galactic surveys astrometry and photometry
TRANSCRIPT
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Carlos Allende PrietoIAC
Galactic Surveys Astrometry and photometry
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Overview
• Astronometry: Hipparcos and Gaia• Photometry: DSS, SDSS, 2MASS …• Fitting data to models
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Basic astronomical measurements (from light)
Astrometry > positions of stars in the sky, proper motions, parallaxes
Photometry > colors and brightness (stellar
properties)
Spectroscopy > radial velocities, line strengths,
stellar properties
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Gaia’s Three ElementsAstrometry (V < 20)
completeness to 20 mag ⇒ 109 starsaccuracy: 10–25 µarcsec at 15 mag scanning satellite, two viewing directionsprinciples: global astrometric reduction (as for
Hipparcos)
Photometry (V < 20)Low-dispersion spectrophotometry 0.3 - 1 µm
Radial velocity (V < 16–17)slitless spectroscopy near Ca II triplet (847–874 nm)
third component of space motion,dynamics, population studies, binariesspectra: chemistry, rotation
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Astrometry• Positions and motions of stars provide full 3D
maps of the near universe around us – first the solar neighborhood, now more distant parts of the Milky Way and even the nearest local group galaxies
• First parallax measured in 1838 by Bessel (61 Cygni, 0.3 arcsec)
• The Hipparcos mission measured parallaxes for 1e5 stars with mas precision and 1e6 stars with lower precision between 1989 and 1993.
• Gaia is Hipparcos successor, with all-around enhancements Tuesday, August 27,
2013
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Hipparcos Gaia
Magnitude limit 12 20 magCompleteness 7.3 – 9.0 20 magBright limit 0 6 magNumber of objects 120 000 26 million to V = 15
250 million to V = 181000 million to V = 20
Effective distance limit
1 kpc 50 kpcQuasars None 5 x 105
Galaxies None 106 – 107
Accuracy 1 milliarcsec 7 µarcsec at V = 1010-25 µarcsec at V = 15300 µarcsec at V = 20
Photometry photometry
2-colour (B and V) Low-res. spectra to V = 20
Gaia: Complete, Faint, Accurate
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Stellar Astrophysics Parallaxes and photometry imply a
comprehensive luminosity calibrationdistances to 1% for ~10 million stars to 2.5
kpcdistances to 10% for ~100 million stars to
25 kpcparallax calibration of all distance indicators
e.g. Cepheids and RR Lyrae to LMC/SMC
accurate parallaxes imply accurate surface gravities and age
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Stellar Astrophysics An unbiased survey implies a detailed
Galactic census
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
rare stellar types and rapid evolutionary phases in large numbers
Statistics on variability across the board (~40 (RVS) - 100 (AS,XP) visits per object)
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One Billion Stars in 6-d will Provide …
in our Galaxy … the distance and velocity distributions of all stellar populations 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)
… 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: γ to 10-7 (10-5
present)
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Π λα ν τε : ρ = 100 µ α σ Π = 18 µ οισ
Exo-Planets: Expected Discoveries
Astrometric survey: monitoring of hundreds of thousands of FGK stars to ~200 pc detection limits: ~1MJ and P < 10 years
masses, rather than lower limits (m sin i) multiple systems measurable, giving relative inclinations
Results expected: ~20,000 exo-planets (~10 per day) orbits for ~5000 systems masses down to 10 MEarth to 10 pc
>1000 photometric transits
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 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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Satellite and System
• ESA-only mission• Launch date: late 2013 • Launcher: Soyuz–Fregat• Orbit: L2• Lifetime: 5 years• 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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Tuesday, August 27, 2013
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Payload
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Payload and TelescopeTwo 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 Photometer CCDs
Sky Mapper CCDs
104.26cm
Red Photometer CCDs
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
42.35cm
Figure courtesy Alex Short
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On-Board Object DetectionRequirements:
unbiased sky sampling (mag, colour, resolution)all-sky catalogue at Gaia resolution (0.1 arcsec) to
V~20
Solution: on-board detection:good detection efficiency to V~21 magFPA CCDs generate Gbps thus windows needed
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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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Astrometric 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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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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Gaia imaging91 CCDs (4000 x 2000 pixels each)Distances for 1.000.000.000 sources!
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The Radial Velocity SpectrometerTDI spectroscopy!
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Stellar motions
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Photometry Measurement Concept
Figures courtesy EADS-Astrium
Blue photometer:330–680 nm
Red photometer:640–1000 nm
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Photometry Measurement Concept
Figures courtesy Anthony Brown
Blue photometer
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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
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Ideal testsShot, electronics (readout) noiseSynthetic spectraLogg fixed (parallaxes will constrain
luminosity)
G=18.5
G=20
S/Nper pixel
Bailer-Jones 2009GAIA-C8-TN-MPIA-CBJ-043
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(Spectro-)photometryILLIUM algorithm (Bailer-Jones 2008). Dwarfs:G=15 ([Fe/H])=0.21
(Teff)/Teff=0.005G=18.5 ([Fe/H])=0.42
(Teff)/Teff=0.008G=20 ([Fe/H])=1.14
(Teff)/Teff=0.021G=20
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RVS S/N ( per transit and ccd)3 window types: G<7, 7<G<10 (R=11,500),
G>10 (R~4500) √ (S + rdn2)Most of the time RVS is working with S/N<1End of mission spectra will have S/N > 10x
higher
G magnitude
Allende Prieto 2009, GAIA-C6-SP-MSSL-CAP-003
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Sample RVS spectra (mission end, black line)
G=10.5 G=12.3 G=15.8
B5V
G2V
Metal-poor
K1III
Allende Prieto 2009
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RVS produceRadial velocities down to V~17 (108 stars)Atmospheric parameters (including overall metallicity) down to V~ 13-14 (several 106 stars)
Chemical abundances for several elements down to V~12-13 (few 106 stars)
Extinction (DIB at 862.0 nm) down to V~13 (e.g. Munari et al. 2008)
~ 40 transits will identify a large number of new spectroscopic binaries with periods < 15 yr (CU4, CU6, CU8)
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RV performance
Spec. for late-type stars
1 km/s at V<13
15 km/s down to V=17
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Atmospheric parameters (Ideal tests)
Solid: absolute fluxDashed: absolute flux, systematic errors
(S/N=1/20)Dash-dotted: relative flux
Allende Prieto (2008)
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Photometry• Gaia will not be the first full-sky photometric
survey• Palomar photographic plates (POSS)• HST needed a full-sky pointing catalog, which
was prepared from digitized photographic plates (DSS)
• 2MASS: first full-sky ground-based near-IR photographic survey (J H Ks filters)
• SDSS provided a large/area (14,000 sqr. deg) optical survey (ugriz system) using CCD detectors
• Others: GALEX (NUV), WISE (IR), UKIDS …
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Usual photometric systems• Johnson (-Cousins) UBVRI • Ströngrem ubvy • Near-IR Y J H Ks• SDSS ugriz• GALEX FUV/NUV• …• System responses usually include (approximate) atmospheric extinction
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SDSS• First massive solid-state optical photometric
survey (some 14,000 deg2 and 150 million stars down to r ~ 22 mag)
• 2% photometry – 1% in stripe 82• Highly-uniform observations (single
site/telescope/instrument)• Carefully designed filters (though issues with
u-band)
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SDSS imaging• 6 x 5 CCDs• Running in TDI
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The world’s biggest picture
• 26 Gigapixels!
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2MASS• 2 automated 1.3m telescopes (one in Arizona,
one in Cerro Tololo, Chile)• 3 channel (J, H, Ks) cameras, each with a
256x256 HgCdTe detector• 7.8s exposures• 4 years of operation• PSC: ~ 300 million stars down to J/H/Ks of ~
16,15,14• About 1 million extended sources
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UKIDSS
started in 2005 some 400 papers already published uses WFCAM on 4-m class UKIRT (four
2048x2048 Rockwell devices) 7500 sqr. Deg down to K~18
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VHS
• 19,000 sqrt. deg• About 4 mag. deeper than 2MASS• Using ESO’s VISTA telescope
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The Future: LSST
• A wide-field 8.4m telescope• A 3.2 Gpix camera• Imaging the whole (accessible sky) every few
nights• Starting in 2018• Tens of TB of data each night
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LSST
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The variable sky• Most of the transients are fairly near, but
most exciting ones are far away
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The variable sky
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The variable sky• We do not know what is out there• Lots of room for classification
algorithms
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Zero-point• Absolute calibration of astronomical sources
is non-trivial• Good lab reference sources hard to observe
through telescopes as if they were at infinity• Atmospheric extinction/distortion gets in the
way• Traditional reference source is Vega, which
sets zero-point tied to lab sources (Tungsten lamps or black bodies; see Hayes 1985, Megessier 1995), but Vega is not easy to model
• Spectrophotometric calibration nowadays tied to DA white dwarf models (Bohlin 2010 and prev. refs.)
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White dwarfs• DA white dwarfs are fairly simple: just two
parameters (Teff,logg), pure-H physics, NLTE but good agreement among models
Allende Prieto, Hubeny & Smith 2009
Examples From SDSS
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HST DAs• 3 DA white dwarf stars constitute the basis
for HST calibration (see papers by Bohlin)• Good to 1-2%• Calibration consistent for VegaV=0.023 +/- 0.008
Allende Prieto, Hubeny & Smith 2009
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HST DAs analyzed with different models
Allende Prieto, Hubeny & Smith 2009
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A-type stars Not pure hydrogen, but spectrum
dominated by it in optical and IR (continuum and lines); exception FeII lines in UV
Three parameters (Teff,logg,[Fe/H]) Reddening needs to be accounted for (also
true for faint WDs) Brighter and more common than WDs
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A-type stars
Not pure hydrogen, but spectrum dominated by it in optical and IR (continuum and lines)
Three parameters (Teff,logg,[Fe/H]) Reddening needs to be accounted for (also
true for faint WDs) Brighter and more common than WDs
Allende Prieto & del Burgo (in prep). Spectra from NGS (Gregg et al.)
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Vega
Allende Prieto & del Burgo (in prep). HST spectrum Gilliland & Bohlin
Fast rotation
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Zero-point• HST flux calibration: using zero-point V
magnitude for Vega (not quite zero, V= 0.023 mag) and 3 DA WDs models
• Vega STIS spectrophotometry calibrated in that way compares well with a model atmosphere for Vega and leads to a consistent zero-point based on photometry performed on the model
• System seems robust to 1-2% level• STIS spectrophotometry (calspec, NGSL) now
being used to set zero points for photometric systems
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Halo turn-off stars F-type, metal-poor: H continuum + lines
and few metal lines (not so few in the blue/UV)
Again 3 parameters (+ reddning) but now higher impact of [Fe/H] due to electrons forming H-
Many of them (just leaving the main sequence), easy to pick up from colors
Choice used for SDSS BD +17 4708 is the prototype
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Halo turn-off stars F-type, metal-poor: continuum H and H-, H
lines and few metal lines (not so few in the blue/UV)
Many of them (just leaving the main sequence), easy to pick up from colors
Choice used for SDSS BD +17 4708 is the prototype
Ramírez et al. 2009; HST data from Bohlin and colleagues
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Fitting models to data• Understanding the structure of the Milky Way
is critical • Starcounts are the most fundamental (and
easy) measurement: just photometry and coarse astrometry (star positions)
• Distances must be estimated, but parallaxes to a few percent available for only some 1e5 stars (Hipparcos)
• Photometric parallaxes derived for dwarfs based on models or semi-empirical relationships (e.g. clusters)
M-m = 5 -5 log(d)
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Color – absolute mag relationships for dwarfs
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Standard candles• Dwarfs outnumber giants in most cases (not
always). Their luminosities depend on metal content but weak dependence on age at a given color
• An alternative is to use stars at specific evolutionary stages that can be identified with certainty, and with reliable theoretical (or semi-empirical) luminosities
• Examples include cepheids, red-clump stars, RR Lyrae
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Standard candles• For example, red-clump giants have been
very useful in the obscured parts of the Milky Way
• An approximateextinction relationbetween colors and passbands can be adoptedCabrera-Lavers
et al. 2008
Babuiaux and Gilmore 2005Tuesday, August 27, 2013
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Red-clumb giants in the bulge
Tuesday, August 27, 2013
Cabrera-Lavers et al. 2008
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Large data sets• One can either fit the data with models, e.g.
Larsen & Humphreys (2003), Robin et al. (2003)
• Or derive density maps, which are subsequently fit to infer the model parameters, e.g. Juric et al. 2008
• Models involve a number of std. Milky Way stellar components: a disk (or two), a halo, and a bulge (plus other non-std. such as a bar or streams as needed)
• Nowadays, more complex orbit-family-type or numerical-simulations available, but parametric models provide a fast and useful path to start
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Density maps• Photometric parallaxes are derived first• Positions on the sky and distances are used to create a binned density map (Juric et al. 2008)• Photometric [Fe/H] estimates can be used
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Density maps• This approach allows to clean-up the density
maps before we fit radially symmetric models
Juric et al. 2008
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Fitting models to data• Typical 3-4 component stellar Milky Way
models involve 6-8 parameters: relative densities, halo exponent, disk scale height(s) and length(s)
• Extinction needs to be included in disk and bulge• Parameters constrained by optimization algorithmalgorithm
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Tools• Besançon model (Gaia universe model)• M. Cohen’s model• TRILEGAL (see Girardi’s lectures)• GALFAST (Juric)• Jordi Molgo’s simulator • …
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What’s next• Back to Gaia…• Starcounts soon to be suplemented with trig.
Parallaxes (replacing photometric ones) • and spectrophotometric metallicities (Gaia)
plus spectroscopic metallicities and more detailed abundances for a fraction of the sample (APOGEE/SDSS, Gaia-ESO, GALAH…)
• Further work on map construction desirable• Idem for tools for evaluating simple,
parametric, Milky Way modelsTuesday, August 27, 2013