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High-Resolution Imaging and Spectroscopy of Exoplanets

Periodic Reporting for period 2 - HiRISE (High-Resolution Imaging and Spectroscopy of Exoplanets)

Reporting period: 2019-06-01 to 2020-11-30

The detection of planets orbiting other stars, known as exoplanets, is one of the greatest astrophysical achievements of the past 30 years. This has recently been recognized by the Nobel committee, who attributed the 2019 Nobel prize in physics to Michel Mayor and Didier Queloz for the discovery of the first exoplanet orbiting a star similar to our own Sun in 1995. Since then, several thousands of exoplanets of various masses and separation to their host star have been discovered using different observational techniques. One of these techniques is direct imaging, which is a method where the photons coming from an exoplanet are directly measured thanks to a combination of various optical and numerical techniques. Since 2014, a new generation of instruments installed on large ground-based telescopes provides highly optimized instrument for the direct imaging of exoplanets. This is the case of the SPHERE instrument installed on the Very Large Telescope (VLT) in Chile, which combines extreme adaptive optics (ExAO) to correct for the atmospheric turbulence, coronagraphy to remove the stellar signal, and two science instruments in the near-infrared (NIR) to measure the photons emitted by young giant exoplanets. One of these instruments is an integral field spectrograph, which provides both spatial and spectral information, but only at very low spectral resolution (R=50). From the astrophysical point of view, spectral resolution more than a 1000 times higher are necessary to answer fundamental questions regarding the formation, composition and evolution of young giant exoplanets. High-resolution spectrographs exists but none of the existing instruments include the ExAO and coronagraphy that are required to perform high-contrast imaging and directly image and characterize exoplanets. One such instrument is the CRIRES+ spectrograph, which has recently been reinstalled at the VLT, on the same telescope as SPHERE.

With HiRISE, we propose to implement a novel system that enables the characterization of young giant exoplanets at high spectral resolution on the VLT. HiRISE aims at implementing a new module inside of SPHERE, which will pick-up the light of a known exoplanet, inject it into an optical fiber, transmit it around the telescope to the CRIRES+ spectrograph, and use the spectrograph to disperse the planet's light at high spectral resolution. HiRISE will use state-of-the-art instrumental concepts to massively augment the capabilities of both SPHERE and CRIRES+. By the end of the project, we aim at having a working prototype instrument installed at the VLT in Chile and use it to characterize a sample of known exoplanet companions.

The astrophysical goal is to obtain detailed spectral information on the composition of the atmospheres of young giant exoplanets. Their atmospheres bear essential markers of their formation mechanism, their internal structure and the chemical and dynamical processes like winds or clouds. Spectroscopic measurements of these atmospheres provide a unique way to answer fundamental questions about exoplanets: where and how did they form in the protoplanetary disk? How does their luminosity evolve as a function of mass and time? What is the influence of dust clouds in their atmospheres? What are the chemical and dynamical processes at play in these atmospheres? Answering these questions on giant exoplanets is a central question for the future search for exolife because giant planets have a major gravitational influence on their planetary systems. They can therefore enhance or inhibit the formation of smaller planets where life could potentially develop on very long time scales.

The goals of the HiRISE project are the following:

1/ Develop innovative instrumental concepts for high-contrast imaging combined with high-resolution spectroscopy

2/ Develop data analysis and signal extraction techniques to detect the signal of planets in data at high spectral resolution, and characterize their physical properties

3/ And finally, design and implement a working prototype at the VLT that enables to combine SPHERE and CRIRES+ for the characterization of young giant exoplanets at high spectral resolution
1/ Development of a prototype system for the VLT

Designing a novel astronomical instrument from scratch, even as a prototype, is a difficult endeavor. For the development of the HiRISE prototype, which will couple SPHERE and CRIRES+ at the VLT, we have pursued several works in parallel since the start of the project. The development of the prototype has been the main activity of the HiRISE team and has explored both theoretical, numerical and instrumental aspects that are detailed below.

The first part concern the development of an end-to-end simulation tool that enables to derive the expected performance of the HiRISE system for the detection and characterization of young giant exoplanets. A postdoc (PostDoc1) hired at the beginning of the project has been responsible for this major development and has recently submitted a peer-reviewed publication presenting a summary of his results. The model is now also used for a side project in which PostDoc1 is associated.

The second part concerns the opto-mechanical design of the prototype system that will be installed and tested at the telescope. The optical design has been taken in charge by an optical engineer associated to the HiRISE project and the preliminary mechanical design has been done by an engineer (MechEng) hired on the project. They have designed the two opto-mechanical systems that will be added in SPHERE and CRIRES+. In parallel, we have selected a large part of the additional hardware that will be necessary to have a working system at the telescope. The design of these systems is now almost finalized. An optical engineer (OptEng) has been hired in the project to work on these activities and follow them through till the end of the project. For this important design work, we have been collaborating with both the SPHERE and CRIRES+ consortia, who have provided essential information for the design, as well as the European Southern Observatory (ESO), who operates the VLT and its instruments.

The third part concerns the test and selection of appropriate optical fibers for the project. An important selection trade off analysis was done based on the simulation model to select fibers that are affordable and still enable to reach the science goals of the project. The fibers have been recently selected and we have now received sample that is going to be tested in the laboratory by PostDoc1.

The fourth part concerns the validation of concepts in the laboratory using MITHiC, the high-contrast imaging test-bed hosted at Laboratoire d'Astrophysique de Marseille (LAM). MITHiC is being used to test injection of the planet signal into the fiber and to validate two different strategies for the centering of the planet's PSF (the image) on the science fiber. This work is being done by a PhD student (PhD2) hired on the project. In the coming months, she will work on validating the centering strategies and publish the results.

The fifth part concerns the development of the ZELDA wavefront sensor, which can be used to measure and manipulate the wavefront of the light in order to optimize the injection of the planet signal in the optical fiber. Developments have been made both on the MITHiC bench and on the SPHERE instrument at the VLT. A paper was published on the results obtained on-sky with SPHERE, and a second one is in preparation. This activity is mainly pursued by the PI.

The final part concerns the practical aspects of the project that are being discussed with ESO for the possible installation of the HiRISE prototype at the VLT. A significant work has been done to produce reference documents presenting all the scientific and technical aspects of the project. Several visits have been organized both at the ESO headquarters in Germany and at the VLT in Chile to discuss the technical aspects. The project has now submitted a formal proposal for the installation of HiRISE as a visitor instrument at the VLT.

2/ Observations with current instruments

In parallel of the technical aspects of the project, we have started investigating scientific observations with existing instruments that provide medium to high spectral resolution. A large part of this work has been done by PhD1, PostDoc1 and the PI.

Several observing programmes have been accepted for the characterization of known companions (instruments: VLTI/GRAVITY, VLT/SPHERE, VLT/ESPRESSO, CFHT/SPIRou), and to look for new planetary companions that could be characterized with HiRISE (instruments: VLT/SPHERE, VLT/MUSE). Due to the Covid-19 epidemic, not all of these programmes have been fully executed, but the data already acquired before is being analyzed.

The PI and PostDoc1 are also involved in external large observing programmes. The PI is involved in the SHINE survey that looks for planets with the SPHERE instrument. The initial results of the survey are being published, and the PI is responsible for the statistical analysis of the results (paper accepted). This survey is particularly important because it has the potential to provide new planets that will be characterized with HiRISE.

The PI and PostDoc1 are also both involved in a large programme with the VLTI/GRAVITY instrument that aims at directly characterizing known planets in the K band. This project is highly complementary to HiRISE, which will work in the H band.

PhD1 has been leading an observing programme to detect the Halpha emission of extremely young giant planets with the VLT/ESPRESSO instrument. In this context, he has been awarded a fellowship to work for 1 year at ESO in Chile.

3/ Future instrumentation on large telescopes

The search and characterization of exoplanets in the future will heavily rely on high-spectral resolution capabilities. In this context, PhD1 has started a work on the use of the high-contrast imaging mode of the HARMONI instrument for ESO's extremely large telescope (ELT). HARMONI will be the first-light instrument of the European extremely large telescope being built in Chile. His work has demonstrated the very high potential of HARMONI for the detection of young giant exoplanets using the medium resolution of the instrument. A publication is now in preparation.
HiRISE has enabled significant advances for the future characterization of exoplanets at high spectral resolution. PostDoc1 has demonstrated that the driving parameter of an instrument characterizing exoplanets at high spectral resolution is the transmission. This is an important discovery that will be a key for the design of future systems. Overall, while designing the HiRISE system for the VLT, the team has gain significant knowledge that will be essential for the development of future instrumentation for the ELT and future space telescopes.

PhD1 has also demonstrated that the use of medium resolution integral field spectrographs, like the one used in ELT/HARMONI, will enable to detect exoplanets much fainter that what current state-of-the-art techniques. His observing programme with VLT/ESPRESSO is also a novel approach that could provide unique information on the sample of planetary companions that he has selected.

The second half of the project will be dedicated to implementing a working system at the VLT. Some important milestones are still ahead, the main one being for the project to be accepted by ESO. Other important steps will be the manufacturing and validation of the full system in the laboratory, before it can be installed at the telescope. Once installed at the telescope, the novel HiRISE system will guarantee major astrophysical results on the characterization of known planets.