HiRISE

HiRISE is funded by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement n°757561).

Context and goals

Atmospheric composition provides essential markers of the most fundamental properties of giant exoplanets, such as their formation mechanism or internal structure. New-generation exoplanet imagers, like VLT/SPHERE or Gemini/GPI, have been designed to achieve very high contrast (>10 mag) at small angular separations (<0.5”) for the detection of young giant planets in the near-infrared, but they only provide very low spectral resolutions (R<100) for their characterization.

High-dispersion spectroscopy at resolutions up to 10⁵ is one of the most promising pathways for the detailed characterization of exoplanets, but it is currently out of reach for most directly imaged exoplanets. The power of high-dispersion spectroscopy lies in the ability to disentangle the stellar and planetary signals using the distinct radial velocity component of the planet that originates from its orbital motion. However, this differential radial velocity can only be measured by overcoming the large contrast ratio between the star and the planet. Self-luminous, young giant planets potentially constitute ideal targets because of their intrinsic brightness in the near-infrared, but current high-dispersion spectrographs in the near-infrared lack coronagraphs to attenuate the stellar signal and the spatial resolution necessary to resolve the planet.

Providing very high spectral resolution to high-contrast imaging instruments is crucial if we want to achieve a major step forward in our understanding of the formation, evolution and composition of directly imaged exoplanets (Snellen et al. 2015; Wang et al. 2017; Mawet et al. 2017). Project HiRISE (High-Resolution Imaging and Spectroscopy of Exoplanets) ambitions to develop a novel demonstrator that will combine the capabilities of two flagship instruments installed on the ESO Very Large Telescope, the high-contrast exoplanet imager SPHERE and the high-resolution spectrograph CRIRES+, with the goal of answering fundamental questions on the formation, composition and evolution of young planets.

Implementation

HiRISE will implement a novel fiber coupling between SPHERE and CRIRES+ to leverage the performance capabilities of both instruments. A self-contained fiber injection module (FIM) will be installed in SPHERE downstream of the coronagraph to pick-up the planetary signal and inject it to CRIRES+ through a fiber relay. Wavefront control using the ZELDA wavefront sensor installed in SPHERE will be used to optimize the injection at the entrance of the fiber and maximize the planetary signal transmission. In CRIRES+, a fiber extraction module (FEM) will be implemented in the calibration stage already available in the warm part at the entrance of the instrument.

As of early 2021, we have finalized the opto-mechanical design of the SPHERE fiber injection module (see image below), the general layout of the fiber bundle around the telescope, and the optical design of the fiber extraction module for CRIRES+. In order to do a first validation step, the system will first be implemented and tested using telecom fibers working in the H-band, up to 1.8 μm. This will enable a faster, cheaper and less risky implementation at the telescope. The fiber bundle and the FEM will be designed for the H-band, but an optimization for the K-band will be considered in the context of the SPHERE+ upgrade.

In parallel of the opto-mechanical design we have been running a set of detailed end-to-end simulations to estimate the final performance of the system (Otten et al. 2021). The simulations show that HiRISE will provide a gain of several magnitudes compared to CRIRES+ in standalone mode for the characterization of known companions in the H band.

This plot shows the detection limits expected for CRIRES+ in standalone and HiRISE for a β Pictoris-like star and 1200K companions in 2 hours of exposure time. The SNR=5 limit for HiRISE is up to 2 magnitudes lower for HiRISE at 400 mas. This gain opens the possibility to characterize fainter companions with HiRISE, for example future objects detected by the ESA/Gaia mission (Perryman et al. 2014).

Finally, we have imlemented an HiRISE-like system on the Marseille Imaging Testbed for High-Contrast (MITHiC; Pourcelot et al. 2021) to test fiber centering and injection strategies for HiRISE. This work is the current focus of the PhD project of Mona El Morsy, who started working on HiRISE in October 2019 and is continuing the work started by a former apprentice student, Raphaël Pourcelot. One of the goals is to test two different strategies for the centering of the planet PSF on the science fiber. The first will be to use feedback fibers, which will surround the science fiber and retro-inject a signal in the system that will be measured on top of the science image with the tracking camera. The second strategy will use a dedicated centering fiber and sattelite spots artificially introduced by the deformable mirror of the system. For this study we are using a dedicated fiber bundle manufactured by Thorlabs with a hexagonal fiber packing and a tip-tilt mirror from Physik Instrumente. The image on the right shows an "injection map" that represents the flux coupled into the science fiber and measured at the output using a flux meter. The Airy pattern of the PSF is easily recognizable. The slight anamorphism is due to the 45° angle of incidence of the beam on the tip-tilt mirror.

Science case

The atmospheres of giant gaseous exoplanets bear important markers of their formation mechanism (Öberg et al. 2011), their internal structure (Madhusudhan et al. 2011) and the chemical and dynamical processes like winds or clouds (Knutson et al. 2007; Moses et al. 2011). Spectroscopic measurements of these atmospheres at high-spectral resolution 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?

The main opportunity enabled by resolutions of several tens of thousands is the detection of the molecular lines of water (H₂O), carbon monoxide (CO), methane (CH₄), and potentially ammonia (NH₃). Access to a direct measurement of spectral lines would enable species-by-species atmospheric characterization, which is of paramount importance to constraint abundances of individual elements. The access to high-spectra resolution also opens the door to radial velocity and rotational velocity measurements directly on the companions.

	Formation & migration
	The core accretion (Pollack et al. 1996) and disk instability (Boss 1998) formation scenarios predict different abundances for C and O, which are related to the processes involved in the formation and to the location of the planet in the protoplanetary disk (Öberg et al. 2011; Piso et al. 2015). Being able to compare the C/O ratio of young giant planets to that of their host star would therefore provide a unique insight into their formation processes.
	Atmospheric chemistry & dynamics
	The detailed composition determined at very high-spectral resolution will provide an insight into the chemistry on-going in young exoplanets atmospheres. Time-resolved measurements of bright companions over several hours will be used to spatially map their photosphere with the Doppler imaging technique (Vogt et al. 1987). This will enable variability and cloud coverage studies on young companions for the first time.
	Mass & internal structure
	At resolutions of 10⁵, the orbital velocity of the planets on their orbit will be determined directly and will enable constructing a full keplerian model of the planetary systems when combined with relative astrometry obtained from imaging. This will lead to the dynamical determination of the planets masses, which are crucially needed to calibrate the sub-stellar evolutionary models (Burrows et al. 1997; Baraffe et al. 2015).

In complement to the scientific preparation of HiRISE, my PhD student Mathis Houllé is investigating the use of medium and high spectral resolution data in different contexts. The first one is the detection of young giant planets with the high-contrast module of ELT/HARMONI. With a dedicated apodizer and NCPA wavefront sensor, the high-contrast module will offer interesting opportunities for exoplanet detection and characterization. Combined with the IFU at resolutions from R=3 500 to R=17 000, HARMONI will enable to use molecular mapping techniques to search for companions in the data. We are now investigating the detection performance of molecular mapping techniques compared to more standard angular differential imaging techniques (Houllé et al. in prep.). We are also looking into the use of very high resolution spectrographs in the visible, like VLT/ESPRESSO, to study known companions.

HiRISE team

LAM or affiliated: Gilles Otten (ERC postdoc: simulations, design), Eduard Muslimov (optical design), Yannick Charles (ERC engineer: mechanical design), Maxime Lopez (ERC engineer: AIT), Mathis Houllé (ERC PhD student), Mona El Morsy (ERC PhD student), Nicolas Tchoubaklian (ERC engineer: mechanical design), Kjetil Dohlen (system engineering, optical design), Anne-Costille (SPHERE expertise), Elodie Choquet (data analysis), Jean-François Sauvage (SAXO expertise), Jean-Luc Beuzit (SPHERE expertise)

University of Göttingen: Ansgar Reiners, Ulf Seemann, Heiko Anwand

ESO: Reinhold Dorn, Gérard Zins, Jérôme Paufique, Markus Kasper, Pedro Figueira

Laboratoire Lagrange: Mamadou N'Diaye, Raphaël Pourcelot, David Mary

IPAG: Alexis Carlotti, David Mouillet

University of Exeter: Isabelle Baraffe, Mark Phillips

LESIA: Anthony Boccaletti, Benjamin Charnay