The Gaia Mission Coryn Bailer-Jones Max Planck Institute for Astronomy Heidelberg, Germany ISYA 2016, Tehran
What Gaia should ultimately achieve high accuracy positions, parallaxes, proper motions e.g. ~20 μas at G=15 entire sky to G~20 ~10 9 sources astrophysical parameters of sources light curves optical spectrophotometry radial velocities to G<15 (1-15 km/s)
Main science goals What is the Galaxy made of? distribution and properties of stars distribution of dark matter How did the Galaxy form? substructure in disk and halo (mergers) star formation history
Astrometry Earth Sun a r! star distant background stars Earth six months later
Astrometry Earth Sun a r! star distant background stars Gaia orbits Sun to create parallax effect Earth six months later Six dimensions of phase space positions: right ascension, declination distance (from parallax) tangential velocities (from proper motions and parallax) radial velocity
Gaia observations star positions as a function of time
Solving the astrometry Source parameters position, parallax, PM, RV Astrometric Global Iterative Solution Satellite attitude CCD calibration
The satellite Graphics: ESA, Astrium
Gaia (minus sunshield) in Kourou cleanroom
Payload Graphics: ESA, Astrium
Instruments Graphics: ESA, Astrium
Launch (19 Dec. 2013)
Earth-Sun L2 orbit
Astrometric accuracy End-of-mission parallax standard error [µas] 100000 10000 1000 100 10 Hipparcos calibration noise floor photon noise non-uniformity over the sky B1V M6V 2 0 2 4 6 8 10 12 14 16 18 20 22 V [mag] Graphic: Anthony Brown (2013) 10-15 μas at V<12; 10-25 μas at V=15; ~350 μas at V=20-22
Gaia parallaxes fpe = fractional parallax error 8kpc 100 000 within 80pc, fpe <0.1% 10 million within 800pc, fpe <1% 150 million within 8kpc, fpe <10% Image: NGC4565 from panther-observatory.com
Parallaxes are noisy: distance 1/parallax PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 127:994 1009, 2015 October 2015. The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A. Estimating Distances from Parallaxes CORYN A. L. BAILER-JONES Max Planck Institute for Astronomy, 69117 Heidelberg, Germany; http://www.mpia.de/homes/calj Received 2015 May 06; accepted 2015 July 08; published 2015 September 21 ABSTRACT. Astrometric surveys such as Gaia and LSST will measure parallaxes for hundreds of millions of stars. Yet they will not measure a single distance. Rather, a distance must be estimated from a parallax. In this didactic article, I show that doing this is not trivial once the fractional parallax error is larger than about 20%, which will be the case for about 80% of stars in the Gaia catalog. Estimating distances is an inference problem in which the use of prior assumptions is unavoidable. I investigate the properties and performance of various priors and examine their implications. A supposed uninformative uniform prior in distance is shown to give very poor distance estimates (large bias and variance). Any prior with a sharp cut-off at some distance has similar problems. The choice of prior depends on the information one has available and is willing to use concerning, e.g., the survey and the Galaxy. I demonstrate that a simple prior which decreases asymptotically to zero at infinite distance has good performance, accommodates nonpositive parallaxes, and does not require a bias correction.
Inferring distance from parallax $ parallax $ error in parallax r distance $/as = 1 r/pc likelihood P ($ r, $) = " 1 p exp 2 $ 1 2 2 $ $ 1 # 2 r posterior P(r $, $) / P ($ r, $) P(r) r = 100 uniform prior $ =0.01 P(ϖ r, σ ϖ ) 0 100 200 300 400 0.005 0.005 0.015 0.025 ϖ σ/ῶ = 0.1 σ/ῶ = 0.2 σ/ῶ = 0.5 P (r ϖ, σ ϖ ) 0.0 0.2 0.4 0.6 0.8 1.0 0 100 200 300 400 500 r
Stellar clusters Hertzsprung-Russell Diagram Hyades Hipparcos stars (colour is dust extinction) (Bailer-Jones 2011) 70 clusters within 500pc distances of individual stars to <1% for G<15 individual transverse velocities to <1km/s cluster ages from luminosities, main sequence position/turn off cluster dispersion from kinematics search for new structures in 6D phase and astro. param. space
Spectrophotometry (BP/RP) T eff = 5000 K, A V = 0 mag photon counts per band 400 500 600 700 800 900 1000 1100 wavelength / nm
Spectrophotometry (BP/RP) T eff = 5000 to 20 000 K photon counts per band 400 500 600 700 800 900 1000 1100 wavelength / nm
Spectrophotometry (BP/RP) galaxies quasars ultracool dwarfs emission line stars photon counts per band x constant stars Teff variation stars A0 variation stars [Fe/H] variation stars logg variation 400 600 800 1000 400 600 800 1000 wavelength / nm 400 600 800 1000 400 600 800 1000
Radial velocity spectra (RVS) [Fe/H] variation logg variation photon counts + offset Teff variation emission line stars 850 855 860 865 870 850 855 860 865 870 wavelength / nm
Science objectives Composition, origin, evolution of the Galaxy Stellar structure and evolution (fundamental parameters) Stellar clusters (formation, dispersion) Astrometric reference frame Cosmological distance scale Binary stars (masses, mass-luminosity relation) Exoplanets (especially Jupiter-mass) Solar system asteroids (including near-earth objects) General Relativity tests (light bending) 3D map of interstellar extinction Galaxies, quasars, supernovae
Summary of the Gaia mission Gaia is the only large-scale, high accuracy astrometric mission It is operating well and producing huge amounts of data Gaia will contribute to many areas of astrophysics For more information see www.cosmos.esa.int/web/gaia First data release is on 14 September 2016
The first Gaia data release
GDR-1 release date: 14 Acknowledgements: September 2016 DPAC, ESA, Airbus 1000 D&S, days after subcontractors launch
What will be released? Results of global astrometric solution 2D positions of 1.1 billion sources down to G 20.7 with a positional accuracy of about 10 mas the secondary set 2D positions, 2D proper motions, and parallaxes of 2 million sources down to G 12 with accuracies of about 0.3 mas and 0.3 mas/yr the primary set or TGAS (Tycho-Gaia Astrometric Solution) Light curves of 3360 variables (RR Lyrae, Cepheids) around the south ecliptic pole over 28 days with around 200 epochs Caveat: this is a preliminary release based on incomplete processing
Photometry in broad G-band throughput 400 600 800 1000 wavelength / nm
TGAS principle Gaia data (~2015) Tycho-2 position (~1991.5)
Astrometric solution Data: 5 25 Gaia transits over 14 (11) months (all sources) Set prior on 5 astrometric parameters for 2 million primary sources Tycho-2/Hipparcos positions and uncertainties broad priors on parallaxes and PMs (Tycho-2/Hipparcos not used) Run iterative solution over primary set get useful 5-parametric solutions fix attitude and CCD parameters Apply solution to secondary set only positions meaningful (with larger errors due parallax/pm neglect)
Limitations of GDR-1 short time span and few observations (11/14 months) no solution for accelerations (e.g. binarity, perspective acceleration) simplified attitude modelling (data gaps, long knot sequence, clanks) use of nominal PSF; no chromaticity correction; CTI neglected no closed loop on photometric astrometric solution simplified fit to Basic Angle Monitor data bright star photometry/astrometry is the hardest to calibrate
These images have been redacted prior to the data release. See http://www.cosmos.esa.int/web/gaia/dr1 for details
Some other aspects of the data Bright limit: G ~ 6 mag Photometric precision: 0.5 10 mmag for sources with 10 transits Maximum source density in secondary set is about 1 million/deg 2 minimum separation to detect equal G binary: 0.23" / 0.70 Some filtering of data sources with too few observations or long data gaps for TGAS: σ(parallax) > 1 mas or σ(pos) > 20 mas for secondaries: σ(pos) > 100 mas or excess noise > 20 mas or N<5
Verification Parallaxes (and positions) solved for 135 000 known quasars assuming them to have negligible proper motions defining quasars used to align GDR-1 with ICRF check on parallax zero point: good to 0.1 mas globally Hipparcos parallaxes for overlap with TGAS independent check on zero point with LMC, Cepheids, clusters, etc. used to inflate formal errors to get parallax accuracy of ~0.3 mas
And after Gaia DR-1? Gaia DR-2 planned for end of 2017 5-parameter astrometry on most Gaia sources (~ 1 billion) 22 months of data Tycho-2/Hipparcos positions will no longer be used G, GBP, GRP (with Teff calibration) for most sources mean radial velocities to GRVS < 12 for stars with constant RV and possibly more
Summary of Gaia Data Release 1 huge amount of work by DPAC, ESA, and industry positions for 1.1 billion sources, G<20.7, accuracy ~10 mas positions, proper motions, parallaxes for 2 million sources, G<12, accuracy ~0.3 mas and 0.3 mas/yr averaging will not give you a 1/ N error reduction (local error correlations) data from ESAC and mirrors; query tools, file download http://www.cosmos.esa.int/web/gaia/dr1