The Subaru Telescope's state-of-the-art exoplanet imaging system – the SCExAO adaptive optics module coupled with the CHARIS integral field spectrograph – has seen two full years of Open Use operation. Now, this new system has gained its first discovery and demonstrated a new approach to best selecting stars with imageable planets and other low-mass companions like brown dwarfs (failed stars).
The newly-discovered object, a brown dwarf named HD 33632 Ab, orbits a 1.5 billion year-old near-twin of the Sun star about 86 light-years from the Earth (Figure 1). It joins one of the few known imaged substellar companions orbiting Sun-like stars on solar system-like (Mercury-to-Pluto) scales.
SCExAO/CHARIS data taken in October 2018 and complemented a month later by Keck Observatory data, revealed a detection of this object at a separation of about 20 AU (astronomical unit, the distance between the Sun and the Earth) from its host star. Follow-up, more intensive SCExAO/CHARIS data taken on August 31 and September 1 of this year, during the COVID-19 pandemic, confirmed that HD 33632 Ab exists and is a gravitationally bound companion, not an unrelated background star. The CHARIS spectrum for HD 33632 Ab has a jagged, sawtooth-like shape, indicative of water and carbon monoxide molecules (Figure 2 left).
"Thanks to SCExAO/CHARIS's incredibly sharp images, we can not only see HD 33632 Ab but get ultra-precise measurements for its position and its spectrum, which gives important clues about its atmospheric properties and its dynamics," said Thayne Currie, an affiliated researcher at Subaru, and lead author of this study.
Unlike nearly all other faint directly-imaged companions, HD 33632 Ab has a directly determined mass instead of a mass inferred from uncertain models predicting a planet/brown dwarf's mass based on its brightness at a given age. All planets or brown dwarfs orbiting their host stars cause the star to accelerate towards it due to the force of gravity. The ultra-sensitive Gaia astrometry satellite and its predecessor astrometry mission (Hipparcos) revealed that the star around which HD 33632 Ab orbits (HD 33632 A) shows an acceleration hinting at the presence of some companion, which SCExAO/CHARIS has now imaged.
"This is the first time we have found a brown dwarf by looking around a star that is being tugged across the sky. Finding a brown dwarf always involves luck, but this time we were able to stack the odds," adds Timothy Brandt, assistant professor of physics at the University of California-Santa Barbara, coauthor, and expert on Gaia/Hipparcos astrometry data.
Modeling the Gaia/Hipparcos absolute astrometry for the star and astrometry for HD 33632 Ab from Keck and Subaru telescope data together provided a precise dynamical mass for the companion of ~46 Jupiter masses (Figure2 right). This mass is significantly higher than the limit usually thought to separate planets from brown dwarfs (13-14 Jupiter masses), although the object also has a low, more planet-like eccentricity.
HD 33632 Ab could be key reference point for understanding the atmospheres of the first imaged and best studied extrasolar planets, which orbit a star called HR 8799 and were discovered from Maunakea in 2008 and 2010. The HD 33632 system is much older than the youthful HR 8799 (40 million years old). While HD 33632 Ab is more massive than the HR 8799 planets and has a higher surface gravity, it likely has a temperature very similar to these planets. Furthermore, we have a direct mass measurement for HD 33632 Ab and also good constraints on the mass for the HR 8799 planets through other analyses. Thus, HD 33632 Ab and the HR 8799 planets together may provide a critical insight into how substellar atmospheres (planets and brown dwarfs) at a given temperature differ at a range of ages and gravities. Their mass measurements then allow us to directly link these observational differences to bulk properties, i.e., masses.
"The atmospheres of planets like HR 8799's are notoriously hard to understand and likely have very peculiar properties like thick clouds, which have proven hard to model. Having a good reference point like our SCExAO-discovered companion is crucial to understanding this and other objects much better," said Currie.
Finally, this program shows the power of approach to identifying stars that likely host imageable planets and brown dwarfs. Most direct imaging searches are 'blind' searches, targeting some subset of stars within some age range or within a common star forming region. Imaging surveys conducted with predecessor instruments like the Gemini Planet Imager on Gemini South telescope in Chile and SPHERE on the Very Large Telescope also in Chile show that the detection rate of companions with these blind surveys is very low (a few percent). The research team is carrying out a different kind of search. Specifically, they are focusing on stars, drawn from a carefully selected sample made by coauthor Timothy Brandt, that show an acceleration seen in Gaia data. This acceleration is indirect evidence that there is a massive orbiting companion that is tugging on the star. HD 33632 Ab's detection represents a proof-in-concept of this approach. While this survey has just started with SCExAO, the team already has identified multiple new candidate companions, with a detection rate significantly higher than from a blind approach.
"These observations could greatly expand the discoveries by the previous successful SEEDS survey with AO188 and HiCIAO. The SCExAO and CHARIS combination will keep the Subaru telescope at the forefront of the direct imaging of exoplanets and brown dwarfs," said Masayuki Kuzuhara and Motohide Tamura from the Astrobiology Center at the National Institutes of Natural Sciences.
This research was published in the Astrophysical Journal Letters on November 30, 2020 (Currie et al. "SCExAO/CHARIS Direct Imaging Discovery of a 20 au Separation, Low-Mass Ratio Brown Dwarf Companion to an Accelerating Sun-like Star".)