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Spectral Analysis of Atmospheres by Nulling Interferometry

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Spectral Analysis of Atmospheres by Nulling Interferometry Molecules in the Atmospheres of Extrasolar Planets ASP Conference Series, Vol. 450 J-P. Beaulieu, S. Dieters, and G. Tinetti, eds. c 2011 Astronomical Society of the Pacific Spectral Analysis of Atmospheres by Nulling Interferometry M. Ollivier and S. Jacquinod Institut d’Astrophysique Spatiale - Université de Paris-Sud 11 and CNRS (UMR 8617) Abstract. Nulling interferometry has often been considered as one of the most promising techniques to get spectra, in the thermal infrared spectral range, of exo- planet and particularly telluric ones. The required performance, in terms of nulling depth, spectral bandwidth and stability begins to be achievable in the laboratory. How- ever, concrete observatory projects have strong difficulties to emerge in space agencies programs, because of mission requirements complexity. In this paper, I present the per- formance various scientific objectives need and the state of the art of this technique, including mission aspects. I finally propose elements that could help defining a first space mission project. 1. Nulling Interferometry : Theoretical Aspects 1.1. Principle of Nulling Interferometry Nulling interferometry has been proposed in the late 70s as a possible technique to detect and spectroscopically characterize planets around nearby stars at wavelengths where diffraction prevents from the use of single aperture telescopes (Bracewell 1978). The principle of nulling interferometry in a 2-telescope configuration is described in figure 1 (left). The idea is to use two small telescopes, pointed in the direction of the star and to combine their beams. On the beam combiner the wavefronts from both star and planet create interferences. If we introduce a π phase-shift in one arm of the inter- ferometer, the stellar interference is destructive (the optical path difference is equal to zero). In the direction of the planet, and because of the angular distance to the pointing direction, the path difference is not equal to zero. Adjusting the distance D between the two telescopes, one can obtain that the path difference (D sin(θ)) introduces a phase- shift that compensates the instrumental one at the observatio× n wavelength. In that way, one gets a constructive interference in the direction of the planet. Thus, the distinction between the stellar and planetary photons is performed by their phase difference. The transmission map of such an instrument is shown in figure 1 (right). 1.2. Key Issues of Nulling Interferometry The main characteristics of a nulling interferometer are : - The depth of the null, i.e. the capability of the instrument to cancel the stellar light. This parameter is quantified by the rejection rate, which is the ratio between off-axis and on-axis transmission of the instrument (ratio between constructive 219 220 Ollivier and Jacquinod and destructive interference). The null depth is also characterize by the reverse of the rejection rate. - The chromatism of the null, i.e. the spectral range over which the rejection rate is performed, - The stability of the null, i.e. the rms amplitude of rejection rate fluctuation, as a fraction of the rejection rate. Figure 1. (left) : principle of nulling interferometer. See the text for description, (right) : transmission map of a two-telescope nulling interferometer The depth of the null directly leads to the level of stellar background that will be seen during the stellar companion observation, and as a consequence to the limiting contrast of the astronomical scene. The chromatism of the null conditions the spectral range and the spectral width of the observation, and as a consequence, the amount of photons that can be used for detection or spectral analysis. The stability of the null conditions directly the noise level of the signal and the ability or not to disentangle the stellar leaks from the companion signal itself. Getting an optimal null requires a perfect matching of the wavefronts to be recom- bined. For instance, in the case of a 2-telescope interferometer, requirements can be given in terms of : - amplitude : If I0 is the intensity of each beam on the beam combiner and δI is their intensity mismatch, then the rejection rate ρ can be computed as, 2 Imax I (2 + δI) 2I √1 + δI δI ρ = = 0 − 0 (1) Imin 4I0 ≃ 16 - phase : if δφ is the phase mismatch between the wavefronts on the beam com- biner, then the rejection rate can be computed as I 2I + 2I cos(π + δφ) δφ2 ρ = max = 0 0 (2) Imin 4I0 ≃ 4 Spectral Analysis with Nulling Interferometry 221 - polarization : polarization effects are more complex to described. They have be studied by many authors in many contexts (see for instance Chazelas et al. 2006). In order to monitor and control the amplitude, phase, and polarization effects, a nulling interferometer is usually made of several optical devices : - optics for beams transportation, - a fine relative angle sensor and associated correcting actuators, - optical delay lines to equalize the optical path difference between the beams at the level of a few nanometers, - an achromatic phase shifter device to cancel the stellar light at every wavelength, - a differential beam intensity matcher, - a differential beam polarization matcher, - optical filtering stages, - a beam combination stage, - a detection stage. 1.3. From Optical Principle to Observatory Concepts The intitial concept of Bracewell has been developed particularly in the thermal infrared spectral range, to take into account the specificities of telluric planets detection : - the size of the planet : the radius distribution of telluric exoplanet has to be de- termined because the planet size (and thus the signal level) is a strong constraint on the size of collecting apertures; - the angular distance from the planet to the star, varying with time (position of the planet on its orbit, relative inclination of the orbit plane with respect to the observer ... ); - low level signal from the planet, typically several tens of photons per second and square metre in the [6-20 µm] spectral range for an earthlike planet at 10 pc; - presence of zodiacal emission in the solar system : in the thermal infrared, the contribution of our zodiacal cloud is several hundred times higher than the ther- mal emission of the Earth; - presence of a zodiacal cloud around the exo-system : at present the level of exo- zodiacal level is not known for nearby stars, but has to be estimated because it is one of the dimensioning parameters for a characterization mission either in the visible or in the thermal infrared spectral range; - instrumental noise associated to the instrument temperature : in the thermal in- frared, the level of the integrated local zodiacal emission and the thermal emis- sion of the instrument reachs several hundred to thousand times the level of the signal to be detected; 222 Ollivier and Jacquinod - the level of stellar leaks : taking into account the way beams are combined, the mean stellar leaks can reach 10 to 100 times the signal level. A trade-off between this level of stellar leaks and the beam combiner complexity has to be found. Figure 2. Principle of the EMMA concept of nulling interferometer. The tele- scopes are located on the parabola and the beam combiner is put at the focus (source : NASA-JPL) All these constraints lead to concepts with several telescopes : - free floating in space (formation flying) in order to allow a fast configuration of the array with respect to target distances and sources characteristics (distance to the star, position on the orbit ... ); rigid (deployable) structures with telescopes on fixed length beams are not considered any longer because of their lack of versatility; - increasing the size of the nulled transmission area on the sky (Angel et al. 1986; Rouan 2003) and the mean depth of the null : this point is particularly important to reduce the level of stellar leaks, taking into accound the non-zero size of the stellar disk. - allowing the creation of sub arrays and partial beam combination between sub- arrays (Mennesson et al. 2005), - internal modulation between sub-arrays, allowing fast planetary signal modula- tion and lock-in detection at a frequency much higher than frequency allowed by only rotation of the whole array (Mennesson et al. 2005; Absil et al. 2003). This point allows disentangling the planetary signal (non centro-symmetric signal with respect to the pointing direction) and the exo-zodiacal background (centro symmetric with respect to the pointing direction). Present mission concepts are nammed TPF-I at NASA (Coulter 2003; Lawson et al. 2007) and DARWIN at Spectral Analysis with Nulling Interferometry 223 ESA (Karlsson & Kaltenegger 2003). The are both based on the EMMA concept (telescopes located on a parabola, with the beam combiner at the focus see figure 2. This concept is a good trade-off between many constraints and requirements (optical and thermal aspects, baseline versatility, imaging capabilities ... ) Whatever the configuration of the array and the beam combination method can be, the signal reaching the detector array is the sum of many contributions : the signal from the planet(s)(modulated), the mean stellar leaks (not mudulated), the mean exo zodiacal light contribution (not modulated because of clever interferometer configuration), the solar system mean zodiacal light contribution (not mudulated), the fluctuation of stellar leaks linked to the instrument stability and asymmetry of the astronomical source (vari- able contribution, that can be considered as noise), the variable contribution of the exo zodiacal light (linked to the cloud asymmetry, modulated by the internal modulation), and other sources of noise (thermal, instrumental and detection noises) that affect all the components mentioned previouly. Huge data processing is thus required to disentangle all the contaminants from the planetary signal itself. Complex algorithms that take into account the spectral nature of the signal and multi baseline information have been pro- posed in that purpose (Mugnier et al. 2006).
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