Globular Clusters in the Large Magellanic Cloud: An Impact from GAIA Photometry

The goal of this work is to assess the expected scientific output from the photometric studies of globular clusters in the Large Magellanic Cloud with ESA's astrometric space mission GAIA. For this purpose we simulate GAIA photometry of individual stars in synthetic cluster populations, covering a large range of cluster ages and metallicities. We find that accurate effective temperatures (Δ T _eff<10%) can be obtained from GAIA photometry down to V ∼ 18 for stars in populations within the studied metallicity range ([M/H] = -0.4 ... -1.7). GAIA will also provide photometric metallicities (Δ [M/H] ≲ 0.3 dex) for the cluster giants brighter than V ∼ 17.5. The knowledge of the effective temperature sand metallicities will allow to obtain accurate ages of stellar populations younger than about 1 Gyr using the usual procedure of main sequence turn-off point fitting. Ages of older stellar populations (≳ 1 Gyr) may be constrained from the isochrone fits to the giant branches in the observed CMDs. We conclude that GAIA will provide excellent opportunities for studying star formation histories far beyond the Milky Way, providing means for better understanding of stellar and galactic evolution in different astrophysical environments. This revised version was published online in July 2006 with corrections to the Cover Date.     

GAIA: An Astrometric and Photometric Survey of our Galaxy

In October 2000, the GAIA astrometric mission was approved as one of the next two `cornerstones' of ESA's science programme, with a launch date target of 2010–12. GAIA will provide positional, radial velocity, and photometric measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars throughout our Galaxy (and into the Local Group), amounting to about 1 per cent of the Galactic stellar population. GAIA's main scientific goal is to clarify the origin and history of our Galaxy, from a quantitative census of the stellar populations. It will advance questions such as when the stars in our Galaxy formed, when and how it was assembled, and its distribution of dark matter. The survey aims for completeness toV =20 mag, with accuracies of 10 μas at 15 mag. This revised version was published online in July 2006 with corrections to the Cover Date.     

Diffuse Interstellar Band at 862 nm as a Reddening Tracer for GAIA

We investigate the plausibility of using diffuse interstellar band at862 nm for tracing interstellar extinction with the ESA's astrometric space mission GAIA. For this purpose we perform numerical tests to simulate the conditions of real observations, covering a wide range of stellar parameters and different amounts of interstellar extinction. Our simulations indicate that with the present Radial Velocity Spectrometer setup the uncertainty in color excess of σ_E(B-V)≤ 0.05 can be achieved only for the interstellar reddening tracers brighter than V ∼ 13. None of the plausible tracers can provide accurate color excesses (σ _E(B-V) ≤ 0.05) at the distances beyond 2 kpc. We therefore conclude that with the currently planned instrumentation onboard GAIA this method can not be used as a stand-alone approach for probing interstellar extinction on the Galactic distance scales within the framework of the GAIA mission. This revised version was published online in July 2006 with corrections to the Cover Date.     

Determination of Stellar Parameters with GAIA

The GAIA Galactic survey satellite will obtain photometry in 15 filters of over 10⁹ stars in our Galaxy across a very wide range of stellar types. No other planned survey will provide so much photometric information on so many stars. I examine the problem of how to determine fundamental physical parameters (T _eff, log g, [Fe/H] etc.) from these data. Given the size, multidimensionality and diversity of this dataset, this is a challenging task beyond any encountered so far in large-scale stellar parametrization. I describe the problems faced (initial object identification, interstellar extinction, multiplicity, missing data etc.) and present a framework in which they can be addressed. A probabilistic approach is advocated on the grounds that it can take advantage of additional information (e.g. priors and data uncertainties) in a consistent and useful manner, as well as give meaningful results in the presence of poor or degenerate data. Furthermore, I suggest an approach to parametrization which can use the other information GAIA will acquire, in particular the parallax, which has not previously been available for large-scale multidimensional parametrization. Several of the problems identified and ideas suggested will be relevant to other large surveys, such as SDSS, DIVA,FAME, VISTA and LSST, as well as stellar parametrization in a virtual observatory. This revised version was published online in July 2006 with corrections to the Cover Date.     

Asymptotic Giant Branch Stars as Tracers of Star Formation Histories: the GAIA Context

We discuss a possible use of the asymptotic giant branch (AGB) stars for tracing star formation histories on the Galactic and extragalactic distance scales with the ESA's astrometric space mission GAIA. Extensive numerical simulations demonstrate that metallicities (Δ [M/H] ≲ 0.3) can be obtained for the AGB stars with GAIA up to the distances of ∼ 200 kpc, if no interstellar extinction is present. Reliable population ages can be also obtained from the AGB stars if their T _eff are constrained precisely. We show that precise effective temperatures can be obtained by fitting observed spectral energy distributions of the AGB stars with theoretical fluxes calculated from the synthetic spectra. A combination of the derived effective temperatures with the bolometric luminosities allows to derive precise population ages for a wide range of ages and metallicities over the large distance scales. This demonstrates that AGB stars can be employed very effectively for tracing star formation histories with GAIA, allowing to refine the global evolutionary scenarios of stellar populations in the Milky Way and the galaxies beyond. This revised version was published online in July 2006 with corrections to the Cover Date.     

3-D Structure of the Galactic Interstellar Matter: A Contribution from GAIA

We investigate the possibilities for tracing interstellar extinction with the ESA's astrometric space mission GAIA. The analysis is based on detailed simulations of the GAIA photometry, which are used to derive the distribution of interstellar matter in a modelled Galaxy. We find that `small' diffuse clouds (diameter D = 4 pc, E <sub>B-V</sub> = 0.06) will be easily traced with GAIA up to the distances of ∼ 800 pc. `Large' diffuse interstellar clouds (D = 10 pc, E _B-V = 0.13) will be located up to the distances of ∼ 2.5 kpc. This holds for the reddening tracers of spectral types O – K2 brighter than V = 17. Inmost cases, due to their low spatial density, the early type stars (O– A2) cannot provide reliable information about the distribution of interstellar matter. None of the reddening tracers measured by GAIA will provide reliable identification of the individual interstellar clouds beyond the distances of ∼ 3 kpc. Therefore, we conclude that the information available from photometric observations will be not sufficient for the detailed reconstruction of the 3-D distribution of Galactic interstellar matter. It is therefore extremely important to define the new strategies which would allow to combine all the available information, including the earlier space- and/or ground-based investigations, together with the information which will be provided by GAIA itself (parallaxes, E _B-V etc.). This revised version was published online in July 2006 with corrections to the Cover Date.     

α-process Elements in the Galaxy: A Possible GAIA Contribution

The sensitivity of stellar spectra to α/Fe abundance changes is investigated with the aim to be detected photometrically and employed for the scientific goals of the GAIA mission. A grid of plane parallel, line blanketed, flux constant, LTE model atmospheres with different [α/Fe] ratios was calculated. As a first step, the modeled stellar energy fluxes for solar-type stars and giants were computed and intercompared. The spectral sensitivity to α/Fe abundance changes is noticeable and has to be taken into account when selecting photometric filters for GAIA. The Ca II H and K lines and Mg I b triplet are the most sensitive direct indicators ofα/Fe abundance changes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Distance indicators from colour-magnitude-diagrams: main sequence, red clump and tip of the RGB

The location of the lower Main Sequence in the Colour Magnitude Diagram, the magnitude of the Red Clump and the magnitude of the tip of the Red Giant Branch are three popular stellar distance indicators. With the present observational capabilities they can be applied to reach distances ranging from the Galactic Disk and Halo populations, to galaxies beyond the Local Group. The techniques devised to exploit these distance indicators are presented, together with a discussion of their calibration, the main sources of systematic errors and the predicted impact of the results from the GAIA mission.     

Programme of the Galactic meridian: Star counts and galactic models. 1. Distributions of stellar magnitudes and colours

A comparison of observed stellar distributions with a three-component model of the Galaxy is presented. The analysis is based on photometric and photoelectric data obtained along the main Galactic meridian and in two fields near the North Galactic pole (programme MEGA). The assumed model considers the Galaxy as composed of the disk (main sequence and disk red giants), the thick disk and spheroid populations. To model the observed colour distribution, we distinguish main sequence stars and disk red giants as the disk subsystem; white dwarfs, subdwarfs and intermediate giants as the thick disk subsystem; extreme subdwarfs, spheroid giants and horizontal branch stars as the spheroid subsystem. A statistical relation between the apparent and absolute magnitudes of stars which make the maximum contribution to the star counts for a given disk subsystem is derived. In order to achieve the best agreement between the model and observations, we fit the values of the ‘dip’ (aw) of the disk luminosity function, the correction to the absolute magnitude of disk red giants (ΔM_V^RG) and the expression for interstellar extinction. As the main result, we obtained aw = 0.6 (logarithmic scale) and ΔM_V^RG = 0.5 mag; the interstellar extinction has to be taken into account by the modified Sandage law.     

GAIA Photometric System: Simultaneous 4-D Parameterization

We present a software package developed for the automatic 4-D stellar parameterization. The package can be adopted to virtually any multicolour photometric system. Procedure of simultaneous determination of T _eff, log g, [M/H] and E _B-V is very flexible and can be performed by applying various photometric parameters(magnitudes, colour indices etc.). Any changes of the photometric systems, weighting schemes or parameterization criteria can be implemented easily. This revised version was published online in July 2006 with corrections to the Cover Date.     

Scale length of the galactic thin disk

This paper presents an analysis of the first 2MASS (The Two Micron All Sky Survey) sampler data as observed at lower Galactic latitude in our Galaxy. These new near-infrared data provide insight into the structure of the thin disk of our Galaxy, The interpretation of star counts and color distributions of stars in the near-infrared with the synthetic stellar population model, gives strong evidence that the Galactic thin disk density scale length,h _R , is rather short (2.7 ± 0.1 kpc).     

A Photometric System for Metallicity Mapping in Galaxies

A new photometric system suitable for deep, precise and quick metallicity mapping in galaxies is proposed. We find a linear correlation between our metallicity index and the Mg₂ index for stellar, globular-cluster, and early-type galaxy spectra, and model spectral energy distributions of the simple stellar populations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Detached eclipsing binaries: analysis of BD–00° 3357

Detached eclipsing binaries constitute potential accurate distance tracers. They are also useful as the test bench of stellar evolution. In BD–00° 3357 eclipses are partial and its orbital period is 1.^d4. Our combined spectroscopic and photometric solution yields secure parameters of this system. The model of the star was obtained using the Wilson‐Devinney method. As result we obtained a semi major axis of 7.65 R_⊙ and a mass ratio of 0.78. The derived masses and radii are M ₁ = 1.73 M_⊙,M ₂ = 1.34 M_⊙R ₁ = 1.78 R_⊙, R ₂ = 1.32 R_⊙, respectively. These values correspond to the slightly evolved F0 and F6.5 components, both slightly less than 1Gyr old. The distance of the star was estimated to be 310 ± 60 pc, and the corresponding photometric parallax is 3.24 ± 0.74 mas. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The main sequence from F to K stars of the solar neighbourhood in SDSS colours

For an understanding of Galactic stellar populations in the SDSS filter system well defined stellar samples are needed. The nearby stars provide a complete stellar sample representative for the thin disc population. We compare the filter transformations of different authors applied to the main sequence stars from F to K dwarfs to SDSS filter system and discuss the properties of the main sequence. The location of the mean main sequence in colour‐magnitude diagrams is very sensitive to systematic differences in the filter transformation. A comparison with fiducial sequences of star clusters observed in g ′, r ′, and i ′ show good agreement. Theoretical isochrones from Padua and from Dartmouth have still some problems, especially in the (r – i) colours. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bandwidth Effects in the 3G Photometric System

Bandwidth effects in the GAIA photometric system 3G are discussed. It is demonstrated that careful account for the bandwidth effects in any proposed GAIA photometric system has to be made, in order to determine the fundamental parameters of heavily reddened stars. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study of the effect of metallicity on the amplitudes of Cepheids

Results concerning the dependence of photometric and radial velocity amplitudes on metallicity are presented based on about 200 Galactic classical Cepheids pulsating in the fundamental mode. The Galactic distribution of the [Fe/H] value of Cepheids is also studied. We show that the photometric amplitude ratio A_I/A_V is independent of metallicity. The observed dependence of this ratio on the pulsation period does not correspond to the theoretical predictions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Milky Way’s stellar disk

A suite of vast stellar surveys mapping the Milky Way, culminating in the Gaia mission, is revolutionizing the empirical information about the distribution and properties of stars in the Galactic stellar disk. We review and lay out what analysis and modeling machinery needs to be in place to test mechanism of disk galaxy evolution and to stringently constrain the Galactic gravitational potential, using such Galactic star-by-star measurements. We stress the crucial role of stellar survey selection functions in any such modeling; and we advocate the utility of viewing the Galactic stellar disk as made up of ‘mono-abundance populations’ (MAPs), both for dynamical modeling and for constraining the Milky Way’s evolutionary processes. We review recent work on the spatial and kinematical distribution of MAPs, and point out how further study of MAPs in the Gaia era should lead to a decisively clearer picture of the Milky Way’s dark-matter distribution and formation history.     

BaSeL: A Library of Synthetic Spectra and Colours for GAIA

The Basel Stellar Library (BaSeL) is a library of synthetic spectra which has already been used in various astrophysical applications (stellar cluster studies, characterization and choice of the COROT potential targets, eclipsing binaries,...). This library could provide useful indications to 1) choose the best photometric system for the GAIA strategy by evaluating their expected performances and 2) apply systematically the BaSeL models for any sample of the GAIA targets. In this context, we describe one of the future developments of the BaSeL interactive web site to probe the GAIA photometric data: an automatic determination of atmospheric parameters from observed colours. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthetic integrated fluxes and colours for elliptical galaxies

Photometric intergrated colours (UBVRI+UV selected bands) of elliptical galaxies are theoretically evaluated, accounting for stellar populations of different metal abundance (0.00001Z0.04). The contribution of different evolutionary phases (from M.S. up to Post-A.G.B.) to the integrated fluxes in the various photometric bands is computed. An explanation for the UV-excess in elliptical galaxies and of its dependence on the metallicity (Mg₂) and mass is also suggested.     

Mapping the substructure in the Galactic halo with the next generation of astrometric satellites

We run numerical simulations of the disruption of satellite galaxies in a Galactic potential to build up the entire stellar halo, in order to investigate what the next generation of astrometric satellites will reveal by observing the halo of the Milky Way. We generate artificial DIVA , FAME and GAIA halo catalogues, in which we look for the signatures left by the accreted satellites. We develop a method based on the standard Friends-of-Friends algorithm applied to the space of integrals of motion. We find this simple method can recover about 50 per cent of the different accretion events, when the observational uncertainties expected for GAIA are taken into account, even when the exact form of the Galactic potential is unknown. The recovery rate for DIVA and FAME is much smaller, but these missions, like GAIA , should be able to test the hierarchical formation paradigm on our Galaxy by measuring the amount of halo substructure in the form of nearby kinematically cold streams with, for example, a two-point correlation function in velocity space.