Gaia A Stereoscopic Census of our Galaxy

http://www.rssd.esa.int/Gaia

May 2008

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Gaia: Design Considerations

• Astrometry (V < 20):– completeness to 20 mag (on-board detection) 109 stars– accuracy: 10–25 μarcsec at 15 mag (Hipparcos: 1 milliarcsec at 9 mag)– scanning satellite, two viewing directions

global accuracy, with optimal use of observing time– principles: global astrometric reduction (as for Hipparcos)

• Photometry (V < 20):– astrophysical diagnostics (low-dispersion photometry) + chromaticity

Teff ~ 200 K, log g, [Fe/H] to 0.2 dex, extinction

• Radial velocity (V < 16–17):– application:

• third component of space motion, perspective acceleration• dynamics, population studies, binaries• spectra: chemistry, rotation

– principles: slitless spectroscopy using Ca triplet (847–874 nm)

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Gaia: Complete, Faint, Accurate

Hipparcos Gaia

Magnitude limit 12 20 mag Completeness 7.3 – 9.0 20 mag Bright limit 0 6 mag Number of objects 120 000 26 million to V = 15 250 million to V = 18 1000 million to V = 20 Effective distance limit

1 kpc 50 kpc Quasars None 5 x 105

Galaxies None 106 – 107 Accuracy 1 milliarcsec 7 µarcsec at V = 10 10-25 µarcsec at V = 15 300 µarcsec at V = 20 Photometry photometry

2-colour (B and V) Low-res. spectra to V = 20 Radial velocity None 15 km/s to V = 16-17 Observing programme

Pre-selected Complete and unbiased

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Stellar Astrophysics • Comprehensive luminosity calibration, for example:

– distances to 1% for ~10 million stars to 2.5 kpc

– distances to 10% for ~100 million stars to 25 kpc

– rare stellar types and rapid evolutionary phases in large numbers

– parallax calibration of all distance indicators

e.g. Cepheids and RR Lyrae to LMC/SMC

• Physical properties, for example:– clean Hertzsprung–Russell diagrams throughout the Galaxy– solar neighbourhood mass function and luminosity function

e.g. white dwarfs (~200,000) and brown dwarfs (~50,000)

– initial mass and luminosity functions in star forming regions

– luminosity function for pre main-sequence stars

– detection and dating of all spectral types and Galactic populations

– detection and characterisation of variability for all spectral types

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One Billion Stars in 3-d will Provide …

• in our Galaxy …– the distance and velocity distributions of all stellar populations

– the spatial and dynamic structure of the disk and halo

– its formation history

– a rigorous framework for stellar structure and evolution theories

– a large-scale survey of extra-solar planets (~20,000)

– a large-scale survey of Solar System bodies (~ few 100,000)

– support to developments such as VLT, JWST, etc.

• … and beyond– definitive distance standards out to the LMC/SMC

– rapid reaction alerts for supernovae and burst sources (~20,000)

– QSO detection, redshifts, microlensing structure (~500,000)

– fundamental quantities to unprecedented accuracy: to 10-7 (10-5 present)

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Exo-Planets: Expected Discoveries

• Astrometric survey:– monitoring of hundreds of thousands of FGK stars to ~200 pc

– detection limits: ~1MJ and P < 10 years

– complete census of all stellar types, P = 2–9 years

– masses, rather than lower limits (m sin i)

– multiple systems measurable, giving relative inclinations

• Results expected:– ~20,000 exo-planets (~10 per day)

– displacement for 47 UMa = 360 μas

– orbits for ~5000 systems

– masses down to 10 MEarth to 10 pc

• Photometric transits: ~5000?

Figure courtesy François Mignard

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• Asteroids etc.:– deep and uniform (20 mag) detection of all moving objects

– ~ few 100,000 new objects expected (357,614 with orbits presently)

– taxonomy/mineralogical composition versus heliocentric distance

– diameters for ~1000, masses for ~100

– orbits: 30 times better than present, even after 100 years

– Trojan companions of Mars, Earth and Venus

– Kuiper Belt objects: ~300 to 20 mag (binarity, Plutinos)

• Near-Earth Objects: – Amors, Apollos and Atens (2249, 2643, 406 known today) – ~1600 Earth-crossers >1 km predicted (937 currently known)– detection limit: 260–590 m at 1 AU, depending on albedo

Studies of the Solar System

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Light Bending in Solar System

Movie courtesy Jos de Bruijne

Light bending in microarcsec, after subtraction of the much larger effect by the Sun

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Satellite and System

• ESA-only mission• Launch date: 2011 • Lifetime: 5 years• Launcher: Soyuz–Fregat from CSG• Orbit: L2• Ground station: New Norcia and Cebreros• Downlink rate: 4–8 Mbps

• Mass: 2120 kg (payload 700 kg)• Power: 1720 W (payload 735 W)

Figures courtesy EADS-Astrium

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Payload and Telescope

Two SiC primary mirrors1.45 0.50 m2 at 106.5°

SiC toroidalstructure

(optical bench)

Basic anglemonitoring system

Combinedfocal plane

(CCDs)

Rotation axis (6 h)

Figure courtesy EADS-Astrium

Superposition of two Fields of View

(FoV)

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Focal Plane

Star motion in 10 s

Total field: - active area: 0.75 deg2

- CCDs: 14 + 62 + 14 + 12 - 4500 x 1966 pixels (TDI) - pixel size = 10 µm x 30 µm

= 59 mas x 177 mas

Astrometric Field CCDs

Blue P

hotometer C

CD

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Sky Mapper CCDs

104.26cm

Red P

hotometer C

CD

s Radial-Velocity Spectrometer

CCDs

Basic Angle

Monitor

Wave Front Sensor

Basic Angle

Monitor

Wave Front Sensor

Sky mapper: - detects all objects to 20 mag - rejects cosmic-ray events - FoV discriminationAstrometry: - total detection noise: ~6 e-

Photometry: - spectro-photometer - blue and red CCDsSpectroscopy: - high-resolution spectra - red CCDs

42.3

5cm

Figure courtesy Alex Short

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On-Board Object Detection• Requirements:

– unbiased sky sampling (mag, colour, resolution)

– all-sky catalogue at Gaia resolution (0.1 arcsec) to V~20

• Solution: on-board detection:– no input catalogue or observing programme

– good detection efficiency to V~21 mag

– low false-detection rate, even at high star densities

• Will therefore detect:– variable stars (eclipsing binaries, Cepheids, etc.)

– supernovae: ~20,000

– gravitational lensing events: ~1000 photometric; ~100 astrometric

– Solar System objects, including near-Earth asteroids and KBOs

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Sky Scanning Principle

Spin axis 45o to SunScan rate: 60 arcsec/sSpin period: 6 hours

45o

Figure courtesy Karen O’Flaherty

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Comments on Astrometric Accuracy

• Massive leap from Hipparcos to Gaia:– accuracy: 2 orders of magnitude (1 milliarcsec to 7 microarcsec)– limiting sensitivity: 4 orders of magnitude (~10 mag to 20 mag)– number of stars: 4 orders of magnitude (105 to 109)

• Measurement principles identical:– two viewing directions (absolute parallaxes)– sky scanning over 5 years parallaxes and proper motions

• Instrument improvement:– larger primary mirror: 0.3 0.3 m2 1.45 0.50 m2, D-(3/2)

– improved detector (IDT CCD): QE, bandpass, multiplexing

• Control of all associated error sources:– aberrations, chromaticity, solar system ephemerides, attitude control …

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Photometry Measurement Concept (1/2)

Figures courtesy EADS-Astrium

Blue photometer:330–680 nm

Red photometer:640–1000 nm

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Photometry Measurement Concept (2/2)

Figures courtesy Anthony Brown

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RP spectrum of M dwarf (V=17.3)Red box: data sent to ground

White contour: sky-background levelColour coding: signal intensity

17Figures courtesy EADS-Astrium

Spectroscopy:847–874 nm

(resolution 11,500)

Radial Velocity Measurement Concept (1/2)

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Radial Velocity Measurement Concept (2/2)

RVS spectra of F3 giant (V=16) S/N = 7 (single measurement)

S/N = 130 (summed over mission)

Field of view RVS spectrograph CCD detectors

Figures courtesy David Katz

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Data Reduction Principles

Sky scans(highest accuracy

along scan)

Scan width: 0.7°

1. Object matching in successive scans2. Attitude and calibrations are updated3. Objects positions etc. are solved4. Higher terms are solved5. More scans are added6. System is iteratedFigure courtesy Michael Perryman

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Scientific Organisation

• Gaia Science Team (GST): – 7 members + ESA Project Scientist + DPAC Executive Chair

• Scientific community:– organised in Data Processing and Analysis Consortium (DPAC)– ~375 scientists in 16 countries active at some level

• Community is active and productive:– regular science team/DPAC meetings– growing archive of scientific reports– advance of simulations, algorithms, accuracy models, etc.

• Data distribution policy:– final catalogue ~2019–20– intermediate catalogues as appropriate– science alerts data released immediately– no proprietary data rights

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Ingestion, pre-processing,data base + versions,

astrometric iterative solutionESAC + Barcelona + OATo

Object processing+ Classification

CNES, Toulouse

PhotometryCambridge (IoA)

+ VariabilityGeneva (ISDC)

Spectroscopicprocessing

CNES, Toulouse

Overall system architecture

ESACData simulations

Barcelona

From ground station

Community access

Data Processing Concept (simplified)

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Status and Schedule

• Prime contractor: EADS-Astrium– implementation phase started early 2006– preliminary design review completed June 2007

• Main activities and challenges:– CCDs and FPA (including proximity electronics)– SiC primary mirrors– high-stability optical bench– payload data handling electronics– phased-array antenna– micro-propulsion– scientific calibration of CCD radiation-damage effects

• Schedule:– no major identified uncertainties to affect cost or launch schedule– launch in 2011– technology/science ‘window’: 2010–12

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Schedule

Catalogue

2000 2004 2008 2012 2016 2020

ESA Acceptance

Technology Development Design, Build, Test

Launch

Observations

Data Analysis

Early Data

Concept & Technology Study (ESA)

Re-assessment:Ariane-5 Soyuz

Cruise to L2

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Gaia

Unraveling the chemical and dynamical history of our Galaxy