A brown dwarf orbiting the Sun-like star HIP 21152 was discovered using the Subaru Telescope’s Extreme Adaptive Optics System. HIP 21152 B was found to be the lightest brown dwarf with an accurately determined mass, approaching the mass of a giant planet. HIP 21152 B is expected to be an important benchmark object for studying the evolution of giant planets and brown dwarfs and their atmospheres.
Brown dwarfs (Note 1) are an interesting type of object not found in our Solar System, with masses somewhere between those of stars and planets. Brown dwarfs are also important for studying the evolution of giant planets and their atmospheres, because Jupiter-like planets and lighter brown dwarfs are expected to have similar characteristics.
Brown dwarfs drift alone in space or orbit around stars. While thousands of brown dwarfs have been found since the first discovery in 1995, companion-type brown dwarfs are rare, with a frequency of only a few per 100 stars. For this reason, astronomers have been racking their brains for an efficient way to find companion brown dwarfs.
An international team including astronomers from the Astrobiology Center; the National Astronomical Observatory of Japan; the University of California, Santa Barbara; and NASA has developed a new method to efficiently discover companion brown dwarfs and giant planets. Furthermore, they applied that method to imaging surveys with the Subaru Telescope. This search adopts information on the “proper motion” of stars in our Galaxy, which is the motion of stars with their own unique velocities. When a companion object orbits a star, the proper motion of the host star is accelerated by the gravity from the companion. However, the velocity change caused by a light companion such as a brown dwarf or planet is very small, making it challenging to measure the change precisely.
However, a turning point came with ESA’s astrometry satellite Gaia (Note 2), the successor to the Hipparcos satellite. By measuring the difference between the measurements from the two satellites, it is now possible to derive minute accelerations in proper motion (Figure 2 left). Using data from both telescopes, the research team analyzed the acceleration of proper motion for stars near the Sun, and selected stars that may be accompanied by giant planets or brown dwarfs. They then proceeded with direct imaging observations using Subaru Telescope’s high contrast instruments, SCExAO and CHARIS, leading to the discovery of a brown dwarf “HIP 21152 B” orbiting the star HIP 21152.
The team determined the orbit of HIP 21152 B by combining a total of four direct imaging observations by the Subaru Telescope and Keck Telescope, line-of-sight velocity observations of the star HIP 21152 by HIDES on the Okayama 188-cm Reflector Telescope, and the proper motion data from Gaia and Hipparcos. The companion’s mass is derived from the orbit, as indicated by Kepler’s law. The actual orbital analysis (Figure 2, right) determined the mass of HIP 21152 B to be 22–36 Jupiter masses. Brown dwarfs with such accurately determined masses are rare (Note 3). HIP 21152 B was also found to be the lightest brown dwarf among those with accurately determined masses, approaching planetary masses (Note 4).
HIP 21152 B will be an important source for characterizing the atmospheres of brown dwarfs and giant planets. The team also obtained the spectrum of HIP 21152 B (Figure 3), showing that its atmospheric characteristics can be classified as being in the transition stage between two brown dwarf spectral types, L-type and T-type. Strong absorption from methane is shown in the atmosphere of a T-type brown dwarf, while an L-type brown dwarf shows little of it in the atmosphere. This spectral transition is strongly related to atmospheric temperature and the presence of clouds. Interestingly, the well-known directly-imaged planets around HR 8799 show a similar spectrum. In this respect, it is again important that the most fundamental characteristics of HIP 21152 B, namely its mass and age, are accurately determined. Masayuki Kuzuhara, a project assistant professor at the Astrobiology Center, who led the research, says, “This result can provide an important clue to understand the atmospheres of giant planets and brown dwarfs based on how and when they show atmospheric characteristics similar to those seen in the planets of the HR 8799 system and HIP 21152 B. It is expected that HIP 21152 B will play an important role as a benchmark for future progress in astronomy and planetary science.”
As this observation project is still ongoing, even more discoveries are expected. The Subaru Telescope’s direct imaging instruments continue to be improved, making new observational capabilities ready for science operation. With the progress in the efficient exploration and the development and improvement of Subaru Telescope’s instruments, various important discoveries will continue to be made in the future.
These results were published in the Astrophysical Journal Letters on July 27, 2022 (Kuzuhara et al., “Direct-imaging Discovery and Dynamical Mass of a Substellar Companion Orbiting an Accelerating Hyades Sun-like Star with SCExAO/CHARIS“.) It was also featured in AAS Nova, which highlights outstanding research in the AAS journals (Featured Image: First Images of a Substellar Companion in the Hyades).
(Note 1) There are several definitions of brown dwarfs, but in general, brown dwarfs are considered to be objects with masses between 13 and 80 times that of Jupiter. Objects with such masses do not fuse hydrogen (unlike stars) but do fuse deuterium (unlike planets). In contrast, heavy planets and light brown dwarfs are very similar, and it is thought that there is no need to distinguish between them except for their mass.
(Note 2) Gaia is a space telescope launched in 2013 for high-precision astrometry. It provides unprecedented positional and radial velocity measurements for about one billion astronomical objects.
(Note 3) So far, the main method used to estimate the mass of brown dwarfs has been the “evolutionary model.” Evolutionary models predict the luminosity and temperature of a brown dwarf as it ages. Then the observed luminosity and temperature are used to determine the mass of the brown dwarf using these models. However, this method could yield an inaccurate mass due to uncertainties in the evolutionary model and the age (generally, the age of the brown dwarf is assumed to be equal to that of the host star or the cluster). HIP 21152 B belongs to the Hyades cluster, so its age is accurately determined, but the evolutionary model remains uncertain. The mass of HIP 21152 B estimated from the evolutionary model is 1.3 times larger than the mass determined from the orbital analysis.
(Note 4) A European research team independently succeeded in imaging HIP 21152 B (myScience article). Meanwhile, the study led by Kuzuhara is the first to prove that HIP 21152 B orbits its host star and to derive its dynamical mass.
(Note 5) A web tool provided by the University of Geneva is used as a reference for displaying the absorption wavelengths of the molecules.
Subaru telescope, Jan. 23, 2023 Press Release
W. M. Keck Observatory January 23, 2023 Press Release