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THE GROUND-BASED EUROPEAN NULLING INTERFEROMETRY EXPERIMENT(DARWIN-GENIE)

P. Gondoin, O. Absil, R. den Hartog, L. Kaltenegger, C. Eiroa, C. ErdM. Fridlund, A. Karlsson, A. Peacock, Z. Sodnik, S. Volonte1 and

R. Wilhelm, M. Schoeller, A. Glindemann2

1European Space Agency, P.O. Box 299, 2200AG Noordwijk, The Netherland ,2European Southern Observatory, D-85748 Garching bei Munchen, Germany

ABSTRACT

Darwin is one of the most challenging space projectsever considered by the European Space Agency(ESA). Its principal objectives are to detect Earth-like planets around nearby stars and to characterisetheir atmospheres. Darwin is conceived as a space“nulling interferometer” which makes use of on-axisdestructive interferences to extinguish the stellarlight while keeping the off-axis signal of the orbitingplanet. Within the frame of the Darwin program,the European Space Agency (ESA) and the Euro-pean Southern Observatory (ESO) intend to builda ground-based technology demonstrator called GE-NIE (Ground based European Nulling Interferome-try Experiment). Such a ground-based demonstra-tor built around the Very Large Telescope Interfer-ometer (VLTI) in Paranal will test some of the keytechnologies required for the Darwin Infrared SpaceInterferometer. It will demonstrate that nulling in-terferometry can be achieved in a broad mid-IR bandas a precursor to the next phase of the Darwin pro-gram.

Key words: Darwin; groundbased interferometry;nulling interferometry.

1. INTRODUCTION

Understanding the principles and processes that cre-ated the Earth, and allowed the development andevolution of life forms to take place is a key sci-entific objective which has been brought as a veryhigh priority to the attention not only of the Euro-pean Science Community but also of the EuropeanSpace Agency. In particular, the successful detec-tion of Earth-like planets possessing environmentsbenign to life would answer central questions suchas “How unique is the Earth as a planet ?” and“How unique is life in the Universe ?”. To achievethese objectives, the Darwin mission of the European

Space Agency (Fridlund 2000; Fridlund & Gondoin2002) will survey a large sample of nearby stars andsearch for Earth-size planets within their “habitablezone”. Darwin will measure their spectra in order toinfer the presence of an atmosphere and search forbiomarkers.

Detection of Earth-size bodies circling nearby starsis extremely difficult because of the weak planetarysignal emitted within a fraction of an arcsecond froman overwhelmingly bright star. A solar type star out-shines an Earth size planet by a factor of more than109 in the visible wavelength range (see Fig.2). In theinfrared spectral range, where the planet’s thermalemission increases and the star’s emission decreases,the contrast is still higher than 106. Only the plan-etary signal, a millionth of the stellar light, shouldremain in the input feed of a spectrograph in order toregister a planet spectrum in a reasonable time. Toaccomplish such an extinguishing of light at the rele-vant spatial scales, the technique of “nulling interfer-ometry” has been selected for Darwin. By applyingsuitable phase shifts between different telescopes inan interferometric array, destructive interference canbe achieved on the optical axis of the system in thecombined beam while interference is constructive forsmall off-axis angles. The principle of nulling inter-ferometry for a simple two telescope Bracewell in-terferometer (Bracewell 1978) is described in Fig. 1.The equivalent transmission map of the nulling inter-ferometer is a set of interference peaks with a sharpnull in the centre. By placing the central star underthis null, and adjusting the interferometer baselineto the required angular resolution, planets can bedetected in the “habitable zone”. The actual shapeand transmission properties of the pattern aroundthe central null depend on the configuration and thedistance between the telescopes.

The Darwin mission concept (ESA 2000) consists ofsix 1.5 m telescopes, each of which is a free flyingspacecraft transmitting its light to a central beam-combining unit (see Fig. 3). Using the nulling in-terferometry technique, the beam combiner extin-guish the stellar light by destructive interferences

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Figure 1. Principle of a two telescopes Bracewell in-terferometer.

Figure 2. Contrast between the solar flux andthe Earth as a function of wavelength in microns.Adapted from Angel et al. (1986).

on-axis and transmit the off-axis signal of the or-biting planet. The light from Earth-like planets atinterstellar distances of up to 25 parsecs can thenbe analysed. The mission concept is based on theability to co-phase telescopes on independent space-crafts to an accuracy better than 20 nm and to per-form nulling interferometry with a rejection factorof ≈ 105 in a wide spectral band extending from5 to 18 µm. In-orbit co-phasing of the free-flyingtelescopes is performed in successive steps combiningfirst a local radio-frequency positioning system withmilli-Newton propulsion devices, then inter-satellitelaser metrology with micro-Newton propulsion de-vices and finally fringe sensors with optical delaylines. These items are currently developed within theESA Technology Research Program (TRP). It is en-visaged to test them in-orbit within the frame of theESA SMART technology demonstration program.

In addition to the co-phasing of free-flying telescopes,a second key issue for Darwin is the nulling in-terferometry technique. Hence, the TRP activitiesalso include the development of Darwin specific op-tical components, namely achromatic phase shifters,wavefront filtering devices and IR single mode fi-bres, integrated optics, IR detectors, electronics andcoolers, optical delay lines and fringe sensors as wellas other components for interferometry. Laboratorynulling breadboards with star-planet simulators arecurrently evaluating the performance of the nullinginterferometric technique (Barillot 2003; Flatscher2003). These innovative breadboards use the narrowtelecommunication band around 1.65 µm where off-the-shelf components are available. Based on theirperformance, a follow-up activity has been identifiedwhich will adapt their design to the mid-IR band andtest the nulling interferometry technique on astro-nomical targets in Darwin representative operatingconditions. The Very Large Telescope Interferome-ter (VLTI; Scholler & Glindemann 2003) at the Eu-ropean Southern Observatory (ESO) is the most ap-propriate infrastructure on-ground to perform such atest. Hence, ESA and ESO initiate a definition studyfor a Groundbased European Nulling InterferometryExperiment which will operate in the central labo-ratory of the VLTI at Mount Paranal (Chile). Thisexperiment is called Darwin-GENIE. The definitionstudy will assess the technical feasibility of the ex-periment and establish its design, performance, pro-grammatics and cost. If successful, the definitionstudy will be the base for ESA and ESO to moveinto the hardware development, integration and ex-ploitation phases of the GENIE project.

2. GENIE OBJECTIVES

2.1. Demonstration of Darwin technology

The primary objective of the Darwin-GENIE nullingexperiment (Gondoin et al. 2003) is to gain experi-ence on the design , manufacture and operation ofa nulling interferometer using Darwin representative

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Figure 3. Artist view of the Darwin Space Inter-ferometer orbiting at Sun-Earth L2 point (courtesyAlcatel Space Division).

Figure 4. The ESO Very Large Telescope Interferom-eter with its four 8.2 meter telescopes and its centralinterferometric laboratory.

concept and technology. Nulling tests with the high-est rejection factor on single stars or close binariesin broad mid-IR spectral bands will achieve this ob-jective with the limitations imposed by the turbu-lence and infrared background of the Earth atmo-sphere. The Darwin-GENIE experiment will com-bine all optical functions foreseen into the futureDarwin Infrared Space Interferometer. It will ben-efit from the existing VLTI infrastructure, includingthe telescopes with chopping and adaptive optics, thedelay lines, the fringe sensors and the beam com-biner laboratory. The overall performance of the in-strument will heavily depend on the performance ofall VLTI subsystems and in particular on the adap-tive optics and co-phasing subsystems. The GENIEoptical bench within the VLTI laboratory will pro-vide the functions specific to the nulling interferom-etry technique, namely photometry and amplitudecontrol, polarization matching, phase shifting, beamcombination and internal modulation, spatial filter-ing, spectrometry, detection, electronics and cryo-genics. The detailed architecture of Darwin-GENIEwill be established during the definition study. It willtake into account the ESO VLTI interface character-istics and the output of the ESA TRP activities.

2.2. Preparation of the Darwin science program

Darwin science program aims to detect Earth-likeplanets around nearby stars, to determine their char-acteristics, to analyse the composition of their at-mospheres and to assess their ability to sustain lifeas we know it. An important information for aproper design of the direct planet detection experi-ment of Darwin is the frequency of Earth-like planetsaround G,K and M dwarfs. The COROT, Eddingtonand Kepler space missions will detect massive bodiesaround cool stars by occultation and will thus pro-vide this information. These missions are thus to beconsidered as true precursors to Darwin. A secondobjective of GENIE is also to prepare the Darwinscience program through a systematic survey of Dar-win candidate targets (Eiroa et al. 2003). The solarzodiacal cloud, a sparse disk of 10−100 µ diametersilicate grains, is the most luminous component ofthe solar system after the Sun. Its optical depth isonly ≈ 10−7, but a patch of the solar zodiacal cloud0.3 AU across has roughly the same emitting areaas an Earth sized planet. Similar and even brighterclouds may be common in other planetary systemsand present a severe obstacle for the direct detec-tion of extra-solar terrestrial planets. A systematicsurvey of Darwin candidate targets will screen-outthose stars for which circumstellar dust prevent thedetection of Earth-like planets. Bright exozodiacalclouds are easier to detect than extra-solar terres-trial planets, but finding an exozodiacal cloud is stilldifficult. The total emission from our zodiacal cloudis no more than 10−4 of the Sun’s at any wavelength.Photometric surveys like the Infrared AstronomicalSatellite (IRAS) survey can only detect exozodiacalclouds that are > 500 times as optically thick as thesolar clouds (Backman & Parece 1993). Attempts to

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Figure 5. Functional description of GENIE.

spatially resolve faint exozodiacal clouds with single-dish telescopes in the mid-infrared and near infrared(Kuchner & Brown 2000) have not yielded better de-tection limits. A Bracewell interferometer using twoESO VLTI 8m Unit Telescopes (UT) could providebetter performance.

3. THE BRACEWELL INTERFEROMETER: APOSSIBLE CONFIGURATION FOR GENIE

The transmission map T(θ,φ) (see Fig. 6) of adiffraction limited Bracewell interferometer with twocircular entrance apertures can be expressed as afunction of wavelength, telescope diameter D, andprojected baseline B as follows:

T (θ, φ) = 2× J1(πθD/λ)πθD/λ2

× sin2(πθcos(φ)B/λ) (1)

θ,φ are respectively the angular distance to the bore-sight and the azimut angle in the plan of the sky withrespect to the projected baseline. Eq.(1) assumesa diffraction limited operation without any residualwavefront error, with a perfect π phase shift betweenthe two arms of the interferometer and equal ampli-tudes of the recombining fields. In practice, the cen-tral null of the transmission map will be degraded

by amplitude and optical path differences (OPD) be-tween the two arms of the interferometer that are notperfectly corrected.

Also, wavefront distorsions induced by atmosphericturbulence will only be partially compensated by theadaptive optics. Spatial filtering of the high frequen-cies wavefronts errors by monomode optical fibresor waveguides will be required (Ollivier & Mariotti1997). The coupling efficiency in the fibre will bevariable (Shaklan & Roddier 1988) depending on thefluctuating wavefront and pointing errors. Hence,the envelope of the transmission map of the interfer-ometer will significantly depart from a Bessel func-tion and will be highly time variable. In the Gaussianapproximation, the optimum injection efficiency intoa fibre is found for an aperture ratio such that thediffraction limited image has approximately the sizeof the core (Ruilier 1998). The geometrical extent ofthe beam seen by the fibre is then SΩ ≈ λ2. TheIR thermal background contribution is then propor-tional to SΩ ≈ λ2 while the signal included into thefield-of-view of the interferometer is proportional tothe collecting area. Hence, the ratio signal to noiseof an exozodiacal dust cloud observed in the mid-IR by a Bracewell interferometer is proportional toits collecting area and the largest telescopes shall beused.

The output signal of a Bracewell interferometerpointing to a star surrounded by a dust cloud canbe expressed as follows:

S(θ, φ) = Aeff

∫

θ

∫

φ

T (θ, φ)O(θ, φ)dθdφ + Bgd (2)

where T (θ, φ) is the transmission map of the interfer-ometer degraded by residual amplitude fluctuationsand optical path differences between the two inter-ferometer arms. O(θ, φ) = Os(θ, φ) + Oz(θ, φ) is thesum of the brightness of the star and of the exozo-diacal dust disk. Bgd is the incoherent backgroundsignal which includes the thermal emission from thesky and from the telescope and instrument optics.

Since the only reliable observations on zodiacal dusthave been carried out in the solar system, spectralproperties of the exozodiacal dust emission can onlybe extrapolated from observations of the solar inter-planetary dust cloud performed by the Diffuse Infra-Red Background Experiment (DIRBE) on board theCOBE satellite. A comprehensive dust disk modelis due to Kelsall et al. (1998) who express the dustdensity n(r, z) as the product of a radial power lawterm and a vertical decreasing exponential term, i.en(r, z) = n0 × r−α × f(z/r). A disk inner cut-off isassumed to account for the sublimation of dust grainclose to the star at temperatures higher than about1500 K. Since the contribution of the scattered stel-lar light is negligible in the infrared, the brightness ofthe cloud is expressed as an integral along the line ofsight of the thermal emission of the dust which tem-perature is a decreasing function of the distance r to

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Table 1. Exozodiacal flux, star/cloud brightness con-trast of an exo-zodiacal cloud 10 times denser thesolar zodiacal dust and located around a G2 V starat 10 pc.

Spectral band L’ NExo-zodiacal flux (mJy) 0.5 1.1Magnitude 14.2 11.3Star/cloud contrast 30000 2500

Figure 6. Transmission map of a diffraction limitedBracewell interferometer with circular entrance aper-tures.

the star. Eq.(1) indicates that the first maximumof the transmission map is located approximately atθ = λ/(2×B). The hot inner dust of nearby solar-likezodiacal clouds, which has a high density, will ther-fore most contribute to the overall signal registeredwith a 50 m baseline interferometer. Flux consider-ations indicate that both the L’ band at 3.8 µm andthe N band around 10.2 µm could be appropriatefor the detection of exozodiacal dust clouds aroundnearby stars with two ESO VLT telescopes operat-ing as a Bracewell interferometer. In principle, theM band could also be considered but can be highlyaffected by the presence of varying amounts of watervapour in the atmosphere. Table 1 shows the fluxof an exozodiacal dust cloud 10 times denser thanthe solar zodiacal dust around a G2 V star at 10 pc.The contrast between the star and the dust cloudbrightness is also given for the different atmosphericbands. The N band present a significant advantageover the L’ band since the contrast between the starand the dust cloud is lower by a factor of 10.

4. THE TECHNICAL CHALLENGES

4.1. Minimization of the stellar leakage

The transmission map of a Bracewell interferometerexhibits a narrow null with a θ2 transmission on axis.Eq. 2 indicates that the high spatial resolution of atwo telescopes interferometer with a ≈ 50 m base-line does not completely reject the starlight on axisspecially when stars are closer than 50 pc. A resid-ual stellar signal Sleak leaks through the transmissionmap. It can be expressed as a function of the oper-ating wavelength λ, interferometer baseline B andangular radius θs of the star as follows:

Sleak = Aeff

∫

θ

∫

φ

T (θ, φ)Os(θ, φ)dθdφ (3)

i.e. for a diffraction limited Bracewell interferometer:

Sleak = Aeff ×Os × (π2/8)× (θsB/λ)2 (4)

In practice the transmission map is distorted andfluctuating rapidling as a function of time. Unequalamplitudes between the arms of the interferometeraffect the depth of the null. Variations of the op-tical path difference (OPD) between the two tele-scopes induce random displacements of the centraldark fringe. Hence the null signal is fluctuating ontime scale that are short compared with typical in-tegration times. When averaged over the detectorframe read-out time (f−1

ro ), one major effect of thestellar leakage is to raise the depth of the null to anaverage stellar leakage < Sleak > that can be esti-mated as follows:

< Sleak >

Fs × f−1ro

=α2

4+

4α∆3π

+(πσpiston

λ)2+σ2

a+∆2

4(5)

where α = πθsλ/B . σa is the rms fluctuation of the

relative amplitude between the two arms of the in-terferometer after spatial filtering. σpiston is the rmsfluctuation of the piston i.e the residual rms OPDfluctuation after correction by a delay line operatedby a fringe sensor unit in a close loop system. ∆ is aphase shift residual depending on wavelength whichdescribe dispersion effects. In addition to an averageoffset of the null depth, a major effect of the stellarleakage is to induce important noise contributions.One contribution is the photon noise of the meanstellar leak < Sleak > which adds onto the photonnoise of the background and of the exo-zodiacal dustclouds. This degrades the ratio signal to noise of theexo-zodiacal dust disk measurements (see Eq. 7). Asecond source of noise is the erratic fluctuation σleak

of the stellar leakage due to residual variations athigh frequencies of the amplitude and optical pathdifference between the two arms of the interferom-eter. The rms fluctuation of the stellar leakage areexpressed as follows:

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σleak = Fs × 8α

3π×

√(πσpiston

λ)2 + σ2

a × f−1ro (6)

The effect of the stellar leakage fluctuations and pho-ton noise on the ratio signal to noise of the dust diskflux measurements can therefore be expressed as fol-lows:

S/N =Sz

√∆λ∆t√

Bgd + Sz+ < Sleak > +σ2leak

(7)

Nulling interferometers using three or four ESO VLTtelescopes could provide theoretically a broader nulland could be therefore less sensitive to stellar leak-age. However, such configurations with three or fourVLTs would be non-linear. The Earth rotation wouldinduce a non-homothetic deformation of the pro-jected bidimensional array on the plane of the sky,so that continuous amplitude adjustments would beneeded to compensate the array deformation duringthe night and to maintain a broad θ4 null. Addi-tional engineering complexity would come from theincreased number of fringe sensors and OPD controlunits. Furthermore, the stellar light leakage throughgroundbased nulling interferometer using more thantwo telescopes would be in practice much larger thantheir theoretical transmission due to their high sen-sitivity to residual OPD and Strehl amplitude fluc-tuations.

Photon noise and fluctuations of the stellar leakageare major limitations for the detection of exozodia-cal dust clouds using a groundbased nulling interfer-ometer. Eq.(4) shows that the stellar leakage of aBracewell interferometer and therefore its contribu-tions to the noise can be reduced by operating at longwavelength with short baselines. According to thiscriterion, the detection of exo-zodiacal dust cloudsat VLTI would be most efficient with a Bracewellinterferometer operating in the N band and combin-ing two large 8.2 m UT telescopes with the shortestVLTI baselines (e.g 46 or 56 meters).

4.2. Subtraction of the thermal IR background

In the N band, the infrared background flux in theinterferometer field of view is about 300 times largerthan the flux from a Sun-like star located at 10 par-secs. In order to measure a null depth of a few thou-sands, the background flux has to be subtracted witha high accuracy. In the L’ band, the requirementon the background calibration accuracy is relaxedsince the background flux is much smaller. The back-ground thermal emission contributes to the noise notonly by its photon noise but also by its temporalfluctuations. Eq. 7 assumes that chopping is effi-cient enough to remove background fluctuations. Inpractice, little is known about the stability of thebackground in the mid-IR both on long and short

Figure 7. Simulation examples of the power spec-tral densities of the optical path difference (top) andStrehl fluctuations (bottom) before and after correc-tion by OPD and amplitude control loops of the GE-NIE experiment.

0

200

400

600

800

1000Time (s)

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Nul

l sig

nal (

e s−

1)

Figure 8. Geniesim simulation example of a cumu-lative averaged null signal in three different spectralchannels.

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time scales. The infrared sky noise in the N bandis mainly due to aerosols and cirrus clouds, confinedto the Earth’s troposphere (Kaufl 1991). Its powerspectrum has been measured by several authors indifferent infrared spectral bands (e.g. Miyata et al.2000). These measurements show that the infraredbackground level and fluctuations greatly depends onthe astronomical site (altitude, humidity, ...), on thesky condition at the moment of the observation andon the instrument and telescope, which contributeto a great extent to the infrared emission. Obtain-ing these information at Paranal is therefore crucialfor designing the GENIE experiment. A measure-ment campaign of thermal infrared emission and at-mospheric dispersion has been initiated (Wilhelm &Gitton 2003) which will benefit from the installationof the MID-Infrared instrument (MIDI; Leinert et al.2003) at the ESO VLTI.

For background subtraction, GENIE could use theVLT chopping sub-system where the target andbackground are alternatively observed by tilting theVLT secondary mirrors. Since the background is notexpected to be spatially uniform, the nodding tech-nique could be applied. The main drawback of thischopping method is that the tip-tilt unit, the adap-tive optics and the fringe sensor cannot remain inclosed loops since they do not see the central guidestar when measuring the background. All these sub-systems have to reclose their loops before each singleinterferometric measurement is made. The overalldetection performance of GENIE will thus depend onthe efficiency of these subsystems which are expectedto close their loops in a few hundred milli-seconds,while a chopping period is expected to last typicallyfor 100 msec. Alternative methods could improvethis chopping efficiency. These include counter chop-ping of a control beam operating in a separate Hor K band or a dual feed interferometer where oneof the beam remains on the guide star to feed thesubsystems. A third possibility is to use an inter-nal chopping method similar to the one proposed inthe context of the Keck nuller (Serabyn 2003) . Inthis method, the exit pupils of each of two telescopesare divided in two parts. Each half-pupil of onetelescope is recombined destructively with the corre-sponding half-pupil of the other telescope, such thattwo Bracewell interferometers with parallel baselinesare formed. Their nulled output are then combinedwith a ±π/2 phase shift. Chopping can thus be car-ried out at high frequencies by alternately registeringthe signal from these two outputs.

Based on the result of IR thermal background mea-surement performed at Paranal, the definition studyof GENIE will trade-off these different choppingmethods and assess the feasibility of detecting ex-ozodiacal dust clouds around nearby stars witha Bracewell interferometer. The thermal infraredbackground level is clearly a major obstacle for thestudy of exo-zodiacal dust clouds in the N band.

4.3. Compensation of the atmospheric dispersion

A wideband interferometer such as GENIE is sen-sitive to the effect of longitudinal dispersion whichaffect the interfering light beams unequally. For in-frared observations, this effect is due to the unbal-anced air paths in the delay line and to random at-mospheric humidity variations in the line-of-sight tothe star and inside the VLTI delay line tunnels. Lon-gitudinal dispersion determines the position of thenull fringe as a function a wavelength, and is there-fore a central problem in producing deep broadbandinterferometric nulls. It produces two deleterious ef-fects, namely (i) an inter-band dispersion betweenthe spectral band where the OPD is controled andthe measurement band where the null is achieved and(ii) an intra-band dispersion within the measurementband.

In the case of the VLTI, the PRIMA fringe sensorunit could be used to correct the differential OPD inthe infrared K band. However, the different amountsof air and water vapour crossed by each light beamwould lead to an offset of the dark fringe position inthe L’ or N band where GENIE will operate. Also,random fluctuations of water vapour content in thepath of the beams will lead to a random motion ofthe dark fringe. This needs to be compensated e.g.by periodically measuring the fringe position in theL’ or N band, or by monitoring the changing differen-tial water column. Dry air has very little dispersionin the infrared. The amount of dry air-induced dis-persion varies slowly with time as the delay lines aremoved to compensate for Earth rotation. The re-fractive index of dry air can be fit fairly well withdispersive solids such as ZnSe. The correction of dryair dispersion is therefore relatively easy, e.g. by in-troducing an adjustable pathlength through ZnSe inone arm of the interferometer. Water vapour, on thecontrary, has a strong dispersion in the infrared, andthe column densities of water vapour above the tele-scopes can differ enough to contribute significantly(Meisner & Le Poole 2003) . Moreover, the amountof water vapour in the two light paths varies ran-domly due to atmospheric turbulence. Therefore, thecorrection of water vapour dispersion is a more diffi-cult task. ZnSe dispersion control devices combinedwith delay lines could still be used (Koresko et al.2003) providing that a high degree of symmetry iskept in the nuller design to minimize unbalanced re-flectivity, transmission or polarisation in the opticalsystem. A pair of wedges is a candidate solution toadjust pathlength through ZnSe while keeping con-stant the direction of the beams.

5. GENIE NULLING PERFORMANCE

A simulation software has been developed (Absil etal. 2003a) in an IDL based code to simulate nullingtests and observation scenarios that could be con-ducted with the GENIE experiment. The code emu-lates the brightness distribution of the target source

8

0

50 100

150 200

Piston rms (nm)

101

102

103

104

105

Max

imum

nul

l

5 pc10 pc20 pc40 pc

Figure 9. Maximum achievable rejection ratio in theL band as a function of the residual rms piston errorfor a G2 V star at 5, 10, 20 and 40 pc. A tho UTstelescope interferometer is assumed with a 46 meterbaseline.

including the central star, its circumstellar dust diskand the eventual presence of a low-mass companion.The noise contribution from the thermal infraredemission of the atmospheric background and of theVLTI is added to the signal. A Kolmogorov powerspectrum describes the effect of the atmospheric tur-bulence on the fluctuation of the optical path differ-ence between the telescopes. The GENIE instrumentis described by the transfer functions of its differentsubsystems. This includes the control loops of theMACAO adaptive optics and of the PRIMA fringesensor unit of the ESO Very Large Telescope Inter-ferometer. Examples of the power spectrum of theOPD and Strehl fluctuations are given in Figure 7 be-fore and after filtering by the OPD control loop andamplitude matching subsystems of the experiment.The output signal of the GENIE simulator consistsin time series of fluxes calculated in different spec-tral channels by folding the brightness distributionof the source with instantaneous transmission mapof the GENIE experiment distorted by OPD, ampli-tude mismatch and background offsets. Examples ofsimulation results are given in Fig. 8 which show thecumulative averaged nulled signal after backgroundsubtraction in three different channels as a functionof integration time. In this example a moderatelystable null of about 1500 is achieved in a thousandsecond.

One major performance limitation of a Bracewellinterferometer operating on nearby stars is due tostarlight leakage through the transmission map. Theresults of a parametric performance analysis modelare summarized in Figure 9. Fig. 9 gives the max-imum achievable rejection ratio in the L band as afunction of the residual rms piston error for a G2 Vstar at 5, 10, 20 and 40 pc in the L. A baseline of 46

meter is assumed between two UT telescopes. In theL band, the nulling performance are severely limitedby stellar leakage for stars closer than 10 parsecs. Formore distant stars, the maximum achievable null israpidly dropping with the degradation of the OPDcontrol accuracy. The OPD control loop is a ma-jor subsystem of the GENIE experiment and drivesthe nulling performance of the interferometer in theL band. This control loop includes a fringe sensorunit and a delay line. Its performance depends onthe power spectral density of the OPD induced byatmospheric turbulence, on the residual fringe mo-tion over the FSU integration time, on the photonflux available from the celestial object and on its vis-ibility factor. The presence of a bright star in thecenter of the field of view of a nulling interferometerenable to control the optical path difference betweenthe two arm of the interferometer with an accuracyhigh enough to achieve a nulling depth greater thana thousand.

In order to avoid a degradation of the nulling per-formance by the wavefront errors, GENIE will usemonomode optical fibers or waveguides. These de-vices will filter the high order wavefront errors thatwill remain after correction by the VLTI adaptive op-tics subsystem. Hence, the coherent flux used for de-structive interference can be assumed to be the frac-tion of the total flux equal to the Strehl ratio. TheStrehl ratio is expected to fluctuate rapidly due to at-mospheric turbulence, therefore affecting the nullingperformance. The amplitude fluctuation will haveto be corrected by an amplitude matching device. Aproper correction will be achieved providing that theerror signal, i.e. the instantaneous filtered intensityof the star, can be measured with a ratio signal tonoise consistent with the required amplitude match-ing accuracy. With an integration time too short, thePoisson statistics will prevent to achieve the mea-surement accuracy. If the photometric integrationtime is too long, fluctuations of the instantaneousstellar signal within the integration time will exceedthe required amplitude matching accuracy. Table 2and 3 quantify the effect of rms OPD variations, rms

Table 2. Effect of RMS OPD fluctuations, amplitudefluctuation and thermal infrared background noise onthe nulling depth and ratio signal to noise of the stel-lar leakage measurement. A 100 seconds nulling testis assumed on a A0 V star located at 50 parsecs.Only one spectral channel of 1 µm width centered at3.8 µm in the L’ band is considered assuming no per-formance degradation by dispersion effect.

σopd σamp bckgd Null S/N(nm) (%) (e- s−1)

0 0 0 2290 15700 0 9.7 × 106 2290 790 2.5 0 1690 6020 0 0 1980 7720 2.5 0 1510 5620 2.5 9.7 × 106 1510 50

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amplitude fluctuations and thermal infrared back-ground noise on the nulling depth and ratio signal tonoise of the stellar leakage measurement. In the Nband (Table 3), a 1 hour nulling test is assumed ona G2 V star located at 10 parsecs. In the L’ band, a100 seconds nulling test is considered on a A0 V starlocated at 50 parsecs. A spectral channel width of1 µm has been used in the calculation assuming noperformance degradation by dispersion effects. Fluc-tuation of the thermal infrared background emissionare not taken into account. Table 2 and 3 indicatethat an amplitude control with a percent accuracy issufficient to maintain a nulling rejection ratio largerthan a 1000. It is anticipated that such a perfor-mance will be easily achievable in the L’ band butmore difficult to obtain in the N band due to the highlevel of the thermal background emission and due toits fluctuations.

Nulling performance in the N band are less sensi-tive to residual OPD fluctuations due to the longeroperating wavelength. Fig. 10 (top) estimates thenulling performance of the GENIE instrument witha 46 m baseline in the N band as a function of thestar distance and spectral type assuming an OPDcontrol accuracy comparable to that achieved withthe VLTI PRIMA fringe sensing unit. A nulling re-jection factor greater than a thousand is obtained fordwarf M, K or G stars at distance greater than 5 par-secs. However, due to the high level of the thermalinfrared background , the integration time needed tomeasure the nulling rejection ratio at a 3 σ accuracylevel rapidly increases with distance. Fig. 11 showsthat hours of integration will be needed to measurethe stellar leakage at a rejection level of 1600–1800on G–K dwarfs at 20 pcs and M dwarfs within 10pcs. This estimate does not take into account ad-ditional performance limitation due sky backgroundfluctuations and atmospheric dispersion that will bedifficult to correct in the N band. The L’ band isconsidered as an alternative operation band for theGENIE ground based nulling demonstrator despiteits tighter requirement on the OPD control accuracy.

Table 3. Effect of RMS OPD fluctuations, amplitudefluctuation and thermal infrared background noise onthe nulling depth and ratio signal to noise of the stel-lar leakage measurement. A 1 hour nulling test isassumed on a G2 V star located at at 10 parsecs.Only one spectral channel of 1 µm width centeredat 10.2 µm is considered assuming no performancedegradation by dispersion effect. Thermal infraredbackground fluctuations are not taken into account.

σopd σamp bckgd Null S/N(nm) (%) (e- s−1)

0 0 0 3810 26100 0 4.4 × 109 3810 40 2.5 0 2390 325

100 0 0 2000 315100 2.5 0 1520 322100 2.5 4.4 × 109 1520 4

0

5 10 15 20 25 30distance (pc)

0

500

1000

1500

2000

Max

imum

nul

l

G2 VK0 VM0 V

0.0 5.0 10.0

15.0 20.0

distance (pc)

0.1

1.0

10.0

Int.

time

for

Sle

ak/N

oise

=3

(hou

rs)

G2 V K0 V M0 V

Figure 10. Maximum achievable null (top) in theN band as a function of star distances and spectraltypes assuming an OPD control accuracy similar tothe ESO VLTI PRIMA fringe sensor unit. Nullingperformance better than 1000 can easily be achieved.However, due to the strong thermal infrared back-ground, the integration time (bottom) needed for anull depth measurement at a 3 σ level accuracy inthe N band increases rapidly with the target distance.

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6. DARWIN-GENIE: A GENERAL USERINSTRUMENT

The nulling interferometry technique in the infraredis well suited to study any faint cool object located inthe immediate environment of a bright astrophysicalsource (e.g. Voit 1997). A wide range of studiescould potentially be conducted using the Darwin-GENIE experiment including infrared spectroscopyof cool dwarfs in spectroscopic binaries, of protoplan-etary disks around T Tauri stars, of Herbig Haro ob-jects (Malbet & Bertout 1995) and of Young StellarObject in general (Kaltenegger et al. 2003). Suchprograms would be important scientific by-productsof the Darwin-GENIE experiment. As an example,a large number of main sequence stars are known toharbor a large quantity of dust. They are mainlyearly-type stars called Vega type systems after thefirst discovery of an excess infrared flux around Vegaby the satellite IRAS (Aumann et al. 1984). Ab-sil et al. (2003b) simulated observations of a typi-cal Vega-type star (ζ Lep) using the GENIE simu-lation software. The results show that GENIE ob-servations could provide an unprecedented accuracyin the description of the disk morphology includingconstraints on the dust density and radial tempera-ture distribution. Both the L’ and N band could beused for such studies with the VLTI UT telescopes.The auxilary telescopes that are dedicated to inter-ferometry would also provide valuable information inthe L’ band. The simulations also show that GENIEcould detect disks as faint as 25 times the solar zodi-acal cloud brightness around nearby solar type stars.This would allow to prepare the Darwin science pro-gram by discarding unsuitable targets for Earth likeplanet detection by IR nulling interferometry fromspace (see Sect. 2.2).

Since 1995, more than 100 extrasolar planets havebeen discovered as companions to solar-type stars(Marcy et al. 2000). All of these planets werediscovered indirectly in high-precision Doppler sur-veys. These surveys measure the radial velocityDoppler shift of the parent star due its periodic mo-tion around the center of mass of the star plus planetsystem. This technique is most sensitive to massiveplanets with short orbital periods. The extrasolarplanets that have been discovered today have massranges from M×sini ≈ 0.15 to 10 Jupiter masses(where i is the orbit inclination onto the observer lineof sight) with orbital periods ranging from 3 days toa few years. Because of their proximity to the par-ent star (<<1 AU), the atmosphere of some of theseplanets is much hotter than that of Jupiter (and ofthe Earth), hence their name of hot Jupiters. Withtemperatures sometimes higher than 1000 K, theirthermal emission is maximum in the near infraredbands. With baselines ranging from 46 to 130 me-ters, the VLTI is optimized for the detection in theL band of planets at about 5-15 milli-arcsec fromtheir parent star which correspond to 0.05-0.15 AUfor a nearby star at 10 parsecs. This is speciallyfortunate since these planets are precisely the hotJupiters of which a significant sample is known. The

contrast between a hot Jupiter and its parent star inthe L band is reduced to less than 104 compared tothe 106 in the N band for Earth-like planets. DenHartog et al. (2003) simulated an observation ofthe τ Boo system using a GENIE experiment op-erating in the L’ band as a Bracewell interferome-ter. They found that for the most suitable baseline,the starlight leakage is much higher than the hot-Jupiter signal. They concluded that one major dif-ficulty in the detection of hot Jupiter from groundwill be to calibrate the stellar leakage with a highaccuracy. One possible solution would be to imple-ment a double-Bracewell configuration (Absil et al.2002; Gondoin et al. 2003) using internal modula-tion between two Bracewell nulling interferometers.This configuration could subtract the IR backgroundand the starlight leakage from the overall nulled sig-nal with an accuracy sufficient to retrieve the sig-nal from the brightest exo-Jupiters. The principleof this double Bracewell technique is to recombinethe nulled output of two Bracewell interferometerswith a ±π/2 phase shift so that new transmisionmaps are obtained which are asymmetric with re-spect to the center of the field of view. A maximumof transmission in one map corresponds to a mini-mum of transmission in the other map. Therefore,an extended source with central symmetry has thesame contribution in both outputs. On the otherhand, a hot Jupiter located on the maximum oftransmission of one map does not give any signalthrough the other map. Thus, the Jupiter signal canbe modulated and therefore retrieved by alternatingthe two outputs. Although possible in theory, directdetection of the hottest exoplanets on-ground usingthis nulling interferometry technique in the L’ bandwould be technically more demanding than a singleBracewell interferometer. It is however worth notingthat three double-Bracewell configurations are pos-sible at VLTI which offer flexibility in adapting thetransmission map to the orbital position of the exo-Jupiters around their parent star.

7. CONCLUSION

The Darwin-GENIE nulling experiment will provideEuropean Scientists and Engineers with a first expe-rience on the design , manufacture and operation ofa nulling interferometer using Darwin representativeconcept and technology. Nulling tests on single starsor close binary systems could be achieved in the L’and N atmospheric spectral bands with a nulling re-jection ratio higher than a 1000. The Darwin-GENIEexperiment will combine all optical functions fore-seen into the future Darwin Infrared Space Interfer-ometer. It will demonstrate that nulling interferom-etry can be achieved in a broad mid-IR band as aprecursor to the next phase of the Darwin program.

GENIE is not only a technology demonstrator butalso a preparatory experiment for the Darwin sci-ence program. The instrument will detect brightexo-zodiacal dust disks around nearby solar type

11

stars. This would allow to establish the Darwin tar-get list and to discard unsuitable sources for Earthlike planet detection from space. Darwin observa-tion scenarios, calibration procedures and data anal-ysis methods will be tested by GENIE. The nullingexperiment could also become a very useful instru-ment for astrophysics that would enable scientist toconduct research programmes on a variety of sourcesincluding Herbig Haro and Young Stellar Objects,protoplanetary disks around T Tauri stars, low masscompanions and may be hot Jupiters around nearbystars.

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