Galactic Surveys Astrometry and photometry

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Transcript of Galactic Surveys Astrometry and photometry

• Astronometry: Hipparcos and Gaia • Photometry: DSS, SDSS, 2MASS … • Fitting data to models
Tuesday, August 27, 2013
Astrometry > positions of stars in the sky, proper motions, parallaxes
Photometry > colors and brightness (stellar
stellar properties
Gaia’s Three Elements Astrometry (V < 20)
completeness to 20 mag ⇒ 109 stars accuracy: 10–25 µarcsec at 15 mag scanning satellite, two viewing directions principles: global astrometric reduction (as for
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, binaries spectra: chemistry, rotation
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,
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
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
Gaia: Complete, Faint, Accurate
Stellar Astrophysics Parallaxes and photometry imply a
comprehensive luminosity calibration distances to 1% for ~10 million stars to 2.5
kpc distances to 10% for ~100 million stars to
25 kpc parallax calibration of all distance indicators
e.g. Cepheids and RR Lyrae to LMC/SMC
accurate parallaxes imply accurate surface gravities and age
Stellar Astrophysics An unbiased survey implies a detailed
Galactic census
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)
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
1 / 0 7 / 0 2 δ(∀ )
0 0 .2 0 .4 0 .6 0 .8 1.0
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
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
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
Payload and Telescope Two SiC primary mirrors 1.45 × 0.50 m2 at 106.5°
SiC toroidal structure
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
Bl ue P h ot o m et er C C D s
Sky Mapper CCDs
R ed P h ot o m et er C C D s
Radial-Velocity Spectrometer
Wave Front Sensor
Sky mapper: - detects all objects to 20 mag - rejects cosmic-ray events - FoV discrimination Astrometry: - total detection noise: ~6 e-
Photometry: - spectro-photometer - blue and red CCDs Spectroscopy: - high-resolution spectra - red CCDs
unbiased sky sampling (mag, colour, resolution) all-sky catalogue at Gaia resolution (0.1 arcsec) to
Solution: on-board detection: good detection efficiency to V~21 mag FPA CCDs generate Gbps thus windows needed
Sky Scanning Principle
Spin axis 45o to Sun Scan rate: 60 arcsec/s Spin period: 6 hours
Astrometric Data Reduction Principles
Sky scans (highest accuracy
Scan width: 0.7°
1. Object matching 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 6. System is iteratedFigure courtesy Michael Perryman
Light Bending in Solar System
Movie courtesy Jos de Bruijne
Light bending in microarcsec, after subtraction of the much larger effect by the Sun
Gaia imaging 91 CCDs (4000 x 2000 pixels each) Distances for sources!
The Radial Velocity Spectrometer TDI spectroscopy!
Stellar motions
Photometry Measurement Concept
AL pixels
wavelength (nm)
Red photometer
AL pixels
wavelength (nm)
spectral dispersion per pixel (nm) .
RP spectrum of M dwarf (V=17.3) Red box: data sent to ground
White contour: sky-background level Colour coding: signal intensity
Ideal tests Shot, electronics (readout) noise Synthetic spectra Logg fixed (parallaxes will constrain
(Teff)/Teff=0.005 G=18.5 ([Fe/H])=0.42
(Teff)/Teff=0.008 G=20 ([Fe/H])=1.14
(Teff)/Teff=0.021 G=20
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<1 End of mission spectra will have S/N > 10x
G=10.5 G=12.3 G=15.8
Allende Prieto 2009
RVS produce Radial 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)
RV performance
15 km/s down to V=17
Atmospheric parameters (Ideal tests)
(S/N=1/20) Dash-dotted: relative flux
Allende Prieto (2008)
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 …
Tuesday, August 27, 2013
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
Tuesday, August 27, 2013
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
The world’s biggest picture
• 26 Gigapixels!
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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Tuesday, August 27, 2013
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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• 19,000 sqrt. deg • About 4 mag. deeper than 2MASS • Using ESO’s VISTA telescope
Tuesday, August 27, 2013
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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The variable sky • Most of the transients are fairly near, but
most exciting ones are far away
The variable sky
The variable sky • We do not know what is out there • Lots of room for classification
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.)
Tuesday, August 27, 2013
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
HST DAs • 3 DA white dwarf stars constitute the basis
for HST calibration (see papers by Bohlin) • Good to 1-2% • Calibration consistent for Vega V=0.023 +/- 0.008
Allende Prieto, Hubeny & Smith 2009
HST DAs analyzed with different models
Allende Prieto, Hubeny & Smith 2009
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
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.)
Allende Prieto & del Burgo (in prep). HST spectrum Gilliland & Bohlin
Fast rotation
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
Tuesday, August 27, 2013
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
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
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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Tuesday, August 27, 2013 Juric et al. 2008
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
Tuesday, August 27, 2013
very useful in the obscured parts of the Milky Way
• An approximate extinction relation between colors and passbands can be adopted Cabrera-Lavers
et al. 2008
Red-clumb giants in the bulge
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Cabrera-Lavers et al. 2008
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
Tuesday, August 27, 2013
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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maps before we fit radially symmetric models
Juric et al. 2008
Tuesday, August 27, 2013
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 algorithm algorithm
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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 • …
Tuesday, August 27, 2013
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 models Tuesday, August 27, 2013
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