Olivier Absil Chargé de Recherches FNRS AEOS group 3 rd ARC meeting – March 4 th, 2011 Imaging faint companions with interferometric closure phases 3 rd 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 Distance [AU], for star at 40 pc 3 rd 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/ rd ARC meeting Δθ

Effect on the (squared) visibility 04/03/ rd ARC meeting Modulation vs baseline  Period = λ/Δθ V 2 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 V 2 Contrast/position ambiguity  Many V 2 measurements 1% contrast at 10 mas

Effect on the (differential) phase 04/03/ rd 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 photocenter M0V at 10 mas of A0V (100 m baselines)

A better solution: closure phase 04/03/ rd ARC meeting Ψ 123 = φ 12 +ε 1 + φ 23 + φ 31 −ε 1  External perturbation removed  ≠ 0 only when object departs from point-symmetry Case of a binary object  Ψ = ρ (sin α 12 +sin α 23 +sin α 31 )  where α ij =2πB ij ∙θ/λ  CP proportional to flux ratio ρ  contrast of 1:100  CP ~ 1°  Contrast/position ambiguity mitigated by spectral dispersion φ 12 +ε 1 φ 23 φ 31 −ε 1 ε1ε1 ε1ε1 turbulence

VLTI/AMBER observations of β Pic 04/03/ rd ARC meeting 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  Fringe tracking 96m 96m 128m

Closure phase stability 04/03/ rd ARC meeting Calibration star  HD (G8III, K=3)  At 1.2° on sky  Unresolved (V 2 ~0.9) Data reduction  Amdlib 3.0  No special options/scripts CP stability  ~0.3° in 2.00 – 2.27 µm

Fitting a (high-contrast) binary model 04/03/ rd ARC meeting Best fit: 1.8 × ± 1.1 × at 14 mas (χ r 2 =0.87)

AMBER performance  Optimal search zone: 2 – 100 mas  Median error bar = 1.2 ×  3σ limit at 50% completeness  1:300 (= 29 M Jup )  3σ limit at 90% completeness  1:200 (= 47 M Jup ) Reduced performance beyond 100 mas  Time smearing  FOV limitation VLTI/AMBER sensitivity profile 04/03/ rd ARC meeting Galland et al Boccaletti et al. 2009

VLTI/PIONIER observations of Fomalhaut 04/03/ rd 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/ rd ARC meeting > 3σ < 3σ

But we do have detections, too! 04/03/ rd ARC meeting Snapshot on δ Aqr  A3V at 50 pc  G5V companion detected  2% contrast  easy

Perspectives with PIONIER 04/03/ rd ARC meeting Survey of young main sequence stars  1:500 around AU Mic (M0V, 10pc, 10Myr)  9 M Jup planet  Fill the AO/corono “blind” hole (< 10 AU)  Explore the shores of the brown dwarf desert Towards hot Jupiters?