Figure 1: Image of the brown dwarf HIP 21152 B, discovered as the
companion of the star HIP 21152. The star mark and arrow indicate the
positions of the host star and HIP 21152 B, respectively. The host star
is masked in the image. HIP 21152 is a young Sun-like star, about 750
million years old, and belongs to the Hyades Cluster, one of the nearest
open clusters, located 160 light-years away in the direction of the
constellation Taurus. As a group of young stars born at almost the same
time, the Hyades Cluster is an important research target for studying
the evolution of stars and planets, and has attracted the attention of
many astronomers. HIP 21152 B is the first confirmed example of a
directly-imaged brown dwarf companion in the Hyades cluster. Click here to see a movie of three imaging observations taken from October 2020 to October 2021. Credit: Astrobiology Center
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 a benchmark object for the study of the evolution of giant planets and brown dwarfs and their atmospheres.
Brown dwarfs (Note 1) are an interesting type of objects that is intermediate between a star and a planet in terms of mass and not found in our Solar System. They are also useful for studying the evolution and the atmosphere of giant planets, 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 tried to establish an efficient way to find companion brown dwarfs.
An international team including astronomers from the Astrobiology Center; the National Astronomical Observatory of Japan; Tokyo Institute of Technology; 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 acceleration caused by 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. The calculation of the difference between the measurements from the two satellites now allows for deriving 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.
Figure 2: (Left) Schematic of the acceleration of proper motion. If a companion is present around a star, it’s gravity accelerates the proper motion of the star, causing a difference in the proper motion measurements between the Hipparcos and Gaia satellites. (Right) Orbit modeling of HIP 21152 B. The open circles and blue circles indicate the predicted and observed positions of HIP 21152 B in the numbered years, respectively. The thick black oval shows the best-fit orbit. Other thin ovals represent other possible orbits, which are color-coded by the derived mass of HIP 21152 B. A magnified view of the area around the observed locations is shown in the lower left. Credit: Astrobiology Center
The team determined the orbit of HIP 21152 B using a combination of a total of four direct imaging observations by the Subaru Telescope and Keck Telescope, line-of-sight velocities of the host star measured 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 help characterize 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 crucial 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."
Figure 3: Spectrum of HIP 21152 B obtained with SCExAO and CHARIS on the Subaru Telescope (blue line). Wavelengths where absorption by water vapor and methane occur are indicated by the horizontal lines above (Note 5). The absorption by those molecules in the atmosphere of HIP 21152 B produces concavities in the spectrum. Credit: Astrobiology Center
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).
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 a benchmark object for the study of the evolution of giant planets and brown dwarfs and their atmospheres.
Brown dwarfs (Note 1) are an interesting type of objects that is intermediate between a star and a planet in terms of mass and not found in our Solar System. They are also useful for studying the evolution and the atmosphere of giant planets, 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 tried to establish an efficient way to find companion brown dwarfs.
An international team including astronomers from the Astrobiology Center; the National Astronomical Observatory of Japan; Tokyo Institute of Technology; 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 acceleration caused by 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. The calculation of the difference between the measurements from the two satellites now allows for deriving 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.
Figure 2: (Left) Schematic of the acceleration of proper motion. If a companion is present around a star, it’s gravity accelerates the proper motion of the star, causing a difference in the proper motion measurements between the Hipparcos and Gaia satellites. (Right) Orbit modeling of HIP 21152 B. The open circles and blue circles indicate the predicted and observed positions of HIP 21152 B in the numbered years, respectively. The thick black oval shows the best-fit orbit. Other thin ovals represent other possible orbits, which are color-coded by the derived mass of HIP 21152 B. A magnified view of the area around the observed locations is shown in the lower left. Credit: Astrobiology Center
The team determined the orbit of HIP 21152 B using a combination of a total of four direct imaging observations by the Subaru Telescope and Keck Telescope, line-of-sight velocities of the host star measured 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 help characterize 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 crucial 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."
Figure 3: Spectrum of HIP 21152 B obtained with SCExAO and CHARIS on the Subaru Telescope (blue line). Wavelengths where absorption by water vapor and methane occur are indicated by the horizontal lines above (Note 5). The absorption by those molecules in the atmosphere of HIP 21152 B produces concavities in the spectrum. Credit: Astrobiology Center
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).
Notes:
(Note 1) There are several ways to define a "brown dwarf" but the most generally accepted is objects with masses as high as 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 measures distances and proper motions of about one billion astronomical objects with unprecedented precision.
(Note 3) So far, the main method used to estimate the mass of brown dwarfs has been the "evolutionary models," which 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 result in an inaccurate estimation of mass due to uncertainties in the evolutionary model and the age (usually, a brown dwarf is assumed to be as young as its host star or the associated cluster). HIP 21152 B belongs to the Hyades cluster, so its age is accurately determined, but the evolutionary model remains uncertain. The evolutionary model inferred mass of HIP 21152 B is 1.3 times larger than the mass determined from the orbital analysis.
(Note 4) A European team independently imaged 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. Very recently, a U.S. team also reported the independent detection of HIP 21152 B.
(Note 5) The absorption wavelengths of the molecules are displayed based on the web tool provided by the University of Geneva.
(Note 2) Gaia is a space telescope launched in 2013 for high-precision astrometry. It measures distances and proper motions of about one billion astronomical objects with unprecedented precision.
(Note 3) So far, the main method used to estimate the mass of brown dwarfs has been the "evolutionary models," which 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 result in an inaccurate estimation of mass due to uncertainties in the evolutionary model and the age (usually, a brown dwarf is assumed to be as young as its host star or the associated cluster). HIP 21152 B belongs to the Hyades cluster, so its age is accurately determined, but the evolutionary model remains uncertain. The evolutionary model inferred mass of HIP 21152 B is 1.3 times larger than the mass determined from the orbital analysis.
(Note 4) A European team independently imaged 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. Very recently, a U.S. team also reported the independent detection of HIP 21152 B.
(Note 5) The absorption wavelengths of the molecules are displayed based on the web tool provided by the University of Geneva.
Relevant Links
- Astrobiology Center January 24, 2023 Press Release
- W. M. Keck Observatory January 23, 2023 Press Release
Source: Subaru Telescope
