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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.
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