Spectral Analysis of atmospheres by nulling interferometry Marc OLLIVIER Institut d’Astrophysique Spatiale - Orsay

Nulling interferometry : principles Molecules nov M.O.2 Beam combiner T2T1 D D.sin   StarPlanet ++ 1 arcsec =10  m, D=10m,  =0.1 arcsec

Nulling interferometry : key-issues : perfect matching of the wavefronts Phase matching Co-phasing of the wavefront : optical delay lines + fringe sensor Perfect coherence of the stellar wavefront : optical filtering + AO if necessary Polarization matching Control of polarization rotation over the optical bench Amplitude matching Molecules nov M.O.3

Planetary observations : requirements Planets around a G0 star at 10 pc V (0.55μm) / L (3.5μm) / N (10.2 μm) Bands Molecules nov M.O.4 Hot Jupiter Super planet Real Earth Real Jupiter Uranus Distance to the star (mas) Telescope diameter (m) Planetary flux (mJy) e-4 1 e e-3 3 e-5 1 e-5 1 e-4 5 e e-5 3 e-5 5 e e-7 2 e-8 Contrast star/planet e+8 1e+6 1 e+9 1e +7 6 e+8 4 e+7 7 e+10

A few interesting photons, a lot of noise Molecules nov M.O.5  Signal : ~ 10 ph s -1 m -2 (in the [6-20  m] spectral range)  background and noise Stellar leaks (star size + pointing stability) Vary as  2 to  6 (and more cf. Rouan) IR background (locl zodi + instrument emission Exo-zodi (global emission x interferometre response) ~ 10 to 100 x Signal ~ 1000 x Signal ~ 300 x Signal

Several configurations Molecules nov M.O.6 Angel et al Mennesson et al. 1997

Internal modulation (Mennesson et al.) Molecules nov M.O.7 Sub-arraysTransmission map  4 transmission central symmetry conjugaison Modulation efficiency - Contribution of Exo-zodi and stellar leaks are the same in the 2 states of modulation - Fast modulation (faster than rotation) post détection. - Inherent modulation (Absil)  /2   /2 Detector Illustration : courtesy O. Absil

What is observed ? Single pixel detector with : Planetary signal (modulated) Mean stellar leaks (not modulated) Mean exo-zodi signal (not modulated) Local zodi (not modulated) Variable stellar leaks : pointing + asymmetry of the star (variable) Variable exo-zodi signal (asymmetry of exo-zodi) Other sources of noise (Thermal noise, instrumental noise, detection noise) Image reconstruction software necessary to get F(α,δ,λ) using multi baseline / λ information (Thiébaut et al.) Molecules nov M.O.8

Scientific products of nulling interferometry Molecules nov M.O.9 (After Mennesson et Mariotti 1997) « Imaging »Spectral analysis

Nulling interferometry in the lab Molecules nov M.O.10 Booth, Martin and Loya, 2008 JPL’s PDT facility

Nulling interferometry in the lab Molecules nov M.O.11 Chazelas, 2006

Stability of the null 1/F noise Need for control loops to stabilize the null Control loop based on the interferometric signal instead of metrology signal (to avoid differential effects) Molecules nov M.O.12

IAS test bench : synapse Molecules nov M.O.13

Stability of the null (Gabor et al;) Molecules nov M.O.14 ddm N + - t t N2N2 N3N3 N1N1 N

Toward a space mission ? Molecules nov M.O.15

Planetary observations : requirements Planets around a G0 star at 10 pc V (0.55μm) / L (3.5μm) / N (10.2 μm) Bands Molecules nov M.O.16 Hot Jupiter Super planet Real Earth Real Jupiter Uranus Distance to the star (mas) Baseline (m) Total flux (mJy) e-4 1 e e-3 3 e-5 1 e-5 1 e-4 5 e e-5 3 e-5 5 e e-7 2 e-8 Contrast star/planet e+8 1e+6 1 e+9 1e +7 6 e+8 4 e+7 7 e+10

2 types of concepts Preparatory science concept : hot jupiters, brown dwarfs, exo-zodi characterisation 2 telescope array, Contrast 10 -4, near IR, stability no internal modulation = state of the art nulling performance Characteristation of exo Earths Multiple telescope arrray, internal modulation, Contrast 10 -6, Thermal IR, stability (In Europe) Only concept 1 has been completely studied using space agencies standards Molecules nov M.O.17

Mission requirements Molecules nov M.O.18 CharacteristicsPrep Sci. ConceptChar. Of Earth concept Spectral bandNear IRThermal IR Cophasing accuracy (residual opd) 2.5 nm rms3 nm rms Interferometric extinction Nulling stability Dephasing accuracy rad10 -3 rad Baseline m m Satellite guidingA few arcsec Fine guiding20 mas8 mas Telescope size30-40 cm1-3 m Instrument T100 K40 K Detector T55 K +/- 1K10 K

Preparatory science from space Molecules nov M.O.19

Main funtionalities of the payload Molecules nov M.O.20 BDT sept Spectro detectors 55 K detection stage +fiber coupling 2.5 nm rms stability zone optical head internal laser metrology  z2 FRAS µm resolution on the sky 30 mas pupil plane 2 beam compressor D 1 Fringe sensor –1.0 µm 2 nm resolution I2I2 I1I1 combining stage  phase shift d ODL 1 1 cm stroke 1 nm resolut. Siderostat 1 O 1 D’ O1O1 Siderostat 2 O2O2 M1M1 O2O2 combiner pupil plane 1 beam compressor 1 G=D/d D

SNR in nulling mode Molecules nov M.O.21 nulling instabilty due to : opd stab. :   nm rms flux balance stab. :   % rms (pointing stability) detector noise et  Td + RON optics thermal noise et  To photon noise integration time  i mi n ma x SNR minin [ min max ] parameters :  i =10h, D=30 cm,  o =0.1,  q =0.6, =55°K,  Td =0.1°K rms, =100°K,  To =1°K rms,  > < 0.01,   =2.5 nm rms,   =0.003 rms, RON 10e -

Payload composition Beam transportation Fine Relative Angle Sensor + actuator Optical delay line Achromatic phase shifter Optical filtering stage Beam combining stage Detection stage Molecules nov M.O.22

Mission requirements Variable baselines : formation flying : 2 siderostats and a beam combining lab satellite cold gas thrusters 2 stage metrology (RF + optical sensor) Fine metrology using the payload signal Thermal control of the instrument : V grooves Launch at L2 Operations at L2 Cost estimate : 300 M€ (mission) + 80 M€ (payload) Molecules nov M.O.23 BDT sept 2008

Preparatory mission and ESA Cosmic Vision process Science poorly considered Feasability estimated as highly risky Cost to high Part of the scientific information provided by other technics Project not selected Molecules nov M.O.24

Characterisation of exo-Earths : why is it difficult to design a mission and to estimate the cost ? Targets are not yet identified (size, distance, central star, zodi level…) Required performance is not well defined Estimated performance is not yet achieved in the lab The payload is not well define Technology is not clearly identified for several key-systems Existing technology should be improved No space experience for several items No such complex systems have already been designed and launched Need for O and A phase studies to define the mission and identify the required technology Molecules nov M.O.25

Conclusion The simpliest nulling interferometry mission is already a big (huge) space mission The ratio science/cost of a precursor (preparatory science) is considered as very low However, the mission concept (payload + system + operations) should be demonstrated A mission to analyse earthlike planets cannot be considered is present (future ?) plans of (European) Space Agency Molecules nov M.O.26

« The end justifies the means » phylosophy When everything appears hopeless call the « A-team » building of a new international structure such as in particle physics domain (LHC and Higgs boson question) with fundings adapted to big projects ? Molecules nov M.O.27