“What if the universe’s most intriguing worlds are those we can barely see?” OASIS, the Observing Accelerators with SCExAO Imaging Survey, aims to pair the accuracy of space-based astrometry measurements with the superior imaging capabilities of the Earth-based highcontrast imaging observational facilities. In its first scientific results, this project, using the Subaru Telescope operated by Mauna Kea, discovered two remarkable companions: a super-Jovian planet and a brown dwarf that had been masked by the brightness of the respective star.
The first discovery is HIP 54515 b, which orbits a star in the constellation of Leo roughly 271 light-years from our planet. This exoplanet has a mass of nearly 18 times that of Jupiter and an orbital radius akin to that of Neptune. It can thus be categorized as a planet on the border of gas giants and brown dwarfs. The angle of separation of HIP 54515 and its exoplanet would require detecting a baseball-sized object from a range of 100 kilometers. This can only be achieved by utilizing its Coronographic Extreme Adaptive Optics system.
The second is HIP 71618 B, a brown dwarf companion of approximately 60 Jupiter masses, which orbits an A-type star in the Bootes constellations at an expected distance of approximately 11 astronomical units. Brown dwarfs lie midway between stars and planets and are formed in the same manner as stars but do not have enough mass for hydrogen fusion. HIP 71618 B is of great importance since it satisfies the very strict requirements for the coronagraph technology demonstration of the Nancy Grace Roman Space Telescope, which is intended by the Roman Coronagraph to reach contrasts of 10⁻⁷ to 10⁻⁹ at visible wavelengths and directly image Earth-like planets.
What sets the approach taken in the OASIS project is its focus on locating targets. By examining years’ worth of data from the ESA Hipparcos and Gaia missions, astrophysicists have been able to identify the subtle acceleration of stellar motions through astrometry, indicating the presence of an unseen companion. What allows this detection is the ~0.1 milliarcsecond astrometric precision offered in the third data release of the Gaia mission. Targets that show large renormalized unit weight error (RUWE) residuals can be pinpointed, indicating that these show indicators of photocenter motions induced by an orbital companion. After being located, they can then be observed using the SCExAO system’s extreme AO capabilities, which can resolve the companion even at large separation.
HIP 71618 B’s confirmation required multi-epoch observations with the Subaru SCExAO/CHARIS integral field spectrograph and Keck’s NIRC2 camera. CHARIS enabled simultaneous JHK spectroscopy at low spectral resolution (R~18), which showed signatures characteristic of a low-gravity M6-M8 dwarf at ~2700 K. Forward modeling corrected for the bias induced by the subtraction of the point spread function, but the dynamical mass determination by means of Markov Chain Monte Carlo orbital fits combined both absolute astrometry from the Hipparcos-Gaia Catalog of Accelerations and imaging astrometry. Its dynamical mass of ~60-70 MJup positions the object just below the hydrogen-burning limit, supporting the object’s status as a brown dwarf.
From an engineering standpoint, HIP 71618 B is an unusual case that will allow us to test the coronagraph in real-world conditions. TTR5 of the Roman Coronagraph Threshold Requirements calls for the 5σ detection of a companion at contrast better than 10⁻⁷ at ≤600 nm within the region 6-9 λ/D of a bright star within 10 hours. HIP 71618 B is expected to fall within the dark hole region during the demo mission in 2027, and predicted contrasts should meet or beat the requirements.
There are bright stars in the region that can be considered as potential point spread functions, which are critical to optimizing the coronagraphic starlight suppression. The detection of HIP 54515 b, which is not a coronagraphic test case, serves as a demonstration of the efficacy of the OASIS. The search criteria of the survey, focusing as it does on stars for which there are astrometric accelerations, serve to extract more massive companions than would a blind survey. The method, further, covers the gaps separating the modes. The astrometry, as a method, is best for the detection of wide orbits, regardless of the inclination, while the high contrast sensing retrieves the direct spectra as well as the positions.
Technologically and scientifically, this discovery is a big step forward. This achievement demonstrates how high precision astrometry from space together with extreme adaptive optics imaging can provide a detection of unseen worlds that can’t be achieved through other approaches. For believers and scientists like this author interested in astronomy, HIP 54515 b and HIP 71618 B are more than newly minted discoveries on the list of exoplanets and brown dwarf companions they are a testing bed for the future of space telescopes that promise direct imaging and analyses of Earth-like planets around a G-type star like our Sun.
