gaia a stereoscopic census of our galaxy may 2008
TRANSCRIPT
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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Planète : = 100 mas P = 18 mois
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