Imaging faint companions with interferometric closure phases

1. Imaging faint companions with interferometric closure phases Olivier Absil Chargé de Recherches FNRS AEOS group 3rd ARC meeting – March 4th, 2011 3rd ARC meeting

2. 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 VLTI VLT (+ adaptive optics) 0.1 1 10 100 Distance [AU], for star at 40 pc 3rd ARC meeting

3. 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 λ/2B B Δθ 3rd ARC meeting

4. Effect on the (squared) visibility 1% contrastat 10 mas • Modulation vs baseline • Period = λ/Δθ • V2deficitw.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 3rd ARC meeting

5. Effect on the (differential) phase λ photocenter photocenter M0V at 10 mas of A0V (100 m baselines) • Absolute phase lost due to turbulence • Wavelength-differential phase can be measured • Non-zero if star and companion have differentspectra • Position/spectrumstillambiguous • Affected by dispersion • Contrastlimited to ~1:100 3rd ARC meeting

6. A better solution: closure phase turbulence ε1 1 • φ31−ε1 φ12+ε1 2 3 φ23 • 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 ambiguitymitigated by spectral dispersion 3rd ARC meeting

7. VLTI/AMBER observations of β Pic 128m 96m 96m • 24 – 28 January 2010 • 10+ hours of observations • VLTI • 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 Pic • FINITO • Fringetracking 3rd ARC meeting

8. Closure phase stability • Calibration star • HD 39640 (G8III, K=3) • At 1.2° on sky • Unresolved (V2~0.9) • Data reduction • Amdlib 3.0 • No special options/scripts • CP stability • ~0.3° in 2.00 – 2.27 µm 3rd ARC meeting

9. Fitting a (high-contrast) binary model Best fit: 1.8×10-3± 1.1×10-3at 14 mas (χr2=0.87) 3rd ARC meeting

10. VLTI/AMBER sensitivity profile Galland et al. 2006 Boccaletti et al. 2009 • AMBER performance • Optimal search zone: 2 – 100 mas • Medianerror bar = 1.2 × 10-3 • 3σlimitat 50% completeness • 1:300 (= 29 MJup) • 3σlimitat 90% completeness • 1:200 (= 47 MJup) • Reduced performance beyond 100 mas • Time smearing • FOV limitation 3rd ARC meeting

11. VLTI/PIONIER observations of Fomalhaut • New visitor instrument at VLTI • Developped in Grenoble • First light: Oct 2010 • Combines 4 telescopes • 4 CPsat 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 3rd ARC meeting

12. Confirmation with double blind test < 3σ > 3σ 3rd ARC meeting

13. But we do have detections, too! • Snapshot on δAqr • A3V at 50 pc • G5V companiondetected • 2% contrast easy 3rd ARC meeting

14. Perspectives with PIONIER • 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? 3rd ARC meeting