High-resolution observations shed more light on a peculiar brown dwarf system

Using the Keck Planet Imager and Characterizer (KPIC), astronomers have performed high-resolution spectroscopic observations of a brown dwarf known as HD 206893 B. Results of the observational campaign, presented Jan. 23 on the arXiv pre-print server, provide more insights into the nature and properties of this object.

Brown dwarfs (BDs) are intermediate objects between planets and stars, occupying the mass range between 13 and 80 Jupiter masses (0.012 and 0.076 solar masses). Although many brown dwarfs have been detected to date, these objects orbiting other stars are a rare find.

HD 206893 B is a substellar companion to HD 206893—a star of spectral type F5V located some 133 light years away. The star is about 40% larger and 24% more massive than the sun, has an effective temperature of 6,680 K, near-solar metallicity and is estimated to be 1.1 billion years old.

HD 206893 is orbited by two exoplanets and also hosts a circumstellar debris disk extending from about 30 to 180 AU from the star. HD 206893 B, estimated to be 26 times more massive than Jupiter, is located inside the debris disk. Previous observations have found that the gravity of HD 206893 B and the self-gravity of a massive debris disk could have had an influence on shaping the inner edge of the debris disk.

A team of astronomers led by Ben Sappey of the University of California, San Diego (UCSD), decided to take a closer look at HD 206893 B and the peculiar architecture of the whole system. For this purpose, they observed this brown dwarf with KPIC at high spectral resolution.

“Using the extracted spectra within a forward-modeled, Bayesian framework, we infer the most likely bulk atmospheric parameters and radial velocity of the companion,” the researchers wrote in the paper.

The observations found that HD 206893 B has a radius of about 1.11 Jupiter radii and its mass is approximately 22.7 Jupiter masses. The effective temperature of the brown dwarf was measured to be 1,634 K, while its age was estimated to be 112 million years.

Based on the collected data, the atmospheric carbon-to-oxygen (C/O) ratio for HD 206893 B was calculated to be 0.57, therefore near the solar value. In general, the C/O ratio is perceived as a potential diagnostic of a system formation and in the case of HD 206893 B it suggests a core accretion or a disk fragmentation scenario.

The study found that HD 206893 B is separated from the host star by approximately 11.62 AU. This, according to the authors of the paper, suggests that the brown dwarf was likely not formed via disk fragmentation, which is expected to form planets at larger orbital distances of about 100 AU.

In concluding remarks, the researchers note that HD 206893 B should be further investigated, with tools like the Near Infrared Spectrograph (NIRSpec) onboard the James Webb Space Telescope (JWST). This instrument would allow them to obtain a carbon-to-sulfur (C/S) ratio, which is a more reliable indicator of formation location than C/O.

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