Looking for Binary Companions with Doppler Measurements of Directly Imaged Exoplanets

Context. As of today, more than 5500 exoplanets have been discovered, and yet no satellite has been confirmed around any of them. Satellites, exomoons or binary companions, are objects that orbit around an exoplanet or brown dwarf. These objects are important to define better the possible formation scenarios of the whole system and its dynamical evolution. Aims. With this project, I will select a group of exoplanets and brown dwarfs detected with a specific technique, called direct imaging, and with conditions suitable for the detection of satellites through the radial velocity technique. Methods. I first selected the sub-group of self-luminous exoplanets and brown dwarfs, detected with the direct imaging technique, as provided by the NASA and European archives. Then, I restricted the sample to those targets that were suitable for radial velocity measurements. This is a cutting-edge technique since I am trying to retrieve radial velocities from the planetary spectrum, not from the stellar one. I thus estimated the radial velocity semi-amplitude of each object in the catalogue to compute detection limits for satellites with the state-of-the-art spectrographs that are available nowadays, the near-infrared high-resolution spectrograph CRIRES+ and its link to the extreme adaptive optics system of SPHERE (HiRISE) at VLT. In this radial velocity precision study, the target’s main characteristics, such as mass, angular separation, effective temperature and its contrast with the host star, were taken into account, along with the properties of the assigned instrument, either CRIRES+ or HiRISE. Moreover, I calculated the fraction of stellar contamination that perturbed the target spectrum. Results. I selected a total of 5 targets, HR 3549 B, HIP 64892 B, and HD 984 B for CRIRES+ and β Pic b and HD 1160 B for HiRISE, with the most promising conditions for the detection of satellites with future observations. Among them, HR 3549 B has the lowest detection limits, pushing down to ∼40 m/s. This translates to a minimum detectable mass for the satellite of 0.06MJ at a separation of 0.035 AU from the host planet. Increasingly more massive satellites might be revealed on wider orbits around the planet. Moreover, 3 additional objects, η Tel B, HIP 78530 B and HD 284149 b are also very promising considering treatments of the CRIRES+ PSF shape at large separations (> 1”), which in this work resulted in an overestimate of contamination of the spectra by the central star. Conclusions. This project is an important baseline for proposals submission and observations aiming to detect satellites around directly imaged exoplanets and brown dwarfs through measurements of the Doppler shifts of substellar spectral lines. Moreover, with the upcoming next-generation instruments such as the ANDES spectrograph at ELT, this kind of study will be even more relevant because it provides the basis for the systematic analysis and possible detection of a wide group of satellites.