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Blazing the Trail: Enabling Exoplanet Imaging in the Habitable Zone with the European Extremely Large Telescope

Periodic Reporting for period 1 - Exoplanet Finder (Blazing the Trail: Enabling Exoplanet Imaging in the Habitable Zone with the European Extremely Large Telescope)

Reporting period: 2018-06-01 to 2020-05-31

The search for life on alien worlds is an exciting and active field of research in modern astronomy. One of the ambitious goals of exoplanet science is to directly image exoplanets in the surrounding of the host star and characterise them by atmospheric spectroscopy. Such a study can enhance our understanding of the mechanisms involved in the formation and evolution of planets and enable comparison of exoplanetary chemical compositions with the solar system planets. At present, we have already confirmed ~ 4000 exoplanets mostly through indirect detections where the influence of a planet on its star is observed. These detections have revealed an unexpected diversity of exoplanets which are unlike the solar system planets. Although these discoveries put new constraints on planet formation scenarios, they are strongly biased towards the population of very short-period planets close to their stars (< 1-5 Astronomical Units (AU)) and are only probing the late stages of planet evolution after migrations. To fully understand planet formation and early stage evolution, exoplanets located at more than a few AU from their stars need to be observed. High-contrast imaging (HCI) is the only feasible technique to approach this regime of separations. However, two major challenges still need to be overcome: obtaining the required spatial resolution and overcoming the large star/planet contrast (defined as the flux ratio of planet to star brightness). For example, imaging mature gas planets at small angles around low-mass stars requires contrast limits of 10^-7-10^-9 in near infrared (NIR). Only the era of Extremely Large Telescopes (ELTs) will enable such detections from ground.

Since exoplanets are 10^4 to 10^10 times fainter than their star, HCI instruments use devices called coronagraphs to reduce the stellar flux without affecting the planet light. However, their performance is affected by optical aberrations of various natures (Earth’s atmospheric turbulence and optical defects). Most of these errors are mitigated by the Adaptive Optics (AO) or Extreme-AO (ExAO) instruments including wavefront sensing, wavefront control techniques and active speckle suppression routines. In addition, the differential errors creating false positive planet signals can be sensed and corrected directly at the focal plane. Several focal plane wavefront sensing (FPWFS) techniques have been proposed and a high and stable planet contrast has been achieved in a space-like steady environment. However, under the AO/ExAO wavefront residuals, imaging and characterising exoplanets at small angles require achieving detection limit at least 10 to 100 times better than the state-of-the-art. No existing ground-based HCI instruments have successfully disentangled the planet signals from stellar residuals at small angles. Addressing this technical challenge, our project tested and characterised different FPWFS techniques in the laboratory and obtained high-contrast in raw science images.

Detecting exoplanets is exciting and understanding the structure and evolution of the circumstellar environment in which these planetary systems flourish provides a broader perspective. To advance the current understanding of how planets emanate from the circumstellar material, the project analysed and modelled the data of one of the known complex debris disk system. This data was obtained with SPHERE, which is a HCI instrument installed at the Very Large Telescope (8-meters) in Chile.
To perform the instrumentation work, end-to-end numerical simulations were executed and realistic laboratory experiments were carried out on a R&D facility (THD2 bench) at Observatoire de Paris. The THD2 bench is unique for its achromatic design, compatibilities with many coronagraphs, and its performance under stable conditions that routinely reaches 10^-8-10^-9 contrast in narrow and broad spectral bands (up to 250 nm bandwidth) in visible/NIR. In this project, we introduced dynamic turbulence on the bench by implementing a phase mask on a rotating wheel that simulated the realistic AO residuals at a typical contrast of 10^-5. This phase mask of 100 mm diameter created 12,000 different phase screens, simulating a typical few minutes of observations executed with the SPHERE/VLT. We then performed an exhaustive study of the impact of residual turbulence on several FPWFSs under the same laboratory conditions. We showed that these techniques are capable of minimising the quasi-static speckle intensity in long exposure science images down to a limitation set by the AO-halo residuals. We achieved 1-sigma raw contrast levels below 10^-6 at small angles (Figure 1). These results were disseminated to the scientific community via peer-reviewed publications, international conferences, workshops and presentations. The significance of these results were also conveyed to the general public by publishing numerous non-scientific articles.

The work related with the analysis of the known debris disk around HD 141569A involved detailed modelling with classical post-processing techniques. The previously published total intensity data (Figure 2) revealed several sharp broken rings of debris around the star HD 141569A:- the most prominent ring is located at about 45 AU featuring a north-south asymmetry. However, the recently obtained polarised data of the same ring exhibits an east-west asymmetry. It was proposed that these brightness asymmetries could be caused due to an azimuthal variation of the dust density. We investigated this hypothesis and modelled the observed data using the GRaTER radiative transfer code and processed it using the Monte-Carlo Markov Chain analysis. We indeed found variations in the dust density in the ring and explored several theories to explain it. This research work opened an exciting avenue to link nascent planetary formation theories with the observational data. An article summarising our analysis is under preparation. These results have already been propagated into the scientific community through poster presentations in national and international conferences.
The significance of the published work related with the FPWFSs is that if the tested techniques are implemented on-sky on the current HCI instruments such as SPHERE/VLT, GPI/Gemini, SCExAO/Suabru, then an improvement in raw contrast by a factor of roughly 10 could be obtainable in long-exposure imaging. This would enable the current instruments to directly image young Jovian-like exoplanets at small angles around nearby stars. This is highly encouraging for the HCI community which could consider the implementation of such techniques on the planned/ongoing technological upgrades (such as SPHERE+ and GPI 2.0) to enable the imaging of exoplanets at small angles in near future.

The article presenting the debris disk analysis question the feasibility of widely-known hypothesis that are used to explain the observed brightness asymmetries. This has a great importance for theorists working on the planet formation and evolution models. The thorough investigation of HD 141569A performed under this project has led to several national and international collaborations with the computational astrophysicists and the experts of debris disk analysts from scattered light to millimeter emission. The goal of this on-going collaboration is to explain the routinely observed features in debris disks, which are emanating from the physical processes not yet understood.
Raw contrast improvement in long exposure Self Coherent Camera (SCC) images (Singh et al. 2019).
Scattered light observation of the innermost ring of debris (~ 40 AU) around HD 141569A with SPHERE.