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High-Resolution Imaging and Spectroscopy of Exoplanets

Periodic Reporting for period 3 - HiRISE (High-Resolution Imaging and Spectroscopy of Exoplanets)

Reporting period: 2020-12-01 to 2022-05-31

The detection of planets orbiting other stars, known as exoplanets, is one of the greatest astrophysical achievements of the past 30 years. 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. Current exoplanet imagers are equipped to correct for the Earth's atmospheric turbulence using extreme adaptive optics and suppress the stellar signal using coronagraphy, but their science spectrographs are limited to spectral resolutions of R=50 to 100. 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 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.

The expected performance of the system has been evaluated using an end-to-end simulation model based on our knowledge of the telescope, the SPHERE and CRIRES+ instruments, and the preliminary design of the HiRISE instrument. This model has been described in Otten et al. (2021) and has established that the main limitation of VLT/HiRISE will be the global throughput of the system. In parallel, in the lab we have tested different observing strategies to ensure that the planet's PSF will be centered as accurately as possible on the science fibre (Pourcelot et al. 2021; El Morsy et al. 2022).

We have also produced a full opto-mechanical design of the system (Vigan et al. 2018), established a control strategy, proposed a design for the fiber bundle, etc. The design has been validated by the European Southern Observatory and HiRISE has been accepted as a visitor instrument for the VLT. The hardware has already been fully purchased and most components have been received at LAM. The system is currently being integrated in a clean room at LAM in Marseille. It is expected to be shipped to the Paranal observatory in early 2023.

As a side activity, we have also continued work on the Zernike wavefront sensor ZELDA, which can help to optimize wavefront and improve the injection of the planet's signal into the fiber (Vigan et al. 2019; Vigan et al. 2022).

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, VLT/CRIRES+), 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. Work is done by PostDoc2, in collaboration with colleagues in Leiden, on the analysis of new CRIRES+ data for the characterization of a known exoplanet.

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 (Houllé et al. 2021)
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 programmes with VLT/ESPRESSO are also a novel approach that could provide unique information on the sample of planetary companions that he has selected.

The final part of the project will be dedicated to the implementation of the HiRISE system on the VLT and the acquisition of science data.
Pre-assembly of the fiber injection module of VLT/HiRISE