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Abstract
Context. Combining the resolving power of long-baseline interferometry with the high-dynamic range capability of nulling still remains the only technique that can directly sense the presence of structures in the innermost regions of extrasolar planetary systems. Aims. Ultimately, the performance of any nuller architecture is constrained by the partial resolution of the on-axis star whose light it attempts to cancel out. However from the ground, the effective performance of nulling is dominated by residual time-varying instrumental phase and background errors that keep the instrument off the null. Our work investigates robustness against instrumental phase. Methods. We introduce a modified nuller architecture that enables the extraction of information that is robust against piston excursions. Our method generalizes the concept of kernel, now applied to the outputs of the modified nuller so as to make them robust to second order pupil phase error. We present the general method to determine these kernel-outputs and highlight the benefits of this novel approach. Results. We present the properties of VIKiNG: the VLTI Infrared Kernel NullinG, an instrument concept within the Hi-5 framework for the 4-UT VLTI infrastructure that takes advantage of the proposed architecture, to produce three self-calibrating nulled outputs. Conclusions. Stabilized by a fringe-tracker that would bring piston-excursions down to 50 nm, this instrument would be able to directly detect more than a dozen extrasolar planets so-far detected by radial velocity only, as well as many hot transiting planets and a significant number of very young exoplanets.
Highlights
The direct imaging of extrasolar planets from the ground remains an incredibly challenging objective that requires the simultaneous combination of high angular resolving power, required to see objects separated by a few astronomical units and located tens of parsecs away, with high-dynamic imaging capability to overcome the large contrast between the faint planet and its bright host star
The structure of the resulting response matrix A is identical to the one for the phase: the same kernel matrix K will simultaneously render the observable quantities robust against piston excursions and small amplitude photometric fluctuations: the uncertainty in the kernel-outputs is proportional to the cube of the input complex amplitude fluctuations, so that even 10% intensity fluctuations on the inputs would translate into errors smaller than 10−3 on the kernel-outputs
Drawing on the idea of kernel, here applied to the outputs of an interferometric nuller, we have described how the design of an otherwise plain four input beam interferometric nuller can be modified to take into account, the possibility of selfcalibration
Summary
The direct imaging of extrasolar planets from the ground remains an incredibly challenging objective that requires the simultaneous combination of high angular resolving power, required to see objects separated by a few astronomical units and located tens of parsecs away, with high-dynamic imaging capability to overcome the large contrast between the faint planet and its bright host star. Very elegant and effective solutions have been devised (Guyon 2003; Soummer 2005; Mawet et al 2010), that can theoretically deliver data where the contribution of the bright star is attenuated to up to ten orders of magnitudes (Trauger & Traub 2007) and a few such coronagraphs are currently in operation on ground based observing facilities Their high-contrast imaging capability is severely affected by the less than ideal conditions they experience when observing through the atmosphere, even (Aime & Soummer 2004) with correction provided by state-of-the-art extreme adaptive optics (XAO) systems like VLT/SPHERE (Beuzit et al 2006), the Gemini Planet Imager (Macintosh et al 2014) or the Subaru Telescope SCExAO (Jovanovic et al 2015). The paper uses a generic recipe that is applied to a four-beam nulling combiner, which is the most relevant case for exploiting the capabilities of the existing Very Large Telescope Interferometer (VLTI), within the framework recently provided by the Hi-5 project (Defrère et al 2018a)
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