Gaia A Stereoscopic Census of our Galaxy

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

November 2003

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Gaia: Design Considerations

• Astrometry (V < 20):– completeness to 20 mag (on-board detection) 109 stars– accuracy: 10-20 arcsec 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)

• 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 (848-874 nm)

• Photometry (V < 20):– astrophysical diagnostics (5 broad + 11 narrow-band) + chromaticity

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

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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 ~3-7 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 1 Mpc Quasars None ~5

Galaxies None 106 - 107 Accuracy ~1 milliarcsec 4 arcsec at V = 10 10-15 arcsec at V = 15 200-300 arcsec at V = 20 Broad band photometry

2-colour (B and V) 5-colour to V = 20 Medium band photometry

None 11-colour to V = 20 Radial velocity None 1-10 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 ~20 million stars to 2.5 kpc– distances to 10% for 150 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 sequences 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 (~10–20,000)– a large-scale survey of Solar System bodies (~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 (10-3 present)

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Planète : = 100 mas P = 18 mois

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:– 10–20,000 planets (~10 per day)– displacement for 47 UMa = 360 as– orbits for ~5000 systems– masses down to 10 MEarth to 10 pc

• Photometric transits: ~5000?

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

– 105–106 new objects expected (65,000 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 (442, 455, 75 known today) – ~1600 Earth-crossers >1 km predicted (100 currently known)– detection limit: 260–590 m at 1 AU, depending on albedo

Gaia: Studies of the Solar System

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Light Bending in Solar System

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Satellite and System

• Mass: 1700 kg (payload 800 kg)• Power: 2000 W (payload 1200 W)

• ESA only mission• Launch date: 2010 targetted• Lifetime: 5 years• Launcher: Soyuz• Orbit: L2• Ground station: Perth or Madrid• Data rate: 1 Mbps

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Payload and TelescopeSiC primary mirrors1.4 0.5 m2 at 106°

Superposition offields of view

SiC toroidalstructure

Basic anglemonitoring system

Combinedfocal plane (CCDs)

Rotation axis

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Astrometric Focal Plane

Total field: - area: 0.6 deg2

- size: 75 60 cm2 - number of CCD chips: 110+70 - CCDs: 4500 x 1966 pixels

Sky mapper: - detects all objects to 20 mag - rejects cosmic-ray events

Astrometric field: - pixel size: 10 30 m2

- window area: 6 12 pixels - flush frequency: 15 MHz - readout frequency: 30 kHz - total read noise: 6e-

Broad-band photometry: - 5 colour

Star motion

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On-Board Object Detection

Requirements: – unbiased sky sampling (mag, colour, resolution)

– no 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 very high star densities

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

– supernovae: 20,000

– microlensing events: ~1000 photometric; ~100 astrometric

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

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Sky Scanning Principle

Spin axis 50o to SunScan rate: 60 arcsec/sSpin period: 6 hours

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Radial Velocity Measurement Concept

F3 giant S/N = 7 (single measurement)

S/N = 130 (summed over mission)

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Comments on Astrometric Accuracy

• massive leap from Hipparcos to Gaia:– accuracy: 2-3 orders of magnitude (1 milliarcsec to 4 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 0.3m2 1.4 0.5m2, 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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Technical Studies (2002-04) and Schedule

• Main activities:– two parallel system studies: Astrium & Alenia/Alcatel– CCD/focal plane development: Astrium + e2v – first CCDs produced– SiC primary mirror: Boostec – mirror prototype under production– high-stability optical bench: Astrium + TPD Delft – testing underway– payload data handling electronics: Astrium-D – breadboard starting– radial velocity instrument optimisation: MSSL/Paris– mission analysis: ESOC– also studied: FEEPs; transmitter; solar array deployment; refocus

mechanism; ground verification/calibration; active optics (backup)

• Schedule:– implementation phase start: May 2005; launch: mid-2010– overall system design advanced and stable since 2000– no major identified uncertainties to affect cost or launch schedule– technology/science ‘window’: 2010-12

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Scientific Organisation

• Gaia Science Team: – 12 members

• Scientific working groups:– 16 groups focused on payload, specific objects, and data analysis– 220 scientists active in the working groups at some level

• Community is active and productive:– regular science team/working group meetings: ~20 in both 2002 & 2003– active archive of scientific working group reports: ~150 since 1 Jan 2003– advance of simulations, algorithms, accuracy models, etc

• Data distribution policy:– final catalogue ~2018

– intermediate catalogues as appropriate

– science alerts data released immediately

– no proprietary data rights

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Data Reduction Principles

Sky scans(highest accuracy

along scan)

Scan width: 0.7°

1. Objects are matched in successive scans2. Attitude and calibrations are updated3. Objects positions etc are solved4. Higher terms are solved5. More scans are added6. System is iterated

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Schedule

2000 2004 2008 2012 2016 2020

Acceptance

Technology Development

Design, Build, Test

Launch

Observations

Analysis

CatalogueEarly Data

Concept & Technology Study ESA SCI 2000(4)

Re-Assessment: Ariane Soyuz

To L2

Assumed start of Phase B2

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Huge and timelyscientific impact

Well-defined payloadand spacecraft

Technology, cost andschedule maturity

Substantial and active ESA-based community