The GAIA photometry

The GAIA mission, the next ESA Cornerstone 6 (launch 2010- 2012), will create a precise three dimensional map of about one billion stars throughout our Galaxy and beyond. To reach the scientific goals, that is to quantify the dynamical, chemical and star formation evolution of the Milky Way, it is crucial to also accurately determine astrophysical parameters through the measured flux for the observed objects (effective temperature, luminosities, global metallicity, ages, chemical anomalies,...). The spectrophotometric instrument on board GAIA, combined with the two astrometric instruments, will provide this information.The medium and broad band photometric systems proposed for GAIA are presented. We discuss their capability to characterize the galactic populations than can be observed. The accuracy expected in the derivation of astrophysical parameters using jointly astrometry and medium band GAIA photometry is also presented.

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)GAIA: Derivation of Stellar Parameters

C. Jordi, J.M. Carrasco, F. Figueras, J. Torra, X. Luri, E. MasanaUniversitat de Barcelona - IEEC, Avda. Diagonal 647, 08028 Barcelona, Spain

Teff = 3500 K(3 filter combinations)

Scientific goals

Accuracies:

4 as at V = 10 10 as 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

Main performances and capabilities

G (~V mag) 10 11 12 13 14 15 16 17 18 19 20 21

Parallax 4 4 4 5 7 11 17 27 45 80 160 500

Position 3 3 3 4 6 9 15 23 39 70 140 440

Annual proper motion 3 3 3 4 5 8 13 20 34 60 120 380

Astrometric accuracy in as

Capabilities:

10 as 10% at 10 kpc 1 AU at 100 kpc

10 as/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

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 dwarfsDistance 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 supernovaeSolar System: 105-106 new minor planets - taxonomy and evolutionExtra-Solar Planetary Systems: complete census of large planets out to 200-500 pc - orbital characteristics of several thousand planetsFundamental Physics: determination of space curvature parameter to 1 part in 5.10-7

The scientific goals of GAIA require complementary astrometry, photometry and radial velocity data

Correct chromatic aberrations in the astrometric focal plane to achieve microarcsec accuracy level (BBP)

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, emssion,…

Solar system: taxanomy classification, variability, ...

QSO: photometric redshif

Galaxies: colours,…

G magnitude accuracy (mag)

G band in the astrometric fields

• 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

Light curves: precision at V~20 as Hipparcos at V~9

Goals Broad band system

Example of a BBP colour-colour diagram for different gravities and metallicities. Arrow indicates reddening for

Av=1. Error bars indicate end-

of-mission errors for a source with G=18.

The four (or five) broad band photometric filters will provide multi-colour, multi-epoch photometric measurements for each object observed in the astrometric field.

Several filter transmision 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 purpouse.

Although somewhat redundant in terms of astrophysical information content , BBP will supply higher S/N and angular resolution than MBP, so useful for QSO and galaxy photometry aplications.

The figures show some examples and the accuracy achivable

Medium band system

Photometric accuracy (in mag) in the spectro telescope in each of the relevant colour indices derived from the 11 medium photometric bands (2F system). The accuracy has been computed for an unreddened star. The abundance of -elements is measured through the MgH reddening free index (QIMg) in the F and G stars and through the QITiO reddening free index for later (K and early M) stars. QICN is used to measure the N abundance of red stars with Teff < 4200 K.

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 atmosheres and spectrophotometric observations), as well as the experience with existing photometric systems.

At present, 2F (shown in the figure) is the base-line photometric system for GAIA (final system by mid-2005).

Temperature determination

Precision of 1-3% in Teff, is achievable at G~19

Several passbands to measure the continuum

(An error of 0.02 mag in E(b-y) is assumed)

Brown dwarfs: Chamaeleon #7 (M8 V)

V= 22.2 , (V-I) = 5.3, G = 18.8 mag

Av=0.26 mag, Teff ~ 2700 K, M= 0.05 Msun Δπ/π = 0.014

σM = 0.030

σTeff= 20-30K

Good derivation of Mass and age

Observed spectra of Chamaeleon #7 (provided by F. Comerón). GAIA 75,78,83,89 filters are overploted

Expected accuracy of the location of Chamaeleon #7 in the HR diagram. Models from Baraffe et al. (1998).

Chemical composition determination

Oxigen rich and Carbon richclassification (variation with phase)

0.2-0.3 dex precision is achievable at G~19

K giant (Teff= 4500 K, log g=3.0) M dwarf (Teff= 3500 K, log g=4.5)

Satellite & system (April 2002 design status)

ASTRO telescopes and focal plane

Astro field #1 Astro field #2

AF1-11ASM2ASM1 BBP

~ 750 mm0.92 deg

600 mm0.737 deg

RVSM(located in telescope

focal plane, in vignetted field)

2 deg = 120 s = 74 mm

MBP #1 field height 1.6 deg(60 mm)

MBP #2 field height 1.6 deg(60 mm)

RVS field height 1.6 deg(60 mm)

Vignetted field

0%

15%

Vignetted field

15%

0%

RVSM(located in telescope

focal plane, in vignetted field)

2 deg = 120 s = 74 mm

MBP #1 field height 1.6 deg(60 mm)

MBP #2 field height 1.6 deg(60 mm)

RVS field height 1.6 deg(60 mm)

Vignetted field

0%

15%

Vignetted field

15%

0%

SPECTRO telescope and focal plane

Entrance pupil 0.5 x 0.5 m2

Optical transmission > 0.92

Pixel size 10 x 15 μm2

Pixel size (angular) 1 x 1.5 arcsec2

MBP RVS

Sample size (in pixels) 1 x 4 1 x 3

Number of CCDs 2 x (1+15) 1+6

TDI integration time per chip 5.5 s 16.8 s

Average total obs./object 2 x 102 102

Spectral range 849-874 nm

Spectral sampling 0.375 Å/pixel

Entrance pupil 1.4 x 0.5 m2

Optical transmission > 0.86

Pixel size 10 x 30 μm2

Pixel size (angular) 44.2 x 133 mas2

Sample size (in pixels) 1 x 10

Number of CCDS in Astro 11 x 10

Number of CCDS in BBP 5 x 10

TDI integration time per chip 3.3 s, 1.9 s

Average total obs/object 2 x 41

ASM: astrometric sky mappers

AF1-11: astrometric field

BBP: broad-band photometer

MBP: medium-band photometer

RVS: radial velocity spectrometer

Mission livetime: 5 years

Mean number of observations per object during mission:

Astrometric field: 82 x 11 CCDs

Broad-band phot: 82 x 4 passbands

Medium-band phot: 204 x 11 passbands

Radial velocity: 102 single observations

• Launch: Proton•Orbit: Sun-Earth L2 (Lissajous)•Continuous scanning

• Two astrometric instruments• Monolithic mirrors• Non-deployable, 3-mirror, SiC optics• Astro focal plane: TDI CCDs• Radial velocity/photometry telescope• Astrophysically driven payload:

• faint stars, to V=20 mag• radial velocities• broad-band photometry: chromaticity• medium-band photometry: astrophysics

• Survey principles:• revolving scanning• on-board detection • complete and unbiased sample