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Stellar parameters derivation from Gaia photometry J.M. Carrasco, C. Jordi, F. Figueras, J. Torra Universitat de Barcelona - IEEC, Avda. Diagonal 647, 08028 Barcelona, Spain The scientific goals of the Gaia mission require complementary astrometry, photometry and radial velocity data. After five years scanning the entire sky, Gaia will have performed measurements with broad (BBP), and medium (MBP) bands, and white light (G magnitude). The main goal of the photometry is the astrophysical parameterization of the stars (temperature, luminosity, chemical composition and age) with an accuracy sufficient for the quantitative description of the chemical and dynamical evolution of the Galaxy over all galactocentric distances. To reconstruct the formation history of the Milky Way and its structure is necessary to determine the abundance of Fe and α-elements separatelly, and with enough accuracy (~0.2 dex) for mapping the galactic chemical evolution. On the other hand, the effective temperature must be obtained to ~5 %. This required accuracies will permit to know to wich population the star belongs (i.e. halo, thin or thick disk, bulge, etc.). In some cases, luminosity can be provided by the astrometric information of Gaia, but, for the majority of stars, the value of the surface gravity must be provided by photometry. This poster reviews the capabilities for the astrophysical parameterization of the stars in several parts of the HR diagram and how this permits to betterunderstand our Galaxy. Scientific goals For more information please contact J.M. Carrasco ([email protected]) or visit http://gaia.am.ub.es/PWG/ Satellite & system Galactic Structure: origin and history of our Galaxy - tests of hierarchical structure formation - inner bulge/bar dynamics - disk/halo interactions – Star Formation and Evolution: dynamics of star forming regions - luminosity function - complete and detailed local census down to single brown dwarfs Distance Scale and Reference Frames: parallax calibration of all distance scale indicators - definition of the local, kinematically non-rotating metric Local Group and Beyond: rotational parallaxes for Local Group galaxies - kinematical separation of stellar populations - internal dynamics of Local Group dwarfs - detection of supernovae Solar System: 105-106 new minor planets - taxonomy and evolution Extra-Solar Planetary Systems: complete census of large planets out to 200-500 pc - orbital characteristics of several thousand planets Fundamental Physics: determination of space curvature parameter γ to 1 part in 5.10-7 • Orbit: Sun-Earth L2 (Lissajous) ASM: astrometric sky mappers AF1-11: astrometric field BBP: broad-band photometer • Continuous scanning MBP: medium-band photometer RVS: radial velocity spectrometer • Mission lifetime: 5 years • Two astrometric instruments • Monolithic mirrors ASTRO telescopes and focal plane • Non-deployable, 6-mirror, SiC optics • TDI CCDs Entrance pupil 1.4 x 0.5 m2 • Radial velocity/photometry telescope Optical transmission > 0.86 • Astrophysically driven payload: Pixel size 10 x 30 µm2 • faint stars, to V=20 mag Pixel size (angular) 44.2 x 133 mas2 • radial velocities Sample size (in pixels) 1 x 12 • broad-band photometry: chromaticity, Number of CCDs in Astro 11 x 12 astrophysics Number of CCDs in BBP 4 x 10 • medium-band photometry: TDI integration time per chip 3.3 s astrophysics Average total observations/object 2 x 41 • Complete and unbiased sample Main performances and capabilities Distances for parallax error ~10% Capabilities: ♦ 10 µas ≡ 10% at 10 kpc ≡ 1 AU at 100 kpc ♦ 10 µas/yr at 20 kpc ≡ 1 km/s ⇒ every star in the Galaxy and Local Group will be seen to move ⇒ GAIA will quantify 6-D phase space for over 300 million stars and 5-D phase-space for over 109 stars SPECTRO telescope and focal plane Entrance pupil Optical transmission Pixel size Pixel size (angular) Sample size (in pixels) Number of CCDs TDI integration time per chip Average total obs./object RVS Spectral range RVS Spectral sampling The scientific goals of GAIA require complementary astrometry, photometry and radial velocity data 0.56 x 0.45 m2 > 0.75 10 x 15 µm2 0.9 x 1.4 arcsec2 1x4 2 x 16 11.95 s 2 x 100 849-874 nm 0.375 Å/pixel Accuracies: ♦ 4 µas at V = 10 10 µas at V = 15 0.2 mas at V = 20 ♦ complete astrophysical sample: one billion stars ♦ 1 km/s radial velocities complete to V = 17.5 ♦ sky survey at ~ 0.25 arcsec spatial resolution to V = 20 ♦ multi-colour multi-epoch photometry to V = 20 ♦ dense quasar link to inertial reference frame dO, V(dO) for null interstellar extinction, dabs, V(dabs) when an average galactic extinction of 0.7 mag kpc-1 is considered. The Gaia photometry Broad band system Goals Medium band system Correct chromatic aberrations in the astrometric focal plane to achieve microarcsec accuracy level (3 bands at least) Characterization of the observed objects in terms of astrophysical parameters. (BBP+MBP) Classification: star (single/multiple), solar system object, galaxy, QSO Stellar astrophysics parameters: Teff, luminosity, chemical composition ([Fe/H], [α/Fe], C/O, ...), peculiarities, emission,… Solar system: taxonomy classification, variability, ... QSO: photometric redshift Galaxies: colours, classification. G band in the astrometric field The five broad band photometric filters will provide multi-colour, multi-epoch photometric measurements for each object observed in the astrometric field. The baseline bands shown in the figure have been proposed by L. Lindegren. The final decision for BBP system will be taken in mid 2005. Considerable effort is being devoted to the design of an optimum system for GAIA, taking into account the spectral energy distribution of the main galactic stellar populations, as derived from model atmospheres and spectrophotometric observations), as well as the experience with existing photometric systems. The current baseline is shown. (Final decision: mid 2005) Several filter transmission curves are being designed and tested to optimize the BBP system for chromaticity calibration. Artificial neural networks (among other techniques) are being used for this purpose. Although somewhat redundant in terms of astrophysical information content, BBP will supply higher S/N and angular resolution than MBP, so useful for crowded regions, QSO and galaxy photometry applications. The bands are named accordingly to their central wavelength. Very broad band: ~ 300-1050 nm • Small bolometric correction • 11 CCDs per passage (3.3s per CCD) • 82 observations • The best S/N for variability detection •Glim~ 20 Vlim~ 20-25 • G-V is a function of SP and reddening • End-of-mission precision for the 4B-system Light curves: precision at V~20 as Hipparcos at V~9 Photometric accuracy (in mag) in the Spectro telescope in each of the relevant colour indices derived from the 12 medium photometric bands (3F system proposed by University of Barcelona and Geneva Observatory teams). The accuracy has been computed for non-reddened star. The abundance of α-elements is measured through the MgH reddening free index (QMgH) in the F and G stars and through the QTiO reddening free index for later (K and early M) stars. QCN is used to measure the N abundance of red stars with Teff < 4200 K. G magnitude accuracy (mag) Astrophysical Parameters Derivation Evaluation principle • Definition of scientific targets for Gaia performances evaluation for the quantitative description of chemical & dynamical evolution of the Galaxy. Uncertainty in parameter determination Error estimation in parameter derivation of the Gaia Scientific Targets for several groups of stars using both BBP and MBP plus parallax. The set of stars for different distances vary accordingly to the galactic population models. (A minimum photometric error of 0.003 mag have been assumed) Galactic Center direction Galactic Anticenter direction Some impacts on Galaxy knowledge Halo: Age galact. gradient (turn-off stars) σage~1.5 Gyr at 2.5 kpc σage~2.2 Gyr at 5 kpc Chemical galact. gradient (G & K giants) σ[M/H] ~0.07 at 5 kpc σ[M/H] ~0.15 at 12 kpc σ[M/H] ~0.20 at 17 kpc Disk: Perpendicular to galactic plane • Simulated photometry using BaSeL2.2 and NextGen new release spectral libraries. • A priori knowledge of parallax. AGB / RGB BHB, HB-A FV, GV, KV, MV GIII, KIII, RR-Lyr AIV, FIV, GIV AV, BV/III, OV/III BIa, Cepheids, FIa, MIa Galactic center Galactic anticenter Age galact. gradient (turn-off stars) σage~1.2 Gyr at 2 kpc σage~4.7 Gyr at 3 kpc σage~1.1 Gyr at 2 kpc σage~2.6 Gyr at 3 kpc Chemical galact. gradient (G,K giants) σ[M/H] ~0.05 at 2 kpc σ[M/H] ~0.20 at 5 kpc σ[M/H] ~0.05 at 2 kpc σ[M/H] ~0.10 at 5 kpc σ[M/H] ~0.15 at 8 kpc Distance Scale: Cepheids: σπ/π < 10% for all observ. σAv < 0.03 for all observ. RR-Lyrae: σπ/π < 10% (up to 10 kpc σMv< 0.20 in galactic pole direction; up to 5 kpc in the disk) σAv < 0.02 at 2kpc, 0.04 at 4 kpc. 1