Olivier ABSIL Université de Liège * Pathways Towards Habitable Planets Barcelona, 14/09/2009

Goal #1: separate planet from star Sun-Earth system at 20 pc: θ = 50 mas – V band: λ /D = 25 mas with 4-m telescope – N band: λ /2B = 25 mas with 40-m interferometer Interferometry required in the thermal regime Goal #2: reach high dynamic range – “Classical” interferometry: closure phases – “Nulling” interferometry

Detecting hot Jupiters with interferometric phases

Star and planet  2 fringe packets – Resulting packet shifted wrt “star only” Problem: atmospheric turbulence – Absolute phase not preserved Base λ /2B Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers

Wavelength-differential phases – Measure contrast variation vs λ Phase referencing (dual feed) – Measure relative phase wrt reference star located within isoplanetic patch Closure phase – Ψ 123 = φ 12 +ε 1 + φ 23 + φ 31 -ε 1 – External perturbation removed –≠ 0 only when object departs from point-symmetry φ 12 φ 23 φ 31 ε1ε1 Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers

Current instruments – VLTI/AMBER (120m) – CHARA/MIRC (330m) Access 1 – 3 mas range State-of-the-art – Error bar ~ 0.1° (wide band) K=3, 1m tel., 1h integration – Contrast ~ reachable – First exoplanet spectrum still to be done Terrestrial planets? – Stellar signal not removed  SNR ~ 10 8 needed! Separation (mas) Contrast Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers Detection efficiency (3T)

How to remove stellar light

Put the 2 beams in phase and lock them Introduce achromatic π phase shift  0 B Constructive Destructive Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers

Interferometer response projected onto the sky Sources of signal – Star – Circumstellar disk – Planets – Thermal background The transmitted flux from all sources within the diffraction spot is summed! 2 λ /D (diffraction spot) λ/B Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers

Stellar nulling ratio: N = 4/ π 2 × ( λ /B) 2 / θ  2 – Sun-Earth system: N ≈ – Hot Jupiter: N ≈ This stellar residual can be (partially) removed – Analytical calibration – Modulation Stellar disk Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers

 0  0 − Symmetric exozodi signal removed, BUT beware of asymmetries! (cf Panel 3 satellite meeting) Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers Requires multi-telescope array

Wavefront quality/stability is critical for deep nulling Piston Turbulence Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers Waveguide

Finding exozodiacal disks

Solar system in mid- infrared – F zodi / F  ~ – F Earth / F  ~ Ground-based nullers  sensitive to bright exozodiacal disks – Keck Interferometer Nuller (N band) – Large Binocular Telescope Interferometer (N band) – ALADDIN (L-band nuller project for Antarctica) Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers

Background removal – 2 nullers – Modulation Sensitivity: ~ 300 zodi Nuller 1 Nuller 2 4 m 10 m D1D1 D2D2 C1C1 C2C2 φ φ 85 m Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers

L-band: better theoretical sensitivity Antarctica – Stable atmosphere – Low background ALADDIN concept – Optimised infrastructure – Movable 1-m telescopes Sensitivity – ~50 zodi Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers © AMOS 2008 Concordia station

FKSI – Telescopes: 50 cm – Baseline: 12 m Structurally connected – Waveband: 3 to 8 µm – Studied by NASA/Goddard Pegase – Telescopes: 40 cm – Baseline: 40 – 500 m Free flying spacecrafts – Waveband: 1.5 to 6 µm – Studied by CNES Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers

Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers Shot noise Asymmetries Tolerable exozodi level for mid-IR Earth-like planet imaging (see Defrère et al. poster + P3 panel meeting)

Characterising Earth-like planets

Mid-IR spectroscopy of habitable planets – Bio-signatures: H 2 O, CO 2, O 3 Need space-based interferometer – At least 3 telescopes for phase chopping NASA & ESA converged towards an optimal concept: “X-array” Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers

6:1 X-array configuration – Out-of-plane design – Size: 120 × 20 ↔ 400 × 67 m – Collectors: 2 m spherical – Waveband: 6-20 µm – Spectral resolution: 25 – Passive cooling to 50 K – Null depth: – Launch: ~8 tons to L2 Free-flyers: cm accuracy Delay lines: nm accuracy Nominal mission: detection (3 yr) + spectroscopy (2 yr) Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers Diameter1m2m4m Screened # F31035 # G # K # M H 2 O, CO 2, O # F012 # G0411 # K2518 # M91139 Survey of ~200 stars for Earth-like planets, possible spectroscopic follow-up for ~20 planets

Formation flying – Space rendez-vous and docking: OK – RF metrology and control: TBD (PRISMA, Proba-3) – Full metrology and control algorithms: OK on ground Space-based validation needed! – Propulsion: TBD (Proba-3) Nulling interferometry – N=10 -5 achieved in lab on wide mid-IR band – Single-mode fibres available for mid-IR – Cryogenic delay lines tested on ground – Full vacuum cryogenic instrument: TBD! Incl. fibres, deformable mirrors, delay lines, adaptive nuller, etc – Planet signal extraction: TBD (incl. instability noise) Not mature  technology development plan needed! Classical interferometryNulling interferometry2-telescope nullersMulti-telescope nullers

Current instruments – VLTI, CHARA: first (closure) phase measurements – Keck nuller: first exozodi finder Near- to mid-term future – VLTI 2 nd generation instruments (hot Jupiters) – LBTI, ALADDIN, FKSI: improved exozodi finders Long-term future – Terrestrial planet characterisation