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Olivier AbsilChargé de Recherches FNRS
AEOS group
3rd ARC meeting – March 4th, 2011
Imaging faint companions with interferometric closure
phases
3rd ARC meeting 04/03/2011
VLTI VLT (+ adaptive optics)
Why interferometry?
Enables physical characterisation Photometry / spectrum temperature, composition 3D orbit mass (when combined with RV)
Complementary to single pupil imaging Search region ~ 200 mas in radius Milli-arcsec resolution access to hot Jupiters
Could replace RV for inappropriate stars
0.1 1 10 100Distance [AU], for star at 40 pc
3rd ARC meeting 04/03/2011
The interferometric view of binaries
Sum of 2 offset fringe packets Global size increased visibility affected Photocentre shifted phase affected “Resolved” when Δθ > λ/2B
B
λ/2B
04/03/20113rd ARC meeting
Δθ
Effect on the (squared) visibility
04/03/20113rd ARC meeting
Modulation vs baseline Period = λ/Δθ
V2 deficit w.r.t. single star Up to 4× flux ratio
Achievable dynamic range Up to ~1:250 (at 3σ) Assumes 0.5% accuracy
on cumulated V2
Contrast/position ambiguity Many V2 measurements
1% contrast at 10 mas
Effect on the (differential) phase
04/03/20113rd ARC meeting
Absolute phase lost due to turbulence
Wavelength-differential phase can be measured Non-zero if star and
companion have different spectra
Position/spectrum still ambiguous
Affected by dispersion Contrast limited to
~1:100
λ
photocenterphotocenter
M0V at 10 mas of A0V
(100 m baselines)
A better solution: closure phase
04/03/20113rd ARC meeting
Ψ123 = φ12+ε1 + φ23 + φ31−ε1
External perturbation removed
≠ 0 only when object departs from point-symmetry
Case of a binary object Ψ = ρ (sin α12+sin α23+sin α31)
where αij=2πBij∙θ/λ CP proportional to flux ratio ρ
contrast of 1:100 CP ~ 1° Contrast/position ambiguity
mitigated by spectral dispersion
1
2 3
φ12+ε1
φ23
φ31−ε1
ε1
turbulence
VLTI/AMBER observations of β Pic
04/03/20113rd ARC meeting
24 – 28 January 2010 10+ hours of
observationsVLTI
ATs: A0 – G1 – K0 Good seeing (0.8")
AMBER MedRes (R = 1500) K1 band (1.93 – 2.27 µm) FOV ~ 420 mas FWHM
Radius ~ 4 AU for β PicFINITO
Fringe tracking96m
96m
128m
Closure phase stability
04/03/20113rd ARC meeting
Calibration star HD 39640 (G8III, K=3)
At 1.2° on sky Unresolved (V2~0.9)
Data reduction Amdlib 3.0 No special
options/scriptsCP stability
~0.3° in 2.00 – 2.27 µm
Fitting a (high-contrast) binary model
04/03/20113rd ARC meeting
Best fit: 1.8×10-3 ± 1.1×10-3 at 14 mas (χr2=0.87)
AMBER performance Optimal search zone:
2 – 100 mas Median error bar = 1.2 × 10-
3
3σ limit at 50% completeness 1:300 (= 29 MJup)
3σ limit at 90% completeness 1:200 (= 47 MJup)
Reduced performance beyond 100 mas Time smearing FOV limitation
VLTI/AMBER sensitivity profile
04/03/20113rd ARC meeting
Galland et al. 2006 Boccaletti et al. 2009
VLTI/PIONIER observations of Fomalhaut
04/03/20113rd ARC meeting
New visitor instrument at VLTI Developped in Grenoble First light: Oct 2010
Combines 4 telescopes 4 CPs at a time! CP stability ~ 0.2°
First test on Fomalhaut 2×2h of observation Dynamic range up to
1:500 Corresponds to ΔK=6.7
Confirmed by double-blind test
Confirmation with double blind test
04/03/20113rd ARC meeting
> 3σ< 3σ
But we do have detections, too!
04/03/20113rd ARC meeting
Snapshot on δ Aqr A3V at 50 pc G5V companion
detected 2% contrast easy
Perspectives with PIONIER
04/03/20113rd ARC meeting
Survey of young main sequence stars 1:500 around AU Mic (M0V, 10pc, 10Myr) 9 MJup planet Fill the AO/corono “blind” hole (< 10 AU) Explore the shores of the brown dwarf desert
Towards hot Jupiters?