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-symmetryCase 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 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 PicFINITO

Fringe tracking 96m

96m128m

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?