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Transcript
How to simulate the Gaia
products
Francesca Figueras
Universidad de Barcelona (Spain)
How to simulate the Gaia products
Generalities:
• Brief description of what Gaia will provide
• Gaia Science Performances
• Few examples of Gaia Scientific Challenges
The Gaia error models
Additional tools for Gaia errors computation:
• Fortran code
• Python code
Application to a sample of Red Clump stars
Gaia focal plane
Astrometry
G band
LR spectra
GBP, GRP
bands
HR spectra
GRVS band, Vr,
Vsini
Astrophysical parameters: Teff, logg, [Fe/H], Ao (interstellar absorption) , [/Fe]
GOG: epoch parameters (per transit)
Epoch data will be provided in the last Gaia releases and will be mostly used
to determine properties of multiple systems and variable sources
GOG: combined parameters
(end-of-mission data)
Gaia Science Performances
A. Brown
Sky map of the mean parallax error
Scanning law (large
number of transits)
The Galactic Center
(large number of faint
stars)
Equatorial coordinates, units: mas (Palmer, Luri et al., 2013, in prep)
CRITICAL
Halo K1III, 4kpc, G= 15  10 % error in parallax
A. Brown
Solar type star, 1kpc, G=15, 3% error in distance, tangential Vel. Error < 1 km/s
Example of the Gaia capabilities
HIP 60350 (Runaway, B-star, 3.5 kpc)
At the moment, the quality of the observational data is insufficient to pinpoint the
precise origin of the star within the spiral arm (cluster birthplace?)
Gaia parallax accuracy ~10 as (G~11), 3 % accuracy in the relative parallax
Was the star originated some ≈15 Myr ago, in the Crux-Scutum spiral arm?
Irrgang, A., et al. 2010
The distance to LMC and SMC
GUMS: Based on a real catalogue:
7.5·106 (LMC), 1.5 106 (SMC) with G<20
Distance estimate from Gaia:
Large error in individual distances (LMC ~ 20 as)
Maximum Likelihood techniques mandatory (Luri et al., 1996)
Relative error in mean distance: 0.5% (LMC), 1.5% (SMC)
No 3D map
SMC with OGLE (Haschke et al., 2012):
Cepheids (2522 stars): 63.1  3.0 kpc , 4.7 % accuracy
RR Lyrae (1494 stars): 61.5  3.4 kpc , 5.5 % accuracy
R136, the star cluster in the Tarantula (30 Doradus)
Gaia (GIBIS)
HST
GIBIS: Gaia Basic Image Simulator
Stellar density at G<20 ~1.4 106 stars /sqdeg
de Bruijne and de Marchi, 2011
GAIA’s view
R136 (LMC)
Transverse velocities
~1-2 km/s accuracy
G=12-15 mag
(~10 as/yr)
de Bruijne and de Marchi, 2011
Gaia error models
Astrometry
Photometry
Spectra
Radial Velocities
Rotational velocities
Astrophysical Parameters
We describe here:
1) GOG approach
2) the receipts published in the Gaia Science Performance website
Gaia Science Performance web page
http://www.rssd.esa.int/index.php?page=Science_Performance&project=GAIA
Astrometry
Astrometric standard errors
Gaia Science Performance website
The mean end-of-mission standard error for parallax includes:
• all known instrumental effects
• an appropriate calibration error
• 20 % margin (results from the on-ground data processing are not included)
It depends sensitively on the adopted TDI-gate scheme (G < 12 mag)
(The decrease of the CCD exposure time to avoid saturation of the pixels)
End-of-mission parallax standard error
For bright stars (G<12 mag) the standard error is dominated by
calibration errors, not by the photon noise
Astrometric End-of-mission errors
Gaia Science Performance website
The end-of-mission performance depends on the scanning law. A more accurate
standard error can be computed by:
1) Multiplying the mean value by a geometrical scaling factor (g), different for each
of the five parameters (see figure and table)
2) Taking into account the individual number of transits the star will have by
multiplying the mean error value by
Both corrections depend on the mean ecliptic latitude β of the source (eclipticlongitude-averaged)
Geometrical scaling factor:
Each particular transit does not carry the same astrometric weight. The weight depends
on the angle between the along-scan direction (where we make the measurement) and
the circle from the star to the sun (the parallax shift is directed along this circle).
Therefore, a large number of transits does not guarantee a small parallax error (Jos de
Bruijne)
Absolute parallaxes
Obtained from differential along-scan measurements between the two FoV.
The error depend on the Γ angle (0.5 μas accuracy)
Parallactic displacement along the great cicle Sun-Star
Sensitivity AL is proportional to sin ξ sin Γ
ξ = Sun-spin axis angle = 45º for Gaia
Γ = basic angle = 106.5º for Gaia
Optimal values between astrometry requirements - that call for a large angle - and
implementation constraints - such as payload shading and solar array efficiency
Geometric factor (g) to be applied to the sky-averaged astrometric errors
for the five astrometric parameters as function of ecliptic latitude β.
Geometric factor (g) to be applied to the sky-averaged astrometric errors
for the five astrometric parameters as function of ecliptic latitude β.
GOG: astrometric Epoch Data
For each transit GOG provides:
-Local plane coordinates (, z)
-Observing time (t), that is mean time per transit
-Angle from local plane coordinates to equatorial coordinates ()
-Precision in the local plane coordinates (, z)
n : along scan AF number of CCDs
pr : relation between AC and AL pixel size (=3)
: line spread function centroiding error
GOG: astrometric epoch data
Example of GOG products:
Orbital motion for a binary system from GOG epoch data astrometry
Units: mass
Astrometric end-of-mission
g = 0.787·g
g = 0.699·g
g = 0.556·g
g = 0.496·g
End-off-mission longitude-proper-motion standard error
respect to the mean (G=20 mag, meand 175 as/yr)
Photometry
Photometry
Magnitudes:
G : broad-band (white-light),
fluxes obtained in the
astrometric instrument
GBp and GRp: integrated
from low resolution
spectra
GRVS: integrated from RVS
spectrometer
Photometric standard errors per transit
Gaia Science Performance website
Includes all known instrumental effects + 20% science margin
This is the single-field-of-view transit, taking into account all CCDs along scan
Photometric standard errors per transit
Gaia Science Performance website
Includes all known instrumental effects + 20% science margin
GOG: Photometric standard error per
CCD-transit
Estimated precisions for G for one transit on the focal plane with respect to
the G The shape for bright stars is due to the decrease of the exposure time
to avoid saturation of the pixels.
End-of-mission photometric standard errors
Gaia Science Performance website
• division of the single-field-of-view-transit photometric standard
errors by the square root of the number of observations (~70 in
average).
• With an assumed calibration error of 30 mmag at CCD-level
Units: mmag
GOG: End-off-mission Photometric
standard error
DPG: factor that takes into account the detection probability. It gives the
probability that a star is detected and selected on-board for observation (TB
implemented in GOG)
Jordi et al. (2010)
BP/RP/RVS spectra
BP/RP/RVS spectra
Low-resolution spectro-photometry obtained in the Blue and Red Photometers
Spectral dispersion & wavelength range:
BP: ~3 to ~27 nm pixel-1 ~330-680 nm
RP: ~7 to ~15 nm pixel-1 ~640-1050 nm
1 CCD along scan each
RVS spectrometer:
Resolving power ~11,500
wavelength range: 847-874 nm.
3 CCDs (strips) along scan
End-of-mission SNR per band
Radial Velocities
End-of-mission radial velocity error
Gaia Science Performance website
Errors are magnitude (V= Johnson Visual) and colour dependent (V-I)
Receipts outlined by JdB-022 (2005)
At the time of the Gaia Mission Critical Design Review (April 2011)
GOG: End-of-mission radial velocity error
From Sartoretti et al. (2007):
-Monte Carlo simulations
-Errors depending on Teff FeH logg vsini Vmag
Rotational Velocities (Vsini)
GOG: End-of-mission rotational velocity error
Astrophysical Parameters (AP)
Remember that one of the most important Gaia goals is to
reveal the formation and evolution of our Galaxy through
chemo-dynamical analysis
Gaia focal plane
Astrometry
G band
LR spectra
GBP, GRP
bands
HR spectra
GRVS band, Vr,
Vsini
Astrophysical parameters: Teff, logg, [Fe/H], Ao (interstellar absorption) , [/Fe]
GOG: AP error model
Astrophysical parameters are derived from (Liu et al., 2012):
1) BP/RP spectra (GOG)
2) the parallax
3) apparent magnitude of the star
Outputs from this work are:
1) Specific uncertainty estimates provided by Aeneas
2) Residuals computed as |estimated –true| values
Teff
Aeneas-pq precision
|estimated values from Aeneaspq – true|
Caution: different vertical scale!
Ao (absorption)
Aeneas-pq precision
|estimated values from Aeneaspq –
true|
Caution: different vertical scale!
Liu et al. (2012), Antiche et al. (2012, in preparation), see Bailer-Jones (2011) for the definition
of Ao
Log g
Aeneas-pq precision
|estimated values from Aeneaspq – true|
Caution: different vertical scale!
[Fe/H]
Aeneas-pq precision
|estimated values from Aeneaspq – true|
Caution: different vertical scale!
The thin disk is metal-rich and covers a wide age range
The other stellar components are all relatively old
(note similarity of [Fe/H] range for thick disk and globular clusters)
Element abundances
[/Fe] vs [Fe/H]
(Meza et al 2006)
The red points are potential omega Cen debris candidates.
Less -enriched than other halo stars : implies a longer history of chemical
evolution, as observed in omega Cen itself
Additional tools for Gaia errors computation:
1.Fortran code
2.Python code
Application to a sample of Red Clump stars
The Red Clump Stars
The Galactic Bar (s) and the Spirals
M. Romero-Gómez et al. 2013
Red Clump Stars: helium burning in the nuclei at a well defined luminosity
Hipparcos distances
Galactic disk space distribution function
1/ is a biased estimates of the
true distance!!
Red clump surface density
Does our Galaxy have one/two bars?
work in the space of the observables!
distances
parallaxes
Romero-Gómez et al. 2013
Does our Galaxy have one/two bars?
work in the space of the observables!
Extinction is critical: A new 3D new map using Gaia and IR
Red Clump: accuracy in tangential velocities
Gaia data
Gaia + IR distances (10%)
Mandatory to combine Gaia and IR data
Gaia and the distance scale
1912: Henrietta Leavitt discovered the key for
the distance scale of the Universe
Period-Luminosity relation
(25 cepheids in the SMC)
63
The Cepheids
Gaia will observe ~9000 Cepheids (extrapolated from Berdnikov et al. cat)
Gaia: Metallicity dependence of the PL relation
Windmark et al., 2011
Hipparcos suspected binarity in Cepheids
~ 50 % binaries, the companion star affect the brightness and motion
Gaia will treat several of them as binaries (astrometric orbit)
Hipparcos: no allowance for
binarity, thus the motion
along the orbital arc could
falsify the deduced parallax
value.
It is remarkable that all
negative parallax values
plotted in the figure belong
to known binaries (open
clicles).
Hipparcos vs. 'ground-based' parallax
(Laszlo Szabados, 2010)