting new evolutionary models that account for the changes in molecular opacities as AGB stars get enriched in carbon via the third dredge-up (Marigo 2002). In fact, simulations that adopt these models are able to reproduce, for the first time, the red tail of C-stars in near-infrared CMDs. Finally, we point out that these simulations also provide useful indications about the efficiency of the third dredge-up process, and the pulsation modes of long-period variables.Comment: To appear in A&A. 14 pages, better if printed in colour. A version with high-resolution figures may be found in http://pleiadi.pd.astro.i

Lifting the Dusty Veil With Near- and Mid-Infrared Photometry: I. Description and Applications of the Rayleigh-Jeans Color Excess Method

The Milky Way (MW) remains a primary laboratory for understanding the structure and evolution of spiral galaxies, but typically we are denied clear views of MW stellar populations at low Galactic latitudes because of extinction by interstellar dust. However, the combination of 2MASS near-infrared (NIR) and Spitzer-IRAC mid-infrared (MIR) photometry enables a powerful method for determining the line of sight reddening to any star: the sampled wavelengths lie in the Rayleigh-Jeans part of the spectral energy distribution of most stars, where, to first order, all stars have essentially the same intrinsic color. Thus, changes in stellar NIR-MIR colors due to interstellar reddening are readily apparent, and (under an assumed extinction law) the observed colors and magnitudes of stars can be easily and accurately restored to their intrinsic values, greatly increasing their usefulness for Galactic structure studies. In this paper we explore this "Rayleigh-Jeans Color Excess" (RJCE) method and demonstrate that use of even a simple variant of the RJCE method based on a single reference color, (H-[4.5um]), can rather accurately remove dust effects from previously uninterpretable 2MASS color-magnitude diagrams of stars in fields along the heavily reddened Galactic mid-plane, with results far superior to those derived from application of other dereddening methods. We also show that "total" Galactic midplane extinction looks rather different from that predicted using 100um emission maps from the IRAS/ISSA and COBE/DIRBE instruments as presented by Schlegel et al. Instead, the Galactic mid-plane extinction strongly resembles the distribution of 13-CO (J=1->0) emission. Future papers will focus on refining the RJCE method and applying the technique to understand better not only dust and its distribution, but the distribution of stars intermixed with the dust in the low-latitude Galaxy.Comment: Accepted to ApJ; 21 pages, 17 figure

ESO Imaging Survey. The Stellar Catalogue in the Chandra Deep Field South

(abridged) Stellar catalogues in five passbands (UBVRI) over an area of approximately 0.3 deg^2, comprising about 1200 objects, and in seven passbands (UBVRIJK) over approximately 0.1 deg^2, comprising about 400 objects, in the direction of the Chandra Deep Field South are presented. The 90% completeness level of the number counts is reached at approximately U = 23.8, B = 24.0, V = 23.5, R = 23.0, I = 21.0, J = 20.5, K = 19.0. A scheme is presented to select point sources from these catalogues, by combining the SExtractor parameter CLASS_STAR from all available passbands. Probable QSOs and unresolved galaxies are identified by using the previously developed \chi^2-technique (Hatziminaoglou et al 2002), that fits the overall spectral energy distributions to template spectra and determines the best fitting template. The observed number counts, colour-magnitude diagrams, colour-colour diagrams and colour distributions are presented and, to judge the quality of the data, compared to simulations based on the predictions of a Galactic Model convolved with the estimated completeness functions and the error model used to describe the photometric errors of the data. The resulting stellar catalogues and the objects identified as likely QSOs and unresolved galaxies with coordinates, observed magnitudes with errors and assigned spectral types by the $\chi^2$-technique are presented and are publicly available.Comment: Paper as it will appear in print. Complete figures and tables can be obtained from: http://www.eso.org/science/eis/eis_pub/eis_pub.html. Astronomy & Astrophysics, accepted for publicatio

Spiral arm kinematics for Milky Way stellar populations

We present a new theoretical population synthesis model (the Galaxy model) to examine and deal with large amounts of data from surveys of the Milky Way and to decipher the present and past structure and history of our own Galaxy. We assume the Galaxy to consist of a superposition of many composite stellar populations belonging to the thin and thick discs, the stellar halo and the bulge, and to be surrounded by a single dark matter halo component. A global model for the Milky Way's gravitational potential is built up self-consistently with the density profiles from the Poisson equation. In turn, these density profiles are used to generate synthetic probability distribution functions (PDFs) for the distribution of stars in colour- magnitude diagrams (CMDs). Finally, the gravitational potential is used to constrain the stellar kinematics by means of the moment method on a (perturbed)-distribution function. Spiral arms perturb the axisymmetric disc distribution functions in the linear response framework of density-wave theory where we present an analytical formula of the so-called 'reduction factor' using hypergeometric functions. Finally, we consider an analytical non-axisymmetric model of extinction and an algorithm based on the concept of probability distribution function to handle CMDs with a large number of stars. A genetic algorithm is presented to investigate both the photometric and kinematic parameter space. This galaxy model represents the natural framework to reconstruct the structure of the Milky Way from the heterogeneous data set of surveys such as Gaia-ESO, SEGUE, APOGEE2, RAVE and the Gaia mission

The solar neighbourhood age-metallicity relation - does it exist?

We derive stellar ages, from evolutionary tracks, and metallicities, from Stromgren photometry, for a sample of 5828 dwarf and sub-dwarf stars from the Hipparcos Catalogue. This stellar disk sample is used to investigate the age-metallicity diagram in the solar neighbourhood. Such diagrams are often used to derive a so called age-metallicity relation. Because of the size of our sample, we are able to quantify the impact on such diagrams, and derived relations, due to different selection effects. Some of these effects are of a more subtle sort, giving rise to erroneous conclusions. In particular we show that [1] the age-metallicity diagram is well populated at all ages and especially that old, metal-rich stars do exist, [2] the scatter in metallicity at any given age is larger than the observational errors, [3] the exclusion of cooler dwarf stars from an age-metallicity sample preferentially excludes old, metal-rich stars, depleting the upper right-hand corner of the age-metallicity diagram, [4] the distance dependence found in the Edvardsson et al. sample by Garnett & Kobulnicky is an expected artifact due to the construction of the original sample. We conclude that, although some of it can be attributed to stellar migration in the galactic disk, a large part of the observed scatter is intrinsic to the formation processes of stars.Comment: 14 pages, 19 figures, A&A TeX, accepted for publication in Astronomy and Astrophysics. Table of ages and metallicities available upon reques

Fashionably Late? Building up the Milky Way's Inner Halo

Using a sample of 248 metal-poor stars (RR Lyraes, red giants and RHB stars) which is remarkable for the accuracy of its 6-D kinematical data, we find a new component for the local halo which has an axial ratio c/a ~ 0.2, a similar flattening to the thick disk. It has a small prograde rotation but is supported by velocity anisotropy, and contains more intermediate-metallicity stars (with -1.5 < [Fe/H] < -1.0) than the rest of our sample. We suggest that this component was formed quite late, during or after the formation of the disk. It formed either from the gas that was accreted by the last major mergers experienced by the Galaxy, or by dynamical friction of massive infalling satellite(s) with the halo and possibly the stellar disk or thick disk. The remainder of the stars in our sample exhibit a clumpy distribution in energy and angular momentum, suggesting that the early, chaotic conditions under which the inner halo formed were not violent enough to erase the record of their origins. The clumpy structure suggests that a relatively small number of progenitors were responsible for building up the inner halo, in line with theoretical expectations. We find a difference in mean binding energy between the RR Lyrae variables and the red giants in our sample, suggesting that more of the RR Lyraes in the sample belong to the outer halo, and that the outer halo may be somewhat younger, as first suggested by Searle and Zinn (1978). We also find that the RR Lyrae mean rotation is more negative than the red giants, which is consistent with the recent result of Carollo et al.(2007) that the outer halo has a retrograde rotation and with the difference in kinematics seen between RR Lyraes and BHB stars by Kinman et al.(2007).Comment: 16 pages, 10 figures, this version accepted by Ap

Variations in disability and quality of life with age and sex between eight Lower and Middle Income Countries:data from the INDEPTH WHO-SAGE collaboration

Background: Disability and quality of life are key outcomes for older people. Little is known about how these measures vary with age and gender across lower income and middle-income countries; such information is necessary to tailor health and social care policy to promote healthy ageing and minimise disability. Methods: We analysed data from participants aged 50 years and over from health and demographic surveillance system sites of the International Network for the Demographic Evaluation of Populations and their Health Network in Ghana, Kenya, Tanzania, South Africa, Vietnam, India, Indonesia and Bangladesh, using an abbreviated version of the WHO Study on global AGEing survey instrument. We used the eight-item WHO Quality of Life (WHOQoL) tool to measure quality of life and theWHO Disability Assessment Schedule, version 2 (WHODAS-II) tool to measure disability. We collected selected health status measures via the survey instrument and collected demographic and socioeconomic data from linked surveillance site information. We performed regression analyses to quantify differences between countries in the relationship between age, gender and both quality of life and disability, and we used anchoring vignettes to account for differences in interpretation of disability severity. Results: We included 43 935 individuals in the analysis. Mean age was 63.7 years (SD 9.7) and 24 434 (55.6%) were women. In unadjusted analyses across all countries, WHOQoL scores worsened by 0.13 points (95% CI 0.12 to 0.14) per year increase in age and WHODAS scores worsened by 0.60 points (95% CI 0.57 to 0.64). WHODAS-II and WHOQoL scores varied markedly between countries, as did the gradient of scores with increasing age. In regression analyses, differences were not fully explained by age, socioeconomic status, marital status, education or health factors. Differences in disability scores between countries were not explained by differences in anchoring vignette responses. Conclusions: The relationship between age, sex and both disability and quality of life varies between countries. The findings may guide tailoring of interventions to individual country needs, although these associations require further study