Transcript GAIA A Stereoscopic Census of our Galaxy

Gaia
A Stereoscopic Census of our Galaxy
http://www.rssd.esa.int/Gaia
November 2003
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
Completeness
Bright limit
Number of objects
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7.3 – 9.0
~0
120 000
Effective distance limit
Quasars
Galaxies
Accuracy
1 kpc
None
None
~1 milliarcsec
Broad band
photometry
Medium band
photometry
Radial
velocity
Observing programme
2-colour (B and V)
None
None
Pre-selected
20 mag
~20 mag
~3-7 mag
26 million to V = 15
250 million to V = 18
1000 million to V = 20
1 Mpc
~5 
106 - 107
4 arcsec at V = 10
10-15 arcsec at V = 15
200-300 arcsec at V = 20
5-colour to V = 20
11-colour to V = 20
1-10 km/s to V = 16-17
Complete and unbiased
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Stellar Astrophysics
• Comprehensive luminosity calibration, for example:
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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…
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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
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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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Planets: Expected Discoveries
• Astrometric survey:
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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:
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1.2
d(")
Planète : r = 100 mas P = 18 mois
1/07/02
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10–20,000 planets (~10 per day)
displacement for 47 UMa = 360 as0.8
orbits for ~5000 systems
0.6
masses down to 10 MEarth to 10 pc
1/01/03
1/07/01
0.4
1/01/02
1/01/01
1/07/00
• Photometric transits: ~5000?
0.2
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1/01/00
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0.2
0.6
0.4
acos d(")
0.8
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1.0
Gaia: Studies of the Solar System
• Asteroids etc:
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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
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Light Bending in Solar System
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Satellite and System
• ESA only mission
• Launch date: 2010 targetted
• Lifetime: 5 years
• Launcher: Soyuz
• Orbit: L2
• Ground station: Perth or Madrid
• Data rate: 1 Mbps
• Mass: 1700 kg (payload 800 kg)
• Power: 2000 W (payload 1200 W)
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Payload and Telescope
Rotation axis
SiC primary mirrors
1.4  0.5 m2 at 106°
Superposition of
fields of view
SiC toroidal
structure
Combined
focal plane (CCDs)
Basic angle
monitoring system
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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
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Star motion
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:
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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 Sun
Scan rate: 60 arcsec/s
Spin 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:
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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:
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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:
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final catalogue ~2018
intermediate catalogues as appropriate
science alerts data released immediately
no proprietary data rights
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Data Reduction Principles
Scan width: 0.7°
Sky scans
(highest accuracy
along scan)
1. Objects are matched in successive scans
2. Attitude and calibrations are updated
3. Objects positions etc are solved
4. Higher terms are solved
5. More scans are added
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6. System is iterated
Schedule
2004
2000
2008
2012
2016
2020
Concept & Technology Study ESA SCI 2000(4)
Acceptance
Re-Assessment:
Ariane  Soyuz
Technology Development
Design, Build, Test
Launch
To L2
Assumed start of
Phase B2
Observations
Analysis
Early Data
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Catalogue
Huge and timely
scientific impact
Technology, cost and
schedule maturity
Well-defined payload
and spacecraft
Substantial and active
ESA-based community
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